AUTOMATIC REGISTRATION OF PELVIC COMPUTED-TOMOGRAPHY DATA AND MAGNETIC-RESONANCE SCANS INCLUDING A DULL CIRCLE METHOD FOR QUANTITATIVE ACCURACY EVALUATION

The purpose of this study is to develop a method for registration of C T and MR scans of the pelvis with minimal user interaction and to obta in a means for objective quantification of the registration accuracy o f clinical data without markers. CT scans were registered with proton density MR scans using chamfer matching on automatically segmented bon e. A fixed threshold was used to segment CT, while morphological filte rs were used to segment MR. The method was tested with transverse and coronal MR scans of 18 patients and sagittal MR scans of 8 patients. T he registration accuracy was estimated by comparing (triangulating) re gistrations of a single CT scan with MR in different orientations in a ''full circle.'' For example, CT is first matched on transverse MR, n ext transverse MR is matched independently on coronal MR, and finally coronal MR is matched independently on CT. The product of the three tr ansformations is the identity if all matching steps are perfect. Devia tions from identity occur both due to random errors and due to some ty pes of systematic errors. MR was registered on MR (to close the ''circ le'') by minimization of rms voxel value differences. CT-MR registrati on takes about 1 min, including user interaction. The random error for CT-MR registration with transverse or coronal MR was 0.5 mm in transl ation and 0.4 degrees in rotation (standard deviation) for each axis. A systematic registration error of about 1 mm was demonstrated along t he MR frequency encoding direction, which is attributed to the chemica l shift. In conclusion, the presented algorithm efficiently and accura tely registers pelvic CT and MR scans on bone. The ''full circle'' met hod provides an estimate of the registration accuracy on clinical data . (C) 1998 American Association of Physicists in Medicine. [S0094-2405 (98)03710-9].