New possibilities of ultrasound surgery
MSU learned how to blow bubbles
Physicists from the Laboratory of Medical and Industrial Ultrasound of Moscow State University together with colleagues from the University of Washington have come up with a new way to remotely destroy tumors inside the human body using tiny bubbles. The scientists described their work in an article published in the IEEE Transactionson Ultrasonics, Ferroelectrics, and Frequency Control journal (Rosnitskiy et al., Design of HIFU Transducers for Generating Specified Nonlinear Ultrasound Fields).
Ultrasound waves are used in a method called boiling histotripsy. During this procedure, the mechanical destruction of the tumor tissue occurs, which is achieved by heating it to a high temperature. However, the diffusion of heat makes the result of exposure unpredictable and, very importantly, with the help of ultrasound, it is impossible to see the result of such irradiation, so you have to use expensive magnetic resonance imaging. In addition, it was unknown how to obtain an ultrasonic wave with a shock front of the required amplitude.
Scientists have established that the main parameter of the emitter is the angle of convergence of the wave beam, which shows how strongly the ultrasound should be focused. The greater this angle, the greater the amplitude of the shock front can be achieved. Numerical methods helped to find the quantitative dependence. So, if you need to get a shock front amplitude of 100 megapascals in focus, then you need an emitter with a convergence angle of 60 degrees.
Michigan researchers have learned how to mechanically destroy tissues by creating a "cavitation cloud" the size of a rice grain in the region using microsecond ultrasonic pulses with a record high amplitude. Scientists from Moscow and Seattle solved the same problem by localized tissue boiling. They used longer pulses, on the order of a millisecond, but of a smaller amplitude.
When such pulses propagate in a very small area (with a diameter of about 0.1 millimeters and a length of 1 millimeter), shock fronts are formed. Under their action, the fabric heats up and boils explosively, growing into a millimeter-sized bubble. This happens so quickly that it begins to block the path of a thin focused ultrasonic "beam" even before the end of the pulse. There are effects of acoustic fountain and ultrasonic spraying. As a result, the tissue is torn into micron-sized particles, which are ejected into the bubble, forming a homogenized mass in its center.
According to scientists, both of these methods of mechanical destruction of tissue open up new possibilities in ultrasound surgery. Their action is predictable, and the cavitation cloud that is born, as well as the bubble that arises during the boiling process, makes itself felt by a response ultrasonic echo, which manifests itself when using a simple ultrasound.
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24.11.2016