Collimator evaluation in the presence of scattering medium

Abstract

In air the line source response function (LSRF) for a focused collimator is, to a good approximation, a pure Gaussian and therefore, the point source response function (PSRF) is also a pure Gaussian of the same shape. Thus, in air the LSRF is sufficient to give an accurate description of the response of a focused collimator to point distributions of activity. In the clinical situation, in the presence of scattering media, the LSRF is not a pure Gaussian but can be approximated by a sum of two or more Gaussians. Hence, in the medium the LSRF does not have the same shape as the PSRF. Isoresponse curves and focal depth measurements for a focused collimator determined from LSRF data obtained in scattering media are therefore in error. In other words, the LSRF in the presence of scattering media does not provide an accurate description of the collimator's response to point distributions of activity. In the clinical evaluation of a collimator, the PSRF, not the LSRF, must be used. Experimentally measured LSRFs and the deduced PSRFs for various depths of a scattering medium for 99Tcm, 85Sr and 198Au were presented. Isoresponse curves determined from both the LSRFs and PSRFs for these isotopes were discussed. Clinical scans with 99Tcm were shown to emphasize the importance of using the PSRF.