Transmitting ultrasonic energy continuously raises tissue temperature at the focal point in the body. The level and duration of this temperature elevation is quantified as the tissue’s “thermal dose.”
Thermal effects can be used to create either a low level thermal rise over several hours (local hyperthermia) or, conversely, a short, highly localized high temperature rise that cooks the tissue (thermal ablation). The graph below illustrates different levels of thermal dose and their biological outcome.
Focused ultrasound’s thermal ablation effect may be used to non-invasively treat a variety of clinical conditions, including symptomatic uterine fibroids; tumors in the prostate, breast, and liver; low back pain; and brain disorders such as essential tremor, Parkinson’s disease, and epilepsy.
Thermal ablation allows for cell death in a targeted area with minimal damage to the surrounding normal tissue. Tissue damage can be accurately controlled, and magnetic resonance imaging allows for the monitoring of temperature in real time. Depending on the equipment and parameters used, high-intensity exposure to focused ultrasound can occur in a volume as small as a grain of rice (10 cubic millimeters). This allows for an extremely localized treatment and a sharp border between treated and untreated areas.
MECHANICAL TISSUE DESTRUCTION
The non thermal effects of focused ultrasound can also be used for the destruction of tissue in a precise location.
As ultrasound propagates through tissue, it interacts with dissolved gases in a process known as cavitation. In its stable form, cavitation forms as oscillating bubbles of gas.
When ultrasound is used at high enough intensities, these bubbles can be made to collapse and release an enormous amount of pressure. This phenomenon, known as inertial cavitation, releases a shockwave capable of causing damage and even liquefying cells. The use of inertial cavitation to destroy regions of tissue is known as histotripsy, and is usually the compounded effect of multiple cavitation collapses.
In procedures, microbubbles will often be injected into a targeted location to negate the need for their spontaneous generation. This lowers the threshold for inertial cavitation and aids in the process of histotripsy.
Histotripsy has been used to generate lesions with sharp borders and completely liquefy tumors. Microbubbles are clearly visible with the use of ultrasound imaging which allows for the accurate targeting of a region of tissue. As thermal effects are kept minimal through the use of pulsed focused ultrasound, the destruction of tissue through inertial cavitation can be a precise process that causes minimal damage to surrounding tissue.