PET images have been reconstructed using filtered back-projection, but iterative reconstruction algorithms are used increasingly for routine clinical imaging. Attenuation correction is done with a transmission scan; it requires perfect alignment of the patient during emission and transmission scanning. If the patient moves between the two scans, attenuation correction will not be accurate, and artifacts will result. Such movement artifacts also occur in combined PET CT imaging when a CT image is used for attenuation correction. A lateral movement of the patient's head will result in an asymmetric appearance of the PET image with one side of the head appearing slightly darker than the other side (Fig. 1A,1B,1C). This discrepancy does not necessarily disturb final image interpretation because malignant lesions usually have intense FDG uptake. Larger head movements may render interpretation of attenuation-corrected images impossible; therefore, the emission scans that are not attenuation-corrected should also be evaluated.

Metal used for dental work will produce a defect in the emission image as a result of excessive photon absorption by the metal (Fig. 2A,2B,2C). In attenuation-corrected images, nonremovable metal dentures, bridgework, and other such items can cause artifacts that mimic FDG uptake (Fig. 2A,2B,2C). These artifacts may look like small lesions in the periodontal space. Artifacts arise in both conventional and CT-based attenuation-correction methods because material with high density can create data inconsistencies in the attenuation map [6]. In patients with nonremovable metal dentures, emission images can be useful in the interpretation of the oral cavity (Fig. 2A,2B,2C). If attenuation correction is performed by means of the CT scan, stripe artifacts on the CT image can be visible on the final attenuation-corrected PET image (Fig. 3A,3B).

Low to moderate FDG uptake occurs in the tonsils and at the base of the tongue because of the physiologic accumulation in the lymphatic tissue in Waldeyer's ring (Fig. 4A,4B). Variable but usually low FDG uptake is visible in the salivary glands, which physiologically secrete low amounts of glucose (Figs. 4A,4B and 5A,5B). A moderately increased FDG uptake can be seen in the anterior part of the floor of the mouth corresponding to the genioglossus muscle, which prevents the tongue from falling back in supine patients (Fig. 6A,6B). Muscular uptake can be seen in the masticator muscles (Fig. 7A,7B,7C), the tip of the tongue, and muscles of the face, neck, and larynx in nervous patients and in patients who speak during the FDG uptake phase (Fig. 8). In nervous patients, muscular uptake can be avoided by using medication, such as diazepam, for muscle relaxation. If patients do not close their eyes during the study, muscles of the eyes and eyelids will also show increased uptake.

Dental problems are common in patients with head and neck cancer. If infectious disease of dental roots and the periodontal space are present, the teeth will generally be removed before radiation treatment is started. This surgery will cause an inflammatory reaction in the jaw, mucosal membranes, and underlying soft tissues. A strong FDG uptake can be visible for several weeks (Figs. 9 and 10A,10B). After chemotherapy, FDG uptake of the bone marrow may increase (Fig. 11). In patients with a tracheal stoma, FDG uptake can occur when inflammation is present at the borders (Fig. 12). Radiation therapy can cause pharyngitis and esophagitis, which can be visible as increased FDG uptake in the mucosal membrane (Fig. 13A,13B).

A lesion with a nodular growth pattern is more visible than a superficially spreading lesion—for example, in the mucous membrane. A lesion with increased FDG uptake in the salivary gland is often from a papillary cystadenoma lymphomatosum (Warthin's tumor) (Fig. 14). This benign lymphoepithelial proliferation is the second most common salivary gland tumor and is often seen in smokers [7] (Fig. 14). A benign lesion, such as sinusitis, can show variable intensity of FDG uptake. A nodular lesion can have very strong FDG uptake even when benign, whereas a malignant lesion with a superficially spreading growth pattern can show low FDG uptake. An example of a mucosal malignant melanoma located in a mucous membrane is shown in Figure 15. Although melanoma avidly takes up FDG, this growth pattern does not allow discrimination of this lesion from a benign inflammation such as sinusitis [8] (Fig. 15).

Image quality in FDG PET and PET CT depends on both patient preparation and the technical quality of image acquisition. Metal parts, such as dental bridgework, should be removed, and patients should be instructed not to speak during the FDG uptake phase. Attenuation-corrected PET studies should be compared with the uncorrected emission scans to check for movement artifacts and artifacts due to dental metalwork. Lesions should be identified in coronal, transverse, and sagittal planes to avoid misinterpretation as muscular uptake. Clinical information is important to correctly identify changes resulting from treatment. Familiarity with normal imaging appearances helps in discriminating pathologic lesions from artifacts.

Address correspondence to G. W. Goerres.