Minimally invasive and non-invasive therapeutic ultrasound treatments can be used to ablate, necrotize, and/or otherwise damage tissue. High-intensity focused ultrasound (HIFU), for example, is used to thermally or mechanically damage tissue. HIFU treatments can also cause mechanical disruption of tissue with well-demarcated regions of mechanically emulsified treatment volumes that have little remaining cellular integrity. For certain medical applications, tissue emulsification may be more favorable than thermal damage because it produces liquefied volumes that can be more easily removed or absorbed by the body than thermally coagulated solid volumes.
A method of non-invasively treating tissue can include positioning a focal plane of an ultrasound source at a target site in tissue. The ultrasound source can be configured to emit HIFU waves. The method can further include pulsing ultrasound energy from the ultrasound source toward the target site, and generating shock waves in the tissue to induce boiling of the tissue at the target site within milliseconds. The boiling of the tissue at least substantially emulsifies the tissue.
HIFU treatments can utilize a sequence of pulses, rather than continuous- wave HIFU exposures, to reduce undesirable thermal effects on the surrounding tissue. HIFU sources operate with low duty cycles (e.g., 1%), use relatively short pulses (e.g., 10-20 microseconds), and deliver high pulse average intensities of up to 40 kW/cm2 to form bubbles that mechanically disrupt tissue. Histotripsy techniques, for example, can induce cavitation by delivering pulses of high-peak negative pressures that are significantly higher than the tensile strength of the tissue. The repetition of such pulses can increase the area of tissue affected by cavitation to create a “cavitation cloud” that emulsifies the tissue. Cavitation is generally stochastic in nature, making cavitation-based HIFU treatments somewhat unpredictable and difficult to reproduce.
This technology is directed toward methods and systems for non-invasively treating tissue using HIFU. A HIFU pulsing protocol can generate shock waves at a target site that induce millisecond boiling to mechanically damage tissue with little to no thermal effect. The pulsing protocol can take into account peak positive pressure, shock wave amplitude, ultrasound frequency, pulse repetition frequency, pulse length, duty cycle, and/or other factors that induce mechanical fractionation of tissue. Additionally, HIFU systems and methods can detect boiling and/or tissue erosion to identify and differentiate mechanical effects from thermal effects of HIFU treatment. These HIFU systems and methods have a number of both therapeutic and cosmetic applications, such as producing bulk ablation of benign and malignant tumors.
Mechanical tissue destruction can occur consistently within localized treatment volumes when the temperature of the tissue reaches 100 ºC, and boiling bubbles form during each pulse or after a series of consecutive pulses. Boiling bubbles can also be much larger (e.g., approximately 2-4 mm) than individual cavitation bubbles that must randomly coalesce to produce any beneficial treatment volume. Therefore, the shock wave beating and millisecond boiling generated by the HIFU system provide a highly repeatable, localizable, and predictable mechanical destruction of the tissue at lower pressure levels as compared to cavitation.