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

摘要

The soft tissue thermal index (TIS), as defined in the AIUM/NEMAnOutput Display Standard, may not be relevant with respect to eyenexposure, primarily because of differences in actual vs. assumednacoustic and thermal properties. Therefore, a theoretical study ofntemperature rise within the eye due to ultrasound insonation wasnundertaken to compare the TIS with more exact calculations. At eachnplane in the direction of propagation, the focused ultrasound beam wasnmodeled as a disc of uniform intensity. Each disc becomes a heat source,nand integration over all discs provides the total temperature rise atnany axial position. Calculations were done assuming the ultrasound beamnintersects the lens of the eye as well as for the case in which the beamndoes not intersect the lens. Results were found for frequencies of 7.0nMHZ to 40 MHZ, transducer diameters of 0.2 cm to 1.0 cm, and focalnlengths ranging from 0.2 cm to 3.0 cm. Perfusion was assumed negligiblenand thermal and acoustic parameters were taken from reported studies.nFor every case, the ratio of maximum temperature rise to the TISn(assuming constant output power) was calculated. For the lens case, thenratio varied from 7.35 to 0.8. For the no-lens case, the ratio variednfrom 4.1 to 0.4. These results indicate that the TIS is not adequate tonrepresent the temperature rise occurring within the eye upon insonation