Usefulness of combined FDG-PET/MR to diagnose active cardiac sarcoidosis.

Objectives Sarcoidosis is a granulomatous disease of unknown etiology that most commonly affects lung and mediastinal lymph nodes. Heart involvement is probably under-diagnosed due to frequent absence of clinical symptoms but poses an increased risk of sudden death. A major current obstacle is the inability to easily diagnose cardiac sarcoidosis (CS) with a non-invasive method. Several imaging techniques are already used to assess CS including myocardial inflammatory activity using 18F-fluorodesoxyglucose (FDG) Positron Emission Tomography (PET) and the pattern of injury using Magnetic Resonance (MR) with late Gadolinium enhancement (LGE). Recent advances now allow combining these 2 techniques to benefit from the best of these modalities. Our aim was to assess the usefulness of FDG-PET/MR in the diagnosis of CS.

Methods Patients with clinical suspicion of cardiac sarcoid involvement were referred in our department for PET/MR imaging (Biograph mMR, Siemens®). Institutional Review Board approved this study and all patients gave written informed consent. PET data were reconstructed using a MR attenuation correction map. Image analysis was performed using 3 methods on fused PET-MR data sets: 1) No correction: maximal standardized uptake value (SUVmax) in the myocardium was recorded; 2) Blood-pool correction: maximum myocardial SUV value was corrected for blood pool measurement in the right ventricular cavity (TBRmax = SUVmax / blood pool uptake; 3) Myocardial correction: maximum myocardial SUV value was corrected for PET activity in the opposing myocardial segment (TNRmax = SUVmax / maximal uptake in a contra-lateral area). A final diagnosis of active cardiac sarcoidosis (CS+) or no active (CS-) was defined by a consensus of clinical experts with access to all clinical, imaging and biopsy data. Mean SUVmax, TBRmax and TNRmax in CS+ and CS- patients were compared using a Student t-test.

Results Thirteen consecutive patients (7M/6F; 55.9 ± 12.3 yo) were included from August to November 2015. One of them did not perform the exam due to claustrophobia. All others underwent PET/MR 70.2 ± 5.4 min after injection of 4.7 ± 0.3 MBq/Kg of FDG. Four patients were considered as CS+ and 8 as CS- by the expert panel. Mean SUVmax were respectively 4.22 ± 1.81 and 3.14 ± 2.11 in CS+ and in CS- patients (p = 0.404). Mean TBRmax were respectively 1.98 ± 0.55 and 1.55 ± 1.08 in CS+ and in CS- patients (p = 0.431). Mean TNRmax were respectively 1.79 ± 0.48 and 1.08 ± 0.05 in CS+ and in CS- patients (p = 0.001). A clear threshold of TNRmax = 1.2 accurately differentiated all patients as being CS+ or CS-. One previously unknown case of sarcoid involvement in bone was also identified. In the CS- group, combined FDG-PET/MR identified an alternative cause for the cardiac symptoms in 3 patients (1 arrhythmogenic right ventricular cardiomyopathy, 1 incidental chronic infarction, 1 aortic valve fibroelastoma).

Conclusions These preliminary results of our prospective study show the usefulness of combined FDG-PET/MR in the diagnosis of CS using TNRmax measurements. Further inclusions of patients are needed to validate an optimal threshold of TNRmax and verify the clinical utility of combined FDG-PET/MR. This series also confirms the ability of PET/MR imaging to evaluate extra-cardiac involvement of sarcoidosis and in assessing alternative myocardial pathologies.