COMPUTER-ASSISTED COREGISTRATION OF MULTISLICE SPECT AND MR BRAIN IMAGES BY FIXED EXTERNAL FIDUCIALS

We have developed and validated in a phantom a method of computer-assi sted coregistration using multislice SPECT and MR images. Reusable fid ucial markers were fabricated from nylon-based plastic and consist of two parts: a base that remains fixed to the skin with adhesive between scans and a removable, spherical cavity insert that can be filled wit h contrast agents appropriate for multiple imaging modalities. Markers external and internal to a three-dimensional brain phantom provided a means of quantifying the method's accuracy. A computer algorithm was used to derive transformation matrices for image sets by minimizing th e root mean squared deviations obtained for multiple permutations (n = 10) of increasing numbers (range 3-11) of external SPECT/MRI point pa irs. As defined by the average +/- SEM mean residual deviations of non coregistered internal fiducials, the minimal accuracy was 2.4 +/- 0.3 mm (no marker >3.6 mm) for three coregistration points and did not imp rove beyond seven to eight fiducials (2.2 +/- 0.1 mm; no marker >3.2 m m). The method's true accuracy is likely to be better than estimates o f minimal accuracy, however, since such measures reflect surmountable random errors in fiducial location. With use of identical MRI (or SPEC T) data sets, measures of intraoperator (0.5 +/- 0.1 and 0.5 +/- 0.1 m m, respectively) and interoperator (0.5 +/- 0.3 and 0.8 +/- 0.1 mm) re liability were also obtained, establishing the method as highly reprod ucible and objective. Preliminary results in a human subject suggest i ts feasibility for clinical studies.