Spectral Multiplexing Multi-Pinhole Collimator Design forHigh Sensitivity Small Animal SPECT

1140 Objectives: Multi-pinhole SPECT is widely used for small animals thanks to its ability to track the real-time distribution of radionuclides in high spatial resolution. A stationary system is usually desirable due to its dynamic imaging capability. However, in a stationary system, the limited number of pinholes may result in artifacts due to inadequate angular sampling. We seek to improve the tradeoff between sensitivity and spatial resolution and angular sampling specific to multi-energy radionuclides, such as Indium-111, by introducing a spectral multiplexing multi-pinhole collimator design. Methods: We proposed a novel spectral multiplexing multi-pinhole collimator that consisted of two sets of pinholes. Each set of pinholes have no overlap between the projections of the designed field of view (FOV). The first set of pinholes consists of 7 rows and the second set of pinholes has 6 rows. Each row includes 14 pinholes distributed equi-angularly. The second set of pinholes interleaves between the first set of pinholes and acquires different angular sampling patterns. The second set of pinholes are covered with a certain thickness of tungsten that filters the emitted gamma-photons of different energy at a different ratio. The energy-discriminating measurements can be decomposed into projections of individual pinhole set without multiplexing effect. The spectral sensitivities (including the energy-dependent spectral sensitivities of isotope abundance and 1 cm CsI scintillator detection efficiency) are implemented in the system matrix. Projection data of a digital phantom that consisted of 6 groups of rods of different sizes were simulated. Poisson noise was added presuming 30MBq total radioactivity of Indium-111 with 300 seconds acquisition time. The noisy projection data were reconstructed into 150*150*46 voxels with 0.2 mm cubic voxel size using 160 iterations of the maximum likelihood expectation maximization (MLEM) algorithm. Four different tungsten filter thickness ([0.1, 0.2, 0.35, 0.5] mm) were investigated and compared with the reconstructions of single-set multi-pinhole collimator through quantitative analysis of contrast and noise in the reconstructed volumes. Results: The reconstructed images with the proposed spectral multiplexing collimator shows superior contrast recovery, lower noise, and lower aliasing artifacts compared with the single-set design results. Among all investigated filter thickness, the 0.1 mm tungsten filter shows the best performance. At the same contrast level at all resolutions, the spectral multiplexing collimator achieves a 40% noise reduction compared with the single-set design results. The 300 seconds scan using a spectral multiplexing collimator with 0.1 mm tungsten filter can achieve higher or equivalent CNR than a 500 seconds scan using a single-set collimator. This allows for 40% time-saving in acquisition protocol design. Conclusions: The proposed spectral multiplexing multi-pinhole collimator can achieve higher contrast to noise ratio or the same contrast to noise ratio with shorter acquisition time. The improved angular sampling is beneficial to improve spatial resolution and reduce aliasing artifacts.