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Registration of planar emission images with reprojected CT data.

UNLABELLED
Planar gamma-camera imaging is still widely used clinically. Alignment of planar images with images from tomographic modalities, such as CT, or with other planar images would be desirable. Here, we present and evaluate a method for such an alignment, using planar transmission images acquired with the emission images and reprojection of the 3-dimensional CT data. This method permits determination of which CT slice corresponds to a particular row of pixels in the gamma-camera image and which column of pixels in that CT slice corresponds to a particular pixel in the emission data.


METHODS
A method based on maximization of the correlation coefficient, previously used for 3-dimensional datasets, was modified to permit 2-dimensional registrations. Planar transmission measurements were obtained using a collimated 99mTc flood source in conjunction with planar emission studies. The CT data were first reprojected to permit the 2-dimensional registration. The registration method was evaluated for its accuracy and reproducibility.


RESULTS
For phantom data, the registration errors were -0.1 +/- 1.0 mm for x-translations, 1.0 +/- 1.3 mm for y-translations, and -0.2 +/- 0.3 degrees for rotations. For patient data, the errors were 1.6 +/- 0.8 mm for x-translations, 1.3 +/- 1.0 mm for y-translations, and 0.5 +/- 0.5 degrees for rotations. An examination of the need for rescaling of the attenuation data (to compensate for the different photon energies used in the respective attenuation measurements) showed no significant impact on registration error. When 5 different regions of interest were used for the correlation coefficient calculation, the mean errors attributable to region-of-interest choice alone were 1.0 mm for x-translations, 2.0 mm for y-translations, and 1.2 degrees for rotations.


CONCLUSION
In almost all instances, translational registration errors were kept to subpixel levels (pixel size, 2.6 mm) and rotational errors to 1 degrees or less. The 1 exception was in the easily avoidable case of "pitch" rotations of the patient of 2 degrees or more. The modified registration method provides a simple yet reliable way to provide cross-modality evaluation of planar emission data.