Analysis of the Scattered Radiation Encountered in Diagnostic Scanning

Abstract

The major disadvantage in the use for diagnostic scanning of radioisotopes which emit low-energy photons stems from the fact that the scattering of lower energy photons takes place with little loss of energy. This condition comes about because Compton interactions of low-energy photons transfer only very small amounts of energy to the recoil electrons and therefore the primary and scattered radiations are of nearly equal energy. Consequently, scattered radiation cannot be as efficiently eliminated from the examination as it can be when higher energy radiation is employed. The spectral distribution and the relative amount of the scattered radiation reaching the detector were determined under conditions similar to those encountered in organ visualization for several different isotopes of varying energies. The isotopes employed in the study were iodine 125, iodine 131, mercury 197, mercury 203, and technetium 99 m. It is apparent from this investigation that, while pulse-height discrimination of scattered radiation is plausible for iodine 131, mercury 203, and technetium 99 m, it is rather inefficient for mercury 197 and com0pletely futile for iodine 125. In addition, it appears from the study that the impossibility of carrying out efficient pulse-height analysis with low-energy gamma-ray and x-ray emitters results in only a slight deterioration of the target/non-target ratio even for relatively deep-seated tumors.