SUBTRACTION ICTAL SPET CO-REGISTERED TO MRI IN PARTIAL EPILEPSY - DESCRIPTION AND TECHNICAL VALIDATION OF THE METHOD WITH PHANTOM AND PATIENT STUDIES

Computer-aided subtraction of the co-registered and normalized interic tal from the ictal single photon emission tomography (SPET) scan, foll owed by co-registration to the magnetic resonance image, may improve t he utility of ictal SPET in the localization of partial epilepsy. This paper describes and technically validates our method. The SPET to SPE T co-registration was tested using six sequential Tc-99(m) brain phant om SPET images of different known positions (15 matches). The registra tion error was determined by multiplying the calculated match transfor mation matrix by the inverse of the known transformation matrix. The ` worst case' co-registration error was less that one voxel diameter in all cases (median 3.2 mm, range 1.2-4.8 mm). For interictal to ictal S PET registrations in 10 consecutive intractable partial epilepsy patie nts, a similar root mean square distance (RMSD) between corresponding points on the matched scans was found as for the phantom studies (medi an 2.2 vs 2.6 mm). The appropriateness of our normalization was studie d by comparing the pixel intensity distributions between the matched s cans, and by analysing the subtraction pixel intensity distribution. T he pixel intensity distribution for both the normalized phantom, and p aired normalized patient studies, were closely matched to each other e xcept for the extreme values, which in clinical situations likely repr esent regions of ictal activation or depression. The subtraction image intensity distributions were symmetrically centred on zero for all va lues up to at least within the 5th to 95th centile range, confirming g ood normalization for the `non-activated' pixels. Also, a linear relat ionship was demonstrated between the measured pixel intensity on the p hantom scans and the true changes in Tc-99(m) activity based on its de cay constant. The results of this study demonstrate that our method pr oduces accurate SPET to SPET co-registration, and appropriate SPET nor malization, thereby allowing a valid ictal subtraction image to be der ived.