Focal destruction of nervous tissue by focused ultrasound: biophysical factors influencing its application.

Ultrasound at frequencies of 1.0 mcps and 2.5 mcps can be focused by a suitable lens system to produce a small region of high vibrational intensity. The concentrated energy within and around the focal region can be used to destroy structures of the central nervous system. The extent of destruction depends upon: (a) the size of the focal region, which varies inversely with the frequency, (b) the ultrasonic intensity, (c) the duration of exposure, and (d) the physical and physiologic characteristics of the tissue under irradiation. With proper choice of ultrasonic dosage, mice were rendered monoplegic by destruction of one-half of the spinal cord without demonstrable injury to the skin or subcutaneous tissues through which the converging ultrasonic beam had been transmitted. In similar fashion, focal lesions were produced in the basal ganglia of living cats by stereotaxic transdural application of a focused ultrasonic beam delivered through the superior aspect of the cerebral hemispheres. Histologic studies of mouse spinal cords and cat brains offered evidence that the fiber tracts of the central nervous system are more vulnerable to ultrasonic irradiation than aggregates of cell nuclei or vascular structures. The destructive action of the ultrasound is apparently a result of mechanical strain combined with a rise in temperature at the focus of the beam. The heating factor was found to assume greater importance under conditions of high intensity and continuous (rather than pulsed) irradiation. Trypan blue staining and radioautography using P(32) have been employed to identify the lesions 1 hour after irradiation. This has been a valuable adjunct in our attempts to determine the accuracy of placement of the lesions and their size. Perhaps more important, however, is the indication from these studies that ultrasonically produced lesions may offer a useful method for investigation of the nature of the blood-brain barrier. "Target studies" were undertaken to determine the precision with which lesions of predetermined size could be placed at predetermined sites in the basal ganglia of the cat. Results to date have been promising, but it is our opinion that further technical improvement will be necessary before ultrasound can be used as an accurate method for placing discrete lesions within the human brain.