Digital superimposition of CT and positive SPECT tumor images. Phantom study and clinical applications.

Digital superimposition of SPECT and CT data was evaluated in a phantom and then applied to patient data. Seven patients were studied. Six patients had pheochromocytomas as evidenced by I-131 or I-123 MIBG localization and one had ovarian cancer imaged by In-111 OC125 MoAb. Anatomic or skin landmarks identified the level of each SPECT transaxial slice. Both SPECT and CT image data sets were transferred to a minicomputer connected to an image processor. Afterwards, a scaled, rotated and translated realignment was performed. Data for each modality were coded in different primary colors and then superimposed. Superimposition of phantom data was checked for the absence of distortion of pinpoint and large structures. For suspected tumor sites, superimposition of the patients' slices were allowed to check for matching SPECT and CT abnormalities to localize a SPECT abnormality without a corresponding CT lesion or to distinguish SPECT abnormalities from those seen on CT. In one case, the technique failed because of very low I-131 MIBG-tumor uptake. The superimposition decreases false positives in SPECT and both false negatives or false positives in CT.