Improvement of image quality of transcranial Doppler ultrasound

Abstract

Transcranial Doppler (TCD) sonography is an ultrasound method that measures blood flow velocity (FV) from the cerebral vessels. The use of TCD sonography is highly attractive as a modality because of its non-invasive nature and portability. Although TCD is recommended in the clinics, its usage is hindered in cases like vasospasm, which requires blood flow rate calculation. Blood flow rate calculation requires accurate measurement of the diameter of the blood vessel, and it can only be calculated when the image quality is good. The problem with TCD lies in the low ultrasonic energy penetrating inside the brain through the skull which leads to a low signal-to-noise ratio. This is because of several effects including phase aberration, variations in the speed of sound in the skull, scattering, the acoustic impedance mismatch, and absorption of the three-layer medium made up of soft tissues, the skull, and the brain. Secondly, there is also an energy loss because of the texture of the temporal bone. This thesis seeks to mitigate such limitations firstly through the development of an analytical model that studies the effect of transmission losses because of the acoustic impedance mismatch on the transmitted energies as a function of frequency. To do so, the wave propagation model will be shown from the ultrasonic transducer into the brain. This model calculates frequency-dependent transmission coefficient for a given skin and bone thickness. This approach was validated experimentally by comparing the analytical results with measurements obtained from a bone phantom plate mimicking the skull. The results showed that there is a possibility to choose an optimized excitation frequency based on the skin and the bone thicknesses improving the image quality of TCD. Based on these results, Doppler experiments were performed on the flat bone phantom plate considering the acoustic impedance mismatch effect. The results allowed the visualization of FV in the blood vessel phantom for the selected frequencies for which there is a high SNR. It was also shown in the results that there were some frequencies for which Doppler measurement was not possible because of low SNR. Blood mimicking fluid flow rates were calculated, which showed great consistency across different experiments. Doppler experimental results showed a major dependency on acoustic impedance mismatch. Surface adaptive ultrasound (SAUL) method, a well-known method in non-destructive testing (NDT) was proposed to mitigate the effect of transmitted energy losses due to the irregular surface of the temporal bone phantom. Firstly, a finite element model (FEM) using POGO is proposed, which implemented the SAUL method to calculate the difference in the transmitted energy with a plane wave and surface compensated wave. The results got from FEM laid a proof of concept for the experiments. Experiments were performed using five different temporal bone phantoms having different thicknesses, attenuation, and texture. The results available from these experiments showed higher transmitted energy with a surface compensated wave. Overall, the results obtained from this work laid a strong background to do an in-vivo study for larger samples to quantify the success rate in TCD using the proposed methods.

Date	30 Apr 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor) & Catherine Laporte (Co-supervisor)