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First row: Attenuation corrected images |
PET vs. PET/CT: What’s the CT part of PET/CT for? Good question—this is confusing even to some radiologists! As mentioned in the PET overview, one of the great strengths of PET imaging is its ability to correct for patient body attenuation. This results in better quality PET images as well as enabling lesion uptake quantitation (SUV). Both stand-alone PET and PET-CT systems can correct for attenuation. Stand-alone PET does this with an external gamma source which revolves around the patient. PET-CT does this with a built-in CT scanner (for a CT is basically a map of patient attenuation). The CT attenuation correction is not better quality than stand-alone PET attenuation correction but is much faster (around 30 seconds as opposed to 5-10 minutes for this portion). However, it also offers the additional significant advantage of much more precise PET lesion localization, combining the metabolic PET image with an anatomic CT one. The CT in our PET-CT unit is a high quality, dual-slice helical (“spiral”) CT scanner. However for purposes of attenuation correction we operate it in a mode optimized to this task which is different from the diagnostic quality CT mode. For examples, no oral or IV contrast is given, x-ray energies used are lower, and patient breath-holding is different from those found in “diagnostic quality” CTs. Nevertheless, this non-diagnostic CT is not bad and leads to significantly improved lesion localization than stand-alone PET. So, if you order a "PET only" at our center, we still have to do some sort of CT to generate the attenuated corrected image. Again this also results in quicker exam time and improved lesion localization. The patient is not charged for a CT and no separate CT report is issued. The nuclear physician will review this CT carefully, however, and incorporate all relevant findings into the PET report. If you would like a PET and a diagnostic CT, including the option of oral/IV contrast, this can also be accomplished during a single patient visit to the PET center. The PET portion is first performed as above (with a non-diagnostic CT) immediately followed by a diagnostic-quality of CT of whatever region is specified (neck, chest, abdomen, and/or pelvis) with oral and/or IV contrast if desired. (We do not perform diagnostic quality brain CTs as PET/CT machines cannot angle their gantries as required.) This CT, while also reviewed by the nuclear physician, is officially interpreted in a separate report by the appropriate radiologist (chest, body, neuro) just like any other CT. That radiologist will be able to make use of the PET portion, however, for improved diagnostic accuracy. The patient will be charged for a separate CT as well as the PET in this scenario. To obtain this type of study ask for a PET with Diagnostic CT (and specify CT neck, chest, abdomen, and/or pelvis and what type of contrast—oral/IV/none). Remember that regardless of the diagnostic CT ordered, the whole body PET will always cover the neck through the pelvis (except for melanoma which is head to toe or if otherwise specified to include a larger region). When is a PET non-diagnostic CT enough and when do you need the additional diagnostic CT? This is a very new area although there is certainly emerging evidence that fused PET/CT in general is more accurate than PET and CT interpreted side by side. As to “diagnostic” versus “non-diagnostic” CT there is still no clear consensus and the answer will vary depending on the clinical scenario. (In addition, diagnostic-CT becomes less important if the PET interpreter is trained in both PET and radiology, as Drs. Pu and Appelbaum are). However, there are areas where diagnostic CT is clearly superior. These include detecting tiny pulmonary metastases, cases where the relationship of tumor to anatomic structures like vessel is important as with lung cancer surgical staging, and tumors with mixed cell types—some lesions of which may have low FDG uptake and are only manifest on the CT portion. Ordering a diagnostic CT may also be required for certain treatment protocols and may also reflect a physician’s experience and comfort level with PET imaging. The bottom line is that PET/CT is in general superior to stand-alone PET, particularly for whole body oncologic imaging. Diagnostic contrast CT may provide additional information in many instances although frequently, the PET/non-diagnostic CT is entirely sufficient. |
