¹⁸F FDG PET CT in Sarcoidosis: Involvement of Skeletal Muscles and Myocardium

Sarcoidosis is a multisystem granulomatous disorder with diverse clinical manifestations, often with occult extrapulmonary involvement. ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) is a valuable imaging modality for identifying active inflammatory disease and assessing disease extent in selected patients with sarcoidosis.^[1,2] We report a 48-year-old Indian woman who presented with a gradually increasing umbilical swelling of three weeks’ duration, without associated systemic symptoms. Excision biopsy of the lesion revealed noncaseating granulomas, consistent with sarcoidosis. Whole-body FDG PET/CT demonstrated multiple discrete foci of increased FDG uptake with subtle bulkiness involving several skeletal muscles Standardised uptake value (SUVmax 5.0) [Fig 1] suggestive of muscular sarcoidosis. Skeletal muscle involvement in sarcoidosis is frequently asymptomatic and often detected incidentally, with asymptomatic involvement reported in up to 80% of patients, whereas symptomatic disease is rare, occurring in approximately 0.5% of cases.^[3]

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Fig 1: ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/ computed tomography (PET/CT) findings in a 48-year-old Indian woman with a history of umbilical swelling which was of 3 weeks duration and slowly increasing in size with no associated systemic symptoms. She underwent excision biopsy with histopathological examination revealing non-caseating granuloma. (a) Maximal intensity projection (MIP) and (b-d) Axial fused PET/CT images demonstrate discrete areas of increased FDG uptake (arrows in b-d) with subtle bulkiness in multiple skeletal muscles and with standardised uptake value (SUVmax) 5.0, consistent with muscular involvement in sarcoidosis. Muscular involvement in sarcoidosis is often asymptomatic and may present with variable clinical features. Asymptomatic muscle involvement across up to 80% of the patients and symptomatic involvement is seen in only 0.5% of the patients^[3]

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In addition, abnormal FDG uptake was noted in the inferior basal septum, apical region, and mid to distal inferolateral walls of the left ventricle (SUVmax 5.8), [Fig 2] consistent with cardiac sarcoidosis. Cardiac involvement occurs in nearly 25% of patients with systemic sarcoidosis and is associated with increased morbidity and poor prognosis.^[4-6] FDG PET/CT plays a crucial role in the diagnosis, prognostication, and follow-up of cardiac sarcoidosis.^[7] Myocardial physiological FDG uptake was suppressed using an 18–24-hour high-fat, no-carbohydrate diet and intravenous unfractionated heparin (10–50 IU/kg) administered 15 minutes prior to FDG injection, with image acquisition performed 90 minutes post-injection.^[7] Although lymph nodes, skin, and eyes are the most common extrapulmonary sites of involvement, cardiac, central nervous system, and skeletal muscle involvement may be more frequent than clinically suspected, highlighting the utility of FDG PET/CT in comprehensive disease evaluation.^[8,9]

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Fig 2: (a) MIP and (b and c) Fused axial PET/CT images shows increased FDG uptake in the inferior aspect of the basal septum, apical region, and mid and distal inferolateral walls of the left ventricle (arrows in a-c) and with SUVmax 5.8, indicative of cardiac sarcoidosis. Cardiac sarcoidosis, an infiltrative cardiomyopathy resulting from granulomatous inflammation, is reported in approximately 25% of patients with systemic sarcoidosis and is associated with a poor prognosis.^[4-6] FDG PET/ CT is an important tool for the diagnosis and management of cardiac sarcoidosis, offering potential for early diagnosis, prognostication, and follow-up. Pre-imaging preparation included an 18-24 hour high-fat/no-carbohydrate diet and intravenous unfractionated heparin (10-50 IU/ kg) 15 minutes prior to FDG injection. Images were acquired 90 minutes post-FDG injection. The most frequent extrapulmonary manifestations of sarcoidosis involve the lymph nodes, skin, and eyes; however, cardiac, central nervous system, and skeletal involvement can occur more frequently than expected. A MIP: Maximal intensity projection, FDG: Fluorodeoxyglucose; SUVmax: Maximum standardised uptake value PET/CT: Positron emission tomography/computed tomography

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