Compensation for the response function of medium energy collimator in 67Ga planar and SPECT imaging

This preliminary study is to improve ⁶⁷Ga planar and SPECT images by compensating for the response function of a medium energy (ME) collimator. The point response functions (PRFs) of a GE ME collimator for the 93, 185 and 300 keV photons of ⁶⁷Ga at 5, 10, 15 and 20 cm from the collimator face were experimentally determined. For small pixel sizes, the PRFs for all distances showed hole pattern effects. Images from the higher energy photopeaks showed increased penetration fractions. For a specified source distance, a Butterworth filter can be designed to eliminate the collimator hole pattern with minimal degradation of the spatial resolution. Compensation for the distance-dependent collimator-detector response was accomplished using iterative reconstruction methods. To evaluate the compensation methods, planar images and SPECT projection data were acquired from a phantom consisting of 3 hot spheres with diameters of 1, 1.3 and 1.6 cm inside a cylindrical phantom. The iterative reconstruction-based compensation method provided improved resolution and fewer artifacts than in images reconstructed with filtered backprojection. The authors conclude that degradation caused by ME collimator in ⁶⁷Ga imaging can be effectively compensated for using these techniques