SMALL ANIMAL IMAGING WITH PINHOLE SINGLE-PHOTON EMISSION COMPUTED-TOMOGRAPHY

Background. High resolution spatial details of the distribution of act ivity in three dimensions is required to evaluate the localization and dosimetric properties of radiolabelled monoclonal antibodies in tumor s and normal tissues. Planar imaging of small animals with a resolutio n of 5-10 mm is usually the imaging modality of choice. The authors in vestigated high resolution single-photon emission computed tomographic (SPECT) imaging, based on a rotating pinhole scintillation camera. Al though the sensitivity of the pinhole collimator is low, several radio nuclides offer suitable decay properties to perform pinhole SPECT, esp ecially in conjunction with high activity levels used in radioimmunoth erapy. Methods. Transverse, sagittal, and coronal sections were recons tructed using a three-dimensional cone-beam algorithm, which is a gene ralization of the two-dimensional fan-beam filtered backprojection alg orithm. Before reconstruction, the pinhole projections were corrected for the decay of the radionuclide, geometric and intrinsic efficiency variations of the camera system, and center of rotation shift. Results . The spatial resolution at 50 mm from the pinhole collimator with 3.3 mm aperture was 3.4 mm, and the sensitivity 7.2 c/s/mu Ci for technet ium-99m. With the 2 mm collimator the resolution was 2.2 mm, and the s ensitivity was 2.6 c/s/mu Ci. To show the spatial resolution in vivo, a rat was injected with 185 MBq of technetium-99m-methylene diphosphon ate or with 5 mCi technetium-99m-hexamethylpropylene amine oxime. The bone structures were well delineated in the methylene diphosphonate im age, and in the hexamethylpropylene amine oxime image, the brain was n icely shown. For comparison a magnetic resonance image for the same se ction was done. Conclusions. High resolution SPECT imaging with the pi nhole collimator provides mapping of the activity in three-dimensions, needed for more detailed biodistribution data and to perform more acc urate dosimetry.