Combined PET-CT in the Head and NeckMuscles Neck and Face.—Physiologic FDG uptake within neck muscles can constitute a diagnostic dilemma in the interpretation of PET scans (7,25)

EDUCATION EXHIBIT 897

Combined PET-CT inthe Head and NeckPart 1. Physiologic, Altered Physiologic,and Artifactual FDG Uptake1

ONLINE-ONLYCME

See www.rsna.org/education/rg_cme.html.

LEARNINGOBJECTIVESAfter reading thisarticle and takingthe test, the reader

will be able to:

� Discuss the com-mon physiologicFDG uptake patternsand variants in thehead and neck.

� List several non-malignant causes ofFDG uptake in theneck.

� Describe the mostcommon artifactsencountered in thehead and neck re-gion, including at-tenuation correctionartifacts.

Todd M. Blodgett, MD ● Melanie B. Fukui, MD ● Carl H. Snyderman,MD ● Barton F. Branstetter IV, MD ● Barry M. McCook, MD ● Dave W.Townsend, PhD ● Carolyn C. Meltzer, MD

Positron emission tomography (PET) with 2-[fluorine-18] fluoro-2-deoxy-d-glucose (FDG) has been effective for the diagnosis, staging,and restaging of malignancies of the head and neck region. However,lack of anatomic landmarks, variable physiologic uptake, and asymmet-ric FDG distribution in several altered physiologic states can confoundimage interpretation. In addition, many benign causes and several arti-facts can simulate physiologic or pathologic FDG uptake in the headand neck. Combined PET–computed tomography (CT) is a uniqueimaging modality that permits anatomic and functional imaging on asingle scanner with nearly perfect coregistration. Combined PET-CTprovides information that cannot be obtained with PET or CT alone.In particular, PET-CT facilitates the interpretation of FDG uptake inthe head and neck, an area that is characterized by dense and complexanatomic structures. An atlas of FDG uptake in this anatomic regionwas compiled on the basis of combined PET-CT findings in 11,000patients. In general, patterns of FDG uptake were variable and oftenreflected patient activity during or immediately preceding the uptakephase. With the growing interest in PET-CT, interpreting radiologistsand nuclear medicine physicians must be familiar with the patterns ofFDG uptake in the head and neck to avoid misinterpretation or mis-diagnosis.©RSNA, 2005

Abbreviation: FDG � 2-[fluorine-18] fluoro-2-deoxy-d-glucose

RadioGraphics 2005; 25:897–912 ● Published online 10.1148/rg.254035156 ● Content Codes:

1From the Departments of Radiology (T.M.B., B.F.B., B.M.M., D.W.T., C.C.M.), Otolaryngology (C.H.S., B.F.B.), Psychiatry (C.C.M.), and Neu-rology (C.C.M.), University of Pittsburgh, 200 Lothrop St, Pittsburgh, PA 15213; and the Department of Radiology, Allegheny General Hospital,Pittsburgh, Pa (M.B.F.). Presented as an education exhibit at the 2001 RSNA Annual Meeting. Received June 26, 2003; revision requested October22; final revision received October 25, 2004; accepted November 3. T.M.B. is a consultant for Petnet Pharmaceuticals; D.W.T. is a consultant forCPS Innovations; all remaining authors have no financial relationships to disclose. Address correspondence to T.M.B. (e-mail: [email protected]).

See Fukui et al (pp 913–930) for Part 2 of this two-part series of articles.

©RSNA, 2005

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IntroductionPositron emission tomography (PET) with2-[fluorine-18] fluoro-2-deoxy-d-glucose (FDG)has been used to successfully diagnose, stage, andrestage head and neck cancer (1–5). FDG PET ismore sensitive and specific than computed to-mography (CT) or magnetic resonance imagingin the detection of recurrent neoplasm. However,if PET alone is performed, limited spatial resolu-tion and lack of anatomic landmarks hinder accu-rate tumor localization, particularly in the denseand complex anatomic structures of the head andneck. In addition, variable FDG uptake in normalstructures such as the nasal turbinates, pterygoidmuscles, extraocular muscles, parotid and sub-mandibular glands, and lymphoid tissue in theWaldeyer ring may confuse interpretation andlead to false-positive results (6). Although FDGuptake in primary neoplasms is usually greaterthan that observed in even the most metabolicallyactive normal structures, overlap between tumorand physiologic uptake may confound interpreta-tion.

Often, the interpreting physician relies onsymmetry or location to differentiate betweenphysiologic and pathologic FDG accumulation.However, symmetry is not a reliable indicator ofphysiologic processes. Several malignancies canmanifest with strikingly symmetric FDG uptake;conversely, physiologic uptake will often be asym-metric.

Furthermore, symmetry is not a reliable indica-tor in postsurgical patients. For example, vocal

cord paralysis from prior neck surgery can causesuperphysiologic FDG uptake on the contralat-eral side, from compensatory effort in the unaf-fected cord. Another interpretive pitfall is seen inpatients who have undergone removal of a glandor muscle. The normal physiologic FDG uptakein the remaining gland or muscle can have anasymmetric appearance.

Patients are generally instructed to remainquiet during the FDG uptake phase to limitphysiologic vocal cord uptake. Other interven-tions, such as sedation or neck collars, help mini-mize physiologic uptake in the head and neck(7,8). In general, however, these protocols areinconvenient for the patient, and they do noteliminate physiologic uptake entirely.

Several limitations of PET in patients with ma-lignancies of the head and neck have been over-come with in-line sequential combined PET-CT.This novel modality allows reliable coregistrationof anatomic and functional data. CT attenuation,rather than transmission scan data, is used forattenuation correction of emission data (9). Al-though combined PET-CT has undoubtedly fa-cilitated the differentiation of physiologic frompathologic processes, there are new artifactsunique to this modality (10,11).

To effectively and accurately interpret PET orfused PET-CT scans, a comprehensive knowl-edge of physiologic and altered physiologic FDGuptake in the head and neck, as well as an aware-ness of potential modality-specific artifacts, is es-sential. In this article, we discuss and illustrate abroad spectrum of these uptake patterns, includ-ing uptake in glands (thyroid gland, salivaryglands), muscles (muscles of the neck and face,

Figure 1. Hypermetabolic thyroid adenoma. Axial CT (a) and fused PET-CT (b) scans show focalintense FDG uptake (arrow) that is localized to the left lobe of the thyroid gland. Subsequent biopsyhelped confirm thyroid adenoma.

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muscles of the oropharynx and nasopharynx, la-ryngeal muscles), fat, lymphoid tissue, and mu-cosa. We also discuss a variety of artifacts that arespecific to PET-CT that may mimic either physi-ologic or pathologic FDG uptake.

We compiled an atlas of FDG uptake in thehead and neck region on the basis of PET-CTfindings in 11,000 patients. PET and CT wereperformed on a prototype or on a Biograph (CTIPET Systems, Knoxville, Tenn) or Reveal (Sie-mens Medical Systems, Florsheim, Germany)

PET-CT scanner. Patients received 6–11 mCi(222–407 MBq) of FDG intravenously and werescanned after a 60-minute uptake period. Stan-dard CT clinical protocols were used, includingoral and intravenous administration of contrastmaterial, as was CT-based attenuation correc-tion. In general, patterns of FDG uptake werevariable and often reflected patient activity (eg,talking, coughing) during or directly precedingthe FDG uptake phase. Part 2 of this two-partseries of articles discusses the use of combinedPET-CT in patients with head and neck malig-nancies.

Patterns of FDG Uptake

Glands

Thyroid Gland.—The thyroid gland has a vari-able appearance on FDG PET scans. It can dem-onstrate diffuse, focal, asymmetric, or virtually noFDG uptake, and these uptake patterns can beseen in physiologic, benign, and pathologic pro-cesses (7,12–14). Focal thyroid uptake is rela-tively nonspecific and has been reported in benignentities such as toxic thyroid adenoma (Fig 1) aswell as in thyroid malignancies (Fig 2) (15). Re-cently, it has been reported that focal thyroid up-take, regardless of the primary indication for theexamination, may represent a second primarymalignancy (16,17).

Diffuse symmetric FDG accumulation withinthe thyroid gland can be seen in normal glandsand is occasionally seen in patients with diffusegoiters (Fig 3).

Figure 2. Hurthle cell carcinoma. Axial CT (a) and fused PET-CT (b) scans help localize a focal abnormality (arrow)to a 2.5-cm cystic thyroid mass. Biopsy of the mass helped confirm a Hurthle cell carcinoma of the thyroid gland.

Figure 3. Physiologic thyroid uptake. (a) CoronalPET scan shows diffuse symmetric FDG uptake local-ized to the thyroid gland (arrow). (b) Axial fusedPET-CT scan helps confirm localization of the uptaketo the thyroid gland (arrow).

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Although the thyroid gland can demonstrateseveral different patterns of FDG uptake, diffuseuptake in the thyroid gland is seen as a normalvariant within normal glands, as well as in benigncauses such as chronic autoimmune thyroiditis orgoiter. The more problematic focal FDG uptake

can be seen as a result of malignancy or adenoma-tous processes (17). However, because focalasymmetric uptake can represent a primary thy-roid neoplasm, we recommend that all patientswith thyroid nodules greater than 10 mm or withintense asymmetric FDG uptake be referred forbiopsy.

Figure 4. Physiologic parotid uptake. Axial CT (a) and fused PET-CT (b) scans show moder-ate symmetric FDG uptake in the parotid glands (arrow).

Figure 5. Physiologic submandibular uptake. Axial CT (a) and fused PET-CT (b) scans dem-onstrate symmetric FDG uptake that is localized to the submandibular glands (arrow).


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Salivary Glands.—FDG is taken up by the sali-vary glands and excreted into the saliva (18). Theparotid and submandibular glands normally dem-onstrate mild to moderate symmetric physiologicuptake (Figs 4, 5). However, in some cases theymay demonstrate little or no uptake. Asymmetricuptake can be seen in patients who have under-gone surgical removal of one of the glands or inpatients with primary or metastatic lesions to theglands. FDG-avid parotid tumors include War-thin tumor, pleomorphic adenoma, and primaryparotid lymphoma (19–22). Nonmalignantcauses of FDG uptake include infectious causesand granulomatous disorders such as sarcoidosis(23).

Benign and malignant parotid tumors cannotbe distinguished with PET-CT alone because ofhigh false-positive rates (21,24). In addition,there are several salivary gland malignancies thathave little or no FDG avidity. Therefore, lack ofFDG uptake within a salivary gland mass doesnot exclude malignancy.

Malignancy can occasionally cause bilateralFDG uptake in the parotid or submandibularglands or in intraparotid lymph nodes, mimickinga physiologic pattern of uptake. In summary, bothunilateral and bilateral FDG uptake within the

salivary glands can be caused by benign or malig-nant processes, and it is necessary to consider allavailable clinical information, including prior on-cologic and surgical history, to make an accuratediagnosis.

Muscles

Neck and Face.—Physiologic FDG uptakewithin neck muscles can constitute a diagnosticdilemma in the interpretation of PET scans(7,25). Frequently, muscle uptake can be distin-guished from malignant nodal uptake by identify-ing the characteristic pattern of linear symmetricuptake. However, muscles often demonstratemore focal uptake patterns, and combinedPET-CT most frequently depicts FDG uptakelocalized to the myotendinous junction. Thesefocal areas of FDG uptake can be difficult to dis-tinguish from abnormal lymph nodes. Several fociof asymmetric uptake can be very difficult to con-fidently distinguish from tumor on PET scansalone (Fig 6). On PET-CT scans, these foci areeasily localized to the strap muscles (particularlytheir insertion sites). Intense asymmetric FDG

Figure 6. Physiologic muscle uptake. (a) Coronal PET scan demonstrates multiple bilateral foci of intenseFDG uptake (arrows). Note also the primary right lung cancer (arrowhead). (b) Axial CT scan of the lowerneck shows foci of intense FDG uptake in the shoulder girdle (arrows). (c) Fused PET-CT scan helps localizethe foci of FDG uptake to muscles and fat in the shoulder girdle (arrows).

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uptake can also be seen within the sternocleido-mastoid muscle (Fig 7) and can mimic an en-larged lymph node. Often, inspection of coronalor sagittal reconstructed images will facilitatecharacterization of muscle uptake by revealing thelinearity on one or more images.

Another muscle frequently demonstratingasymmetric radiotracer uptake is the inferiorobliquus capitus muscle (Fig 8). Uptake withinthis muscle may appear focal on coronal images,but its linear nature is evident when the uptake isviewed in an axial plane. The extreme posteriorposition of these paraspinal muscles is also oftenhelpful in identifying the source of the FDG up-take. The muscles of facial expression may alsodemonstrate linear FDG uptake (Fig 9).

Because most muscles in the neck can displayasymmetric FDG uptake, both physiologically

and after strain, close inspection of images ob-tained in all three orthogonal planes is essential toavoid misdiagnosis. Although several methodshave been used to minimize muscle uptake, in-cluding the use of soft collars and benzodiaz-epines during the uptake phase, these techniquesare inconvenient to the patient and are often notcompletely effective (8).

Figure 7. Asymmetric sternocleidomastoid muscle uptake. Coronal PET demonstratedasymmetric FDG uptake in the right side of the neck. (a) Axial CT scan shows focal FDGuptake in the right sternocleidomastoid muscle (arrow). (b) Corresponding fused PET-CTscan helps accurately localize the FDG uptake to the right sternocleidomastoid muscle (ar-row). Note also the symmetric FDG uptake within the prevertebral muscles (arrowheads).

Figure 9. Physiologic facial muscle uptake. Axialfused PET-CT scan helps localize linear FDG up-take to the muscles of facial expression in the lowerpart of the face (arrow), a finding that is consistentwith physiologic FDG uptake.


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Insulin mediates the entry of glucose as well asFDG into both muscle and adipose tissue. In hy-perglycemic patients, insulin can be administeredbefore FDG administration help decrease bloodglucose levels. Unfortunately, insulin administra-tion drives both glucose and FDG into the mus-cles, which may interfere with the evaluation ofsurrounding structures. Although some authorsadvocate administering a small dose of insulin toreduce the blood sugar level in hyperglycemicpatients, in our opinion, this will not improve im-age quality and will, in fact, often cause imagedegradation from increased FDG accumulationin muscle and fat (Fig 10). At our institution, pa-tients with mildly elevated glucose levels (�200

mg/dL) are scanned without intervention, andpatients with markedly elevated glucose levels(�200–250 mg/dL) are rescheduled for laterscanning if reasonably convenient for the patient.

Oropharynx and Nasopharynx.—Severalmuscles of the oropharynx and hypopharynxdemonstrate symmetric physiologic FDG uptake,including the pterygoid muscles (Fig 11) and themuscles of the oral floor (Fig 12). When asym-metric, uptake in these locations may mimic focalareas of malignancy (Figs 13, 14).

Figure 8. Asymmetric uptake in the inferior obliquus capitus muscle. (a) Coronal PET scan shows a hypermeta-bolic focus in the left side of the neck (arrow). (b) Axial CT scan reveals no correlative abnormality. Arrow indicatesthe left inferior obliquus capitus muscle. (c) Axial fused PET-CT scan helps confirm prominent asymmetric physi-ologic FDG uptake in the left inferior obliquus capitus muscle (arrow).

Figure 10. Physiologic effect of insulin on FDG muscle uptake. Axial CT (a) and fused PET-CT (b) scans showprominent linear FDG uptake within the muscles of the neck and upper back (arrows). The patient had received in-sulin prior to the injection of FDG.


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Figures 11–13. (11) Physiologic lateral pterygoid muscle uptake. Coronal PET demonstrated bilateral foci of in-tense physiologic FDG uptake in the infratemporal fossa and adenoids. CT findings were normal. Axial fusedPET-CT scan helps localize foci of uptake to the superior portion of the lateral pterygoid muscles (arrow). (12) Sym-metric physiologic mylohyoid muscle uptake. Axial fused PET-CT scan helps accurately localize FDG uptake to themylohyoid line (the insertion site of the mylohyoid muscle) (arrow). (13) Asymmetric mylohyoid muscle uptake.Coronal fused PET-CT scan demonstrates precise localization of FDG to the right mylohyoid muscle (arrow).

Figure 14. Asymmetric uptake in the right medial pterygoid muscle. (a) Coronal PET scanshows a large, asymmetric hypermetabolic focus in the right infratemporal fossa (arrow). (b) AxialCT scan demonstrates no abnormality. Arrow indicates the right medial pterygoid muscle. (c) Ax-ial fused PET-CT scan helps accurately localize uptake to the right medial pterygoid muscle (ar-row), a finding that is consistent with asymmetric physiologic muscle uptake.


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Lingual FDG uptake is also common and mayappear diffuse or as focal bilaterally symmetricuptake. Lingual uptake is often indistinguishablefrom the slightly more superior palatal mucosaluptake on PET scans alone but can typically bedifferentiated on fused PET-CT scans (Figs 15,16).

Larynx.—It is well recognized that talking dur-ing the FDG uptake phase will cause radiotraceraccumulation in the muscles of phonation and thevocal cords (Fig 17). Another muscle that can

Figures 15, 16. (15) Lingual uptake. Axial fused PET-CT scan demonstratesphysiologic lingual FDG uptake (arrow). (16) Palatal mucosal uptake. Axial fusedPET-CT scan shows intense FDG uptake in the superior oropharynx (arrow).FDG uptake within the mucosa of the soft palate is often indistinguishable from theslightly more inferior lingual uptake (cf Fig 15).

Figure 17. Physiologic vocal cord uptake. Axial CT (a) and fused PET-CT (b) scans help identify FDG uptake inthe neck as symmetric physiologic activity in the true vocal cords (arrow). The patient was talking during the uptakephase.


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appear as a focal area of FDG uptake is the crico-pharyngeus muscle (Fig 18) (26). Coughing dur-ing the uptake phase produces FDG activity inthe pharyngeal constrictor muscles as well as inthe vocal cords (Fig 19). Pronounced uptake inthese muscles can interfere with interpretation ofPET scans in any patient, but especially in pa-tients with head and neck malignancies, thyroidcancer, or lymphoma, in whom it can be very dif-ficult to distinguish physiologic muscle uptakefrom pathologic uptake (27,28).

FatUntil recent reports of FDG uptake by brown fat,most of the linear and focal uptake in the neckthat had the characteristic appearance of musclewas attributed to muscle alone. After the imple-mentation of combined PET-CT, it became ap-parent that uptake previously attributed to musclehad no definite soft tissue–muscle correlationwhen the fused PET-CT scans were inspected.Several authors have recently reported intenseFDG uptake within fat, presumably brown fat(Fig 20) (29–31). The question of whether theFDG uptake is in fat or muscle has little clinical

relevance; however, the foregoing discussion illus-trates how mislocalization of physiologic FDGaccumulation can be rectified with combinedPET-CT. The way to determine whether an areaof FDG uptake correlates with fat is to measurethe attenuation, which should be between �50and �150 HU (ie, fat attenuation).

Figure 18. Physiologic cricopharyngeus muscle uptake. (a) Coronal PET scan demonstrates ahypermetabolic focus (arrow). (b) Axial fused PET-CT scan helps localize the area of uptake pre-cisely to the cricopharyngeus muscle (arrow).

Figure 19. Pharyngeal constrictor muscle up-take. Axial fused PET-CT scan demonstratesphysiologic FDG uptake within the pharyngealconstrictor muscles (arrow). The patient wascoughing during the uptake phase.


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Postoperative AlterationsKnowledge of prior surgeries and of surgical com-plications is essential for properly interpretingFDG uptake in the head and neck. Several casesof asymmetric vocal cord uptake following injuryto the recurrent laryngeal nerve have been re-ported (32,33). This phenomenon is the result ofaltered physiologic or superphysiologic uptakedue to compensation of the contralateral vocal

cord during phonation (33). Asymmetric FDGuptake within a single vocal cord can mimic ma-lignancy, especially at PET, if direct anatomiccorrelation is not available. Localizing asymmetricFDG uptake within a vocal cord is much less of adiagnostic problem when using PET-CT (Fig21).

Figure 20. Physiologic brown fat uptake. (a) Coronal PET scan shows bilateral foci ofintense FDG uptake in the posterior superior thorax (arrow). (b) Axial CT scan shows nocorrelative abnormality. Arrow indicates an area of fat attenuation. (c) Axial fused PET-CTscan helps localize the foci precisely to areas of fat attenuation seen at CT (arrow).

Figure 21. Asymmetric superphysiologic uptake in a vocal cord following paralysis of thecontralateral cord caused during thyroid surgery. (a) Axial fused PET-CT scan helps local-ize asymmetric FDG uptake to the right true vocal cord (arrow), a finding that simulates alaryngeal neoplasm. (b) Laryngoscopic image helps confirm superphysiologic metabolic ac-tivity in the normal vocal cord (arrow) and paralysis of the contralateral cord. (Figure 21 re-printed, with permission, from reference 33.)


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Lymphoid TissueThere are many lymphatic structures in the headand neck, including the Waldeyer ring (adenoids,palatine tonsils, lingual tonsils), lymph nodes,and lymphatic channels (Figs 22, 23). PhysiologicFDG uptake can be seen in any lymphatic struc-ture in the head and neck, due at least in part toaccumulation of FDG within macrophages andlymphocytes. Several authors have describedphysiologic FDG uptake within these structureson dedicated PET scans (4,7,34,35). The inter-pretation is straightforward in most cases, espe-cially when there is symmetric uptake. However,malignancy or hyperplasia may have a similarsymmetric appearance.

Uptake in the Waldeyer ring will often beasymmetric, even from benign conditions such asinfection or inflammation. Malignancy will usu-ally manifest as asymmetric FDG uptake as well,with or without significant anatomic abnormali-ties (Fig 24).

When it infiltrates into the subcutaneous tis-sues, FDG can be transported through the lym-phatic system, causing a lymphangiographic effectat PET. If FDG infiltrates into the soft tissues ofthe upper extremity, it may accumulate in axillaryor supraclavicular lymph nodes, which may ne-cessitate repeat scanning (Fig 25).

Figures 22, 23. (22) Physiologic adenoid uptake. Axial fusedPET-CT scan helps localize intense bilateral nasopharyngeal FDGuptake to the adenoids (arrow). (23) Physiologic palatine tonsil up-take. Axial fused PET-CT scan shows intense symmetric physiologicFDG uptake within the palatine tonsils (arrow). This finding can alsobe seen in tonsillar hyperplasia.

Figure 24. Squamous cell car-cinoma of the nasopharynx in apatient with squamous cell metas-tases to the neck and no knownprimary malignancy. Axial fusedPET-CT scan shows asymmetricFDG uptake in the left side of thenasopharynx (arrow). Subse-quent biopsy helped confirm thediagnosis.

Figure 25. Lymphangiographic effect in apatient with known infiltration of FDG intothe left antecubital vein. (a) Coronal PETscan shows accumulation of FDG within theaxillary and left paratracheal lymph nodes (ar-row). (b) Axial fused PET-CT scan of the leftarm shows indistinct areas of FDG accumula-tion within the lymphatic channels (arrow).Repeat fused PET-CT performed a few weekslater showed no evidence of nodal uptake.


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MucosaThe mucosa of the oropharynx and nasopharynxoften demonstrates physiologic FDG uptake.This uptake does not usually cause problems withinterpretation because it is almost invariably lo-cated superficially along the mucosal plane andlinear in configuration (Fig 16).

PET-CT ArtifactsSeveral artifacts inherent in PET have been de-scribed, including prominent skin activity on non-attenuation-corrected images and relatively pho-topenic areas due to prosthetic devices (12,36).These artifacts are present on fused PET-CTscans as well. However, there are a number of“new” artifacts that are unique to combinedPET-CT. These artifacts are not generated onPET scanners, which use point source–based sys-tems for attenuation correction, because they aregenerated by CT-based attenuation correctionprotocols.

Metallic devices, including dental implants,create areas of photopenia on PET scans. Theseareas remain photopenic on attenuation-cor-rected images when Germanium point sourcesare used for attenuation correction. In contrast,with use of current CT-based attenuation correc-tion algorithms, these areas can demonstratefalsely elevated FDG uptake (Fig 26). This arti-fact is a limitation of the current CT attenuationcorrection protocols.

Intravenous contrast material can also causeartifacts on attenuation-corrected images whenCT-based attenuation correction protocols areused. Dense contrast material is present in venousstructures during CT but not during the PETportion of the examination. This mismatch causesareas of linear artifact (mimicking intense FDGaccumulation) on the attenuation-corrected PET

Figure 26. CT attenuation artifact from an implantable catheter port. (a) Coronal PETscan demonstrates focal intense FDG accumulation in the right supraclavicular area (arrow).(b) Axial fused PET-CT scan helps localize the FDG activity to an implanted catheter port(arrow). (c) Axial uncorrected emission image helps confirm that the abnormality (arrow-head) is an attenuation correction artifact caused by the extremely high attenuation of theport.

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scans (Fig 27). Occasionally, this artifact can ap-pear focal and mimic a metastatic lymph node inthe axilla or supraclavicular area (Fig 28) (10).Rarely, a small malignant lymph node or smallsoft-tissue abnormality can lie within or directly

beside a contrast material–related artifact, whichthen partially or completely obscures the abnor-mality (Fig 29). A relatively simple means of re-solving any diagnostic uncertainty regarding thepresence of a CT-based attenuation artifact isinspection of the emission data set (ie, non-at-tenuation-corrected PET scan). Unfortunately,

Figure 27. Linear CT attenuation artifact due to high-attenuation intravenouscontrast material. (a) Coronal PET scan shows a focal area of intense FDG accu-mulation located in the left axilla (arrow) and mimicking an abnormal axillarylymph node. (b) Axial fused PET-CT scan helps localize the uptake to an area ofdense contrast material in the left subclavian vein (arrow). (c) Axial uncorrectedemission image reveals that the area of apparent FDG uptake in the axilla (arrow)is an artifact generated during the attenuation correction process. (Figure 27 re-printed, with permission, from reference 37.)

Figure 28. Focal CT attenuation artifact due to high-attenuation intravenous contrast material. (a) Coronal PETscan shows symmetric FDG uptake in the thyroid gland and a focal area of intense uptake in the right supraclavicularregion (arrow) mimicking an abnormal lymph node. (b) Axial fused PET-CT scan helps localize the apparent FDGactivity precisely to the brachiocephalic vein (arrow). (c) On an axial uncorrected emission image, no apparent FDGactivity is seen (arrow), indicating that the area of uptake seen at fused PET-CT represents an artifact of CT-basedattenuation correction.


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with some viewing systems it can be cumbersometo switch between the attenuation-corrected PETimage sets and the emission-only data sets. Inaddition, some systems will not allow side-by-sidecomparison of corrected and uncorrected images.

Although one group has reported that the useof oral contrast material does not cause attenua-tion artifacts, another has reported that areas ofhighly concentrated barium-based oral contrastmaterial within the bowel can cause similar arti-facts and overestimation of FDG activity in thebowel by as much as 20% (38,39). The samegroup has proposed a complex region-growingand segmentation CT-based attenuation correc-tion algorithm that appears to correct most, if notall, of the CT-based attenuation artifacts.

ConclusionsCombined PET-CT is a unique imaging modal-ity that permits anatomic (CT) and functional(PET) scans to be acquired with perfect or near-perfect coregistration using a single device. Com-bined PET-CT offers additional information thatcannot be obtained with PET or CT alone. Inparticular, PET-CT facilitates the interpretationof FDG uptake in the head and neck, an area ofparticular difficulty due to dense and complexanatomic structures.

We have presented an index of FDG uptake inthe head and neck, covering physiologic FDGuptake and uptake due to altered physiologicstates, and have discussed artifacts in the headand neck that are unique to PET-CT scanners.With the burgeoning interest in these scanners, itis imperative that interpreting radiologists andnuclear medicine physicians be familiar with thepatterns of FDG uptake in the head and neck toavoid misinterpretation or misdiagnosis.

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Figure 29. Abnormal lymph node adjacent to high-attenuation intravenous contrast material. (a) Coronal PETscan shows a small focal abnormality in the supraclavicular region (arrow). (b, c) Axial CT (b) and fused PET-CT (c)scans help localize the abnormality to the right brachiocephalic vein (cf Fig 28). In this case, however, uncorrectedemission PET still showed the abnormality, indicating that the abnormality did not represent an attenuation correc-tion artifact, but rather a small node adjacent to the brachiocephalic vein.


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This article meets the criteria for 1.0 category 1 credit toward the AMA Physician’s Recognition Award. To obtaincredit, see www.rsna.org/education/rg_cme.html.


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Combined PET-CT in the Head and NeckMuscles Neck and Face.—Physiologic FDG uptake within neck muscles can constitute a diagnostic dilemma in the interpretation of PET scans (7,25)