Towards Non-invasive Ultrasonic Characterization of Carotid Atherosclerotic Plaque

Vulnerable atherosclerotic plaques are thought to be prone to rupture due to various compositional and morphological factors. One key characteristic is thought to be the presence of a soft, lipid rich core in the plaque. Acoustic radiation force impulse (ARFI) and thermal strain imaging (TSI) are non-invasive ultrasound-based imaging modalities. ARFI imaging measures the tissue response to an ultrasonically generated mechanical perturbation. In TSI, the tissue temperature is increased and image contrast is a result of the temperature and composition dependence of the speed of sound. Initial efforts to develop a TSI system utilized two separate ultrasound transducers for heating and imaging. We developed signal processing to improve estimates of thermal strain obtained from this system and showed that TSI could be used to detect lipids in ex vivo human arterial tissue samples. However, the translational obstacles encountered by this system outweighed the potential imaging utility. In order to address these challenges, we developed temporally interleaved multi-foci beamforming which could be implemented on a standard imaging array to generate a broad, homogeneous ultrasound beam for either ARFI pushing or TSI heating. We showed that this beamforming approach could enable simultaneous acquisition of ARFI and thermal strain data while substantially improving the frame rate for ARFI imaging. In order to better understand the factors that affect signal quality in TSI and ARFI imaging, we conducted separate phantom studies for each imaging modality. We showed that with a temperature rise <1oC, TSI could differentiate between phantoms with different lipid percentages. Additionally, we showed that pulse inversion harmonic imaging could be used to improve TSI signal quality in the presence of clutter. Finally, we showed that high frame rate ARFI imaging was able to achieve a 45-fold improvement in frame rate at the cost of increased estimation bias and jitter, and decreased image contrast. These studies indicate that multi-foci beamforming can be used to enable simultaneous TSI and ARFI imaging on current clinical systems. This imaging sequences developed in this dissertation facilitate non-invasive assessment of both the composition and mechanical properties of tissue which might be especially useful for characterization of vulnerable plaques.