Ultrasound has been used in medicine since 1951 for diagnostic purposes. In imaging applications, ultrasound is sent to tissues and returns an echo. The recording of the returned echo allows for the creation of ultrasound images. Since the initial development of ultrasound imaging, the principle has remained the same, but systems have improved with advancements in computing power and the arrival of Doppler, 3D imaging, and recently elastography. The power of the ultrasound energy sent to the tissue for imaging is very low in order to not induce any tissue damage.
If ultrasound is emitted with a power increased by a factor of 10, it becomes capable of inducing tissue heating that can either activate or destroy the cells. Ultrasound for thermal therapy is called High Intensity Focused Ultrasound (HIFU). In order to not destroy all of the tissues in the path of the ultrasound beam, the ultrasonic waves are emitted by a large aperture transducer and converge at a focal point, where the intensity is 10-1000 times higher than along the path. HIFU is in clinical development for the destruction of tumors throughout the body, including in the prostate, liver, brain, and breast.
The application of HIFU to the brain currently poses a significant problem: the skull limits the passage of ultrasound energy. The skull absorbs about 80% of the emitted energy. In addition, the inhomogeneous nature of the skull leads to a defocusing of the waves and therefore a loss of the focal spot. CarThera® has managed to avoid the problem of the skull bone by using devices that deliver ultrasound energy directly to the brain tissue, without traversing the bone.