Page 33 - Volume 8, Issue 4 - Winter 2012

Fig. 6. Experiment demonstrating the role of shocks in forming cavitation clouds. (Top) Pulses focused without
an acoustic filter have significant shock amplitude (blue) while the filtered waveforms have reduced shock ampli-
tude and p (upper left). Both waveforms have the same p . In the frequency domain (upper right), this trans- +-
lates to a reduction in the high-frequency harmonics of the pulse while the lower frequency content remains. (Bottom) The reduction in shock amplitude results in significant reduction in the probability of generating a cloud during a pulse (Pcloud), although single microbubble cavitation from the negative pressure of an incident wave is still observed.
propagating wave. This cluster acts as a scatterer for the next shock in the incident pulse, which creates a second cluster of cavitation in front of the first. The reflected shock incites a larger region of cavitation, and this cavitation serves to scat- ter more effectively the following shock, in a cascade effect which is only terminated when no additional shocks are available (either the pulse ends or the cloud grows outside the focal region where the shocks do not occur). This seeding effect causes the cloud to emerge from a single bubble and grow explosively back toward the transducer. The cloud takes
on a layered appearance, each layer occurring from one shock front of the pulse.
This mechanism explains the “all- or-nothing” behavior of the cloud. If a single bubble nucleus is not serendipi- tously positioned within the center of the focal region where the shocks form, there is no scatterer to initiate the cloud. While the incident p- is respon- sible for forming the single bubbles, the shocks, which only form locally within the high pressure focal region, are responsible for initiating the cloud. One experiment which illustrates this particularly well is the introduction of an acoustic low-pass filter between the transducer and focal zone. In this experiment, the filter was constructed from thin sheets of brass which effec- tively attenuate the high-frequency harmonics that create the shocks. Meanwhile, the fundamental frequency transmission that determines p- is min- imally altered (Fig. 6). With the filter in place, the shock amplitude was reduced from 85 to 38 MPa, while p- was 19 MPa in both cases. This filter sup-
pressed the probability of generating a cloud during a pulse fromP=0.72toP=0.01.
The acoustics at the initial phase of this process can be modeled by the classic problem of a plane wave scattering
27,28
Such a model demonstrates the importance of the characteristic of the shock front to the process. The individual seed bubbles are considerably small- er than the fundamental wavelength (λ = 1.5 mm in the gel) and scattering at this frequency is weak. However, the high frequency components of the waveform scatter strongly and
from a spherical fluid target.
Fig. 7. Photographs (top) and simulations (bottom) of a shock scattering from a bubble. The shock is focused in the experiment but is approximated as a plane wave in the simulation. The constructive interference of the scattering wave and the rarefaction phase of an incident wave creates a region where the tensile pressure is greatest, which coincides with the location cavitation is observed in the field. The 5 frames of simulations on the left show the transient pressure distribution and the image on the right shows the peak negative pressure achieved throughout the field over 1 cycle.
Disintegration of Tissue Using HIFU 29