Quantifying Image Quality Improvement Using Elevated Acoustic Output in B-Mode Harmonic Imaging

Abstract

Tissue harmonic imaging has been widely used in abdominal imaging because of its significant reduction in acoustic noise compared with fundamental imaging. However, tissue harmonic imaging can be limited by both signal-to-noise ratio and penetration depth during clinical imaging, resulting in decreased diagnostic utility. A logical approach would be to increase the source pressure, but the in situ pressures used in diagnostic ultrasound are subject to a de facto upper limit based on the U.S. Food and Drug Administration guideline for the mechanical index (<1.9). A recent American Institute of Ultrasound in Medicine report concluded that an effective mechanical index ≤4.0 could be warranted without concern for increased risk of cavitation in non-fetal tissues without gas bodies, but would only be justified if there were a concurrent improvement in image quality and diagnostic utility. This work evaluates image quality differences between normal and elevated acoustic output hepatic harmonic imaging using a transmit frequency of 1.8 MHz. The results indicate that harmonic imaging using elevated acoustic output leads to modest improvements (3%-7%) in contrast-to-noise ratio of hypo-echoic hepatic vessels and increases in imaging penetration depth on the order of 4 mm per mechanical index increase of 0.1 for a given focal depth. Difficult-to-image patients who suffer from poor ultrasound image quality exhibited larger improvements than easy-to-image study participants.

Keywords: Elevated acoustic output; Harmonic imaging; Mechanical Index journal: Ultrasound in Medicine and Biology.

Fig. 1

Hydrophone measurement results for the 4C1 transducer with a lateral focus at 6 cm. (a) Pressure waveform corresponding to a mechanical index of 1.4. (b) Pressure waveform corresponding to a mechanical index of 2.8.

Fig. 2

Top: Matched tissue harmonic images using mechanical index (MI) values of 1.6 (left) and 2.6 (right) in an easy-to-image (category 1) volunteer with a body mass index of 20.9 kg/m². The arrows indicate structures in which the contrast-to-noise ratio (CNR) was computed. Both images are shown with a dynamic range of 60 dB. Bottom: CNR versus MI for each structure. The error bars reflect the variability of CNR among the three pairs of measurements at each MI.

Fig. 3

Matched tissue harmonic images using mechanical index (MI) values of 1.6 (left) and 2.0 (right) in a medium-image-quality (category 2) participant with a body mass index of 26.0 kg/m². The arrow indicates the structure in which the contrast-to-noise ratio (CNR) was computed. Both images are shown with a dynamic range of 60 dB.

Fig. 4

Number of hepatic hypo-echoic structures that exhibited a contrast-to-noise ratio (CNR) increase between matched low-and high-mechanical index B-mode imaging across all 25 participants. Of the 129 identified structures, 54% had a CNR increase in harmonic images, whereas only 14% of had a CNR increase in fundamental imaging.

Fig. 5

Percentage contrast-to-noise ratio (CNR) change as a function of subjective assessment of overall image quality for hepatic harmonic images. The mean CNR change is positive for all levels of image quality. The CNR increase for image quality level 1 is significantly lower than the CNR increase for image quality levels 2 and 3 (p ≤ 0.005).

Fig. 6

Percentage contrast-to-noise ratio (CNR) change versus focal depth between and low- and high-MI harmonic imaging.

Fig. 7

(a) Example harmonic image using a focal depth of 6 cm and a mechanical index value of 1.6 from an overweight participant (body mass index = 25.7 kg/m²). The image is shown with a dynamic range of 60 dB. The correlation coefficient (CC) was calculated from the M-mode data of the center beam of the image as highlighted by the blue line. (b) CC as a function of depth. With use of a CC cutoff of 0.8, the penetration depth of this image was determined to be 10.4 cm.

Fig. 8

(a) Penetration depth (PD) as a function of mechanical index (MI) computed from harmonic images in an overweight patient (body mass index of 25.7 kg/m²). An increase in MI results in an increase in penetration depth for all focal depths. The dashed lines represent the linear fit between MI and mean PD. The error bars reflect the standard deviation from eight different spatial locations. (b) Slope of the linear fit between MI and PD as illustrated in (a) combining data from all the study subjects (R² ≥ 0.95) as a function of focal depth. The error bars represent inter-participant variability.

Fig. 9

Matched tissue harmonic images using a typical mechanical index (MI) value (MI = 1.4, left) and an elevated MI value (MI = 3.4, right) focusing at 5 cm in an obese volunteer (BMI = 30.4 kg/m²) with a fatty liver. The image intensities are in decibels. The arrows point to vessels in which the contrast-to-noise ratio (CNR) was computed. The cyan and magenta arrows in the right image indicate structures deep to the focus that are visible only in the elevated-MI configuration.