Registration of planar emission images with reprojected CT data

Planar gamma-camera imaging is still widely used clinically. Alignment of p lanar images with images from tomographic modalities, such as CT, or with o ther planar images would be desirable. Here, we present and evaluate a meth od for such an alignment, using planar transmission images acquired with th e emission images and reprojection of the 3-dimensional CT data. This metho d permits determination of which CT slice corresponds to a particular row o f pixels in the gamma-camera image and which column of pixels in that CT sl ice corresponds to a particular pixel in the emission data. Methods: A meth od based on maximization of the correlation coefficient, previously used fo r 3-dimensional datasets, was modified to permit 2-dimensional registration s. Planar transmission measurements were obtained using a collimated Tc-99m flood source in conjunction with planar emission studies. The CT data were first reprojected to permit the 2-dimensional registration. The registrati on method was evaluated for its accuracy and reproducibility. Results: For phantom data, the registration errors were -0.1 +/- 1.0 mm for x-translatio ns, 1.0 +/- 1.3 mm for gamma-translations, and -0.2 +/- 0.3 degrees for rot ations. For patient data, the errors were 1.6 +/- 0.8 mm for x-translations , 1.3 +/- 1.0 mm for gamma-translations, and 0.5 +/- 0.5 degrees for rotati ons. An examination of the need for rescaling of the attenuation data (to c ompensate for the different photon energies used in the respective attenuat ion measurements) showed no significant impact on registration error. When 5 different regions of interest were used for the correlation coefficient c alculation, the mean errors attributable to region-of-interest choice alone were 1.0 mm for x-translations, 2.0 mm for gamma-translations, and 1.2 deg rees for rotations. Conclusion: In almost all instances, translational regi stration errors were kept to subpixel levels (pixel size, 2.6 mm) and rotat ional errors to 1 degrees or less. The 1 exception was in the easily avoida ble case of "pitch" rotations of the patient of 2 degrees or more. The modi fied registration method provides a simple yet reliable way to provide cros s-modality evaluation of planar emission data.