SPECT imaging of 131I (364 keV): importance of collimation

A low sensitivity medium energy collimator (LSMEC) designed with thick septa and long bore (theoretical leakage less than 3% 364 keV), was evaluated using 131I for a SPECT system operated in both planar and tomographic imaging modes. The collimator was designed to minimize the influence of photon penetration on spatial resolution, in particular the resolution index FWTM. Overall spatial resolution for the planar imaging mode at 10 cm from the collimator face was found to be 11.6 mm FWHM and 21.6 mm FWHM. The corresponding transverse plane and slice thickness resolution was of the order of 17 mm FWHM and 31 mm FWTM, for a radius of rotation of 16 cm. A SPECT resolution phantom was imaged. Two quadrants of cold rods were well resolved, with rod dimensions of 16 and 12.7 mm respectively, the resolution being comparable to that obtained using 99Tcm (140 keV) and a low-energy high-resolution collimator. NEMA sensitivity obtained was 75 cpm/microCi 131I. The resolution measurements obtained suggest that this collimator should be useful for SPECT imaging with 131I in either radioimmunoimaging or radioimmunotherapy.