Validation of fully automatic brain SPET to MR co-registration

Fully automatic co-registration of functional to anatomical brain images us ing information intrinsic to the scans has been validated in a clinical set ting for positron emission tomography (PET), but not for single-photon emis sion tomography (SPET). In this paper we evaluate technetium-99m hexamethyl propylene amine oxime to magnetic resonance (MR) co-registration for five f ully automatic methods. We attached six small fiducial markers, visible in both SPET and MR: to the skin of 13 subjects. No increase in the radius of SPET acquisition was necessary. Distortion of the fiducial marker distribut ion observed in the SPET acid MR studies was characterised by a measure ind ependent of registration and three subjects were excluded on the basis of e xcessive distortion. The location of each fiducial marker was determined in each modality to sub-pixel precision and the inter-modality distance was a veraged over all markers to give a fiducial registration error (FRE). The c omponent of FRE excluding the variability inherent in the validation method was estimated by computing the error transformation between the arrays of MR marker locations and registered SPET marker locations. When applied to t he fiducial marker locations this yielded the surface registration error (S RE), and when applied to a representative set of locations within the brain it yielded the intrinsic registration error (IRE). For the best method, me an IRE was 1.2 mm, SRE 1.5 mm and FRE 2.4 mm (with corresponding maxima of 3.3, 4.3 and 5.0 mm). All methods yielded a mean IRE <3 mm. The accuracy of the most accurate fully automatic SPET to MR co-registration was comparabl e with that published for PET to MR. With high standards of calibration and instrumentation, intra-subject cerebral SPET to MR registration accuracy o f <2 mm is attainable.