Hybrid imaging in planar scintigraphy: new implementations and historical precedents

Fusion of tomographic radionuclide studies with anatomical examinations has become standard practice in positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging. Nonetheless, fusion of planar scintigraphic images with an anatomical modality remains distinctly uncommon, although methods to do so have appeared sporadically in the literature during the past 2 decades. In this article we review several techniques that have been used to combine planar scintigraphic images with radiographs and visual (photographic) images. Rigid or affine transformations have been performed to co-register the planar images with each other using custom, commercial, or public domain software. Display of the hybrid images has been achieved primarily with nonselective color-fusion methods. Promising efforts are underway to develop a technique of fusing planar lymphoscintigraphic images with CT topograms (scout images) obtained on the SPECT-CT camera in a manner that compensates for position-dependent variation in magnification that affects the CT scout. An advantage of this approach is that both of the component images are acquired on the same gantry, without need for repositioning of the patient. It is instructive to note that techniques of fusing rectilinear scans with radiographic and visual images were first developed more than 50 years ago. The revisiting of these methods after many decades reflects a fundamental need for spatial orientation in nuclear medicine that fusion imaging can also bring to planar scintigraphic studies.