A new coregistration software package, Neuro900 Image Coregistration s
oftware (Strichman Medical Equipment), has been developed specifically
for nuclear medicine. With this algorithm, the correlation coefficien
t is maximized between volumes generated from sets of transaxial slice
s. No localization markers or segmented surfaces are needed. The coreg
istration program was evaluated for translational and rotational regis
tration accuracy. A Tc-99m HM-PAO split-dose study (0.53 mCi low dose,
L, and 1.01 mCi high dose, Il) was simulated with a Hoffman Brain Pha
ntom with five fiducial markers. Translation error was determined by a
shift in image centroid, and rotation error was determined by a simpl
ified two-axis approach. Changes in registration accuracy were measure
d with respect to: 1) slice spacing, using the four different combinat
ions LL, LH, HL, HH, 2) translational and rotational misalignment befo
re coregistration, 3) changes in the step size of the iterative parame
ters. In all the cases the algorithm converged with only small differe
nce in translation offset, theta and phi. At 6 mm dice spacing, transl
ational errors ranged from 0.9 to 2.8 mm (system resolution at 100 mm,
6.8 mm). The converged parameters showed little sensitivity to count
density. In addition the correlation coefficient increased with decrea
sing iterative step size, as expected. From these experiments, we foun
d that this algorithm based on the maximization of the; correlation co
efficient between studies was an accurate way to coregister SPECT brai
n images.