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[Therapeutic technology utilizing focused ultrasound]

Ultrasound has two major biological effects potentially to be utilized for tumor treatment: heating and cavitational effects. In coagulation treatment, the tissue in the focal spot is heated above the coagulation temperature in a relatively short period of time with high intensity focused ultrasound so that the irreversible change in tissue is completed much earlier than the temperature distribution blurs due to heat conduction. However, the smallness of the coagulation volume formed by an ultrasonic shot causes the problem of low treatment throughput. The split-focus technology, in which the focal spot is enlarged in the lateral rather than longitudinal direction, multiplies the coagulation volume so as to solve this problem. Sonodynamic treatment was proposed based on the recent in vitro and in vivo experimental findings that ultrasonic cavitation can activate certain porphyrins and thereby induce significant antitumor effects. It was also found that the ultrasonic intensity threshold for producing cavitation can be significantly reduced by superimposing the second harmonic onto the fundamental. The threshold to form a focal lesion in murine liver tissue was reduced by orders of magnitude, especially in combination with administration of a certain xanthene dye.