Isolated Gate Driver Matlab

Isolated Attosecond Pulses

S. Haessler1, 2, T. Balčiūnas2, G. Fan2, L. E. Chipperfield3 & A. Baltuška2 Isolated attosecond pulses (IAP) generated by high-order harmonic generation are valuable tools that enable dynamics to be studied on the attosecond time scale. The applicability of these IAP would be widened drastically by increasing their energy. Here we analyze the potential of using multi- colour driving pulses for temporally gating the attosecond pulse generation process. We devise how this approach can enable the generation of IAP with the available high-energy kHz-repetition-rate Ytterbium-based laser amplifiers (delivering 180-fs, 1030-nm pulses). We show theoretically that this requires a three-colour field composed of the fundamental and its second harmonic as well as a lower-frequency auxiliary component. We present pulse characterization measurements of such auxiliary pulses generated directly by white-light seeded OPA with the required significantly shorter pulse duration than that of the fundamental. This, combined with our recent experimental results on three-colour waveform synthesis, proves that the theoretically considered multi-colour drivers for IAP generation can be realized with existing high-power laser technology. The high-energy driver pulses, combined with the strongly enhanced single-atom-level conversion efficiency we observe in our calculations, thus make multi-colour drivers prime candidates for the development of unprecedented high-energy IAP sources in the near future.

More than a decade ago, the first observations of isolated attosecond light pulses1 (IAP) or attosecond pulse trains (APT)1,2 launched the field of “attosecond physics”, for which these optical tools became the technological base. The high-order harmonic generation (HHG) process in atoms, at the origin of these attosecond light pulses, can be described by a semi-classical three-step model3,4, where an electron is first released by tunnel ionization, then accelerated by the strong laser field and finally driven back to the core.

The recombination leads to the emission of an ultrashort burst of XUV light. In a multi-cycle laser pulse, this process is repeated coherently every half cycle, leading to the emission of an APT. In order to isolate a single attosecond pulse and thus greatly facilitate the implementation of tradi- tional pump-probe schemes for time-resolving processes on the sub-femtosecond time scale, it is nec- essary to temporally gate the HHG process and limit it to a single laser half-cycle5,6 For example, using carrier-envelope-phase (CEP) controlled near-single-cycle driver pulses 4 fs for an 800-nm carrier wave- length), the highest XUV photon energies are only produced by a single driver half-cycle. Spectrally selecting this cutoff region then selects an IAP1,7 Slightly longer driving pulse envelopes and a wider selectable XUV spectral range can be used with polarization-shaping, such that the driver pulses have elliptically polarized wings and a linearly polarized center. This allows recollision of the laser-driven electron trajectories only in a single-half cycle in the pulse center8,9 Using driver pulses with very high intensity leads to a rapidly rising degree of medium ionization already during the rising edge of the pulse.

With few-cycle envelope duration, this may leave a narrow time-window of about one driver half-cycle 1Laboratoire d’Optique Appliqueé, ENSTA-Paristech, Ecole Polytechnique, CNRS, Université Paris-Saclay 91761 Vienna, Austria. 3The Blackett Laboratory, Imperial College London, South Kensington Campus, London SW7

2

during which the efficient XUV generation remains possible (through a combination of efficient tunnel ionization launching electron trajectories, transient phase-matching with a tolerable free-electron den- sity, and depletion of the ground state)10,11 Finally, a sufficiently fast wavefront rotation of the driver pulses leads to the individual attosecond pulses of an APT to be emitted at varying angles, such that an IAP may be selected by a spatial mask in the far field12,13.

All these techniques require extremely short  2-cycle driver pulse durations. Due to gain-narrowing, these cannot be achieved directly from laser amplifiers, so an additional post-compression stage after the amplifier is required, where the laser spectrum broadens by self-phase modulation during the propaga- tion through a gas-filled hollow-core fiber and is subsequently compressed with chirped mirrors14,15 This typically limits the driver pulse energy to a few mJ16 due to the onset of ionisation. Even with the best optimization effort, the attainable IAP energy is thus limited to the nJ- level simply through the limited driver pulse energy.

While very advanced OPCPA schemes with careful phase control do permit the generation of a sub- 3-cycle pulses with very high pulse energy17 or average power18, this complex technology remains rather inefficient.

The resulting low power/pulse energy of current IAP sources hampers a much wider applicability of current attosecond science methods to lower sample densities (like molecules, clusters or nanoparticles that have to be evaporated or ablated to be brought into the gas phase, or excited species that only exist as a small fraction of the target medium) or to processes with low cross-sections. It also makes it very hard to reach a non-linear interaction regime for the XUV attosecond pulses19,20.

The gating technique known as multi-colour gating has lead to a breakthrough by enabling IAP gen- eration with ≈10-cycle drivers. It is based on the coherent mixing of two or more laser pulses with dif- ferent carrier wavelengths (and usually parallel linear polarization directions). Already in 2006, it was demonstrated that the broken symmetry of a two-colour field composed of a fundamental with its second harmonic doubles the spacing of attosecond pulses from one half-cycle to one full laser cycle (2.7 fs in the case of an 800-nm fundamental), which correspondingly allows generating IAP with twice as long (i.e. ≈4-cycle) driver pulses21,22. The usable driver duration was further increased to ≈10-cycles by either adding polarization shaping to this scheme to perform “double optical gating”23, or by detuning the second harmonic24, which further increases the periodicity of the two-colour field.

While such “incommensurate” two-colour combinations turned out to be barely realizable by optical harmonic generation with its very limited tunability, optical parametric amplification (OPA), creating a frequency down-converted additional colour, lends itself ideally to this task. The price to pay is the fact that the phase-delay between the fundamental and the auxiliary colour component is not passively stable, as in optical harmonic generation. Using a white-light-seeded OPA25 together with an actively CEP-locked fundamental (pump) laser does however yield a stable phase delay between the pump and the signal/ idler wave and thus stable multi-colour waveforms26. Without CEP-locking, the phase delay fluctuates and the gating efficiency thus varies shot-to-shot27,28 Conditions can however be found, notably including sufficiently short pulse envelopes, which yield an IAP for a very wide CEP-range20,29 thus relaxing the CEP-locking requirement, albeit for the price of strong shot-to-shot fluctuation of the IAP intensity. The latter method yielded record-high µJ-level energy per IAP, constituting a 100-times enhancement com- pared to previous achievements11 resulting mainly from the fact that the longer driver pulses can be taken directly from laser amplifiers or OPA. Note that multi-colour gating is much more energy-efficient than polarization- or ionization-based gating techniques. The latter “waste” a lot of field-energy in parts of the pulse where HHG is shut off by elliptical polarization or excessive ionization, whereas the multi-colour beating automatically concentrates the available field energy in a few very strong field oscillations which then efficiently drive the desired HHG process. There are even indications that undesired ionization might be reduced29, which would allow reaching higher saturation intensities and thus higher HHG efficiency and cutoff.

Here, we are going to expand on the temporal gating aspect of our recent work on optimizing HHG at the single-atom-level through shaped multi-colour driver waveforms26. We derive a clear understanding from a simple time-domain picture and then demonstrate, using single-atom-level numerical simulations with the Lewenstein model of HHG30, how the multi-colour gating technique combined with (rough) CEP-control so that stable multi-colour waveforms are realized enables the generation of IAP based on 180-fs long pulses typically supplied by Yb-based laser amplifier technology. The extremely efficient temporal gating is accompanied by strongly enhanced IAP intensities compared to a single-colour driver, achieved through advantageous shaping of the central driving waveform cycle as experimentally demon- strated in our recent work26. While the single-atom-level calculations we discuss here cannot quantita- tively simulate an experimental HHG source, recent experimental work demonstrated that the qualitative predictions of such a simplified theoretical treatment are indeed realized in the lab29,26. Our results thus suggest that high-energy/high-flux sources of isolated attosecond pulses based on Ytterbium-laser tech- nology without a need for pulse-post-compression are within immediate reach. Finally, we discuss the experimental implementation.

Theoretical Analysis

Temporal gating by two-colour “few-cycle-pulse trains”.

A Formal Dynamical Symmetry Analy-

sis31 shows that incommensurate two-colour combintations, i.e. of fields with frequencies ω0

Η

.

0 5

1, leads in the spectral domain to a densification of high-harmonic peaks—independently of the colour-components’ phase delay. The derived spectral selection rules have been experimentally verified32 and the potential for temporal gating of HHG has been underlined.

While this potential has already been examined in a number of (parametric) numerical studies pub- lished during the last years33–35 we will here, in contrast, base our reasoning on an intuitive time-domain picture, inspired by a brief mention in Ref. 31. We find this picture to give a very accessible under- standing of the temporal gating effect and its dependence of the colour components’ phase delay, which enables us to devise and understand very easily the optimal colour combinations for generating an IAP over a broad CEP-range.

Coherently combining two laser waves with equal strength and frequencies ω0 and ω1 leads to a beat-

1

i.e. the factorization into a slowly oscillating “envelope” term and a faster oscillating “carrier” term. We

Η

.

<

<

.

0 9 Since These Produce Waveforms That Can Be

considered as a train of few-cycle pulses with carrier frequency ω

0

1 .

1

i.e. twice the number of carrier-cycles per few-cycle sub-pulse. For integer N, the few cycle pulses repeat with constant CEP when N is odd (i.e. the waveform is strictly π

1 -Periodic, See Fig. 1B), Or

with a π CEP flip between successive pulses when N is even (see Fig. 1c). For non-integer N, there is a corresponding fraction-of-π CEP slip between successive few-cycle pulses (see Fig. 1a). For targeting a

1

1

will be useful. Since we consider the ω1-auxiliary-component to be generated from a white-light-seeded OPA

0

1 , is CEP-dependent. The “envelope” term in equation

T

0, and the “carrier” term has a CEP of ϕCEP. This means that the phase delay variation caused by a drift of ϕCEP can also be thought of as a CEP-drift of the few-cycle sub-pulses.

Figure  1b shows an illustrative example of two-colour pulses composed of 1030 nm and 1545 nm,

N

5. We obtain a train of near-single-cycle sub-pulses with 1236 nm carrier wavelength (corresponding to a 4.1-fs oscillation period) spaced by a 10.3-fs period. Figure 1.  Electric field (red line) of two-colour pulses composed from a 1030-nm fundamental and a 1400-

=

.

=

.

4 1) (C) Auxiliary Component, Each With A 60-Fs

FWHM intensity envelope and a peak intensity of .

. For Comparison, The Gray

dashed lines show the field-envelope of a 60-fs single colour pulse with ×

14

2 peak intensity.

4

Due to the odd N, the sub-pulses all have the same CEP, ϕCEP. Even with the rather long 60-fs pulse envelopes, the large sub-pulse spacing leads to a significant variation of the peak field strength from one sub-pulse to the next, which underlines the potential for efficient temporal gating.

Detuning the auxiliary wavelength has a number of coupled consequences. Bringing ω1 closer to the fundamental ω0 (see Fig. 1a) increases the spacing of the sub-pulses (e.g. to 13 fs for a 1400-nm auxiliary wavelength), as well as their effective duration, while decreasing their carrier wavelength (e.g. to 1187 nm for a 1400-nm auxiliary wavelength)—the sub-pulses thus contain more cycles. Bringing ω1 farther away from ω0 has the opposite effect (see Fig. 1c). For temporal gating this means that there is an optimum auxiliary wavelength to be found where (i) the sub-pulses are neither too close, meaning that adjacent ones would contribute to the near-cutoff photon energy range, thus forming an APT, nor (ii) where the sub-pulses contain too many cycles of the carrier wave, meaning that within the central sub-pulse, more than one attosecond pulses in the near-cutoff energy range would be generated. Point (i) can be mitigated by an ω1-value correponding to a non-integer N, so that the adjacent sub-pulses experience a CEP-shift.

If the central sub-pulse has an optimal CEP for efficiently generating high photon energies, the adjacent ones would then be sub-optimal and their contribution suppressed (cp. Fig. 1a). This picture is complementary to the one proposed in36 where a “phase accordance” parameter of the two colour components is defined, which takes small values in gating windows corresponding to our sub-pulses. The authors of36, however, did not study the experimentally very important CEP-dependence of the gating and generation efficiency.

Numerical simulations. Our numerical simulations treat HHG on the single-atom level with the non-adiabatic quantum path analysis of the Lewenstein model for HHG30,37 (see Methods-section).

For quantitative predictions on the performance of an experimental source, one has to also treat the propagation of the driving and generated XUV fields in the macroscopic ionizing HHG medium, as done, e.g. by Jin et al. 38,39. This additional layer adds a large number of additional parameters that need to be optimized, which is beyond the scope of the present work. Instead, we focus here on the single-atom level dynamics, from which we select the short-trajectory contribution to the HHG emission (see Methods-section), as this is typically the one macroscopically phase-matched in an experimental attosecond pulse source.

For our numerical simulations, we start with 60-fs FWHM-intensity envelopes for all colour compo- nents before exploring even longer durations. We consider a total peak intensity (i.e. the sum of the peak

2. This Means That, Unless Stated Other-

wise, we always compare driver pulses of the same total pulse energy. While we consider the ionization

=

.

P

eV, note that hydrogenic dipole matrix elements d k

[ ] Are Used So

as to avoid an influence of the particular atomic structure on our conclusions. The driving fields are always linearly polarized and all colour components have parallel polarization directions. The attosecond emission is calculated from the HHG spectra after application of a 15-eV wide spectral filter selecting the region of highest generated photon energies near the cutoff.

As a reference, we show in Fig. 2 the HHG spectra generated by single-colour drivers with varying carrier wavelengths, as well as the temporal profile of the attosecond emission. Unsurprisingly, even for the longest wavelength of 1800 nm, the many-cycle pulse duration of the driving fields leads to the emission forming an APT. The well-known increase of the attainable photon energy cutoff accompanied by a rapid drop of conversion efficiency40,41 is also clearly apparent.

Two-colour gating.

With The Intensity-Fwhm

duration τ = 60 fs. We consider a fundamental frequency ω0 corresponding to the emission wavelength of Yb-based lasers of 1030 nm.

R

1 the generated high-harmonic spectra and attosecond emission as a function of ϕCEP, i.e. of the phase delay between the two colour components, for a range of auxiliary wavelengths, λ1, from 1200 nm to 1800 nm. As the carrier-wavelength of the two-colour waveform increases (with increasing auxiliary wavelength), the achievable HHG cutoff photon energy increases accompanied by a drop in conversion efficiency, as expected from the well-known wavelength scaling of HHG40,41 As for temporal gating, the trends expected from the discussion of equation (1) are clearly observed: (i) For the shorter auxiliary wavelengths, the central sub-pulse becomes so long that it generates, for most CEP values, two attosecond pulses separated by a half-period of its carrier wave. As the auxiliary wavelength gets longer, the transition to a single-cycle central sub-pulse is observed, effectively limiting the attosec- ond pulse generation to a single half-cycle. (ii) For the shorter auxiliary wavelengths, the long spacing of the sub-pulses effectively suppresses the attosecond emission by the sub-pulses adjacent to the central one. (iii) For the auxiliary wavelengths corresponding to (close-to) integer N, 1545 nm and 1700 nm, the

5

sub-pulses all have the same CEP and the attosecond emission is correspondingly strongest at the same CEP-values for the central sub-pulse and its adjacent ones. In contrast, the CEP-shift between subsequent sub-pulses for non-integer N is beneficial for temporal gating since it shifts the maxima of the attosecond emission of the adjacent sub-pulses to different CEP values as that of the central sub-pulse (e.g. compare Fig. 3d,e,f).

We now select optimal CEP-values from the scans shown in Figure 3 as follows: if there are CEP-values for which the contrast ratio between the intensity I max of the strongest attosecond pulse and that of the second-strongest one is ρ > 10, then only these are considered, and the CEP with the highest “fitness”

I Max

4 is selected. This ensures that we enforce a good contrast ρ > 10 while giving preference to the highest attosecond peak intensity. If no CEP-value provides ρ > 10, we relax the intensity prefer- ence and selected the CEP value with the highest fitness

Ρ

I max . The optimal CEP-values so selected are marked by red dashed lines in Fig. 3; lineouts of the HHG spectra and attosecond emission for these CEPs are shown in the Supplementary information. With the

1

nm (due to an unfavorable combination of small driving sub-pulse spacing and near-integer N), all considered auxiliary wavelengths permit, within a limited CEP-range, to generate an IAP. Here we define IAP as a single attosecond pulse which dominates over potential pre- or post-pulses by more than an order of magnitude, i.e. ρ > 10. The highest attosecond pulse intensities are found for

1

nm and 1400 nm, of which the latter will be the practically most advantageous because it offers

Cep

rad) within which an IAP is retained, i.e. ρ > 10 (cp. Fig. 3b,c). The obtained attosecond pulse intensities are two orders of magnitude higher than in the APT with similar photon energy obtained with single-colour drivers (to be generated by OPA) with the same total pulse energy and peak intensity (cp. 2). This underlines the extremely useful temporal gating action of well-designed two-colour drivers: we do not force the isolation of a very weak attosecond pulse by picking an ever so narrow spectral slice in the very cutoff region, but we obtain, from ⪆10-eV-wide spectra, IAP orders of magnitude more intense than the APT generated with a single-colour driver.

Furthermore, reaching higher photon energies, like the ≈100 eV in the examples presented here, with “efficient” HHG gas media offering higher recombination cross sections, like argon, generally requires Figure 2.  High harmonic spectrum (left) and attosecond pulses (right) for single-colour driver pulses with

Cep

, 60-fs duration and 1030-nm (a), 1300-nm (b), 1545-nm (c), and 1800-nm (d) carrier wavelength. The red line superposed to the spectra shows the 15-eV wide normalized super-Gaussian filter centered at the cutoff frequencies 53 eV (a), 70 eV (b), 90 eV (c), and 115 eV (d), which selects the spectral window corresponding to the shown attosecond pulses. The dashed line marks the 10%-level of the intensity of the strongest attosecond pulse.

6

longer driving wavelengths that need to be generated by OPA. It is thus certain that a two-colour field composed of the OPA signal and part of the pump laser emission can be generated with higher total pulse energy. Not only does the two-colour field reach the same high HHG cutoff with higher efficiency, but it will contain more driving energy to begin with, further increasing its advantage in terms of attain- able attosecond pulse energy and/or intensity.

For a practical implementation, it will be interesting to know how much the relative weight of the auxiliary colour component can be decreased while still keeping the temporal gating effect intact. While the factorization in equation (1) is valid only for equal amplitudes of the beating colour-components, we find that the conclusions drawn from it remain valid guides also for a much weaker auxiliary component.

Having repeated the same numerical calculations as shown in Fig. 3 for

1 2 (Results Shown In The

Supplementary Information), we find that, except for the lowest λ = 1200

Nm, Which No Longer Leads

Figure 3.  CEP-dependence of the generated high harmonic spectrum (left) and attosecond pulses (right) for the combination of a 1030-nm fundamental with an auxiliary 1200-nm,

=

.

=

.

=

.

=

.

=

.

=

.

=

.

N

3 7 (h) pulse, both with a 60-fs duration. The pulses have peak intensities of

=

.

=

.

14

2, respectively, i.e.

1. For The Attosecond Pulses,

the cutoff region has been selected by a 15-eV wide normalized super-Gaussian filter centered at 100 eV (a- c), 105 eV (d-f) or 110 eV (g,h). The optimal CEP-values (see text) are marked by red dashed lines. Outside of the shown time window, the attosecond emission is suppressed to below a

10 4-Fraction Of The Strongest

attosecond pulse’s peak intensity.

7

to an IAP, the results are very similar to those obtained for =

R 1 (Attosecond Pulse Intensities Remain The

same within ±10%, the CEP range for IAP generation for λ = 1400

Rad), If

one adapts the position of the spectral filter for the attosecond pulses to the small ≈5 eV decrease of the achieved cutoff photon energies. Upon further lowering the peak intensity ratio to

1 9 (Results

shown in the Supplementary Information), we find the temporal gating to become significantly less effective, i.e. the CEP-range for which the central driving sub-pulse generates a single attosecond pulse only is now significantly reduced and the emission from the adjacent sub-pulses is less strongly sup- pressed. Nonetheless, with the spectral filter positions adapted to the further lowered cutoff photon energies, we can still find optimal CEP-values that allow generating an IAP for the auxiliary wavelengths

1

nm, 1400 nm, 1600 nm and 1800 nm, with IAP peak intensities reduced by a factor ⪅3 com-

Nm Is Still Identified As The

best choice, uniting highest attosecond pulse intensity and suppression of satellites. The requirement on

Cep

rad) is now more stringent than for the cases of higher relative intensity of the auxiliary colour component, but still well achievable in real-world experimental setups26. It will thus be possible to relax the required OPA output energy and retain the majority of the driving energy in the fundamental laser pulse.

Having shown the effectiveness of two-colour temporal gating for 60-fs pulses and having identified

Nm, 1400 Nm And 1600 Nm As Most Promising

choices, we can now explore even longer pulse durations approaching realistically achievable values for Ytterbium-based amplifiers15, 42–46. Since we have found the temporal gating to be very robust against variations of the relative strength of the auxiliary colour component, we can very well consider different pulse envelope durations for the two colour components. This is experimentally relevant since the OPA output can be significantly shorter than the pump pulses (see section “Experimental feasibility” below).

Comparing calculations for 180-fs fundamental and 60-fs auxiliary pulses (with superposed envelope

R

1 so as to maximise the gating efficiency (results shown in the Supplementary Information), to the case where both colour components have 60-fs envelopes (Fig. 3) , we find, unsur- prisingly, that the action of the central sub-pulse is barely modified while the suppression of the attosec- ond emission from the adjacent sub-pulses is less efficient. It is however still sufficient for the generation

1

nm. Since a 1300-nm auxiliary wavelength is not easily generated in an OPA pumped by a 1030-nm laser (see section “Experimental feasibility” below), and the CEP-ranges found for the longer auxiliary wavelengths are incompatible with real-world CEP-jitter, further enhancement of temporal gating will be required for IAP generation with experimentally realistic driver parameters.

Three-colour gating and enhancement. Adding a third colour component—the second harmonic of the fundamental—will provide this required enhancement. We extend equation (3) to

Cep

i.e. we assume for simplicity that the second harmonic has the same pulse envelope,

0 With Respect To The Fundamental Wave. Since

it is independent of the CEP, it can be considered perfectly stable even in realistic experimental condi- tions47. The enhanced gating with this third colour component is illustrated in Fig. 4 for different auxiliary wavelengths around the value of 1400 nm, which we had found to provide a good compromise between wide inter-pulse spacing of the few-cycle-pulse train and short few-cycle duration. On the scale of the

2

0-component functions as a “detuned second harmonic” of the carrier wave24, effectively “de-activating” all but a single half-cycle of the carrier wave for HHG. This doubles the periodicity of the CEP-dependence of HHG from π (as in Fig. 3) to π

2 (See Fig. 5). On The Time-Scale

of the “envelope-term” of the few-cycle-pulse train, the effect depends on N. For integer N (Fig. 4a,c),

2

0-component has the same phase at every few-cycle-pulse maximum. Therefore, for odd N, its phase relative to the carrier-wave of the few-cycle-pulse train is also the same at each few-cycle-pulse maximum, whereas it changes by π from one few-cycle-pulse maximum to the next for even N. This means that if for a given phase ϕ2, for which the ω

0-Component Enhances Hhg In The Central Few-Cycle

pulse, it will have a quenching effect in the adjacent few-cycle-pulses if the auxiliary wavelength is chosen such that N is even. This is obviously beneficial for temporal gating since it effectively results in the generation of an IAP only every other few-cycle pulse in the train.

In the following calculations, we will thus consider three-colour drivers with the even-N auxiliary

6). We Set A 180-Fs Intensity-Fwhm Dura-

tion for the fundamental and its second harmonic, while for the auxiliary colour component, we set a 100-fs duration. The peak intensity of the second harmonic pulse is chosen to be 10% of that of the

Authors:

Peder EZ Larson 1, 2,* , Jenna ML Bernard1, James A Bankson 3, Nikolaj Bøgh 4, Robert A Bok1, Albert P. Chen 5, Charles H Cunningham 6,7, Jeremy Gordon1, Jan-Bernd Hövener 8, Christoffer Laustsen 4, Dirk Mayer 9,10, Mary A McLean11 12, Franz Schilling13, James Slater1, Jean-Luc Vanderheyden5, 14, Cornelius von Morze 15, Daniel B Vigneron1, 2, Duan Xu1, 2, and the HP 13C

94143, Usa.

Denmark. 5 GE Healthcare, Menlo Park, California, USA. 6 Physical Sciences, Sunnybrook Research Institute, Toronto, Ontario, Canada.

8 Section Biomedical Imaging, Molecular Imaging North Competence Center (MOIN CC), Medicine, Baltimore, MD, USA. Cambridge, United Kingdom.

14Jlvmi Consulting Llc, Dousman, Wi, Usa

#See Acknowledgements for a list of all HP 13C MRI Consensus Group Members This work was supported by the ISMRM Hyperpolarized Media MR Study Group, the ISMRM Hyperpolarization Methods & Equipment Study Group, and the Hyperpolarized MRI Technology Resource Center (NIH/NIBIB grant P41EB013598).

Abstract

MRI with hyperpolarized (HP) 13C agents, also known as HP 13C MRI, can measure processes such as localized metabolism that is altered in numerous cancers, liver, heart, kidney diseases, and more. It has been translated into human studies during the past 10 years, with recent rapid growth in studies largely based on increasing availability of hyperpolarized agent preparation methods suitable for use in humans. This paper aims to capture the current successful practices for HP MRI human studies with [1-13C]pyruvate - by far the most commonly used agent, which sits at a key metabolic junction in glycolysis. The paper is divided into four major topic areas: (1) HP 13C-pyruvate preparation, (2) MRI system setup and calibrations, (3) data acquisition and image reconstruction, and (4) data analysis and quantification. In each area, we identified the key components for a successful study, summarized both published studies and current practices, and discuss evidence gaps, strengths, and limitations. This paper is the output of the “HP 13C MRI Consensus Group” as well as the ISMRM Hyperpolarized Media MR and Hyperpolarized Methods & Equipment study groups. It further aims to provide a comprehensive reference for future consensus building as the field continues to advance human studies with this metabolic imaging modality.

Keywords: Hyperpolarized MRI, metabolic imaging, carbon-13, pyruvate, dissolution dynamic

Introduction

MRI with hyperpolarized 13C agents, also known as hyperpolarized (HP) 13C MRI, has shown great potential as a novel imaging modality, particularly for its ability to probe metabolic processes in real time. The first human studies with HP [1-13C]pyruvate were performed in 2011 in prostate cancer patients (1).

Since then, there have been over 60 papers published with imaging results of human subjects from 13 different sites, with applications including prostate cancer, brain tumors, breast cancer, kidney cancer, pancreatic cancer, metastatic disease, liver disease, ischemic heart disease, diabetes and cardiomyopathies. The vast majority of these studies used [1-13C]pyruvate (1–63), where [2-13C]pyruvate (64) and 13C-urea (56) have been demonstrated too.

As clinical HP 13C MRI advances, there is a growing need to build consensus for best practices, which are critical for comparing data across sites, performing multi-site trials,deploying methods to new sites, partnering with vendors, and potentially for obtaining broader regulatory approvals.

In March 2022, we initiated an effort to build consensus within the HP 13C MRI community with this opportunity in mind, and it was greeted with strong enthusiasm. The “HP 13C MRI Consensus Group”, containing over 55 members from 27 sites, identified the area of greatest need and opportunity for consensus building to be HP [1-13C]pyruvate human

●

Pyruvate is the most mature and widely used HP agent and has the most significant translational evidence emphasizing the potential clinical impact.

●

Clinical trials, particularly multi-site trials, have the strongest need for consensus methods to ensure that data can be combined across sites. This work is a Position Paper for which the goal is to describe current successful practices and study methods for HP [1-13C]pyruvate human studies along with justification to support those practices. This is divided into four major topic areas: (1) HP 13C-pyruvate preparation, (2) MRI system setup and calibrations, (3) data acquisition and image reconstruction, and (4) data analysis and quantification (Fig. 1). The current successful practices and study methods include a literature review of published peer-reviewed journal papers showing human HP [1-13C]pyruvate study data, up to September 2022 (1–63), as well as new unpublished information from surveys of HP 13C study sites. Based on this information, we also highlight the evidence gaps, strengths, and limitations of current practices which are summarized at the end of each section.

Figure 1: Illustration of the HP 13C MRI human study process, including the 4 major areas covered in this paper: Hyperpolarized 13C-pyruvate preparation, MRI system setup and calibration, Acquisition and Reconstruction, and Data Analysis and Quantification.

Figure 2: Anatomical targets of HP [1-13C]pyruvate MRI human studies published up to September 2022.

Hyperpolarized 13C-Pyruvate Preparation

This section covers the processes for creating the HP agent, 13C pyruvate, and will include many aspects and considerations that are needed to safely and effectively prepare doses for metabolic imaging studies in human subjects. These include material, personnel, equipment and facility, fluid path preparation, quality control, and release.

It is helpful to understand that the specifications of a dose of 13C pyruvate suitable for in vivo MR HP metabolic imaging were shaped in part by early preclinical studies performed by GE HealthCare summarized in Ref. (65). In short, the safety of the two novel drug components, 13C pyruvate and the electron paramagnetic agent (EPA) AH111501, were demonstrated in those studies. The more precise formulation of the dose suitable for human use was then determined from clinical studies (66) that included two Phase 1 clinical trials in young and elderly healthy volunteers without hyperpolarization of the 13C nuclei and another Phase 1/2a dose escalation and imaging feasibility study with HP 13C pyruvate in 31 prostate cancer patients at the With the exception of the first HP 13C imaging clinical trial, which utilized a prototype device in a cleanroom (1), all HP 13C studies performed in humans to date have utilized the SPINlab polarizer (manufactured by GE HealthCare). Consequently all doses of the HP 13C pyruvate delivered by SPINlab have been produced using the “SPINlab Pharmacy Kit” that serves as the container-closure system for the various drug components (13C pyruvic acid and EPA mixture, dissolution medium, and neutralization and dilution medium) during sample polarization, dissolution and quality control (QC) processes. Thus many aspects of the HP sample preparation considerations discussed below are related to the SPINlab instrument and the consumables designed to be used with it (67).

General Considerations

While more than 860 patients or healthy subjects having been injected with HP 13C pyruvate as of January 2022 without reports of any serious adverse events (68), HP 13C pyruvate injection remains an investigational MR contrast agent and can only be administered by those with Investigational New Drug (IND) exemption from the Food and Drug Administration (FDA) in the USA, a Clinical Trial Application (CTA) in Canada, approval from National Research Ethics Committee Services in the UK, or approval from the relevant local regulatory body. Thus, methods and processes involved to produce a dose should have patient safety as the first priority. Since utilizing dissolution dynamic nuclear polarization (dissolution-DNP) for human use is still a relatively new development, there are no existing published regulatory guidelines specifically for this method.

There are two major production styles that determine how various sites approach the agent preparation. In the US, the most common approach is to rely on a sterilizing filter (“Terminal Sterilization”) to ensure sterility of the final product, akin to PET tracer production, where a starting molecule with a radioisotope is processed using various other ingredients to make the final, desired and injectable contrast agent within a necessarily short amount of time (69). For these sites, sterilization of the components and accessories upstream of this filter are not required, although many of them were manufactured and tested following Good Manufacturing Practice (GMP) or Good Laboratory Practice (GLP) requirements. The filling process is usually performed under an ISO 5 laminar flow hood, but a clean room or an isolator is not required.

This approach is typically accompanied by testing the integrity of the sterilizing filter prior to release of the dose for injection. Typically, post release endotoxin and sterility tests are performed using an aliquot reserved from each released dose.

In the UK and EU, the most common approach is to more-closely follow sterile pharmaceutical compounding guidelines (70), where all components and ingredients are required to be sterile or manufactured under GMP guidelines and are assembled and filled within a clean room environment or an isolator system (“Sterile Preparation”). Typically a batch of Pharmacy Kits for HP 13C pyruvate injection are prepared together. The sterility of the final dose is also ensured by batch validation testing, in addition to the sterility of the ingredients and the sterile compounding process. The endotoxin and sterility testing are performed for the process validation but are not performed for each injected dose.

Some institutions fill and assemble the Pharmacy Kit required for a specific study on the same day or the day prior to polarization, dissolution, and patient administration, but others have also demonstrated the feasibility of preparing a batch of kits, keeping them in a -20ºC freezer and using them over a period of a few months.

Beyond the obvious requirements that the process and the facility has to ultimately produce a dose that is safe to inject into a human, regulatory authorities will also focus on the question “Are you in control of your processes?”. To be in control of your process requires an in-depth and broad understanding of all processes involved in pre, post, and during the production process.

Personnel

It is typical and may be required to have licensed personnel involved in the production process depending on local regulations.Typically a pharmacist, radiopharmacist or other similarly qualified person (QP), in charge of the facility where the Pharmacy Kit filling and preparation is taking place, is responsible for the overall process and the release of the injectable dose.

Qualified cleanroom technicians are often involved in the Pharmacy Kit filling under the supervision of the pharmacist or QP. As is required for pharmaceutical compounding or PET tracer production, training requirements and training records for all personnel need to be maintained and available for audit by the FDA or equivalent.

Equipment And Facility

The facility and all equipment need to have standard operating procedures (SOPs) that describe how equipment is used, maintained, and calibrated to comply with relevant legislation. Currently, almost all the filling of the Pharmacy Kit takes place within a compounding laminar flow hood or isolator (typically ISO 5). At some sites, the filling is conducted within a cleanroom, while at others, it is conducted in a dedicated non-cleanroom space, reflecting differences in cleanroom approach and specifications between regulators worldwide (71). Some equipment or facilities, such as the compounding hood or cleanroom, may require external certified laboratories for testing.

Material Handling

Material handling guidelines (69,70) require SOPs detailing a system to track all of the materials involved in the HP production process for a particular patient dose, similar to current good manufacturing practice (cGMP) requirements for material handling for drug compounding. This includes acceptance standards, storage conditions, amount used in the patient dose for each ingredient and materials used in the assembly of the fluid path and Pharmacy Kit. Currently some users choose to open and inspect and sometimes modify the Pharmacy Kits upon arrival, but some users keep them in the sealed packaging until they are required for dose preparation.

Pharmacy Kit Filling And Assembling

As required by an IND or its equivalent, the preparation of the doses of HP 13C agent are detailed in the Chemistry, Manufacturing, and Control (CMC) section of an applicable regulatory submission; an example of this has been made available (72). It describes the processes of filling the Pharmacy Kit with the different components that make up the final drug product, and of assembling the final kit for either storage or immediate use in the polarizer. Special attention should be given to the laser welding process in order to satisfy installation qualification (IQ) and operational qualification (OQ). Typically, the final developed process is validated by process qualification (PQ) runs, during which 3 or more Pharmacy Kits are filled and used and the final HP 13C products are tested for endotoxin and sterility and to confirm that they meet the dose specifications for injections (usually including pyruvate concentration, residual EPA concentration, pH, liquid state polarization level and dose temperature). The data from 3 consecutive PQ runs are submitted as part of the IND submission (or its equivalent), and are often also reviewed by the Institutional Review Board (IRB) where the studies are conducted.

Quality Control And Dose Release

The quality control (QC) and dose release can be separated into two aspects: one is the QC and release of the filled Pharmacy Kit, and second is the QC and release of the HP 13C agent for injection, after polarization and dissolution. For institutions filling a batch of kits and storing them to use over a period of time, typically the batch can be released based on initial validation, environmental monitoring data from the day of kit production, and if filters are used during preparation of any of the components, filter integrity testing. But in some cases one or more kits are used for validation before the batch of kits are released for future use. For institutions that fill only the kits required for specific studies shortly before the experiment, the filled kits often do not go through separate release tests before they are used.

The quality control of the HP 13C pyruvate solution post dissolution is primarily performed to ensure that the agent meets the dose specifications (Table 1) before it is administered to the subject. These specifications target both safety (pH, residual EPA, temperature) and efficacy (pyruvate concentration, polarization, volume). Typically, the pyruvate concentration, residual EPA concentration, pH, dose temperature, dose volume, and liquid state polarization are measured by the QC accessory associated with the SPINlab polarizer. Some users perform a secondary measurement for one of the parameters, such as pH, using a different instrument or pH paper. For sites that do not go through a separate release testing process for batch filled kits, the integrity of the sterilization assurance filter, a part of the Pharmacy Kit, is typically tested as a part of the dose release. It is also common for these users to preserve an aliquot of the final HP 13C pyruvate solution for post-release endotoxin and sterility testing. This testing cannot be completed fast enough to test an individual dose prior to injection, but this is why other processes such as PQ runs and validation testing are done to minimize the chance a subject could be injected with a contaminated dose.

The Final Dose Release And Injection

should be done under the supervision of a licensed professional, based on local regulations.

Some Key Challenges

Many of the challenges associated with HP 13C pyruvate preparation can be attributed to the conditions required for the dissolution-DNP method of high magnetic field (~3-7 T) and very low temperature (~1 K) during polarization, with pressurized and superheated water necessary for the rapid dissolution event. These extreme conditions are quite challenging for the design of the container-closure and fluid path system. In particular, the cryogenic temperature in the polarizer requires special attention to any moisture or ambient (moist) air introduced into that portion of the fluid path, which can form an ice block at ~1 K. This ice can lead to flow restriction during the dissolution event and reduce the strength of the laser welded bond between the cryovial and its cap. This can ultimately produce failures in the dissolution step, including variations in final pyruvate concentration and pH that may fail to meet QC release criteria as well as fluid path ruptures that provide no available dose and result in polarizer down-time.

The polarization of the HP 13C pyruvate sample decays quickly over the span of a few minutes after dissolution, and thus the process of dissolution, QC for release, and injection should be completed as fast as possible to preserve the high polarization level achieved. Any delays in the preparation process, such as transportation time or equipment malfunction, can significantly reduce the final polarization and result in lower quality imaging data.

Current Practices

A summary of data collected from all sites performing clinical trials with HP 13C-pyruvate is shown in Fig. 3 and Table 1, including the specification of the final dose and how the quality control and release of the final dose are performed. There is a split in the Production Style, described in the General Considerations section above, with 8/13 sites using Sterile Preparation versus 5/13 using Terminal Sterilization. While many of the dose specifications show notable differences in acceptable ranges, all of these variations listed in tables have been successfully and safely been used to perform HP 13C pyruvate studies in humans. Their differences depend on the institutions’ preferences, resources and their particular regulatory situation. There is high similarity in pyruvate ranges, temperature ranges, EPA limits, and volume limits. There is modest variability in pH ranges and large variability in the endotoxin test limit. There is a 3-fold difference in acceptable polarization levels, which are measured to ensure a futile dose is not injected since the polarization is directly proportional to SNR. This reflects the decision by several sites to believe that useful data can be still be obtained with suboptimal polarizations.

Figure 3: Hyperpolarized agent preparation methods reported by sites currently performing HP

In House

Table 1: HP 13C-pyruvate preparation parameters, methods, and dose specifications used for quality control testing and release as well as validation. These were obtained from a survey of all sites performing clinical trials with HP [1-13C]pyruvate. The parameters used for product release are noted in bold text, otherwise these parameters are measured for batch validation or other QC measurements. The endotoxin and sterility testing are performed during process validation of the batch and/or post-injection, and largely depends on the agent production approach.

Summary

The overall safety record of HP 13C-pyruvate has been very strong, and the SPINlab hyperpolarizer has proven to provide high polarizations at human sized doses while meeting numerous QC and release criteria. A weakness remains the failure modes of the SPINlab Phamacy Kits (e.g. ice blocks, path ruptures), which are placed under extreme requirements particularly during dissolution. The preparation process still requires a high degree of expertise.

Therefore, there is a significant need to improve the reliability, robustness, and ease of operation for generating HP 13C-pyruvate doses for human studies. Furthermore, there is a divide between manufacturing and sterile compounding style preparation as well as other site-specific practices, resulting in variations in SOPs and justification required to relevant regulatory bodies. There have also been no comparisons between these approaches. It is also unclear what release criteria and QC parameters are truly required to ensure patient safety.

However, all of the reported methods are acceptable and approved by the appropriate regulatory authorities, and have led to the rapid expansion of successful human studies in recent years.

Mri System Setup And Calibrations

This section covers the MRI system setup, including the imaging system, RF coils, phantoms, and prescan calibration methods.

Imaging System

The main prerequisite for a given MRI scanner to be capable of supporting studies with HP 13C is its “broadband” capability to transmit and receive radiofrequency (RF) signal at the frequency of 13C, which is around 4 times lower than 1H. This does not come as a default on clinical MR devices. The transmit power of the broadband amplifier should also be sufficient to support the intended flip angle and RF pulse shape with the employed transmission RF coil(s) for 13C. Most studies to date use relatively low flip angles (< 90 degrees) for HP 13C in order to preserve polarization for time-resolved imaging. The capability to receive 13C signal on multiple channels is also desirable to increase SNR, as discussed further in the “RF coils” section.

The choice of magnetic field strength is primarily dependent on the metabolites’ frequency separation due to chemical shift dispersion and 1H imaging. High field strengths do not enhance hyperpolarized 13C signal as they do for 1H because the signal strength in a HP experiment relies on manipulating the population of quantum energy states outside of the MRI scanner.

However, the injected HP 13C-pyruvate and its metabolic products have greater frequency separation at higher fields, and it may thus be easier to separate and quantify these resonances at higher fields. This comes at the cost of a reduction in the achievable T2* and often reduced T1. As the initial polarization is independent of the imaging field strength it has been proposed that the increased T2* at 1.5T can potentially be exploited to increase SNR by adapting the acquisition bandwidth or reduce off-resonance imaging effects in cases when the decay of the transverse magnetization is dominated by T2* (73). In practice, 3T has been used in all published human 13C-pyruvate studies surveyed (Supporting Table S1), and comprises the majority of scanners currently in use for human studies (Table 3). A field strength of 3T is well-suited for 1H MRI anatomical reference and correlative imaging.

Stronger and more rapidly slewing magnetic field gradients support more rapid spatial encoding, particularly for metabolite-specific single-shot imaging using echo-planar imaging (EPI) or spiral imaging (See “Acquisition and Reconstruction”). Although the spatial resolution acquired for HP 13C imaging is typically much coarser than for 1H MRI, the factor of ~4 in gyromagnetic ratio leads to the same reduction factor in performance of the gradient system, so 13C experiments are potentially more limited by gradient hardware performance. To date, all human studies have used the commercially-available integrated gradient systems provided in clinical MRI scanners.

Optimization of scanner design has understandably focused on minimization of artifacts in 1H MRI, where devices such as room lights, the gradient amplifiers, and the motors driving the patient bed are checked to ensure that they do not produce RF interference at the 1H frequency, but artifacts may arise at other frequencies. Eddy current compensation is also not always appropriately adjusted for nuclei at other frequencies (74). In order to optimize for 13C, many sites have performed checks on phantoms for RF interference, gradient artifacts, and eddy currents (74), including the use of post-hoc gradient impulse response function characterisation and correction, and some vendors have fixed these issues as well.

Rf Coils

For HP 13C imaging studies in humans, RF coils for both 1H and 13C nuclei are needed, with 1H MRI providing an anatomical reference for registration and optional additional multiparametric MRI readouts. At the Larmor frequency of 13C nuclei, the relative contributions from coil noise compared to sample noise increase compared to 1H (73,75), although sample noise still is likely the dominant contributor for human-sized coils at 32.1MHz - the resonance frequency of 13C nuclei at 3T.

The key requirement for human 13C-pyruvate RF coils are that the coil geometry and sensitive volume must cover the volume of interest in the subject. Table 2 and Figure 4 shows coil configurations that have been used and optimized for applications in different anatomic regions.

Volume resonators are most commonly used for transmit, as they surround the subject to

Provide B1 Transmit Across The Fov (B1

+). While 1H relies on a large birdcage (“body”) coil built into the scanner, 13C transmit coils must be placed inside the bore. This takes up valuable space within the magnet, and also has led to the use of designs with relatively inhomogeneous

B1

+. Many human studies have used Helmholz pair resonators for transmit, including the “clamshell coil”, which has a notably inhomogeneous B1

+ Profile But Has Been Used Because Of

relatively easy integration into the scanner bore. B1

+ Variation Results In Variations In The Flip

angles that control the use of the hyperpolarized magnetization and creates errors in common HP metrics (9,76). The exception are head coils, where birdcage designs with highly

Homogeneous B1

+ can be placed around the head while easily fitting inside the bore. As with 1H MRI, higher SNR can typically be achieved by smaller receive coil elements, such as surface coils or phased arrays, and the majority of 13C receive coils used have layouts similar to 1H phased arrays.

RF coil quality control is important to ensure proper functioning of the coils to provide consistent imaging quality, especially with limited natural abundance 13C signal in vivo. It typically involves 1) a physical integrity check of the coil cables and connectors and 2) phantom SNR tests to check the coil’s performance and to monitor it over time (see Phantoms below). An useful reference for RF coil quality control is outlined in the MRI accreditation program of the American College of Radiology (77) and can be adapted for 13C coils.

Notably, configurations for brain and prostate studies used dual-tuned 1H/13C coil designs, which greatly simplify workflow and registration of 1H and 13C images, as no switching of coils is needed.

(1)

Table 2: RF coil configurations reported for human HP [1-13C]pyruvate studies.

Tx = Transmit

coil, RX = receive coil. The commonly used “clamshell” TX coil is a Helmholz pair design. For 1H RF configurations, all used the Body coil for TX unless otherwise noted, and “repositioned” indicates the 13C coil was removed for 1H imaging. One representative reference is listed for each configuration. The RF coil configurations reported in the reviewed papers are shown in Supporting Table S1.

Figure 4: Examples of RF coil configurations used for human HP [1-13C]pyruvate brain studies. (A,B) 13C Clamshell TX (Helmholz pair) and 2× 4-channel paddle RX arrays. (C) 13C Birdcage volume TX and 32-channel RX array (RX array slides into TX coil). (D) 13C Birdcage volume TX and 24-channel RX array, combined with a 1H 8-channel RX array. Image reproduced with permission from Ref (16).

Phantoms

Since hyperpolarized magnetization is non-renewable, phantoms containing 13C nuclei are important to: 1) test the multi-nuclear capabilities of the imaging system, including all parts of the signal excitation and receive chain; 2) perform calibration measurements before a scan with hyperpolarized nuclei; and 3) perform necessary pre-scan adjustments (see “Prescan Calibration” section). The phantoms currently in use are listed in Table 3. Their composition must provide sufficient 13C signal, with additional considerations of conductivity, stability, chemical shift(s) present, potential for dynamic imaging, and cost. The phantom geometries are typically either compact, in order to be used alongside the subject during a HP scan, or large enough to mimic the inner volume of a RF coil for system testing.

One popular compact design contains enriched 13C-urea at high concentration, typically 8 M, which provides a single resonance, placed inside a small container ~1 mL. The most common recipe mixes 13C-urea in a 90% water/10% glycerol solution, with glycerol used to increase the urea solubility and doping with a Gd-based contrast agent to shorten T1 which increases the potential SNR per unit time. For example, when Dotarem is added at a 3:1000 volume ratio the 13C-urea T1 is around 500 ms and T2 is around 100 ms. However, when testing pulse sequences influenced by T1 and T2, doping should be used carefully. This phantom is suitable for frequency calibration, transmit gain calibration, sequence testing, and as a fiducial marker when placed next to a patient. However, enriched 13C-urea has a relatively high cost compared to natural abundance compounds.

For larger volumes (>100 ml), the phantoms most often used contain undiluted ethylene glycol, glycerol, or dimethyl silicone. These compounds have sufficiently high carbon concentrations to provide sufficient 13C signal even with the 1.1% natural abundance of 13C. These larger phantoms matching the inner volume of an RF coil are useful for coil testing, including transmit

+) And Receive (B1

-) coil profile mapping, as well as to mimic acquisitions using in vivo FOV requirements. In this case, size and conductivity should match the expected subject size in order to mimic coil loading and get a realistic estimation of B1+. Large-volume natural abundance urea phantoms have also been used by some sites, but suffer from higher conductivity compared to biological tissues. Typically, it is easier to increase the conductivity and hence coil loading of the non-conductive phantom by adding NaCl to match physiological loading (16,78).

Dynamic phantoms that aim to mimic metabolite kinetics have also been developed (79–81), and have the potential to more closely mimic the HP experiment, but so far these are not widely used.

Prescan Calibration

Prior to performing an MRI acquisition, the so-called prescan procedure is used to set the shim parameters to maximize B0 homogeneity over the field of view (FOV) or a specific region of interest (ROI), the scanner center frequency (CF), the RF transmit gain, and the receiver gain.

While this calibration procedure is usually automated for 1H, the lack of sufficient natural abundance 13C signal prevents use of automated methods. (Although natural abundance 13C lipid signal has been detected, there are so far no reports on using this signal for prescan.) Table 3 shows current practices across sites.

Maximizing B0 homogeneity is independent of the nucleus and is therefore performed prior to 13C imaging using the 1H water signal and existing shimming tools, such as by a standard automated process (“Auto Shimming”) or using high order shimming routines. Similarly, the 13C CF can be calculated from the 1H CF using a predetermined scaling factor that depends on the target chemical shift (82). Another common approach used is to have a small, high-concentration 13C phantom, e.g. 8M 13C-urea, integrated in the RF coil or placed next to the scan subject (1). The reference frequency can also be based on real-time measurements after the HP injection but prior to imaging (83). Both the CF and B0 shimming are critical when using spectrally-selective RF pulses, as inmetabolite-specific imaging methods, where the desired excitation bandwidths are typically very narrow and frequency offsets can lead to a failure mode that is only apparent after injection.

The calibration of the RF transmit power is typically performed on a small, high-concentration 13C phantom placed near the region of interest during the scan or on a large 13C phantom of similar size and coil loading as the subject, prior to the subject scan. Reference power is often done by sweeping the power in a pulse-acquire sequence (53,62), or the Bloch-Siegert method (52,84). When using a small phantom, the location of the phantom, B1

+ Inhomogeneity As Well

as any shielding effects, e.g., when the phantom is integrated into a coil (1), may degrade the accuracy. Other methods include real-time Bloch-Siegert method measurements after the HP injection (83), and using the stronger natural abundance 23Na signal that is close enough to the 13C resonance frequency to be detected by 13C coils (82).

The receiver gain is predetermined, either systematically based on independent phantom measurements and assuming the dose and polarization of the HP compound is known prior to injection, or based on past HP imaging studies.

Power [Kw]

Phantom(s) - during study Phantom(s) - before study 13C Frequency

8

13C-bicarbonate doped with dimethyl silicone, various

Maximum Values

Table 3: Summary of the imaging systems, phantoms, and prescan procedures used at sites currently performing HP 13C-pyruvate human studies. These were obtained from a survey of all sites performing clinical trials with HP [1-13C]pyruvate. *Previously performed studies with a Siemens 3T Tim Trio. The imaging systems, phantoms, and prescan procedures reported in the reviewed papers are shown in Supporting Table S1.

Summary

Commercially available 3T MRI systems are by far the most commonly used for human HP 13C-pyruvate studies, although a systematic investigation of the impact of B0 has only recently been investigated (73). The multi-nuclear RF transmit and receive chain has proven sufficient for current acquisition strategies, although many sites have observed artifacts due to RF interference, gradient interference, and residual eddy currents when operating at the 13C frequency. A variety of 13C RF coils, tailored for numerous anatomical targets, have been successfully demonstrated, with the main limitation that most transmit coils take up a lot of additional space inside the bore and provide relatively inhomogeneous B1

+ Profiles. The

phantoms used have converged into generally 2 categories - small phantoms containing 13C-enriched compounds that can be used during the study and human-sized phantoms containing compounds with high carbon concentrations but without 13C enrichment that are used to test and calibrate the coils. There are no standardized compositions or geometry, and dynamic phantoms that recapitulate in vivo kinetics would be desirable but are still an emerging area. Prescan calibration procedures were not well defined in most publications, so we surveyed individual sites to determine current practices. Calibration procedures for the B0 field (13C CF and shimming) for most sites take advantage of 1H signal and methods, while methods

For Calibration Of B1

+ is more variable across sites, likely a reflection of remaining challenges in how to perform this calibration. Standardization of both phantoms and calibration procedures would synergistically improve the robustness and reproducibility of HP 13C studies.

Acquisition And Reconstruction

Data acquisition strategies in human HP [1-13C]pyruvate MRI studies must account for multiple chemical shifts, efficiently utilize the non-renewable HP magnetization, and acquire data quickly relative to metabolism and relaxation decay processes. These studies require spectral encoding to separate metabolites, necessitating pulse sequences that efficiently encode up to 5D data (3 spatial + 1 spectral + 1 temporal dimension). RF pulses must efficiently sample without immediately saturating the non-renewable HP magnetization, and sequences must acquire data quickly and be robust to both experimental and physiologic variation (e.g. B1

+ Inhomogeneity,

variation in perfusion) to ensure reproducibility and minimize scan-to-scan variability. This section covers current successful practices for data acquisition in human [1-13C]pyruvate studies, and accompanying 1H imaging, from different anatomic regions, including scan parameters and image reconstruction.

Acquisition And Reconstruction Methods

The acquisition methods used in human [1-13C]pyruvate studies can be classified into 3 categories: 1) MR spectroscopy or MR spectroscopic imaging (“MRS/I”), 2) chemical shift encoding methods, and 3) metabolite-specific imaging (Fig. 5).

Mrs/I Methods Specifically

resolve a spectrum that can be analyzed to extract expected as well as unexpected resonances, making this approach very robust. It was used in many initial studies (1).

Chemical Shift

encoding methods, most commonly the Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation (IDEAL) method, use imaging sequences acquired with multiple TEs and rely on a model-based separation of expected chemical shifts (85).

Metabolite-specific imaging methods use specialized RF pulses that are spatially and spectrally selective to excite individual metabolites which are then typically imaged with fast k-space trajectories such as echo planar imaging (EPI) or spirals (86).

Their Application To Different

organ systems is described below. The image reconstruction methods used in human [1-13C]pyruvate studies have typically been conventional methods (e.g. FFT, non-uniform FFT, or equivalent). The incorporation of accelerated imaging and advanced reconstruction methods including parallel imaging (4,57,87) and compressed sensing (7) has also been applied in human studies for improved spatial resolution, temporal resolution and coverage, but have the potential for additional artifacts as well as SNR losses due to ill-conditioning of the reconstruction (e.g. g-factor).

The Majority Of

published studies do not use accelerated imaging indicating the resolution and coverage achievable without acceleration is currently adequate for successful data collection. Performing coil combination, even with fully sampled data has also been shown to have specific challenges for HP human images: using naive sum-of-squares methods suffer from high noise amplification in the relatively low SNR regime of HP [1-13C]pyruvate (compared to 1H), motivating several HP 13C-specific methods that include data-driven coil sensitivity estimation which have shown obvious improvements over sum-of-squares (11).

More recently denoising techniques have been applied as post-processing of human HP data(41,42,44). The techniques applied are based on spatial-temporal singular value decomposition for unsupervised estimation of signal and noise components. They have shown improvements in apparent SNR in the brain and liver, while care must be taken to choose parameters such as the rank threshold to avoid oversmoothing and overfitting to the estimated signal components.

Prostate Studies

Prostate cancer was the first human application of HP [1-13C]pyruvate (1), and data was acquired with MRS/I methods: 1D dynamic MRS, single-slice 2D dynamic echo-planar spectroscopic imaging (EPSI), and single time point 3D EPSI. Advances in imaging strategies led to the development and application of new acquisition schemes, including undersampled 3D EPSI with compressed-sensing (7), model-based chemical shift encoding methods that use a priori information (47,59), and metabolite-specific EPI (10), all of which can provide volumetric whole-organ coverage and dynamic acquisitions.

The pyruvate bolus arrival in the prostate can vary by ± 10 s between patients, necessitating dynamic imaging to reliably and consistently capture the pyruvate bolus (18). For this reason, all currently ongoing studies acquire dynamic data. While MRS/I, chemical shift encoding, and metabolite-specific imaging can all achieve dynamic imaging, chemical shift encoding and metabolite-specific imaging provide greater dynamic and volumetric coverage (85). For scan prescriptions, the FOV is designed to provide full prostate coverage and typically to match the orientation of the anatomic imaging used for registration. Flip angles used in current studies are constant through time, as quantification with a variable-through-time flip scheme is highly sensitive to bolus timing (8) and errors in the RF transmit (B1 +) field (76).

Heart Studies

Data acquisition methods for 13C imaging in the heart must be designed to meet the demands of significant cardiac motion and blood flow. To cope with the periodic cardiac motion, most human heart studies to date used gating to the diastolic window, the longest cardiac cycle interval, which has reduced motion (2,22,28,30,35,36,38,45,52). The duration of the diastolic window limits the available data sampling time, making cardiac acquisitions the most time-constrained of the HP 13C MRI applications. The most common acquisition approach is metabolite-specific imaging with spiral k-space trajectories (2). Their single-shot imaging capability makes these methods particularly robust to motion effects. Furthermore, spiral k-space trajectories provide rapid k-space coverage and relatively benign flow and motion artifacts. The majority of studies have used 2D multi-slice acquisitions, but 3D encoding has also been used successfully (35).

Brain Studies

For HP 13C MRI of the human brain, the majority of studies have also used 2D (slice selective) acquisitions (10–12,14,16,28,33,40,41,44,51,53,60), with a trend toward volumetric coverage using 2D multi-slice metabolite-specific imaging. 3D metabolite-specific imaging of the whole brain, with phase encoding of the slice direction (34,57), has been shown to provide similar SNR efficiency (88) compared with multislice imaging. A number of studies have employed MRS/I (5,6,29,31–33,50,55) resulting in a spectrum from each voxel, which has the advantage of not requiring a priori information about which peaks to encode. This was important in early brain studies when it was not known which peaks would be detectable. Chemical shift encoding, using a set of images with different echo times and an iterative reconstruction of the individual resonances (i.e. the IDEAL approach (85)), has also been used (12,49,54), with the drawback that coverage in the slice direction was limited due to the time required to acquire multiple echo time images.

Abdomen And Breast Studies

The fundamental approaches to data acquisition and reconstruction in the abdomen and breast are largely similar to the aforementioned applications, but demand attention to particular challenges associated with these anatomic regions, especially relating to respiratory motion.

Although it has been shown that a basic 2D MRSI approach based on phase encoding and FID readout can be successfully applied for HP 13C imaging in breast (15) and kidney (13), major advantages in terms of spatiotemporal resolution and coverage have been realized using tailored approaches based on metabolite-specific imaging (43,62) and chemical shift encoding (43), which have facilitated multi-slice or 3D dynamic acquisitions over large FOVs in the abdomen (4,37,46).

The significant respiratory motion encountered in these regions can directly blur 13C images, and has further favored these rapid acquisition strategies. Motion also degrades B0 homogeneity, which can shift frequency-selective excitation profiles and introduce artifacts into rapid imaging readouts. This makes accurate determination of the acquisition center frequency and shimming essential in these regions which often cover large FOVs. (See “Prescan Calibration” section for more information). In some studies, breath-holding was used to minimize motion effects and enforce frame-to-frame data consistency (42). A pragmatic and reasonably effective approach for dealing with respiratory motion during 13C data acquisition is an initial breath-hold (as long as can be tolerated), followed by free-breathing (46,62).

1H Imaging

Collection of 1H imaging data is essential both for prescribing the 13C acquisition and for interpretation of the resulting 13C data. Multi-planar 1H scouts are acquired prior to 13C acquisition to enable graphical prescription of the 13C imaging region. All human HP 13C-pyruvate imaging studies acquire conventional MRI scans (e.g. T1- and T2-weighted volumes) for anatomic reference, aiming to cover at least the full 13C FOV. Acquiring these anatomic scans as close as possible to the time of 13C imaging (immediately before or after) minimizes potential misregistration between the data sets. Depending on the application, other advanced 1H sequences are also acquired (e.g. diffusion-weighted imaging for cancer imaging).

When contrast-enhanced data is acquired, it is done after 13C imaging, as paramagnetic contrast agents will accelerate 13C relaxation.

Reported Study Parameters

Figures 5 and 6, and Supporting Table S2 shows the reported acquisition study parameters for human HP [1-13C]pyruvate studies published as of September 2022. Figure 5 shows a mixture of MRS/I, metabolite-specific imaging, and chemical shift encoding methods have been successfully used, where spectroscopy-based methods have become less prevalent in recent studies. Figure 6 shows the acquisition timing, including the important start time and interval/temporal resolution, is quite variable across studies.

Figure 5: Acquisition methods used in published HP [1-13C]pyruvate human studies published up to September 2022, classified into: MR spectroscopy and spectroscopy imaging (MRS/I); chemical shift encoding methods, such as IDEAL, that use multiple TEs and model-based reconstructions; and metabolite-specific imaging methods that use spectrally-selective excitation to image a single resonance at a time.

Figure 6: Temporal acquisition characteristics reported in HP [1-13C]pyruvate human studies published up to September 2022. (a) Reported referencing of acquisition start times.

(B)

Acquisition start times reported when using dynamic imaging and when timing was reported relative to the end of the injection. (c) Temporal resolutions. “Not Applicable” indicates dynamic imaging was not used.

Summary

Three general categories of acquisition strategies have been used successfully for human HP 13C-pyruvate studies: MRS/I, model-based chemical shift encoding (e.g. IDEAL) methods, and metabolite-specific imaging methods. These have enabled successful studies in the prostate, heart, brain, abdomen, and breast. Recent studies increasingly have used the imaging-based strategies of metabolite-specific imaging and chemical shift encoding which are the fastest methods, although a heads-to–head comparison between techniques has not been performed.

Metabolite-specific imaging is quite popular because of its speed and compatibility with single-shot imaging, but is sensitive to B0 field variations and thus requires careful calibrations. Nearly all studies surveyed acquired data dynamically, allowing measurement of the bolus and metabolite kinetics. The exact timings and associated flip angles vary quite widely across reported studies, with no consensus yet as to how to choose these parameters. Image reconstruction is typically done directly using Fourier Transform methods, and accelerated imaging strategies are uncommon.

Data Analysis And Quantification

This section covers the analysis of data from human HP [1-13C]pyruvate studies, including modeling and metrics, visualization, as well as considerations for how to store data and metadata. Depending on study design, the analysis may need to give quantitative or semi-quantitative output reflecting a biological process or may just reflect a contrast between different regions of interest for quantitative evaluation.

Metrics

Figure 7: HP [1-13C]pyruvate raw data (A) have typically been quantified using four categories of metrics depending on the acquisition. Data acquired as a single time point are often quantified using normalized metabolite images or metabolite ratios (B). Dynamic data can be quantified using normalized metabolite images or metabolite ratios (B), or with metabolite timings such as time-to-peak (TTP) or pharmacokinetic (PK) models (C). The latter two require the data to be time-resolved. [1-13C]alanine and 13C-bicarbonate are analyzed similarly to [1-13C]lactate but omitted here for display.

Metabolite images are commonly used as summary metrics for HP MRI data, often including some form of normalization as well as summed over time as an area under the time curve (AUC) (17). These are analogous to the visual evaluation that is most used for routine clinical work (89,90). In these metabolite images, we expect that the [1-13C]pyruvate AUC signal is predominantly weighted towards perfusion and uptake, while [1-13C]lactate, [1-13C]alanine and 13C-bicarbonate AUCs represent metabolic conversion. The strength of this approach lies in its simplicity and relatively few underlying assumptions. Limitations to the use of single-metabolite images or AUCs include sensitivity to inhomogeneous coil profiles (57,87,91), the acquisition strategy and acquisition parameters, pyruvate polarization and concentration level, and signal relaxation rates (92). Further, the reader must be careful to interpret all the images in conjunction to better understand the underlying biology; for example, increased [1-13C]lactate in the presence of decreased [1-13C]pyruvate delivery can have a very different meaning compared to increased [1-13C]lactate with increased [1-13C]pyruvate delivery.

In an attempt to address variations in coil sensitivity, polarization level, and pyruvate delivery, AUC images are often computed by normalizing to a specified parameter, such as the maximum pyruvate or average lactate signals, or presented as a ratio such as lactate/pyruvate or divided by “total Carbon” - the sum total of HP 13C signal observed across all metabolites. The AUC ratios between metabolites and pyruvate are proportional to the corresponding forward kinetic rates (81,93), but are not directly comparable to rate constants when magnetization loss rates (e.g. relaxation and losses due to signal excitation) differ between studies. Similarly, the ratios between the produced metabolites (e.g. bicarbonate/lactate) can reflect the balance between downstream metabolic pathways (12,55). Care must be taken to consider how AUC images are calculated and normalized before comparing values between studies.

To further quantify the interpretation, pharmacokinetic (PK) modeling approaches were developed to compute the apparent kinetics of pyruvate-to-metabolite exchange (92,94–99). These yield semi-quantitative to quantitative apparent rate constants, given in s-1. Some models require a vascular input function, while others avoid this requirement (95). PK models can explicitly account for acquisition-specific details such as excitation angle and repetition time, and thus may reduce the effects of these details on quantification. An input-less model, provided in the Hyperpolarized-MRI-Toolbox (https://github.com/LarsonLab/hyperpolarized-mri-toolbox) (100) and thus frequently employed for human data, has been shown to fit well and robustly to prostate and brain data (8,20). PK models are quantitative in nature, arguably provide more relevant biological information (8,20), and appear to be reproducible across sites (51). However, rate constants derived from PK models are still apparent rates, and likely do not reflect a single biological characteristic.

Some additional considerations include whether complex or magnitude data is used, as the noise behaviors will impact the analysis differently. Additionally, cut-off thresholds or other criteria may be used to identify and avoid voxels with insufficient SNR before analysis to improve robustness (20,41).

Regardless of the analysis approach, the underlying biology is not always clearly represented by the data; instead, the metrics may be influenced by perfusion, barrier permeability, intercellular shuttles, enzyme activities, co-substrate concentrations, or combinations thereof, depending on the organ and disease of interest (19,43,94,101–103). This may be addressed by incorporating complementary information. As an example, HP 13C pyruvate data is influenced by perfusion, and thus addition of perfusion MRI could be important for interpretation (98,104,105).

All the methods outlined above have been explored in clinical studies, described in Supporting Table 3 and summarized in Figure 8. As of September 2022, approximately 52% of studies involving human subjects report rate constants derived from a PK model with a few different models reported. A nearly equal fraction (51%) of the studies report AUC ratio values.

Approximately 66% of these studies report metabolite-specific images or AUC values. About 40% report SNR values; this metric is particularly frequent in manuscripts that describe technical developments for clinical HP MRI. Approximately 16% of these studies summarize model-free metrics, and 10% report measurements from a single timepoint. Most studies report a combination of quantities.

Figure 8: Reported metrics used for analysis in HP [1-13C]pyruvate human studies published up to September 2022.

Visualization

A wide variety of approaches have been used for visualizing data from human HP 13C-MRI studies. The challenges and practical considerations are: 1) choosing the appropriate metrics to display, 2) how to encode the parameters (e.g. the colormap), and 3) choosing how to provide anatomical context and other multi-parametric data. The choice of visualization also depends on the goal which could be for diagnostic interpretation, but also quality control, reproducibility among readers and publication.

Metrics

The choice of HP 13C metrics is described in detail above. At this stage in HP 13C development where there is no standardized metric, often a combination of metabolite images and ratios or PK model parameters are shown.

Parameter Encoding

The mapping function chosen should provide an adequate, often quantitative, impression of the parameter mapped. There is a consensus in the visualization field that perceptually uniform maps are best suited to visualize continuous parameters, like the greyscale typically used by radiologists as well as other monochrome (black to blue) and color ranges (fire-type, rainbow-type) (106,107). Multi-color heatmaps have been the most frequently employed method for HP 13C data, while greyscale has infrequently been used but it ensures there is no coloring-based bias as well as facilitating later reuse (Fig. 9a). Among the color schemes employed in the clinical HP 13C literature, fire-type scheme seems to be the most common [similar to “Plasma” or “Inferno” in matplotlib.org]. Next most commonly employed is the rainbow-type scheme [similar to “Rainbow” in matplotlib.org].

Anatomical Context

HP MRI faces the challenge that it does not necessarily depict the anatomical features, similar to PET, and thus requires an anatomical reference. Most often, a grayscale anatomical image is overlaid with a HP colormap (Fig. 9c,d). This approach is very intuitive, but can skew perception as the grey-scale anatomical reference may affect the brightness of the HP data (e.g. signal in the skull). This bias does not occur when showing adjacent maps (Fig. 9a, b). Here, anatomical outlines may help to provide reference (Fig. 9b).

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