Ph.D Thesis

Ph.D StudentLevy Yoav
SubjectInvestigation of New Techniques for Ultrasonic Imaging
DepartmentDepartment of Biomedical Engineering
Supervisors PROF. Haim Azhari
PROF. Yehuda Agnon
Full Thesis textFull thesis text - English Version


Ultrasonic imaging offers a valuable non-invasive diagnostic tool. The purpose of this study was to investigate new techniques for ultrasonic imaging in order to:
(a) Introduce a new ultrasonic imaging contrast which may contribute to tissue characterization and tumour detection.
(b) Improve the performance of current methods. The chosen strategy to achieve both challenges was to combine novel spectral analysis methods with the transmission of special signals.

            While most imaging techniques have focused on dominant properties such as tissue echogenity and attenuation, speed of sound dispersion (SOSD) phenomenon is very weak and difficult to measure, and hence has not been used for imaging. In this study, three new methods for measuring SOSD which are sensitive for the weak dispersion in soft tissues are introduced. Using the new techniques, SOSD is utilized as a new imaging contrast source.

            Spectral analysis applied to backscattered ultrasound signals is used in many applications such as attenuation mapping, tissue characterization, temperature monitoring and mean scatterer spacing estimation. Furthermore, it plays a major role in velocity estimation since velocity is associated with the frequency dependant Doppler shift. Signal to noise ratio (SNR) is a great concern in such applications. Hence, it is desirable to utilize high energy transmitted signals. The signal energy can be augmented by increasing the intensity of the transmitted signal. However, this approach is limited by safety aspects. Alternatively, one can increase the transmission duration, but this approach commonly decreases the axial resolution. A method for performing localized spectral analysis using long structured signals was developed.

            In this study, it was shown that the combination of long structured signals and appropriate algorithms yields benefits in terms of SNR, measurement accuracy and acquisition rate.

            With respect to the measurement of SOSD, this study has clearly demonstrated the feasibility of SOSD projection imaging and that SOSD may serve as a new contrast 1

            source. Images, based on SOSD projections, and measurements of SOSD in soft tissues in pulse-echo mode were presented for the first time. These new imaging techniques may contribute to tissue characterization, tumour detection and breast diagnosis. One of the methods developed for SOSD measurement was also found suitable for estimation of target velocity in single ultrasonic transmission.