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DEPENDENCE OF BOILING HISTOTRIPSY TREATMENT EFFICIENCY ON HIFU FREQUENCY AND FOCAL PRESSURE LEVELS
DEPENDENCE OF BOILING HISTOTRIPSY TREATMENT EFFICIENCY ON HIFU FREQUENCY AND FOCAL PRESSURE LEVELS
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In this case, other parameters could be altered to minimize the incidence of ghost lesions, e.g., using longer (10~20 ms at 1.2 MHz or 2~10 ms at 1.5 MHz) pulses with lower focal pressures, so that the prefocal negative pressure does not induce cavitation shielding effects. On the basis of the observed efficacy of longer pulses (between 1.2~1.5 MHz), this frequency range appears to be a good compromise between treatment reliability and ablation rate for BH ablation of deep tissues such as the kidney, liver and pancreas. An important question in the investigation of BH parameter space that has been overlooked in past experiments is whether the size of the BH lesion is determined by the total "HIFU on" time or by the number of pulses delivered to a given focal spot, provided that each pulse induces boiling. The BH exposures of ex vivo tissues performed here at pulse durations of 1~10 ms and the same number of pulses per spot produced lesions of the same size. This indicates that the number of pulses delivered per spot can be considered as an appropriate metric for BH exposure "dose". In addition, we have demonstrated that the exposure time can be reduced 10-fold by decreasing the pulse duration (and increasing the PRF and pressure amplitude at the same time) and still produce the same effect on tissue. In using shorter pulse durations and higher amplitudes, care must be taken to avoid the occurrence of the cavitation cloud shielding the focus that leads to the formation of the ghost lesion. This effect is also frequency-dependent: at 1.5 MHz, ghost lesions were occasionally observed only for the highest output power tested (corresponding to the shortest pulses of 1ms duration), at 1.2 MHz—at the intermediate output power, but at 1 MHz—already at the lowest power (corresponding to the longest pulse duration of 10 ms). As shown in Table 4, the estimated time to reach boiling temperature was in good agreement with the experimental observation of hyperechoic region on B-mode ultrasound for 10-ms and 5-ms pulse durations, but not for the shorter pulses (1 ms and 2 ms). For the latter cases, the estimated time to reach boiling temperature was consistently longer than the pulse duration, yet liquefied lesions still formed. Several factors may contribute to this effect. The tissue at the focus may, indeed, fail to reach the boiling temperature within the first pulse, yet the focal pressure is large enough to produce a cavitation cloud that appears similar to a boiling bubble on B-mode ultrasound. However, the larger PRFs (5~10 Hz) used in these exposures can lead to some heat accumulation between pulses, and, according to the simple estimation of heat diffusion between pulses (following Parker 1983), a combination of 2-3 pulses may lead to 100C temperatures.
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