IN VIVO COUNTING OF THORIUM DIOXIDE PATIENTS: SPECIAL TECHNIQUES AND PRELIMINARY RESULTS *

Since the small shielded room at our disposal rules out the linear scan technique described by Dudley, we rely upon the use of a h e d crystal detector at a distance of one meter from the patient for the measurement of total body burden. It is well recognized, however, that these measurements are too limited for the appraisal of radiation effects in Tho, cases, unless we can supplement them with measurements of radioactivity distribution, which is already known to be complex: For instance, approximately 80% of the total 228Th but only 30% of the total zizBi and z08Th body burden is expected to be deposited in liver and spleen,’ in a still not very definitely known proportion. An attempt to determine these distributions with an uncollimated crystal centered over the diaphragm is hampered by the fact that the crystal will also “see” large portions of the other principal deposition sites, namely, the lungs, lymph nodes, blood, and soft tissues. This suggests that the detector be positioned as close as possible to organs of interest. The FIGURE 1 depicts the detectors available when this work was commenced, and the geometry employed. The upper crystal is 29 cm in diameter by 10 cm thick; the lower, 17.8 by 8.9 cm. (1 1.5 X 4 inches and 7 X 3.5 inches respectively). For a given individual, the size, shape and position of the liver and spleen are known only roughly; but the detector calibration at such close spacing is very sensitive to these factors, particularly to uncertainties in the distance of the source to detector. It thus appears that a reasonably precise estimate of the position of these organs, though not particularly useful in itself, would greatly aid in determination of the absolute amount of activity present therein. A fundamental assumption is now made-one that seems intuitively reasonable and is subject to experimental verification-namely, that the response of these detectors to radioactivity distributed (not necessarily uniformly) in an extended and irregularly-shaped source (i.e., the liver and/or spleen) may be duplicated by that from a point source of equal activity at some uniquely determined and more readily specified position. This would not be valid, of course, for a scanning technique. This assumption was tested by an extended series of data runs employing a 16-mm-thick presdwood phantom. A bottle of thorotrast was employed as a first approximation to a point, until it was realized that settling of precipitated mate-