Theoretical study of steady-state temperature rise within the eye due to ultrasound insonation

The soft tissue thermal index (TIS), as defined in the AIUM/NEMA Output Dis play Standard, may not be relevant with respect to eye exposure, primarily because of differences in actual vs. assumed acoustic and thermal propertie s. Therefore, a theoretical study of temperature rise within the eye due to ultrasound insonation was undertaken to compare the TIS with more exact ca lculations. At each plane in the direction of propagation, the focused ultr asound beam was modeled as a disc of uniform intensity. Each disc becomes a heat source, and integration over all discs provides the total temperature rise at any axial position. Calculations were done assuming the ultrasound beam intersects the lens of the eye as well as for the case in which the b eam does not intersect the lens. Results were found for frequencies of 7.0 MHZ to 40 MHZ, transducer diameters of 0.2 cm to 1.0 cm, and focal lengths ranging from 0.2 cm to 3.0 cm. Perfusion was assumed negligible and thermal and acoustic parameters were taken from reported studies. For every case, the ratio of maximum temperature rise to the TIS (assuming constant output power) was calculated. For the lens case, the ratio varied from 7.35 to 0.8 . For the no-lens case, the ratio varied from 4.1 to 0.4. These results ind icate that the TIS is not adequate to represent the temperature rise occurr ing within the eye upon insonation.