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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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DISCUSSION. Some of the most promising potential clinical applications of BH involve ablation of tumors in the liver, kidney and pancreas. The considerable depth of these targets within the body warrant the use of relatively low HIFU frequencies in the range of 1-2 MHz, as are used in most commercial HIFU ablation instruments. Historically, most studies in optimization of BH, both in vivo and ex vivo, were performed at 2 MHz, and the acoustic parameter space within the 1-2 MHz frequency range and focal pressure amplitude was not thoroughly explored. In this study, we investigated the efficiency of BH ablation within this frequency range using a broadband HIFU transducer in conjunction with a 26 kWpower amplifier. In accord with previous findings, the size of the lesion and, therefore, the ablation rate generally increased with decreasing frequencies. At the higher frequency of 1.9 MHz the lesion dimensions in ex vivo tissue agreed with those produced in our previous studies at 2.1 MHz to within the experimental error. The use of lower frequencies (1–1.2 MHz) resulted in a higher incidence rate of ghost lesions—a prefocal area of tissue that appears to be mechanically disrupted, with a narrow thermally denatured area at the focus. We speculate that this effect can be attributed to the lower cavitation threshold at lower frequencies, and the increased probability of forming a prefocal bubble cloud that distorts the acoustic field and prevents the onset of boiling at the focus. The heating at the focus may still be sufficient to form the narrow denatured area in ghost lesions, but not to reach boiling temperature. Indeed, the formation of a cavitation bubble cloud that visibly distorted the heating pattern within the 10-ms BH pulse was directly observed in all exposures at 1 MHz in transparent gel phantoms. In tissue, the activity of the prefocal cavitation cloud alone appeared to be insufficient for full liquefaction within the 30 pulses delivered, only for partial tissue disruption. It is unclear however, whether the activity of this cavitation cloud would eventually produce a fully liquefied void provided it was given a longer exposure duration, i.e., a larger number of delivered pulses. This is an intriguing question that warrants further exploration. On real-time B-mode imaging, the appearance of a cavitation cloud was indistinguishable from that for a boiling bubble that produced a liquefied BH cavity both were seen as a transient hyperechoic region. The confusion between these two phenomena may be deceiving in BH treatment planning. Such confusion could be avoided by operating in a parameter range that is unlikely to produce cavitation-induced shielding. Above 1.5 MHz, we observed no cloud cavitation, which suggests the use of a higher frequency. However, the use of lower frequencies is still desirable, because the greater ablation volume would allow the treatment to proceed much more quickly.
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