Safety Considerations and Clinical Benefit Analysis for the Use of Elevated Acoustic Output in Diagnostic Ultrasound Imaging

Diagnostic ultrasound imaging is sometimes unable to yield clinically useful data. This is caused by the presence of body walls. This thesis examines how body walls impacts the propagation of ultrasound in the human body and searches for ways to improve image quality.Previous works have demonstrated remarkable improvements in tissue stiffness quantification with the use of elevated Mechanical Index (MI) during ultrasonic harmonic shear wave elasticity imaging (SWEI). Since ultrasound SWEI sequences consist of a long push pulse and a short tracking pulse, it remains unclear which one of the two pulses is impacted more by the body wall and benefits more from elevated MI. In Chapter 3, an opposing window experiment is devised and built to isolate the impacts of the body wall on push and track beams. Track beams are found to be more affected by the presence of body walls and to benefit from higher MI transmits. In Chapter 4, 3D nonlinear ultrasound simulations and experimental measurements were used to estimate the range of in situ pressures that can occur during transcutaneous abdominal imaging, and to identify the sources of error when estimating in situ peak rarefaction pressures (PRP) using linear derating as specified by the MI guideline. Using simulations, it was found that for a large transmit aperture (F/1.5) MI consistently overestimated in situ PRP by 20-48%, due primarily to phase aberration. For a medium transmit aperture (F/3), the MI accurately estimated the in situ PRP to within 8%. For a small transmit aperture (F/5), MI consistently underestimated the in situ PRP by 32-50%, with peak locations occurring 1-2 cm before the focal depth, often within the body wall itself. The large variability across body wall samples and focal configurations demonstrates the limitations of the simplified linear derating scheme. The results suggest that patient specific in situ PRP estimation would allow for increases in transmit pressures, particularly for tightly focused beams, to improve diagnostic image quality while ensuring patient safety. Tissue harmonic signal quality has been shown to improve with elevated acoustic pressure. The peak rarefaction pressure (PRP) for a given transmit, however, is limited by the FDA guideline for the mechanical index (MI). In Chapter 4, We demonstrated that the MI overestimates in situ PRP for tightly focused beams in vivo due primarily to phase aberration. In Chapter 5, we evaluate two spatial coherence-based image quality metrics, short-lag spatial coherence (SLSC) and harmonic short-lag spatial coherence (HSC), as proxy estimates for phase aberration and assess their correlation with in situ PRP in simulations and experimentally when imaging through abdominal body walls. We demonstrate strong correlation between both spatial coherence-based metrics with in situ PRP (r2 = 0.77 for HSC, r2 = 0.67 for SLSC), an observation that could be leveraged in the future for patient-specific selection of acoustic output.