Experimental study of histotripsy in ex vivo bovine liver tissue exposed to pulsed high intensity focused ultrasound

High intensity focused ultrasound (HIFU) has been used in clinical treatment for a variety of diseases, and achieved good results. Former related researches are mostly focused on either consecutive HIFU or HIFU with a low duty cycle, which is similar to the damage caused by consecutive HIFU mainly through thermal effect. However, there are few researches on tissue damage caused by non-thermal effect. Actually, the cavitation effect of HIFU can also cause serious tissue damage. So, to avoid the thermal effect caused by HIFU irradiation, this experiment will use high intensity HIFU of low duty cycle to irradiate bovine liver tissue in vitro, so as to make use of its non-thermal effect, especially the tissue damage directly caused by cavitation effect. A Commercial HIFU tumor therapeutic system, with 500W sound power, 4Hz pulse repetition rate and 1~4% duty cycle parameter, is used to irradiate the degassed fresh bovine liver tissue, with a depth of 20 mm. During the experiment, the temperature of focal area is measured by thermocouple, and the cavitation activity is analyzed based on data gained by passive cavitation acquisition system. Then the irradiated bovine liver tissues in vitro were sliced into 1~2 mm pieces to observe its damage form, and get the specimen for H&E staining to see the result of damaged tissue. After the experiment, it is discovered that the damage caused by irradiation of high intensive HIFU of low duty ratio is a cavernous injury without content (the content can be liquid), rather than containing necrotic tissue coagulation necrosis. What's more, during the experiment, the temperatures in focal region are 41.19±1.42°C, 45.73±1.92°C, 53.07±2.09°C respectively, no more than 56°C. If the duty ratio is higher (4%), the tissue damage caused by the irradiation of pHIFU will be coagulation necrosis, which is the same result caused by the irradiation of traditional cHIFU, and the temperature in the focal region is 64.13±2.56°C, surpassing 56°C. Cavitation can be clearly observed in all irradiation. After H&E staining, there is no intact cell at the edge of the visible hole-shape damage under microscope, cell nucleus vanishing and cytoplasmic homogeneously forming. We can conclude that: the irradiation of high intensive HIFU of low duty ratio can cause tissue damage, which is different from coagulation necrosis of the cavernous injury, and the damage formation involves cavitation. The result of this experiment implies that HIFU cavitation damage tissue can be a new method of treatment in the future.