A new, simple, and accurate ultrasonic method is elaborated for thyroid volume determination with commercially available high-resolution real-time equipment. The method takes into account the special and variable shape of the thyroid. The measurements were taken in three perpendicular sections. Volume calculations were done by applying the corrected ellipsoid model and computer-aided numerical integration, as well as by the conventional simple ellipsoid model. The accuracy of the method was tested by comparing direct measurements of a silicone rubber phantom with ultrasonography of a tissue-equivalent thyroid phantom. Both phantoms were modeled as casts of the same thyroid, which was obtained at autopsy. From the phantom measurements the deviation between the volume calculated by numerical integration or the corrected ellipsoid method and the real volume determined directly with the silicone rubber phantom was less than 5% on average, while volume calculations using the conventional ellipsoid model involved about +20% systematic error. In vivo studies in 40 subjects gave similar results: the ratios of the volumes calculated by simple and corrected ellipsoid method and by numerical integration [V(E), V(CE), and V(NI), respectively] were [V(E)/V(CE)] = 1.19 +/- 0.11 and [V(NI)/V(CE)] = 1.02 +/- 0.11. In conclusion corrected ellipsoid method is suggested for the clinical practice.
Long-term body retention was measured in six workers who accidentally inhaled 60Co aerosols during manipulation with a high-activity 60Co source. Improved whole body counting and calibration techniques provided good conditions to follow body clearance over 5 y. A two-detector profile scanning arrangement was used to measure the activity distribution in vivo over the lung region. The observed whole body retention followed a two-exponential time function between 10 and 1,850 d. Based on the profile measurements, the shorter exponent of 25-78 d was associated with activity leaving the pulmonary region while the long-term exponential should be interpreted as the clearance of the slowest component of the systemic burden with a biological half-time of 500-1,100 d. These observations classify the 60Co aerosols encountered in this incident as inhalation class W. The agreement of the measured retention pattern with the ICRP inhalation model was investigated assuming different aerosol size distributions characterized by an activity median aerodynamic diameter = 0.25 micron, 1 micron, and 4 microns. It was found that the ICRP model with an activity median aerodynamic diameter = 4 microns could describe the retention of our investigated cases reasonably well.
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