Normal Physiologic FDG Uptake Patterns



Normal Physiologic FDG Uptake Patterns


Todd M. Blodgett, MD

Alex Ryan, MD

Barry McCook, MD










Multiple images (A, B, C) show the typical appearance of a bladder diverticulum image.






Graphic shows a representation of a right-sided bladder diverticulum image, similar in structure to the one depicted in the previous images.


TERMINOLOGY


Definitions



  • FDG activity associated with normal anatomical structures or benign processes


IMAGING FINDINGS


General Features



  • Best diagnostic clue



    • FDG uptake in primary neoplasms is usually greater than that observed in even the most metabolically active normal structures



      • However, overlap does occur & may confound interpretation


  • Morphology: Any physiologic FDG activity should correlate with otherwise normal-appearing structures


Imaging Recommendations



  • Best imaging tool: Correlation with CT is absolutely essential to minimize misinterpretation


  • Protocol advice



    • Sedation and neck immobilization may help reduce physiologic FDG activity in the skeletal muscles



      • Sedation is inconvenient for patients and may not completely eliminate physiologic uptake


    • Patients generally instructed to remain quiet during FDG uptake phase to limit vocal cord uptake


Nuclear Medicine Findings



  • Symmetry



    • Interpreting physician often relies on symmetry or location to differentiate between physiologic and pathologic FDG accumulations


    • Symmetry is not always a reliable indicator of physiologic processes



      • Physiologic uptake will often be asymmetrical


      • Several malignancies can present with strikingly symmetrical FDG uptake


    • Post-surgical patients often demonstrate anatomic asymmetry



      • Example: Vocal cord paralysis from prior neck surgery can cause compensatory effort and FDG uptake in unaffected cord



      • Example: Patients who have undergone removal of gland or muscle may have asymmetrical physiologic activity in normal remaining gland


DIFFERENTIAL DIAGNOSIS


Head and Neck Structures with Variable FDG Uptake



  • Nasal turbinates


  • Pterygoid muscles


  • Extraocular muscles


  • Parotid and submandibular glands


  • Lymphoid tissue in Waldeyer throat ring


Chest Structures with Variable Uptake



  • Thymus


  • Heart


  • Thorax muscles


  • Many are age- or activity-dependent


Thyroid Gland



  • Normal thyroid has variable appearance using FDG PET



    • May demonstrate diffuse, focal, asymmetric, or virtually no uptake


    • Each of the above patterns may be seen in physiologic, benign, and pathologic processes


  • Focal uptake



    • Relatively nonspecific



      • Malignancy, including second primary


      • Adenomatous processes


      • Toxic thyroid adenoma


    • Recommended that all patients with nodules ≥ 10 mm and all those with intense asymmetric FDG uptake be referred for biopsy


  • Diffuse symmetric uptake



    • Seen in normal thyroid


    • Occasionally seen with diffuse goiter


    • Chronic autoimmune thyroiditis


Salivary Glands



  • FDG is taken up by salivary glands and excreted into saliva


  • Parotid and submandibular glands normally demonstrate symmetric mild-moderate uptake



    • Normal glands may also show no uptake


  • Asymmetric uptake seen in



    • Patients who have undergone surgical removal of a gland


    • Patients with primary or metastatic lesions to the glands


    • FDG-avid parotid tumors include



      • Warthin tumor


      • Pleomorphic adenoma


      • Primary parotid lymphoma


  • Nonmalignant uptake



    • Infectious etiologies


    • Granulomatous disorders (e.g., sarcoidosis)


  • Benign and malignant parotid tumors cannot be distinguished with PET/CT alone because of high false positive rates



    • In addition, several salivary gland malignancies have little or no FDG avidity


    • Lack of FDG uptake does not exclude malignancy


  • Malignancy can sometimes cause bilateral FDG uptake in the parotid or submandibular glands or in intraparotid lymph nodes



    • Mimics a physiologic pattern of uptake


Muscles of Neck and Face



  • Neck muscle uptake a common diagnostic dilemma


  • Muscular uptake can frequently be distinguished from malignant nodal uptake by identifying characteristic pattern of linear symmetric uptake


  • However, muscles often have more focal uptake patterns



    • Fusion images often localize FDG uptake to the myotendinous junction


    • Can be difficult to distinguish from abnormal lymph nodes



    • Intense asymmetric uptake may be seen in sternocleidomastoid muscle, mimicking enlarged node


  • Inspection of coronal or sagittal reconstructions may reveal linearity


  • Inferior obliquus capitis muscle frequently demonstrates asymmetric tracer uptake



    • Uptake may appear focal on coronal images


    • Linear nature evident in axial plane


    • Extreme posterior position of these muscles often helpful in identifying them as the source of FDG uptake


  • Muscles of facial expression may also demonstrate linear FDG activity


  • Close inspection of 3 orthogonal planes is essential to avoid misdiagnosis


Muscles of Oropharynx and Nasopharynx



  • Symmetric physiologic uptake seen in pterygoid muscles and muscles of oral floor



    • May mimic malignancy when asymmetric


  • Lingual uptake is common and may appear as diffuse or bilateral symmetrical focal uptake



    • Often inseparable from slightly more superior palatal mucosal uptake


    • Can be differentiated on PET/CT images


Laryngeal Muscles



  • Talking during FDG uptake period causes tracer accumulation in vocal cords and muscles of phonation


  • Cricopharyngeus muscle can also appear as focal area of uptake


  • Coughing during uptake period produces activity in pharyngeal constrictor muscles and vocal cords


  • In patients with head and neck malignancies, thyroid cancer, or lymphoma, it can be very difficult to distinguish physiologic from pathologic uptake


Fat



  • Brown fat is metabolically active and may demonstrate FDG uptake



    • Theorized to be useful for heat generation


  • Generally easily localized by PET/CT


  • Brown fat can be distinguished from other tissue if Hounsfield units measure fat attenuation, -50 to -150 HU


  • Warming patients may reduce uptake in this tissue


  • More common in women and observed more commonly in winter months


  • Commonly seen in the following areas



    • Neck


    • Retrocrural


    • Perirenal


    • Left paratracheal


Post-Operative Altered Physiologic States



  • Knowledge of prior surgeries and surgical complications is essential for properly interpreting foci of FDG in the head and neck



    • e.g., recurrent laryngeal nerve damage and intense FDG uptake by compensatory effort of contralateral vocal cord


Lymphoid Tissue



  • Lymphatic structures in head and neck



    • Waldeyer throat ring (adenoids, palatine tonsils, and lingual tonsils)


    • Lymph nodes


    • Lymphatic channels


  • Physiologic uptake can be seen in any lymphatic structures in head and neck



    • Related to uptake in macrophages and lymphocytes


    • Malignancy and hyperplasia may have similar symmetric appearance


  • Malignancy usually presents with asymmetric FDG uptake



    • May appear with or without significant anatomic abnormalities


    • Uptake in Waldeyer ring will often be asymmetric


  • When accidentally infiltrated into subcutaneous tissue, FDG can be transported through lymphatic channels and produce lymphangiogram effect



    • May accumulate in lymph nodes of axilla and supraclavicular area


Mucosa



  • Mucosa of oropharynx and nasopharynx often demonstrates physiologic FDG uptake


  • Rarely causes diagnostic problems because almost invariably superficial along mucosal plane in linear configuration


Esophagus



  • Generally not very FDG avid


  • Several benign infectious/inflammatory processes can cause spectrum of FDG uptake



    • Distal esophagitis: Mild focal uptake


    • Radiation injury (and other etiologies that affect entire esophagus): Diffuse linear intense FDG uptake


  • Great majority of esophageal malignancies are visualized as focal to short segment areas of intense FDG uptake



    • Exception: Some gastroesophageal junction adenocarcinomas that arise in cardia of stomach


Lung



  • On attenuation-corrected (AC) images, lungs appear very light with little FDG uptake


  • Benign lung lesions may mimic a malignant pulmonary nodule



    • Tuberculosis


    • Pneumonia (viral, bacterial, fungal)


    • Collagen vascular diseases


    • Vasculitides


    • Sarcoidosis


    • Silicosis


  • Most malignant lesions > 1 cm tend to have higher SUVs (> 2.5) than benign lesions



    • However, there are reports of benign lesions with SUVs well above 5


    • Some malignancies are poorly FDG avid, including bronchoalveolar cell carcinoma


  • PET/CT imaging allows anatomic correlation that can help secure diagnosis



Thymus



  • Often visible in pediatric population as V-shaped structure just above heart on coronal image, with mild to intense FDG uptake


  • In adult patients, thymus typically not visible on FDG PET or CT


  • Causes of thymic uptake in adult



    • Thymic carcinoma


    • Thymic rebound after illness or chemotherapy


    • Thymic hyperplasia


Heart



  • Most important factor determining heart uptake is whether patient has fasted



    • Most protocols instruct patient to fast for 4-6 hours prior to scan


    • Fasting reduces cardiac glucose dependence


  • In non-fasting patient, left ventricular FDG uptake can obscure a lesion directly adjacent to left ventricle



    • Left ventricle is the only chamber with appreciable activity on PET scan


Abdominal Muscle



  • Muscle generally exhibits more FDG uptake when exercised during or preceding FDG injection


  • Muscles that may appear asymmetrical include crus of diaphragm and strap muscles


Stomach



  • Variable physiologic FDG activity ranging from minimal to fairly intense


  • Uptake typically distributed throughout gastric mucosa


  • Inflammatory processes such as gastritis can increase uptake


Liver/Spleen



  • Both organs have similar physiologic FDG activity, usually mild and diffuse


  • Focal areas worrisome for neoplastic uptake


  • Causes of diffuse splenic uptake



    • Erythropoietin


    • Chemotherapy


    • G-CSF


Bowel



  • One of the most difficult structures in which to differentiate physiologic from pathologic uptake


  • Typical appearance is linear in 3 orthogonal planes, ranging from mild to intense


  • Focal bowel activity



    • Can be physiologic


    • Should raise suspicion of neoplastic process (particularly if the rest of the bowel has no activity)


    • Most patients should have correlation with colonoscopy or sigmoidoscopy


    • Polyps may have focal uptake irrespective of degree of malignancy


  • Focal lesions adjacent to normal linear bowel uptake often cannot be separated as distinct structures


Uterus



  • Patient history is crucial to interpretation


  • Intense endometrial activity is seen during menstruation



    • With correct menstrual history, no follow-up is warranted


    • In postmenopausal patient, intense uptake very concerning for endometrial carcinoma


  • Fibroids can have focal FDG uptake ranging from minimal to very intense in the setting of degeneration


Ovary



  • Most patients do not have visible FDG uptake within ovaries


  • Cases of physiologic uptake have been reported


  • Benign structures such as corpus luteum cyst can have intense FDG uptake



    • Correlation with CT is important


    • Typical appearance of corpus luteum cyst: Thick rind of enhancement in otherwise normal-appearing ovary


Urinary Collecting System



  • Unlike glucose, FDG is normally excreted in the urinary collecting system


  • Background uptake in kidneys makes it difficult to distinguish renal masses (such as renal cell carcinoma) from background excretory FDG


  • Focal ureteral stasis may appear as focal area of FDG uptake and mimic appearance of lymph node



    • Helpful to have patient void and then repeat 1-2 bed positions through area of uptake


CLINICAL ISSUES


Demographics



  • Gender: Brown fat more common in women during the winter months



SELECTED REFERENCES

1. Chang JM et al: False positive and false negative FDG-PET scans in various thoracic diseases. Korean J Radiol. 7(1):57-69, 2006

2. Rosenbaum SJ et al: False-positive FDG PET uptake–the role of PET/CT. Eur Radiol. 16(5):1054-65, 2006

3. Truong MT et al: Pitfalls in integrated CT-PET of the thorax: implications in oncologic imaging. J Thorac Imaging. 21(2):111-22, 2006

4. Blodgett TM et al: Combined PET-CT in the head and neck: part 1. Physiologic, altered physiologic, and artifactual FDG uptake. Radiographics. 25(4):897-912, 2005

5. Fukui MB et al: Combined PET-CT in the head and neck: part 2. Diagnostic uses and pitfalls of oncologic imaging. Radiographics. 25(4):913-30, 2005

6. Heiba SI et al: The distinctive role of positron emission tomography/computed tomography in breast carcinoma with brown adipose tissue 2-fluoro-2-deoxy-d-glucose uptake. Breast J. 11(6):457-61, 2005

7. Nakamoto Y et al: Normal FDG distribution patterns in the head and neck: PET/CT evaluation. Radiology. 234(3):879-85, 2005

8. Subhas N et al: Imaging of pelvic malignancies with in-line FDG PET-CT: case examples and common pitfalls of FDG PET. Radiographics. 25(4):1031-43, 2005






Image Gallery









DDx: Focal FDG Activity






(Left) Axial CECT shows no abnormalities. (Right) Axial fused PET/CT shows symmetrical intense FDG activity within the lingula tonsils image, compatible with normal physiologic activity.






(Left) Coronal PET shows symmetrical intense FDG activity within the palatine tonsils image, compatible with physiologic activity. (Right) Axial images show normal anatomy on the axial CT (left) with corresponding focal intense FDG activity on fused PET/CT (right) in the palatine tonsils image, compatible with normal physiologic activity.







(Left) Axial CT (A, B) and fused PET/CT (C, D) show physiologic activity within the lingual tonsils bilaterally image as well as the palatine tonsils image, compatible with normal physiologic activity. (Right) Coronal PET (A) shows intense FDG activity along the midline image, corresponding to the lingual tonsils image on the fused PET/CT image (C). Note that no abnormalities are seen on the CECT (B).






(Left) Axial CECT shows no obvious abnormalities. (Right) Axial fused PET/CT shows diffuse FDG activity within the adenoids bilaterally image, compatible with normal physiologic activity.







(Left) Axial CECT shows no obvious abnormalities. (Right) Axial fused PET/CT shows focal asymmetrical FDG activity in the right lingual tonsil image. A subsequent biopsy demonstrated hyperplasia.






(Left) Axial CECT shows no obvious abnormalities. (Right) Axial fused PET/CT shows bilateral areas of linear intense FDG activity that correspond to the sublingual glands image and are compatible with physiologic activity.







(Left) Axial CECT shows no obvious abnormalities. (Right) Axial fused PET/CT shows intense diffuse symmetrical activity within both parotid glands image, compatible with physiologic activity.






(Left) Axial CECT shows no obvious abnormalities. (Right) Axial fused PET/CT shows intense symmetrical activity within the submandibular glands image, as well as within the sublingual glands image, compatible with physiologic activity.







(Left) Axial CT (A, B) and fused PET/CT (C, D) demonstrate intense FDG activity within the submandibular glands image, as well as the parotid glands bilaterally image, compatible with normal physiologic activity. (Right) Coronal PET (A) shows asymmetrical mild to moderately increased FDG activity within the right neck image, corresponding to asymmetrical physiologic activity within the right parotid gland image on PET/CT (C) in this patient who had a history of a left-sided parotidectomy. Axial CT (B) is normal.






(Left) Coronal PET (A) shows diffuse intense FDG activity throughout the thyroid gland image, corresponding to an otherwise normal-appearing thyroid image on axial CT (B) and fused PET/CT (C). Subsequent correlation with thyroid function test showed no abnormalities, suggesting physiologic activity. (Right) Axial fused PET/CT (bottom) shows diffusely increased FDG activity image that corresponds to an enlarged left lobe of the thyroid image on axial CT (top), findings compatible with a multinodular goiter.







(Left) Coronal PET (A) shows diffusely increased FDG activity throughout both lobes of the thyroid gland image, corresponding to an unenlarged gland with multiple nodules image on axial CT (B) and fused PET/CT (C), compatible with multinodular goiter. (Right) Graphic shows a multinodular goiter with multiple colloid cysts image, as well as slightly more nodular soft tissue areas image.






(Left) Axial CECT shows no obvious abnormalities. (Right) Axial fused PET/CT shows multiple foci of increased FDG activity that correspond to areas of fat attenuation image, compatible with physiologic brown fat.







(Left) Coronal PET (A) shows extensive foci of increased FDG activity in the neck and supraclavicular/axillary areas image, corresponding to areas of fat attenuation image on CT (B) and fused PET/CT (C), compatible with brown fat. (Right) Axial fused PET/CT (bottom) shows multiple foci of intense FDG activity in the supraclavicular area image, corresponding to areas of fat attenuation image on the axial CT (top), compatible with physiologic brown fat.

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Sep 22, 2016 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Normal Physiologic FDG Uptake Patterns
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