EDITORIAL

E this issue, Yu et al. (l) evaluate the bility to register the emission and transmission scans of patients undergo ing thoracic PET studies. They utilize a surface-fitting image correlation tech nique developed by Pelizzari et al. (2) and conclude from phantom and patient studies that internal structures (lungs) can be registered to approximately 3 mm and 2 degrees. The ability to re peatedly register PET studies obtained at different times is important in the attenuation corrections applied to these scans to extract quantitative data. Image correlation has many poten tial applications in nuclear medicine. Indeed it isvery natural to want to com bine the high-resolution anatomical delineation of structures as visualized on MRI or CT with the less spatially resolved but functional biochemical images from PET and SPECT. Both modalities benefit from this correlation. Examples of successful clinicalapplica tions of correlating nuclear medicine studies with high-definition anatomical studies include correlation of structure and function studies such as registra tion of PET and MRI (3), three-dimen sional displays for surgical planning (4), registration of SPECT and MRI (5) and the use of SPECT/CT/MRI registration for localization of tumors by radioim-munoconjugates (6). The most important consideration of an image registration technique is its accuracy in its clinicalutilization. Many algorithms can register two three-dimensional data sets to a mean value whose magnitude is less than the RMS value of the sum of the squares of the pixel sizes (2,7,8). For PET and CT, this value is approximately 3-4 mm. A second consideration is the ability to retrospectively register image data sets