Quantifying the Impact of Imaging with Elevated Acoustic Output in Diagnostic Ultrasound
Ultrasound imaging is one of the most widely used diagnostic imaging modalities. Abdominal ultrasound is typically used for screening liver diseases, and it is the recommended modality for six month screening in patients at risk for hepatocellular carcinoma (HCC). The major drawback of abdominal ultrasound is poor image quality that is insufficient for diagnosis, which is reported in 25-60% of patients, and is often correlated with obesity. Tissue harmonic imaging (THI) has become the default imaging mode for most abdominal imaging exams. It has also been applied in motion tracking in ultrasound elastrography. THI provides better image quality through decreased sidelobe energy and decreased reverberation clutter in the abdominal wall compared to fundamental imaging. However, THI can be both signal-to-noise ratio (SNR) and penetration-depth limited during clinical imaging, resulting in decreased diagnostic utility.
A logical approach to increase the SNR of harmonic imaging is to increase the acoustic source pressure, but the acoustic output of diagnostic imaging has been subject to a de facto upper limit based upon the Food and Drug Administration (FDA) guideline for the Mechanical Index (MI < 1.9). This value was derived from historic values, rather than being linked to scientific evidence of bioeffects. A recent report from the American Institute of Ultrasound in Medicine (AIUM) concluded that exceeding the recommended maximum MI given in the FDA guidance up to an estimated in situ value of 4.0 could be warranted without concern for increased risk of cavitation in non-fetal tissues without gas bodies, if there there were concurrent improvement in diagnostic utility.
This thesis presents the preliminary work of evaluating the potential diagnostic benefit of employing acoustic output beyond the FDA guideline of MI = 1.9 in the context of hepatic imaging. Three clinical studies were performed with goals of: (1) quantifying the image quality improvement of using elevated acoustic output in B-mode harmonic imaging; (2) assessing penetration depth changes in harmonic imaging; and assessing elevated MI in (3) harmonic motion tracking and in (4) acoustic radiation force impulse (ARFI) excitation of shear wave elasticity imaging (SWEI). High MI B-mode harmonic imaging resulted in modest increases in the contrast-to-noise ratio of hypoechoic hepatic vessels. Difficult-to-image patients who suffer from poor ultrasound image quality demonstrated larger improvement than easy-to-image subjects. The imaging penetration depth increased linearly with increasing MI, on the order of 4 - 8 cm per unit MI increase for a given focal depth. High MI harmonic motion tracking resulted in considerable increase in shearwave speed (SWS) estimation yield, by 27% and 37% at focal depths of 5 cm and 9 cm respectively, due to improved harmonic tracking data quality through increased SNR and decreased jitter of tissue motion data. In addition, SWS estimation yield was shown to be linearly proportional to the push (ARFI excitation) energy. SWEI measurements with elevated push energy were successful in patients for whom standard push energy levels failed. These studies suggested that liver capsule depth could be used prospectively to identify patients who would benefit from elevated output in push energy. These results indicate that using elevated acoustic output has the potential to provide clinical benefit for diagnostic ultrasound.
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