Passive cavitation imaging with ultrasound arrays

Abstract

A method is presented for passive imaging of cavitational acoustic emissions using an ultrasound array, with potential application in real-time monitoring of ultrasound ablation. To create such images, microbubble emissions were passively sensed by an imaging array and dynamically focused at multiple depths. In this paper, an analytic expression for a passive image is obtained by solving the Rayleigh-Sommerfield integral, under the Fresnel approximation, and passive images were simulated. A 192-element array was used to create passive images, in real time, from 520-kHz ultrasound scattered by a 1-mm steel wire. Azimuthal positions of this target were accurately estimated from the passive images. Next, stable and inertial cavitation was passively imaged in saline solution sonicated at 520 kHz. Bubble clusters formed in the saline samples were consistently located on both passive images and B-scans. Passive images were also created using broadband emissions from bovine liver sonicated at 2.2 MHz. Agreement was found between the images and source beam shape, indicating an ability to map therapeutic ultrasound beams in situ. The relation between these broadband emissions, sonication amplitude, and exposure conditions are discussed.

Figure 1

Computer simulations for passive images created using 64-element (14.08 mm) subapertures are shown here. The top row contains examples of simulations where an “idealized” array approximation is used for each subaperture, with single point-sources at depths (a) 20 mm, (b) 55 mm, (c) and 90 mm. Simulated images in the bottom row were created through time-delay focusing at 16 equally spaced depths (6.2 mm) with single point-sources located at (d) 20 mm, (e) 55 mm (e), and (f) 90 mm. All images are plotted with a 40 dB dynamic range.

Figure 2

Computer simulations for passive images created using a constant f-number (7.1) subaperture are shown here. Except for the subaperture sizes, images were created using the same simulation methods, focus positions, and source locations as the corresponding panels in Fig. 1.

Figure 3

Point-sources represented in this figure were assumed to emit acoustic energy between 6.3 and 6.7 MHz (64-element subapertures). (a) Simulated image of five point-sources at (azimuth mm, range mm): (−12,40), (−10,40), (0, 10), (0, 40), (0, 70). (b) Simulated image of a cluster of 25 point-sources placed randomly in the image plane between −7 and −3 mm in azimuth, and 3141 mm in depth. (c) Comparison between depth-integrated simulated image brightness as a function of azimuth, and the azimuthal distribution of point-sources. (d) Comparison between azimuth-integrated simulated image brightness as a function of range, and the range distribution of point-sources. Passive cavitation images are plotted with a 40 dB dynamic range.

Figure 4

Experimental setup: CW ultrasound sources sonicate a 1-mm steel wire, PBS and bovine liver, while a 192-element linear array captures passive images.

Figure 5

(a) Passive images of 520-kHz ultrasound scattered from a 1-mm steel wire located at 20-mm depth, (b) 55 mm and (c) 90 mm. A constant f-number (7.1) subaperture was employed. Energy in the source harmonics (5.2–9.36 MHz) was integrated. Passive cavitation images are plotted with a 40 dB dynamic range.

Figure 6

Image magnitude along the array axis, plotted with respect to the range dimension, for representative simulation (---) and experiment (—) cases. The depth location of simulated point-sources and wire target (in experiment) were (a) 20 mm, (b) 55 mm, and (c) 90 mm.

Figure 7

Acoustic emission spectra from PBS solution sonicated with 520-kHz, cw ultrasound at 0.125 MPa (peak-negative pressure). Power spectra measured in dB relative to the measured noise floors were computed from (a) rf data acquired by the L7 array and (b) acoustic emission signals recorded by the single-element (10 MHz) detector.

Figure 8

Representative passive cavitation images in saline solution due to 520-kHz CW ultrasound 0–0.15 MPa (peak-negative): (a) B-scan showing a cavitating bubble cloud, (b) co-registered passive cavitation image formed from ultrahar-monic emissions (6.5 MHz), and (c) co-registered passive cavitation image formed from broadband emissions (6.3–6.7 MHz). (d) Comparison between B-scan and passive image brightness levels integrated between 31 and 41-mm depth, across all azimuths. Passive cavitation images are plotted with a 30 dB dynamic range.

Figure 9

(a) Representative passive cavitation image using broadband emissions (8–10 MHz) from bovine liver sonicated with 2.2-MHz, CW, focused ultrasound at 0.8 MPa peak-to-peak pressure amplitude (0.38 MPa peak-negative pressure). (b) Spatially integrated emission energy as a function of sonication amplitude, plotted as mean ± st. dev. (c) Comparison of emission amplitude at 20-mm depth with measured beam profile at 0.80 MPa (peak to peak) sonication pressure. (d) Comparison of emission amplitude at 20-mm depth with measured beam profile at 1.44 MPa (peak to peak) sonication pressure. Passive cavitation images are plotted with a 40 dB dynamic range.