ACCURACY OF A REGISTRATION PROCEDURE FOR BRAIN SPET AND MRI - PHANTOMAND SIMULATION STUDIES

Phantom experiments and simulations were performed to evaluate the sig nificance of different error sources in a clinical registration proced ure for brain SPET and MRI based on external markers. The results from the phantom experiments were used to adjust the error model for simul ations. In the phantom experiments, 13-14 external markers were attach ed to the surface of a three-dimensional brain phantom for computing r egistration. Three internal test markers were used to estimate the acc uracy of registration. The phantom was imaged with two different SPET and MRI devices. The mean root-mean-squared (RMS) residual of the loca tions of the test markers after registration using different combinati ons of four external markers varied from 3.5 +/- 1.0 to 5.2 +/- 1.3 mm depending on the imaging equipment and parameters used. The accuracy improved with an increasing number of external markers, from 3.2 +/- 0 .5 to 4.9 +/- 0.5 mm for 6 markers and from 3.1 +/- 0.1 to 4.7 +/- 0.1 mm for 13 markers. In simulations, the external markers had an error comparable to the corresponding error in the phantom experiments. The error in the test markers was varied independently of that of the exte rnal markers. When the locating error of the test markers was removed, about 2 mm of the residuals of the test markers were found to come fr om this source. When an error comparable to the resolution of the orig inal images (7-10 mm for SPET, 2 mm for MRI) was included in the test markers, the largest mean RMS residual after registration was smaller than the resolution error (8.8 +/- 1.1 mm). This was due to the accura cy of localization of the external markers and the fact that the direc tion of the error was random for each marker. The size of the registra tion error of an image volume was site-dependent, being minimal near t he centre of mass of the external markers. When comparing the error wi th the spatial resolution of SPET, it was concluded that the accuracy of registration is not the limiting factor in region-of-interest analy sis of registered images, provided that the design and attachment of t he marker system are appropriate.