PET/CT Protocols and Artifacts in the Head and Neck
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. The general protocols for CT are different with PET/CT, however, 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 or not IV contrast is used and how and when it is administered. There are different CT scan protocols for combined PET/CT that are performed in clinical practice today: (1) noncontrast with low current (approximately 40 mA used for AC and localization only), (2) noncontrast with normal current (approximately 140 mA), (3) normal current with IV or oral contrast, or (4) both low-dose (for AC) and full-current (for diagnostic interpretation) CT.4
Reimbursement of the CT portion of a PET/CT
New PET/CT codes have recently been developed, including 78814 (limited area), 78815 (skull base to midthighs), and 78816 (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 78811, 78812, and 78813, reflecting the increased capital cost to purchase the combined device.
In general, diagnostic (full-dose noncontrast or contrast-enhanced) CT studies should not be performed unless medically necessary
Suggested protocols for patients who have head and neck malignancies
CMS and most third-party payers reimburse for PET and PET/CT for several malignancies, including head and neck cancer, to evaluate initial diagnosis, staging, and restaging; however, it is not clear if all patients require a contrast-enhanced 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 appropriate
PET/CT artifacts
There are several artifacts inherent to FDG-PET imaging that have been reported in the literature, including photopenic areas resulting from metallic devices or other high-attenuation materials.1 There also are several PET/CT artifacts; however, their appearances are different from those seen on dedicated PET scanners and there are several new artifacts that are unique to combined PET/CT scanners. Most of these unique artifacts are generated by the CT-based AC protocol that is currently in use
CT attenuation correction and related artifacts
There are several AC methods for PET and PET/CT scanners.2, 3, 4 One of the advantages of using PET/CT is that the AC is easily performed using the CT portion of the examination rather than having to perform a separate transmission scan, which is necessary to perform AC on dedicated PET systems. Using CT for AC thus permits a significant reduction in the amount of time it takes to complete an examination. Although counterintuitive, it may take up to 40% less time to complete a whole-body PET/CT
Intravenous contrast and attenuation correction artifacts
One of the reasons why the use of contrast agents with PET/CT is controversial is that they may cause AC artifacts on the corrected PET images when using CT for AC.6, 8, 9, 10, 11 When dense contrast material is present in venous structures during the CT acquisition there tends to be an overcorrection of the PET data. This mismatch causes areas of linear artifact (mimicking intense FDG accumulations) on the AC-PET images (Fig. 1). Occasionally, these artifacts are clinically significant when
Ports and other high-attenuation devices
Metallic objects, including various orthopedic devices and chemotherapy ports, typically cause areas of photopenia on images obtained on a dedicated PET scanner. When correcting for attenuation with Germanium or other point sources, these areas remain photopenic on the AC images. In contrast, when using the current CT-based AC algorithms, these areas usually demonstrate falsely elevated FDG uptake (Fig. 4).11, 21, 22, 23 It usually is easy to correlate the area of apparent FDG uptake with the
Calcified lymph nodes and attenuation correction artifacts
Perhaps the most clinically significant but under-reported CT-based AC artifacts are ones caused by calcified lymph nodes. This is true particularly, for example, in patients who are being evaluated for lung cancer and have falsely elevated uptake of FDG in a normal calcified lymph nodes. In patients who have a right-sided primary squamous-cell carcinoma of the lung and a single contralateral calcified paratracheal lymph node with falsely elevated FDG uptake, this could lead to nonsurgical
Lymphangiogram effect
The FDG used for PET studies is injected IV into patients. Occasionally, a portion or the entire dose may be infiltrated accidentally into the subcutaneous tissues. When this occurs, FDG may be taken up into the lymphatic system. This can be problematic because it can cause eventual uptake within axillary or mediastinal lymph nodes and make a study essentially nondiagnostic because of the inability to exclude nodal disease (Fig. 9).30 This often requires a short-term follow-up examination to
Arm positioning and image quality
Unlike dedicated CT, where short scan times allows for routine scanning with the arms kept out of the field of view, with PET/CT the arms often may be kept in the field of view to minimize patient discomfort and the potential for motion artifacts. When the arms are positioned at a patient's side, there can be significant beam hardening and streak artifacts in the CT images (Fig. 10), which can be especially problematic when the artifact overlaps with the area of interest. One potential solution
Summary
There are several artifacts unique to combined PET/CT imaging, including AC-based artifacts and protocol-based artifacts. For experienced readers, many typical appearances of these artifacts are not clinically significant. Atypical patterns of artifacts, however, even for experienced readers, can be challenging. For inexperienced readers it is imperative to become familiar with the common and atypical appearances of the most common artifacts and be diligent about inspecting the non-AC–PET
References (30)
- et al.
Normal physiological and benign pathological variants of 18-fluoro-2-deoxyglucose positron-emission tomography scanning: potential for error in interpretation
Semin Nucl Med
(1996) - et al.
Positron emission tomography/computed tomography—imaging protocols, artifacts, and pitfalls
Mol Imaging Biol
(2004) - et al.
Applications of positron emission tomography/computed tomography image fusion in clinical positron emission tomography-clinical use, interpretation methods, diagnostic improvements
Semin Nucl Med
(2003) - et al.
Artifactual 2-deoxy-2-[(18)F]fluoro-D-glucose localization surrounding metallic objects in a PET/CT scanner using CT-based attenuation correction
Mol Imaging Biol
(2003) - et al.
Role of attenuation correction for fluorine-18 fluorodeoxyglucose positron emission tomography in the primary staging of malignant lymphoma
Eur J Nucl Med
(1999) - et al.
Attenuation correction for a combined 3D PET/CT scanner
Med Phys
(1998) - et al.
PET attenuation coefficients from CT images: experimental evaluation of the transformation of CT into PET 511-keV attenuation coefficients
Eur J Nucl Med Mol Imaging
(2002) - et al.
Impact of patient weight and emission scan duration on PET/CT image quality and lesion detectability
J Nucl Med
(2004) - et al.
Focal tracer uptake: a potential artifact in contrast-enhanced dual-modality PET/CT scans
J Nucl Med
(2002) - et al.
Acquisition protocol considerations for combined PET/CT imaging
J Nucl Med
(2004)