Computer simulations are being performed to model the temperature patterns produced during ultrasonically induced hyperthermia of ocular tumours. The software package for these simulations incorporates operator interaction and uses tissue geometry obtained from B-mode data. Previous studies used geometric approximations for the incident beams used for hyperthermia. In the current study, these beams were computed using diffraction analysis to obtain more realistic simulations of clinical exposures.Ultrasonically induced heating of ocular tissues has been studied because of its importance in evaluating ultrasonic safety and its potential role in therapeutic ultrasound systems. At sufficiently high intensities, ultrasound can produce thermally mediated ocular damage such as cataracts and chorioretinal lesions, as described in early reports by Purnell, Sokullu, and their coworkers (1964, 67) and by our laboratories (Lizzi et al. 1978 a,b).Ultrasonically induced heating can also produce lesions that are potentially useful in therapeutic applications. As an example, to treat refractory glaucoma, we have employed a highly focused beam that produces trans-scleral lesions of the ciliary body (1985). A recent report (Silverman et al. 1991) on over 1000 clinical cases of refractory glaucoma demonstrated that this approach can provide high efficacy rates while maintaining side effects at low rates of incidence. We have also been evaluating ultrasonically induced hyperthermia, applied conjointly with radiotherapy, for treating choroidal malignant melanomas (Coleman et al.
1988).In order to delineate the various physical factors that influence desired tissue heating, we have developed computer simulations of ultrasonically induced heating in various types of ocular tissue.For studies of glaucoma therapy and chorioretinal lesions, we developed a thin-layer model and examined relatively short ultrasonic exposures (up to several seconds) (Lizzi et al. 1984, 87). Computed results demonstrated excellent agreement with experimental data; for example, the computed sizes of scleral lesions, as determined from damage-integral analysis, were within 15% of measured lesion sizes.To examine tumour therapy, we are expanding our computer model to include thick tissue segments, intervening acoustic attenuation, and blood-flow cooling. The model has been used in initial studies of heating produced by two therapeutic exposure modes. The first mode involves hyperthermia in which the distal, defocused segment of a beam is used to encompass a large tumour volume: low spatial-average (SA) intensities (e.g., 1 W/cm2) are used to produce sustained temperature elevations (e.g. to 45" C ) for relatively long periods (e.g., 30 min). The intensities within the focal zone, which is located in the vitreous, have not been found to produce deleterious changes. The second mode involves ablation, in which the focused segment of the beam is placed within the tumour to achieve immediate focal destruction of exposed tissue volumes. In this mode, high SA intensiti...