Coalescence and Extinction of Residual Cavitation Nuclei by Means of the Secondary Acoustic Field while Performing Histotripsy with Electronic Focal Steering
Following the collapse of a cavitation bubble cloud, residual microbubbles can persist for up to seconds and function as nuclei for subsequent cavitation events in a phenomenon known as the cavitation memory effect. The cavitation memory effect can hinder tissue fractionation efficiency and homogeneity during histotripsy treatment. One strategy for mitigating the cavitation memory effect is to allow sufficient time between pulses at a given location such that residual nuclei dissolve under the Laplace pressure. Another, as Duryea et al. have demonstrated, is an active strategy termed “bubble coalescence” (BC) whereby ~1 MPa, ~1000-cycle pulses are applied to drive residual nuclei together using the secondary Bjerknes force. Histotripsy combined with BC can significantly improve ablation efficiency. Previously, BC has been performed using separate acoustic sequences for high-amplitude therapy pulses and low-amplitude BC pulses. The present study demonstrates that by using electronic focal steering (EFS) to direct the therapy focus throughout a trajectory consisting of many discrete spatial positions, it is possible to use the acoustic secondary field to accomplish BC during EFS without any additional pulse sequence. The authors define the acoustic secondary field to be regions of low, but nonzero, gain outside the main lobe and side lobes.
Histotripsy pulses with approximately 1.5 cycles and 50 MPa P- were generated by a 250 kHz , 256-element hemispherical array with a 15 cm focal distance. All experiments were performed in agarose hydrogel degassed to approximately 15% O2 saturation. First, the size and passive dissolution behavior of residual nuclei were monitored over time by a high-resolution optical camera. Cavitation bubble clouds were generated at a position 3 mm offset in the lateral direction from the geometric focus, (0, 3, 0), with 5 s pulse repetition period to allow time for residual nuclei to dissolve between pulses. This focal location was selected so that high-amplitude signals emitted by the cavitation bubble cloud, which are reflected and refocused by the surface of the transducer, would be refocused to a position mirrored about the acoustic axis, (0, -3, 0), thereby reducing the influence of these signals on the residual nuclei population at (0, 3, 0). Second, to demonstrate proof of principal regarding BC using the secondary acoustic field during EFS, a special 50-point steering sequence was constructed such that the first position in the EFS sequence lay at (0, 3, 0). The remaining 49 EFS positions were distributed in a plane 12 mm beyond the geometric focus with a minimum of 5 mm of separation. As a result, the first EFS position experienced P+ = 0.9 ± 0.2 MPa and P- = 0.7 ± 0.1 MPa (mean ± s.d.) over the course of the remaining 49 EFS pulses. The PRF of the 50-pulse EFS sequence was varied between 50 Hz to 5 kHz with a burst repetition period of 5 s. A high speed optical camera was used to monitor the activity of residual nuclei at the first EFS position.
For the passive dissolution experiment, the mean and maximum radii of residual nuclei at 10 ms following the histotripsy pulse were found to be 4.5 ± 0.3 and 11.6 ± 1.9 µm, respectively. The largest residual nucleus for each pulse required 0.4-2.3 s to dissolve passively below the ~2 µm detection limit of the camera (fig. 1). For the second experiment, all residual nuclei were coalesced into a single bubble or dense bubble cluster no more than 0.8 mm across with 100% efficacy for PRF ≤ 1 kHz and ~99% for PRF > 1 kHz (fig. 4a). Coalescence was achieved within 3.9 ± 1.6 pulses at 50 Hz PRF (79 ± 32 ms) and within 19.9 ± 5.9 pulses (4 ± 1 ms) at 5 kHz PRF, the number of pulses required to achieve BC increasing with the PRF of the EFS sequence. In some cases, after consolidating residual nuclei into a single bubble, the bubble was observed to disappear entirely from subsequent frames. This sudden extinction of the bubble (fig. 3) was observed commonly for PRF ≤ 1 kHz and uncommonly for PRF > 1 kHz (fig. 4a). In the cases where extinction of residual bubbles was observed during BC using the secondary field, the time-scale was up to ~100X faster than that of passive dissolution.
Using BC pulses separate from therapy pulses has been shown to increase the histotripsy ablation rate dramatically but at the expense of increased deposition of acoustic energy. The results from this study show that histotripsy with EFS can achieve the same BC effect without employing a separate BC pulse sequence. Thus, this technique has the potential to accelerate histotripsy ablation of volume tissue while minimizing energy deposition. Additionally, the rapid coalescence observed here with a small number of pulses also suggests that only a limited EFS range and an array with a modest number of elements would be required.
Figure 1. a) Characteristic images of passively dissolving residual nuclei, the largest of which persist for up to seconds after cavitation collapse. Time of image capture after expansion of cavitation cloud appears in upper left corner. Scale bar = 100 microns. b) Theoretical (blue) and experimentally observed (grey) dissolution curves. Experimental data show the radius of the largest residual nucleus for each pulse (N=100).
Figure 2. Representative “secondary field”: averaged acoustic waveform (N=2000) measured by fiber optic probe hydrophone (FOPH) at first focus in EFS sequence while exciting cavitation in a plane 12 mm beyond the geometric focus, at a position 14.3 mm from the first focus in the sequence.
Figure 3. High speed images of cavitation activity at the primary focus throughout the EFS sequence (last 10 frames omitted for brevity). The first frame shows bubble cloud expansion at the first EFS position. Frames 2-40 show activity of residual nuclei at EFS position (0, 3, 0) throughout EFS pulses 2-40 with frame-capture timed to coincide approximately with the maximum radius of re-excited residual nuclei. All frames were captured 113 µs after firing earliest phased element and used a 10 µs exposure. Time-stamp appears in the lower left corner of each frame and corresponds to time following the first pulse in the EFS sequence. Global PRF is 1 kHz. Scale bar = 1 mm.
Figure 4. a) Success fraction of bubble coalescence to single bubble or small, dense bubble cluster ≤ 0.8 mm across (blue squares) and complete extinction (grey circles). N=10 samples and 100 cycles/sample for each PRF. b) Number of secondary field pulses required to achieve coalescence of residual bubbles into a single bubble or dense bubble cluster no more than 0.8 mm across (blue squares). Number of pulses required to achieve complete extinction of residual bubbles (grey circles). Error bars represent standard deviation of successful samples.