Inflammatory Disease: Sarcoidosis, Large-Vessel Vasculitis & IBD
Beyond infection, FDG PET/CT maps active inflammation. In cardiac sarcoidosis it detects active myocardial inflammation (pooled sensitivity ≈89%, specificity ≈78%) and guides immunosuppression — provided dietary preparation suppresses normal myocardial uptake. In large-vessel vasculitis (giant cell arteritis, Takayasu) it shows vessel-wall uptake — ≈90% sensitivity and ≈98% specificity for large-vessel inflammation in GCA in one meta-analysis, lower for Takayasu activity — and EULAR 2023 lists it as an alternative to ultrasound (GCA) or MRI (Takayasu). In inflammatory bowel disease it is a second-line tool (pooled sensitivity ≈85%, specificity ≈87%), with endoscopy and MR/CT enterography remaining the mainstays.
FDG accumulates in activated macrophages and lymphocytes, so PET/CT images disease activity and extent, not just structure. That makes it valuable for detecting active granulomatous or vasculitic inflammation, staging its extent, targeting biopsy and monitoring response to therapy.
Cardiac sarcoidosis
- FDG PET detects active myocardial inflammation and is read with a rest perfusion study: focal (or focal-on-diffuse) FDG uptake indicates active inflammation, with or without a perfusion defect, whereas a perfusion defect without FDG uptake suggests scar; diffuse or isolated basal-lateral uptake usually reflects poor suppression.
- Pooled sensitivity ≈89%, specificity ≈78% (2012 meta-analysis, 7 studies); a larger 2022 meta-analysis (26 studies) reported ≈84% and ≈82%.
- Strict high-fat/low-carbohydrate preparation with prolonged fasting (± heparin) is essential to suppress physiological myocardial uptake.
- Used to guide and monitor immunosuppressive therapy.
Large-vessel vasculitis
- Giant cell arteritis and Takayasu arteritis show smooth, linear FDG uptake in the walls of the aorta and its large branches.
- In GCA a meta-analysis reported sensitivity ≈90% and specificity ≈98% for large-vessel inflammation versus controls (Takayasu activity: ≈87% / ≈73%). EULAR 2023 recommends ultrasound first-line in suspected GCA, with FDG-PET or MRI as alternatives, and MRI first in Takayasu arteritis, with FDG-PET, CT or ultrasound as alternatives.
- Best performed before, or within 3 days of starting, glucocorticoids; after about 10 days uptake falls markedly and sensitivity is lost.
- Also maps disease extent and assesses treatment response.
Inflammatory bowel disease
- FDG uptake reflects active bowel inflammation but is non-specific; endoscopy and MR/CT enterography are the mainstays.
- EANM/SNMMI v2.0 lists disease extent at diagnosis, early therapy assessment and inflammatory versus fibrotic stricture as indications; labelled-WBC scintigraphy is the established second-line nuclear test when radiology/endoscopy are inconclusive.
- Physiological bowel uptake limits specificity.
Protocol & preparation
- Cardiac sarcoid: dietary preparation as above; combine with rest myocardial perfusion imaging.
- Vasculitis: vertex-to-knees FDG PET/CT after at least 60 min (preferably 90–120 min) uptake, ideally before glucocorticoids.
- Interpret against physiological uptake and prior treatment.
Diagnostic performance
| Condition | Sensitivity | Specificity | Note |
|---|---|---|---|
| Cardiac sarcoidosis | ≈89% | ≈78% | Needs strict diet prep |
| Large-vessel GCA | ≈90% | ≈98% | EULAR alternative to US/MRI; image early |
| Inflammatory bowel disease | ≈85% | ≈87% | 2013 meta-analysis; second-line |
Pitfalls
- Inadequate cardiac preparation → physiological myocardial uptake mimics or masks sarcoid.
- Glucocorticoids rapidly reduce vascular/inflammatory uptake — image early.
- FDG is non-specific — correlate with clinical and other findings.
- Brown fat and bowel activity can confound.
Evidence & guidelines
- Cardiac sarcoidosis: SNMMI/ASNC expert consensus on FDG PET; pooled sensitivity ≈89%, specificity ≈78%.
- Large-vessel vasculitis: EULAR 2023 imaging recommendations place FDG-PET as an alternative to ultrasound (GCA) or MRI (Takayasu); meta-analytic sensitivity ≈90%, specificity ≈98% for large-vessel GCA.
- IBD: FDG PET/CT is a second-line tool (pooled sensitivity ≈85%, specificity ≈87%) listed among EANM/SNMMI v2.0 indications, not a first-line diagnostic test.
In depth
- FDG PET/CT for cardiac sarcoidosis: pooled sensitivity 89% / specificity 78% (specificity likely underestimated by older reference criteria).
- FDG spans pure inflammation (uptake only) to scar (perfusion defect only); concordant focal or focal-on-diffuse uptake with a matching perfusion defect suggests active disease.
- Physiological myocardial suppression uses at least two high-fat (>35 g), low-carbohydrate (<3 g) meals the day before and a 4–12 h fast; with structured dietary instructions about 91% of studies achieve adequate suppression, and the added value of IV heparin is uncertain.
- Grade FDG uptake visually (none, diffuse, focal, focal-on-diffuse) and quantify intensity with SUVmax and extent with the volume of FDG-positive myocardium (or volume × mean activity); on follow-up, a change is likely significant when both move in the same direction by at least 20%.
- ⁶⁷Ga-citrate is highly specific but insensitive (under 50%) for cardiac sarcoidosis, partly because cardiac uptake is hard to separate from lung and mediastinal uptake, yet it remains in the JMHW and HRS diagnostic criteria.
- Perfusion defect plus abnormal FDG confers a four-fold risk of death/ventricular arrhythmia, and RV FDG uptake strongly predicts events.
- ⁶⁸Ga-DOTATATE binds somatostatin receptors expressed in sarcoid granulomas and needs no dietary preparation because normal myocardium takes up little tracer; early studies show high patient-level agreement with FDG but lower signal-to-background. Cardiac involvement is clinically evident in under 10% of patients with sarcoidosis, but autopsy and imaging series report more than 20% in the USA and over 50% in Japan.
Sources: Youssef et al. J Nucl Med 2012 (PMID 22228794) · SNMMI/ASNC expert consensus 2017 (PMID 28770463) · SNMMI/ASNC expert consensus on FDG PET in cardiac sarcoidosis 2017 (PMID 28770463) · Christopoulos et al. J Nucl Cardiol 2021 (PMID 31111450) · Blankstein et al. J Am Coll Cardiol 2014 (PMID 24140661) · Lee et al. J Nucl Cardiol 2023 (PMID 36266526)