Evaluating the Benefit of Elevated Acoustic Output in Harmonic Motion Estimation in Ultrasonic Shear Wave Elasticity Imaging

Abstract

Harmonic imaging techniques have been applied in ultrasonic elasticity imaging to obtain higher-quality tissue motion tracking data. However, harmonic tracking can be signal-to-noise ratio and penetration depth limited during clinical imaging, resulting in decreased yield of successful shear wave speed measurements. A logical approach is to increase the source pressure, but the in situ pressures used in diagnostic ultrasound have been subject to a de facto upper limit based on the Food and Drug Administration guideline for the mechanical index (MI <1.9). A recent American Institute of Ultrasound in Medicine report concluded that an in situ MI up to 4.0 could be warranted without concern for increased risk of cavitation in non-fetal tissues without gas bodies if there were a concurrent clinical benefit. This work evaluates the impact of using an elevated MI in harmonic motion tracking for hepatic shear wave elasticity imaging. The studies indicate that high-MI harmonic tracking increased shear wave speed estimation yield by 27% at a focal depth of 5 cm, with larger yield increase in more difficult-to-image patients. High-MI tracking improved harmonic tracking data quality by increasing the signal-to-noise ratio and decreasing jitter in the tissue motion data. We conclude that there is clinical benefit to use of elevated acoustic output in shear wave tracking, particularly in difficult-to-image patients.

Keywords: Elevated acoustic output; Harmonic imaging; Liver; Mechanical index; Shear wave elasticity imaging.

Figure 1

Example B-mode images from easy (a), medium (b), and difficult-to-image (3) subjects.

Figure 2

Percent yield of successful SWS measurements across all study subjects for low and high MI tracking. Going from low to high MI tracking, the percent yield increased from 43.3% to 70.7%.

Figure 3

Example shear wave trajectories using low and high MI tracking focused at 5 cm. (a) An easy-to-image volunteer (BMI = 20.9 kg/m²), where both low and high MI tracking yield successful SWS estimates and similar SWS values. (b) A difficult-to-image patient (BMI = 31.8 kg/m²), where low MI tracking failed to produce a successful SWS estimate.

Figure 4

Correlation coefficients (CC) estimated from locations away from the shear wave where zero motion is expected. CC was averaged across all acquisitions from all 25 subjects at low and high MI tracking. The ‘x’ and error-bars reflect the median and interquartile range (IQR) from inter-subject variability, which was smaller at the higher MI. High MI tracking results in significantly higher CC with a p-value < 0.01.

Figure 5

Displacement jitter computed from low and high MI tracking. The ‘x’ and error-bars reflect the median and IQR from inter-subject variability. The title of each sub-figure in the top row indicates the data/filtering type. In all cases, high MI tracking results in significantly lower jitter with p-values < 0.01.

Figure 6

Percent SWS yield at various image qualities. The image quality is represented by scores 1–3 (1 - easy, 2 - medium, 3 - difficult).