Unmasking Cardiac Sarcoidosis with PET-CT and CMR fusion: A Comprehensive Diagnostic Tool

Description of Clinical Presentation: A 58-year-old female with no prior cardiac history developed complete heart block (CHB) during a vitrectomy procedure after receiving local bupivacaine. She was admitted to the cardiac stepdown unit, where her electrocardiogram (EKG) revealed deep inferior T-wave inversions and rate-related right bundle branch block (RBBB). An initial cardiac MRI, performed on a 1.5T Siemens scanner, revealed a low-normal left ventricular ejection fraction (LVEF) of 50% with extensive subepicardial and mid-wall late gadolinium enhancement (LGE) involving the inferior wall, consistent with fibrosis or scarring.
The fusion of 18F-FDG PET with MRI in this case enabled precise colocalization of inflammation and fibrosis. Specifically, the regions of LGE on MRI were colocalized with areas of increased FDG uptake on PET, further supporting the diagnosis of cardiac sarcoidosis. The patient was diagnosed with cardiac sarcoidosis and was started on methotrexate and prednisone, later replaced with infliximab due to prednisone-induced glaucoma.

Diagnostic Techniques and Their Most Important Findings: The patient underwent a hybrid PET/CT scan using nitrogen-13 ammonia (13N-NH3) for perfusion and fluorodeoxyglucose (18F-FDG) for metabolism. The PET/CT revealed a large area of FDG uptake with a perfusion-metabolic mismatch pattern (increased FDG uptake with reduced perfusion), a hallmark of active cardiac sarcoidosis.
The fusion of 18F-FDG PET with LGE MRI was pivotal in this case. The areas of LGE on MRI, especially in the inferior wall, colocalized with regions of increased FDG uptake on PET, providing strong evidence of active myocardial inflammation and fibrosis. This fusion of modalities allowed for the precise localization of diseased tissue, enhancing diagnostic confidence.
In addition to the cardiac findings, FDG PET revealed uptake in scattered lung nodules and bilateral hilar lymph nodes, indicative of extra-cardiac sarcoidosis. Despite a negative cardiac biopsy, the positive pulmonary lymph node biopsy confirmed the diagnosis of sarcoidosis.
Six months after treatment, a follow-up FDG PET scan showed a significant reduction in FDG-avid volume, indicating decreased inflammatory activity and a positive therapeutic response, which was further corroborated by improvements in the patient’s clinical condition, including no recurrence of heart block.

Learning Points from this Case:

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  Fusion of 18F-FDG PET-CT and MRI for Diagnostic Precision: The combined use of PET for detecting metabolic activity and MRI for structural imaging allowed for precise colocalization of inflammation and fibrosis, leading to an accurate diagnosis of cardiac sarcoidosis.
  


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  Perfusion-Metabolic Mismatch: The PET/CT scan identified a characteristic perfusion-metabolic mismatch (increased FDG uptake with reduced perfusion), which is highly suggestive of active cardiac sarcoidosis.
  


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  Therapeutic Monitoring: The reduction in FDG uptake on follow-up PET/CT scans demonstrated the utility of hybrid imaging not only in diagnosis but also in monitoring treatment efficacy over time.
  

Figure 1. 18F-FDG and CMR Hybrid Imaging for LGE Co-localization.

Figure 2. 18F-FDG metabolic and 13N-NH3 perfusion scan showing perfusion-metabolic mismatch at the inferior wall suggesting active sarcoidosis.

Figure 3. Follow-up18F-FDG metabolic and 13N-NH3 perfusion scan at 6 months showing inferior wall scar with resolution of inflammation.