An investigation of the stationarity of the 3D modulation transfer function of SPECT

The authors evaluated the rotational symmetry and stationarity of the three-dimensional (3-D) modulation transfer function (MTF) of single-photon-emission computed tomography (SPECT). Point spread functions (PSF) in scattering media were obtained by placing a point source at varying radial distances from the center of three differently shaped water-filled phantoms. Results indicate that at the center of a nonisotropic phantom, the 3-D MTF is anisotropic. For off-axis source locations the 3-D MTF becomes increasingly anisotropic as the point source is moved radially off center. 3-D MTFs obtained using arithmetic mean averaging of opposing projections showed less low- to mid-frequency but more high-frequency variation with position than 3-D MTFs obtained using geometric mean averaging. It is concluded that the postreconstruction 3-D MTF is anisotropic and nonstationary, and that the inconsistency of the projection data with angle due to nonisotropic attenuation is the major cause of the low- and mid-frequency variation in the transfer functions.>