Ma. King et al., AN INVESTIGATION OF THE FILTERING OF TEW SCATTER ESTIMATES USED TO COMPENSATE FOR SCATTER WITH ORDERED SUBSET RECONSTRUCTIONS, IEEE transactions on nuclear science, 44(3), 1997, pp. 1140-1145
With the Triple Energy Window (TEW) scatter correction method, scatter
is estimated by interpolation between narrow energy windows placed on
either side (or from a single window just below) the photopeak window
. The use of narrow windows results in a noisy estimate which requires
filtering to prevent significant amplification of noise in the correc
ted images. Using Monte Carlo simulated projections of a digital model
of the anatomy of the torso, we investigated two-dimensional pre-reco
nstruction filtering of the scatter estimate by the Wiener low-pass fi
lter, and Butterworth low-pass filters with various cutoff frequencies
. The scatter estimate was either subtracted from the photopeak window
projection or used directly in ordered-subset maximum-likelihood (OS-
ML) reconstruction. Using the normalized mean square error (NMSE) betw
een the estimated and true slices as the criterion, it was observed th
at: 1) low-pass filtering of the TEW scatter estimate dramatically dec
reases the NMSE compared to that of no filtering of this estimate; 2)
the cutoff frequency of the Butterworth filter used to filter the scat
ter estimate is lower than that typically used for photopeak window im
ages; 3) the cutoff frequency of the Butterworth filter has a broad ra
nge of values over which the NMSE is near its minimum value; 4) the cu
toff frequency at which the Butterworth reaches its minimum value depe
nds on the number of counts in the TEW window(s), and source distribut
ion; 5) the Wiener low-pass filter adapts to produce a low, but not ne
cessarily the minimum, NMSE; and 6) the inclusion of the scatter estim
ate directly into OS-ML reconstruction results in a lower NMSE than su
btraction of the scatter estimate from the photopeak window prior to r
econstruction.