Fast, Low-Frequency Plane-Wave Imaging for Ultrasound Contrast Imaging

Abstract

Plane-wave ultrasound contrast imaging offers a faster, less destructive means for imaging microbubbles compared with traditional ultrasound imaging. Even though many of the most acoustically responsive microbubbles have resonant frequencies in the lower-megahertz range, higher frequencies (>3 MHz) have typically been employed to achieve high spatial resolution. In this work we implement and optimize low-frequency (1.5-4 MHz) plane-wave pulse inversion imaging on a commercial, phased-array imaging transducer in vitro and illustrate its use in vivo by imaging a mouse xenograft model. We found that the 1.8-MHz contrast signal was about four times that acquired at 3.1 MHz on matched probes and nine times greater than echoes received on a higher-frequency probe. Low-frequency imaging was also much more resilient to motion. In vivo, we could identify sub-millimeter vasculature inside a xenograft tumor model and easily assess microbubble half-life. Our results indicate that low-frequency imaging can provide better signal-to-noise because it generates stronger non-linear responses. Combined with high-speed plane-wave imaging, this method could open the door to super-resolution imaging at depth, while high power pulses could be used for image-guided therapeutics.

High-Frequency Multipulse, Plane-Wave Acoustic Contrast Imaging

“…The relative change in CBR between standard imaging and an MP approach helps normalize the results because phantoms and experimental parameters vary across reports in the literature. Although there are differences in exposure parameters, phantoms, and measurement methods, the results reported here are consistent with past experimental work [11], [13], [17], [26].…”

“…Filtering in the bandwidth of the transducer still revealed nonlinear residuals in the PI and AM receive signals. Viti et al [11] reached a similar conclusion using a 4.6-MHz linear array with 104% bandwidth as have others [13], [27]. Fig.…”

“…These results were very close to the 6-dB relative improvement. Kusunose and Caskey [13] observed for PI also using a Verasonics system with plane-wave imaging, but no AM results were reported. The relative change in CBR between standard imaging and an MP approach helps normalize the results because phantoms and experimental parameters vary across reports in the literature.…”

“…We are not aware of any plane-wave MP excitation results in the 15-MHz range. Kusunose and Caskey [13] used in-house UCAs with 2-MHz plane waves and saw a CBR of 10 dB with PI, while the standard plane-wave imaging had a CBR around 4 dB for a relative change of 6 dB. Viti et al [11] used Bracco BR-14 (Bracco Research S.A., Geneva, Switzerland) with 3.5-MHz plane waves and saw a CBR of 25 dB using AM but did not use a PI scheme.…”

“…Frame-rate limitations imposed by MP nonlinear imaging modes can be overcome by using plane-wave approaches [11]- [13]. The unfocused nature of coherent wavefront transmissions reduces peak pressures relative to traditional line-by-line image formation, but nonlinear imaging generally requires lower pressures in order to avoid UCA destruction.…”