Intrinsic Uniformity Requirements for Pinhole Spect

Pinhole SPECT is a fully 3-dimensional tomography technique. Uniformity requirements for gamma-cameras have been studied for 2-dimensional SPECT performed with parallel-hole collimators. This study investigated reconstruction artifacts in pinhole SPECT arising from intrinsic uniformity defects of the gamma-camera in the case of a pinhole aperture (5 mm) and a rotation radius (10 cm) suitable for human studies. METHODS: A cylindric phantom was filled with water and 99mTc. The count density in the pinhole SPECT projections largely exceeded the density that would be expected in human studies. Two-dimensional gaussian-shaped positive uniformity defects of various heights and various full widths at half maximum were simulated at 5 locations on the 64 projections. All these sets of projections were reconstructed using an iterative 3-dimensional ordered-subset expectation maximization (OSEM) algorithm tailored to the pinhole geometry. The influence of the number of OSEM iterations (2, 5, or 8 for 8 subsets) was also investigated. The height of the ring artifacts generated by the uniformity defects was measured on the reconstructed transverse slices and was compared with the noise in the noncorrupted slices. The uniformity defects were also generated on a 30-megacount flood image from the camera. These images were inspected visually, and the National Electrical Manufacturers Association (NEMA) differential and integral uniformities were calculated. RESULTS: Defects centered on the line corresponding to the orthogonal projection of the camera axis of rotation onto the detector plane generated artifacts whose magnitude-to-noise ratio exceeded 1% when the defect height was larger than 5%-10%, depending on the defect full width at half maximum. These defects were clearly visible on the slices and the flood images. Defects located elsewhere on the camera detector generated ring artifacts of magnitude-to-noise ratio smaller than 1%. They were visually observable both on the reconstructed slices and on the flood images for defect heights larger than 5%. The increase in the number of OSEM iterations conducted to a decrease in the artifact magnitude-to-noise ratio. CONCLUSION: Although hardly visible on the flood images and only slightly increasing the NEMA differential or integral uniformity, a detector uniformity defect of 3% height is able to generate a visible artifact on the reconstructed transverse slices. The study was conducted on a relatively high-count pinhole SPECT acquisition. Considering that far fewer counts would accumulate in clinical practice, any camera that fulfils these uniformity requirements should not lead to the presence of visible uniformity artifacts in the reconstructed slices.