A rotating parallel hole collimator for high resolution imaging of medium energy radionuclides

A novel rotating parallel hole collimator has been designed and built for high resolution imaging of medium energy radionuclides. The collimator has thick septa and was designed for imaging 50-100 mCi I-131 radiolabeled monoclonal antibodies being administered intratumorally for brain tumor therapy. Projection data from different collimator rotation positions are summed to create a composite projection dataset with more dense spatial sampling than projection data at a single position. Measured SPECT resolution with I-131 was 5.5-7.5 mm FWHM for a 13 cm radius of rotation compared with 9.9-13.9 mm with a conventional medium energy collimator. SPECT resolution for the rotating collimator was only slightly greater with I-131 than with Tc-99m, indicating good attenuation by the septa. Sensitivity was 56 counts/sec/mCi, as determined with a tight region of interest around the projection of an I-131 line source 12 cm from the collimator surface. SPECT studies of 3.7 cm and 6.2 cm diameter shell-core tumor phantoms filled with I-131 were made. The lower activity core and higher activity shell were well resolved in a SPECT study using the rotating hole collimator. SPECT shell to core activity concentration ratios were more accurate with the rotating collimator than with a medium energy collimator. These results show the potential of this rotating collimator for high resolution human tumor studies with I-131 tracers