High-Intensity Ultrasound Facility

Research dealing with therapeutic treatment with ultrasound includes its use for hypothermic treatment of tumors and for surgery. These applications and many biological effect studies require moderate to high ultrasound intensities. The BRL is particularly well equipped to generate the high intensity ultrasound fields needed for these studies.

Unfocused transducers capable of generating many watts per centimeter squared (W/cm2) of acoustic intensity and several focused transducers capable of generating ultrasound intensities as high as several kW/cm2 are available. These can be mounted on special positioning systems so that the location of the axis of the unfocused beam, or the focal region for a focused transducer, can be positioned and moved with respect to a sample. The samples and transducers can be mounted in a tank containing water or some other acoustical coupling liquid. The temperature of the coupling medium can be controlled over the approximate range 10 - 40 oC. One special purpose tank allows for a change in the hydrostatic pressure, a feature that is useful in studying the effects of small bubbles. Transducers are available that can operate at several discrete frequencies over the range from about 0.7 - 3 MHz.

Over the years this facility has been used to study the effects of ultrasound, over a very large intensity range, on the mouse fetus in utero, the gonads, the brain and the liver. Results from these studies have provided valuable information about the acoustical levels at which effects occur and the mechanisms responsible for the effects. This same facility has been used to measure the absorption of ultrasound in many different biological tissues, by measuring the rate of temperature rise using very small thermocouples embedded in the tissue sample as detectors.

One specific application of these high intensity systems has been the production of well defined volumes of damage (lesions) within brain and liver. In the brain it is possible to produce reversible effects or irreversible effects by changing the exposure conditions. Thus, the function of a small volume of the brain can be disrupted temporarily or permanently. Historically this was used to treat human patients and to study the neuronal pattern of the brain (see History of BRL). The latter was accomplished by creating small lesions in the brain and tracing the degenerated axons histologically using serial brain sections. These techniques still offer a noninvasive means of creating a lesion of controlled size within tissues. The use of such systems for nonivasive surgery is growing rapidly. Current research within BRL is directed toward the development of transducer systems capable of electronically steering the ultrasound focus while maintaining a small focal volume.