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We have studied, by a nonisotopic in situ end-labeling(ISEL) technique, frequency of apoptosis in the external granular layer(EGL) of the cerebellum of immature mice by Υ-rays irradiation from ^(60)Co or diagnostic ultrasound exposure. The total number of normal cells and cells showing morphological features of apoptosis were counted. The frequency of apoptotic cells was expressed as a percentage of the total number of cells in EGL. The extent of changes following 200 cGy(1090 cGy/min) was studied at 2, 4, 6, 8, 12, or 24 hours after exposure. The maximal frequency was found 6-8 hours after exposure. The immature mice that received 18, 36, 54, 108, 198, 396 cGy of Υ-rays or diagnostic ultrasound(7.5MHz, 4.2mW, I_(SPTA) = 7.9mW/㎝, I_(SPTA)= 114.3W/㎝) for 10 or 30 minutes were examined 6 hours after irradiation. Measurements performed after Υ-ray irradiation showed a dose-related increase in apoptotic cells in each of the mice studied. The dose-response curves were analyzed by a linear-quadratic model ; frequency of apoptotic cell in the EGL was y = (0.1349±0.01175)D+(-0.0001522±0.0000334)D²+0.048(r² = 0.981, D = dose in cGy). In the experiment of ultrasound exposure, the frequency of apoptotic cell was 0.106±0.130(10 minutes exposure) and 0.167±0.220(30 minutes exposure). We estimated the relative dose of the yield from the experiment with ultrasound by substituting the yield from ultrasound exposure into the curve from the Υ-irradiation. The relative dose of ultrasound exposure compared with Υ-irradiation were 0.432 cGy(10 minutes exposure) and 0.885 cGy(30 minutes exposure). We have found that there is no evidence to indicate that diagnostic ultrasound involves a significant risk.
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