JOURNAL OF NUCLEAR MEDICINE 6:747-753, 1965 Scintillation Spectra From Point Sources in a Scattering Medium1

In 1958, Rundo ( 1) suggested that changes in scintillation spectra might be used as a method of determining the depth of a point source in a human body. Over a number of years, experiments have been carried out in this laboratory to examine the shape of scintillation spectra from thick sources with particular application to the effects observed in the counting of humans in a whole body counter ( “steelroom―). The present paper reports results particularly pertinent to the estimation of the depths of point sources of gamma radiation in a human body using only a single sodium iodide crystal. It is clear that the location of sources is more straightforward if scanning of the body in both horizontal and vertical planes is carried out. However, at the present time there are many laboratories where single detector whole body counters exist without scanning facilities (2). The most noticeable effects of the scattering of originally monoenergetic radiation on the shapes of the scintillation spectra observed are a widening of the photopeak, a filling of the gap between the photopeak and the Compton plateau, and a build-up of low energy pulses. Of these three, the second is the most val uable for characterizing the changes that occur (3,4); the valley-to-peak ratio, V/P, defined as the number of counts in the lowest channel between the photo peak and the Compton plateau divided by the maximum count in the photopeak, is a useful parameter for describing this effect. This valley-to-peak ratio, and the change in apparent resolution are used in the present paper to measure changes in the shapes of spectra obtained from gamma ray sources of energies from 0.28 to 1.53 MeV when they are at different positions in a water phantom simulating the human body