eliminated is called the cut-off frequency (c). As the cut-off frequency is
increased, spatial resolution improves and more image detail can be seen
up to a certain frequency. At a too high cut-off value, image detail may be
lost due to inclusion of inherent noise. Thus, a filter with an appropriate
164 12. Single Photon Emission Computed Tomography
Fig. 12.9. Different windows for reconstruction filters in SPECT. Different windows
suppress the higher spatial frequencies to a variable degree with a cutoff Nyquist
frequency of 0.5 cycle/pixel.
Fig. 12.10. Different filters for SPECT that are obtained by multiplying the respec-
tive windows by the ramp filter with cutoff at Nyquist frequency of 0.5 cycle/pixel.