AJR 2002; 179:1337-1343
© American Roentgen Ray Society
Positron Emission Tomography and PET CT of the Head and Neck: FDG Uptake in Normal Anatomy, in Benign Lesions, and in Changes Resulting from Treatment
Gerhard W. Goerres1,
Gustav K. von Schulthess and
Thomas F. Hany
1 All authors: Division of Nuclear Medicine, University Hospital Zurich,
Raemistr. 100, CH-8091 Zurich, Switzerland.
Received January 9, 2002;
accepted after revision April 10, 2002.
Address correspondence to G. W. Goerres.
Introduction
During recent years, positron emission tomography (PET) with FDG has become
an established imaging method for the evaluation of patients with head and
neck cancer
[1,2,3,4].
However, the anatomic information acquired with PET is limited, and
correlation with structural imaging such as CT is important. Recently,
combined in-line PET CT devices that allow the coregistration of functional
and anatomic information have been introduced into clinical practice. On
combined PET CT cameras, conventional transmission scanning using the built-in
germanium-68 sources can be replaced by CT
[5]. This capability can
influence the quality of coregistered PET CT images and of
attenuation-corrected PET images. We present the spectrum of PET and PET CT
imaging of the head and neck for normal anatomy, benign lesions, and changes
resulting from treatment.
Technical Aspects of Imaging
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.
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Fig. 1C. —53-year-old man with hypopharyngeal carcinoma.
Attenuation-corrected positron emission tomography image (reconstructed using
iterative ordered subset expectation maximization algorithm) at same level as
B. Slight movement of patient's head between emission and transmission
scanning caused asymmetric appearance and slightly darker left side of head
(arrows).
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Dental Metalwork
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).
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Fig. 2A. —57-year-old woman with bronchogenic carcinoma. Transverse
positron emission tomography (PET) emission image shows normal appearance of
oral cavity. Dental metalwork generates white spots (arrows) where
information is lacking.
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Fig. 2B. —57-year-old woman with bronchogenic carcinoma. Same PET image
as A with attenuation correction shows artifacts arising adjacent to
metal (long arrows) that mimic increased FDG uptake. Moderately
increased uptake (short arrow) due to inflammation is visible in
right tonsil.
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Fig. 3A. —72-year-old man with bronchogenic carcinoma. Transverse
coregistered positron emission tomography (PET) CT image illustrates
beam-hardening stripe artifacts (arrows) from CT image that were
caused by dental metalwork.
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Fig. 3B. —72-year-old man with bronchogenic carcinoma. PET image
corresponding to A shows that such artifacts (arrows) can be
introduced into final PET image if CT data are used for attenuation
correction.
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Normal Tissues of the Head and Neck
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.
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Fig. 4A. —49-year-old woman with cancer of large intestine. Transverse
positron emission tomography image shows normal appearance of parotid gland
with moderate FDG uptake (short arrows) and low to moderate uptake
(long arrows) at border and base of tongue and soft palate.
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Fig. 6A. —56-year-old man with liver cancer. Transverse positron
emission tomography image shows FDG uptake at insertion site of genioglossus
muscle (short arrows), which keeps tongue in position in supine
patients. Muscular uptake in longus capitis muscle (long arrow) can
mimic lymph node involvement in transverse PET image.
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Fig. 7C. —36-year-old man with seminoma. Transverse PET image shows FDG
uptake in longus capitis muscle (lcm), which can mimic uptake in epipharyngeal
lymph node (Rouvière's node or Rosenmüller's cavity).
Differentiation of this uptake from lymph node involvement can be difficult.
Muscular uptake is usually not nodular and can be easily discriminated from
lymph node involvement with proper image analysis. Asterisk indicates
cerebellum.
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Fig. 8. —76-year-old woman with pancreatic cancer. Coronal PET image
shows laryngeal FDG uptake at vocal cords (arrows) because patient
spoke during FDG uptake phase. In patients with laryngeal nerve palsy due to
Pancoast's tumor or nerve damage after surgery, unilateral uptake (not shown)
can be visible.
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Treatment Effects
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).
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Fig. 9. —65-year-old woman with hypopharyngeal cancer. Transverse
positron emission tomography image obtained 1 week after surgical removal of
teeth in patient scheduled for radiation treatment shows strong bilateral FDG
uptake in lower jaw. Dental extraction is often performed in patients with
head and neck cancer before radiation treatment, and FDG uptake, which can
mimic uptake in malignant lesion, continues to be visible for several
weeks.
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Fig. 11. —62-year-old man who received chemotherapy for non-Hodgkin's
lymphoma. Sagittal positron emission tomography image shows increased FDG
uptake in bone marrow caused by rebound of marrow cells. Bone marrow uptake
could be decreased if vertebral column (not shown) were in field of
irradiation.
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Fig. 12. —53-year-old man with tracheostoma after laryngectomy.
Transverse positron emission tomography image shows increased FDG uptake at
borders of tracheostoma. Such inflammation can mimic local recurrence.
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Fig. 13A. —42-year-old man who received combined radiation and
chemotherapy for hypopharyngeal carcinoma. Primary carcinoma and lymph node
metastases were successfully treated. Sagittal positron emission tomography
(PET) CT image shows increased FDG uptake in mucosal membrane and adjacent
soft tissues of hypopharynx and esophagus 5 weeks after treatment. Mucositis
is common inflammatory reaction of mucosal membrane of pharynx and esophagus
soon after therapy.
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Fig. 13B. —42-year-old man who received combined radiation and
chemotherapy for hypopharyngeal carcinoma. Primary carcinoma and lymph node
metastases were successfully treated. Sagittal PET image shows involvement of
esophagus. Clinical information helped us to correctly interpret this finding
as treatment effect.
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Growth Pattern and Visibility of Benign Lesions
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).
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Fig. 14. —44-year-old man with bronchogenic carcinoma. Transverse
positron emission tomography image reveals focal uptake in right parotid gland
(arrow). This lesion was Warthin's tumor, a benign lymphoepithelial
proliferation with nodular growth pattern.
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Fig. 15. —72-year-old woman with mucosal malignant melanoma. Transverse
positron emission tomography (PET) image shows moderate FDG uptake in
maxillary sinus (arrows). PET was performed for N- and M- staging of
metastasis [9]. Amount of FDG
uptake alone cannot be used to discriminate between benign and malignant
lesions. Melanoma cells avidly take up FDG, but because this lesion has
superficially spreading growth pattern, uptake appears to be low.
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Conclusion
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.
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Introduction
Technical Aspects of Imaging
