Caracterizing the MTF in 3D for a quantized SPECT camera having arbitrary trajectories

The modulation transfer function (MTF) in 3D is examined for a compact SPECT system capable of moving in different trajectories. A novel phantom has been developed to allow simultaneous measurements of the MTF oriented along three orthogonal axes for dedicated SPECT. The phantom consists of three capillary tubes filled with radioactivity, positioned nearly orthogonally to each other. The evaluated imaging system is a 16×20cm² CZT-based compact gamma camera with 2.5 mm square pixels having a hexagonal close-packed collimator on its front surface. Projection data are acquired with the phantom positioned in air and in various uniformly cylindrical and non-uniformly breast-shaped water scattering media in the camera´s field of view. Various source-to-collimator distances are evaluated using simple and more complex orbital trajectories about the phantom including: vertical axis of rotation, tilted parallel beam, circle-plus-arc, and a sinusoid projected onto a hemisphere. Using data collected with an ±8% energy window about the ^99mTc 140 keV photopeak, images are reconstructed with an iterative ordered subsets expectation maximization algorithm for different iterations, reconstructed voxel sizes and sampling schemes. After reconstruction, each line is rotated into its own plane that is mutually orthogonal to the other two lines, and reoriented with coordinates obtained from 2D Radon transform. One dimensional line spread functions and corresponding MTFs are evaluated along the various segments of each capillary tube. Results indicate small variations in the MTFs, with complex orbits having slightly better performance in scatter conditions.