Experimental measurement of the energy-dependent point-source response function in Tc-99m SPECT imaging

An experimental measurement of the ^99mTc point-source response function (PSRF) for a SPECT system is reported in 40 energy windows (80-160 keV). Many researchers have proposed methods for improving SPECT reconstruction by using scattered radiation. Implementation of such reconstruction algorithms requires detailed knowledge of the energy-dependent PSRF, including both the scattering within the patient body and the imaging characteristics of the camera. The authors have measured this PSRF experimentally using a cylindrical phantom filled with water and having a movable point-source immersed inside. The measurements were made using a gamma-camera with a special xyE acquisition interface card that provided both the x-y coordinates and the energy of each event. A spherical capsule filled with ^99m Tc was mounted on a geared armature which moved the source without opening the phantom. Measurements, exceeding 4 million counts at each source position, were made at radial intervals of 1 cm (0-16 cm) and at angular separations of 11.25 degrees. This idealized phantom (cylindrical symmetry and uniform attenuating medium) approximates whole-body imaging in SPECT and provides data for validating Monte Carlo simulations and testing reconstruction algorithms. The authors report fit parameters for an empirical analytic model of the PSRF. The singular value decomposition (SVD) of this SPECT imaging system is computed. From the SVD, energy weighting functions are derived as an alternative to the usual energy windows