ae whee fy dPak a be 97 bvhere the matrices A and B correspond to the comFponents K, and K,of the response kernel, multiplied by appropriate quadrature weights. The regularization unwlding is based on the physically meaningful assumption that (except possible at a few irregular locations) the magnitude of the activity f(z,y) cannot change abruptly between one point and its immediate neighbors ou the grid or between two adjacent grids; in other words, we require the surface f(x,y) of the distribution to be sufficiently smooth. The solution of the matrix equation (3) subject to our smoothness requirements becomes F = (A774 + yt S|A7GB* (BB? + yS2Ss]"', (4) whore 8; and S_ are appropriate smoothing matrices and vy, and yz are corresponding weights. Usually it suffices to compute Eq. (4) for a few values of +; and +2 in order to estimate the acceptable values of these weights y: anil y:. The acceptable values of y1 and yz are those which would lead to a new spectrum G’ with an error comparable to the expected statistical and experimental error in the given spectrum. Figure 82 shows a typical surface studied and Figure 83 compares both the usual (unsmoothed) solution (y1 = ye = 0) and the smoothed solution (y1 = y2 = 0.04) with the exact distribution for two sets of three typical cross sections, each parallel to the x and y axis, respectively. Because of symmetry, the sets of Figure 83 correspond to only one of the peaks of Figure 82. The errorin the spectra g(z’,y’) in this case was between 24% and 28%. A detailed analysis of the two dimensional regularization unfolding described above is currently being prepared for publication. REFERENCES 1. Marinelli, L. D., Clemente, G. F., Abu-Shumays, I. K., and Steingraber, O. J. Argonne National Laboratory Radiological Physics Division Annual Report, July 1967-June 1968. ANL-7489, p. 1. 2. Clemente, G. F., Marinelli, L. D., and Abu-Shumays, I. K. This report. 3. Phillips, D. L. J. Assoc. Comput. Mach. 9, 1 (1962). METABOLISM OF 28°Th DECAY SERIES RADIONUCLIDES IN MAN AND OTHER ANIMALS FOLLOWING INTRAVASCULAR ADMINISTRATION OF THOROTRAST RM. Parr*, H. F. Lucas, Jr., and M, L. GriemT —_— Tho setivities of °Th decay series radionuclides have been Sludicd in tissue samples from 6 human patients and 9 experinienttal animals to whom Thorotrast had been administered intrivascularly. In favorable cases where analyses could be colmenced before extensive in vitro radioactive growth or decay had occurred, the tn vivo activities of seven nuclides, *Th. “Ra, Ac, “Th, “Ra, **Pb, and “Bi, were determined. The nvailable information on the translocation of these nu- chiles within the bodyis reviewed, and current “best-estimates” pare derived for the distribution of activities and consequent [done rates in the tissues of typical long-term Thorotrast patitnts, The anomalous behavior of “*Ra, which is retained in th reticuloendothelial system in high concentrations, is ex- Pliined by a model which postulates that atoms are trapped Within the Thorotrast particles unless the recoil distance thiough which they move at the moment of their creation is Sillicient to allow their escape. INTRODUCTION Thorotrast is a commercial preparation of colloidal thorium dioxide which was introduced as a contrast jmetum for radiography around 1931. Because of its | ats of th Present address: Department of Research and Isotopes, 1 rnational Atomic Energy Agency, Vienna. -\rgonne Cancer Research Hospital, Chicago. excellent contrast properties and its ease of administration, it was widely used for many years, mainly for angiography but also in other applications such as retrograde pyelography. Around 1945, however,it came into disrepute when the possibility was recognized of undesirable delayed sequelae resulting from its radioactivity and negligible excretion rate. Since then, its use in man has been largely discontinued. Despite this diminished importance in radiography, interest in the metabolic characteristics of Thorotrast has tended to increase. As was pointed out by Marinelli,“ the many thousandsof patients to whom it was administered intravascularly during the years that it was in common usage now constitute an invaluable and irreplaceable potential source of data on the effects of low level and long-term irradiation in man. This is particularly the case for those patients to whom it was administered for cérebral angiography since they have a long life expectancy, and any observable effects are not likely to be related to the need fortreatment. Of fundamental importance in any study of the radiation effects of Thorotrast is, of course, the accurate