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Effects of nonlinear propagation, cavitation, and boiling in lesion formation by high intensity focused ultrasound in a gel phantom
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Image of FIG. 1.
FIG. 1.

Experimental arrangement for video imaging of HIFU-induced single lesions and lesion stripes in gel phantom. The transducer of frequency, aperture, and focal length was translated vertically at a constant velocity while HIFU was on.

Image of FIG. 2.
FIG. 2.

Experimental arrangement for video imaging of HIFU-induced single lesions in gel under elevated static pressure. The transducer was of frequency, aperture, and focal length.

Image of FIG. 3.
FIG. 3.

Single lesions formed in degassed gel with (a), (b), and (c) acoustic power of the HIFU source after (left-hand set) and (right-hand set) exposure. The HIFU transducer was on the right-hand side. Slight increases in acoustic power cause accelerated lesion growth, then boiling, and then lesion distortion and migration toward the transducer.

Image of FIG. 4.
FIG. 4.

Images of HIFU-induced overheated lesions in excised degassed bovine liver. Both an axial tadpole section (left-hand side) and a transverse section (right-hand side) of the lesions show vaporized cavities along the HIFU axis.

Image of FIG. 5.
FIG. 5.

Numerical results of modeling HIFU in gel with (solid lines) and without (dash lines) accounting for the effects of nonlinear propagation, axial waveforms at the focus; axial time-average intensity, heat deposition, and temperature rise after exposure. Acoustic power of the HIFU source in simulations was which corresponded to spatial peak intensity in gel of calculated linearly. Accounting for nonlinear propagation predicts a shocked waveform, higher intensity and heat deposition, as well as boiling temperatures at focus as compared with the linear modeling.

Image of FIG. 6.
FIG. 6.

Lesion stripes formed in degassed gel sample by moving the HIFU transducer upwards with constant velocity , constant time-average acoustic power of the transducer , and different duty cycle (and peak power) from 6.25% ( peak power, 1) to 8.35% (2), 12.5% (3), 25% (4), 50% (5), 67% (6), and 100% ( peak power, 7). Ultrasound was applied from the front of the sample as indicated by HIFU arrows. The location of HIFU focal peak is shown by the straight dashed lines, and the general shape of the HIFU beam is shown by the curved dashed lines on the views of the sample from above (A) and from the left (B). Although the average power was held constant for each stripe, the stripes have very different sizes. The highest amplitude, shortest duty cycle pulses created the largest lesion stripe (1).

Image of FIG. 7.
FIG. 7.

Comparison of lesion stripes formed in degassed and nondegassed gel samples by moving the HIFU transducer upwards with different scan velocities of 0.5 and . Insonation was from the right. Peak acoustic power was , the increase of the transducer velocity was compensated by increased duty cycle from 12.5% to 100% to maintain equal acoustic energy radiated into the sample to create one stripe. Dashed lines indicate the location of the HIFU focal peak. There are more visible bubbles sustained in the nondegassed gels and for faster scan velocity.

Image of FIG. 8.
FIG. 8.

Sequence of selected video frames illustrating different stages of lesion development in nondegassed gel: lesion inception ( after HIFU was on), thermal symmetric growth , asymmetric tadpole change of lesion shape due to boiling (20 and ), and shrinkage of the lesion soon after HIFU exposure . Two gel samples were sonicated from the right at ambient static pressure for with equal acoustic power of and different illumination either from the left-hand side toward the transducer (frames on the left) or with scattered backlight (frames on the right). Small bubbles of the order of were present and visible as dark shadows in the lesion under backlighting conditions at , but large boiling bubbles of the order of were obvious after .

Image of FIG. 9.
FIG. 9.

Simultaneous visualization of the effect of HIFU on nondegassed gel with B-mode imaging and CCD video capture (inset) during exposure, no overpressure, average acoustic power, and 72% duty cycle. The HIFU transducer was on the right-hand side, lighting was from the left-hand side, and the diagnostic probe was on the back side of the chamber (on the top in the images). The HIFU focal region in the gel is initially hypoechogenic. During HIFU, interference covers all but the center of the image. At the focus, first a weakly brightened zone is seen, and then a strongly echogenic region appears as bubbles become visible in the lesion and the lesion begins to distort dramatically.

Image of FIG. 10.
FIG. 10.

Development of superheated lesion in degassed gel during exposure to HIFU under slightly elevated static pressure of , and boiling explosion of bubbles from the lesion caused by overpressure (OP) release 5 seconds after HIFU was turned off. The HIFU transducer was on the right-hand side. Acoustic power of the source was with 100% duty cycle. An LED in the upper right-hand corner of the frames indicates when HIFU was being applied. The explosion of bubbles is seen as the superheated lesion suddenly boils out.

Image of FIG. 11.
FIG. 11.

Comparison of final lesions obtained in degassed gel after exposure to HIFU (cw regime) under atmospheric static pressure (frames on the left) and under overpressure of 100 bars (frames on the right) with 41.5 and acoustic power chosen below and above the shock formation transition level, respectively. The HIFU transducer was on the right-hand side. Under overpressure the lesions grew symmetrically around the focus to the similar size but different shape than the ones formed without overpressure which were distorted by the effects of boiling bubbles.

Image of FIG. 12.
FIG. 12.

Final lesions formed in degassed gel under standard atmospheric static pressure (1 bar) and under overpressure (100 bars) after exposure to HIFU of lower (with 100% duty cycle) and higher (with 50% duty cycle) peak pressure and equal average acoustic power of . The HIFU transducer was on the right-hand side. Corresponding waveforms are modeled at focus with and without accounting for the effects of nonlinear propagation. The largest change in lesion size is observed by comparing the cases with a change in acoustic pressure, rather than the cases for which only the static pressure was changed. The first comparison isolates the role of acoustic nonlinearity (for high static pressure), whereas the latter comparison isolates the role of cavitation; these observations indicate that nonlinear propagation played a larger role in increasing HIFU heating than did cavitation.


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Chemical composition of polyacrylamide gel with 7% BSA concentration.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Effects of nonlinear propagation, cavitation, and boiling in lesion formation by high intensity focused ultrasound in a gel phantom