Transcranial Doppler — Biomedical Instrumentation & Clinical Sensing

Abstract — Transcranial Doppler

relatively quick and inexpensive tests are growing in popularity. [ citation needed ] The tests are effective for detecting sickle cell disease , ischemic cerebrovascular disease, subarachnoid hemorrhage , arteriovenous malformations , and cerebral circulatory arrest . The tests are possibly useful for perioperative monitoring and meningeal infection . The equipment used for these tests is becoming increasingly portable, making it possible for a clinician to travel to a hospital, to a doctor's office, or to a nursing home for both inpatient and outpatient studies. The tests are often used in conjunction with other tests such as MRI , MRA , carotid duplex ultrasound and CT scans . The tests are also used for research in cognitive neuroscience (see Functional transcranial Doppler, below).

Sensor Interfacing & Analog Front-End Conditioning

Two methods of recording may be used for this procedure. The first uses "B-mode" imaging , which displays a 2-dimensional image of the skull, brain, and blood vessels as seen by the ultrasound probe. Once the desired blood vessel is found, blood flow velocities may be measured with a pulsed Doppler effect probe, which graphs velocities over time. Together, these make a duplex test . The second method of recording uses only the second probe function, relying instead on the training and experience of the clinician in finding the correct vessels. Current TCD machines always allow both methods. [ citation needed ] The ultrasound probe emits a high-frequency sound wave (usually a multiple of 2 MHz ) that bounces off various substances in the body. These echoes are detected by a sensor in the probe. In the case of blood in an artery , the echoes have different frequencies depending on the direction and speed of the blood because of the Doppler effect . If the blood is moving away from the probe, then the frequency of the echo is lower than the emitted frequency; if the blood is moving towards the probe, then the frequency of the echo is higher than the emitted frequency. The echoes are analysed and converted into velocities that are displayed on the unit's computer monitor. In fact, because the probe is pulsed at a rate of up to 10 kHz, the frequency information is discarded from each pulse and reconstructed from phase changes from one pulse to the next.

Bio-Signal Waveform Analysis & Simulation Findings

Because the bones of the skull block most of the transmission of ultrasound, regions with thinner walls (called insonation windows), which offer the least distortion to the sound waves, must be used for analyzing. For this reason, recording is performed in the temporal region above the cheekbone / zygomatic arch , through the eyes, below the jaw, and from the back of the head. Patient age, sex, race, and other factors affect bone thickness and porosity, making some examinations more difficult or even impossible. Most can still be performed to obtain acceptable responses, sometimes requiring using alternative sites from which to view the vessels.

Scope & Clinical Applications

• Real-time multi-parameter vital signs telemetry (ECG, Heart Rate, SpO2, Temperature, Respiration)
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• Digital signal processing in MATLAB: Pan-Tompkins QRS detection, FIR/IIR notch, and Continuous Wavelet Transforms
• Finite element tissue bio-electromagnetics and orthopedic mechanical simulations in COMSOL & ANSYS
• Complete Arduino firmware .INO sketches, circuit schematics, and university dissertation reports