13. PET and CT Correlation. An Important Explanation for Discordant Findings

Purpose: The purpose of this investigation is to compare and explain discordant findings on high quality PET and CT where no corresponding CT abnormality is seen despite a significant appearing PET abnormality.Methods: The methods involved the review of forty sequential oncologic cases. State-of-the-art helical post contrast CT scans and state-of the-art attenuated corrected and uncorrected PET images were examined. Discordant findings were classified as: Type A Obvious PET abnormalities fail to reveal CT anatomic abnormalities. Type B CT anatomic abnormalities show no PET abnormalities (usually benign disease). Only Type A discordance was evaluated and repeat CT scans were obtained if more than two weeks separated the exams.Results: The results showed that there were four cases of Type A discordance. Case I showed a large PET abnormality with a negative CT. Repeat CT was again negative. The suspicion of a mis-registration artifact on PET due to motion was confirmed. Cases II, III, & IV had prominent PET abnormalities with normal CT scans within two to six weeks preceding the PET. Repeat CTs showed remarkable new findings corresponding exactly with the PET abnormalities.Conclusion: In conclusion, when a conspicuous hypermetabolic focus has no corresponding CT anatomic structure on a high quality CT exam, extra evaluation is in order. Aside from technical artifacts, a relatively short time delay between CT and PET may account for development of new findings on CT in the face of aggressive disease.