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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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Dependence of BH lesion formation on frequency. On the basis of the observations in the PA gel phantoms, the 1-MHz, 1.2-MHz and 1.5-MHz frequencies were selected for further investigation in ex vivo tissue, as they included the range that demonstrated the presence or absence of prefocal shielding by cavitation, as well as likely providing the largest ablation rate. For comparison, the 1.9-MHz frequency was also included in some of the experiments, because it was the closest to the 2.1-MHz frequency which we used extensively in our previous studies (Canney et al. 2010; Khokhlova et al. 2011). Figure 6a shows representative photographs of bisected BH lesions induced in bovine liver and myocardium at the frequencies of 1 MHz, 1.2 MHz, 1.5 MHz and 1.9MHz, at lowoutput power level. Table 3 lists the width and the length of the lesions in bovine liver averaged in the number of samples tested, with variabilities mostly caused by inaccuracies in the tissue bisection rather than the natural variability of the process, on the basis of our prior experience (Wang et al. 2013). As expected, the overall trend of the sizes being inversely proportional to the HIFU frequency was observed. Both transverse and longitudinal sizes of the lesions at 1.9 MHz, to within the measurement error, were consistent with those reported in Khokhlova et al. (2011) at 2.1 MHz. Another observation (illustrated in Figure 6b) is the formation of "ghost" lesions instead of liquefied voids in the liver at 1MHz (dashed white box),whichwas observed in 25% of these exposures. The ghost lesion consisted of a prefocal region of tissue that appeared mechanically disrupted. On the basis of darkened coloration, but not liquefied (i.e., could not be irrigated out), and a small region of thermally denatured tissue at the focus. However, hyperechoes on B-mode images corresponding to ghost lesions were not distinguishable fromhyperechoes observedwhen boiling at the focus yielded a typical liquefied lesion. For ghost lesions, B-mode hyperechoes most likely corresponded to the prefocal cavitation bubble cloud also observed in PA gel at 1 MHz that shielded the focal area and prevented the onset of boiling. In bovine myocardium, such ghost lesions were formed at 1 MHz in all cases (Fig. 6c). The liquefied lesions formed at 1.5 MHz in bovine myocardium were somewhat smaller than those at 1.2 MHz, which follows the same trend observed in liver tissue. Dependence of BH lesion formation on the shock amplitude. In this set of BH exposures of ex vivo tissue, the HIFU frequency was fixed at either 1 MHz, 1.2 MHz or 1.5 MHz, and the focal pressure amplitude was gradually increased while reducing the pulse duration accordingly, and keeping the duty factor and the number of pulses per spot the same (1% and 30 pulses, correspondingly).
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