Positron Emission Tomography/Computed Tomography: Protocol Issues and Options
Section snippets
Standard PET/CT Protocol
One of the major benefits of combined PET/CT is the ability to acquire accurately coregistered PET and CT images in a single imaging session. However, the general protocols for CT are different with PET/CT because, unlike PET imaging, which typically is a neck-through-pelvis survey of the body, CT traditionally has been performed for regional evaluation (eg, head, neck, chest, abdomen, and pelvis). PET/CT imaging protocols therefore must be adjusted to adequately evaluate the primary area of
Protocol Options
One important PET/CT protocol decision is whether IV contrast will be used and how and when it will be administered. There are different CT scan protocols for combined PET/CT that are performed in clinical practice today: (1) noncontrast with low current (∼40 mAs used for AC and localization only) (Fig. 4A), (2) noncontrast with normal current (∼140 mAs), (3) normal current with IV and/or oral contrast (Fig. 4B), or (4) both low-dose (for AC) and full-current (for diagnostic interpretation) CT (
IV Contrast AC Artifacts
As mentioned previously, one of the stated reservations about the use of contrast media is that they may cause artifacts on the AC PET images when using CT for AC.9, 37, 38, 39, 40 When dense contrast material is present in central venous structures during the CT acquisition, but not during the PET portion of the examination, there tends to be an overcorrection of the PET data. This mismatch causes an area of linear artifact (mimicking intense FDG accumulation) on the AC PET images (Fig. 6).41
Reimbursement Issues
New PET/CT codes recently have been developed, including 78,814 (limited area), 78,815 (skull base to mid-thighs), and 78,816 (whole body). These are used for PET acquired on a PET/CT scanner, and the charges/reimbursement are slightly higher than for the dedicated PET codes 78,811, 78,812, and 78,813, reflecting the increased capital cost to purchase the combined device.
In general, diagnostic (contrast-enhanced in most instances) CT studies should not be performed unless medically necessary
Suggested Protocols
PET and PET/CT are covered by CMS and most third-party payers for several malignancies to evaluate initial diagnosis, staging and restaging; however, it is not clear whether all patients require a diagnostic CT as part of their PET/CT examination. At the University of Pittsburgh, we have attempted to identify potential patient populations that might be adequately evaluated by a low-dose CT as part of the PET/CT in an attempt to optimize the appropriate use of PET/CT, as well as to decrease
Who Should Interpret PET/CT
The issue of who is qualified to interpret a PET/CT scan is controversial and beyond the scope of this review. However, certain observations warrant consideration. Physicians interpreting imaging studies generally are held responsible for recognizing and reporting abnormalities that are present on the images, even if the explicit reason for obtaining the study is outside the expertise of the interpreting physician. For example, orthopedic surgeons interpreting musculoskeletal magnetic resonance
Future Direction of CT Protocols
Very few studies have been performed actually comparing noncontrast with contrast-enhanced PET/CT. Even fewer studies have addressed the issue of whether multiphasic enhancement of the CT portion of a PET/CT offers any potential benefit. However, a single phase of contrast enhancement for CT may not be optimal or adequate in some settings. For example, some hepatic tumors are variably FDG–avid and are well known to be reliably detected only on a particular phase of a contrast-enhanced CT scan.
Conclusion
There is no single “correct” protocol for performing a PET/CT scan. Important variables to be considered include the specific type of tumor and the likelihood of encountering viable abdominal and pelvic malignancy. The use of oral and intravenous contrast media may improve the diagnostic value of the CT component, but can give rise to artifacts that may interfere with interpretation. Artifacts can be minimized by attention to technique and the use of newer faster PET/CT devices.
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