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prove001{at}mc.duke.edu
CONTINUING MEDICAL EDUCATION
The immediate advantage that PET/CT provides to the medical community is that the physiological information available through PET now can be complemented by the anatomical imaging provided by CT. Thus, in the case of 18F-fluorodeoxyglucose (18FDG) PET, foci of altered glucose metabolism now can be localized definitively to an anatomic structure, organ, or specific region within an organ. This advance has a clear impact on facilitation of diagnosis, identification of optimal biopsy site, and improvement in monitoring of therapeutic response.
The addition of physiological information from PET to anatomic information provided by another imaging technique is not a new phenomenon. At the author's institution, MRI scans routinely have been coregistered with PET images in a subpopulation of our brain tumor patients for years. Comparison of the MRI and PET images in such patients often has proven illuminating. For instance, it generally is understood that low-grade tumors show either normal or low metabolism on PET studies. Therefore, low-grade brain tumors such as gangliogliomas would be expected to show only normal or low-glucose metabolism on PET studies. In one study of patients with gangliogliomas who underwent both PET and MR imaging within a few days of one another, assessment of unregistered PET images appeared to indicate that only regions of normal or low-glucose metabolism were present in the tumors [1]. However, after PET images were coregistered with MR images, many foci of elevated glucose metabolism (more consistent with a high-grade tumor) were found in regions of the tumor as seen on MRI.
It is clear from the article written by Blodgett et al. in this issue that a number of unresolved controversies remain. For instance, at many institutions, PET scans are interpreted by nonradiologists trained in nuclear medicine whereas CT scans are interpreted by radiologists. Therefore, as pointed out by Blodgett et al., a number of issues reading interpretations of PET/CT scans arise, including whether (and how) training guidelines for interpretation of such scans will be devised and how cross-training in the technique will be accomplished for physicians who are trained in interpretation of solely CT or solely PET. An additional issue that needs to be addressed is whether a single standard of practice should be adopted for performance of PET/CT studies (e.g., use of oral contrast material and IV contrast material). Furthermore, CT scans presently are obtained with a variety of imaging parameters, such as at different mA values, thereby providing images of differing quality with regard to ability to interpret anatomic images.
As noted above, the immediate perceived advantage of combined PET/CT is the addition of anatomic information to physiological data. However, one can foresee a further improvement by the addition of physiological information from CT (e.g., from CT perfusion) to the algorithm, thereby allowing CT to add both anatomic information and physiological information. To date, CT perfusion has been used primarily for assessment of acute cerebral infarction. However, it may prove to be valuable in assessment of other conditions, for example, brain tumors, as well as disease processes involving organ systems other than the central nervous system. For instance, it might be fruitful to compare two different forms of physiological information, for example, hemodynamic information from tumors (using CT perfusion) and information about glucose utilization (from PET). Coregistration of the data sets would be facilitated given that both are obtained from the same imaging system.
In summary, some of the advantages of the basic combination of CT anatomical information and PET physiological information already are very clear. However, many real challenges remain in terms of further mining the full capabilities of these two imaging techniques. It may be that the combination of these techniques will yield much more than we presently envision. If that is the case, it is quite possible that PET/CT will be followed by the introduction of other dual-imaging techniques to clinical practice.
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