FIG. 5.

Measured and theoretical NPS and MTF at four locations within the uniform Circular disk phantom, illustrating the nonstationary, anisotropic characteristics of each for FBP and PL. Reconstruction parameters were chosen to “match” spatial resolution (in terms of the radial average f₅₀) in FBP and PL at the center of the image. In each plot, the left half shows theoretical prediction, and the right half shows measurement from simulated images, with excellent agreement between the two. The anisotropic nature of the NPS is similar in both FBP and PL and is consistent with attenuation of rays traversing different pathlengths through the phantom. The MTF of FBP is isotropic and independent of locations, whereas that of PL is anisotropic and roughly “complementary” to the shape of the NPS at the corresponding location.