Case 15 cont'd
Image findings: - In the heart, there is an intense heterogeneous FDG uptake involving the anterior aspect of the bioprosthetic aortic valve, more intense at the level of the left cusp. Uptake seems to extend into the basal septum – may be indicative of an abscess. In the chest, there are mildly hypermetabolic right lower paratracheal and bilateral hilar lymph nodes. There atelectatic changes within the left lower lobe. - Intense heterogenous uptake involving anterior aspect of the bp av highly suspicious of endocarditis. Possible extension into the basal septum. Intense focal uptake in the basal to mid anterolateral wall is indeterminate and could correspond to physiologic variant versus septic embolization. - Hypermetabolic mediastinal lymph nodes are likely reactive - Increased splenic and bone marrow uptake are suggestive of bone marrow activation secondary to infection.
A. Sarcoidosis is an inflammatory disease that can affect multiple organs (mostly lung and lymph nodes), including the heart. It is characterized by infiltration of tissue by granulomas (bundles of immune cells). These granulomas may evolve into fibrosis of the involved tissue. Cardiac involvement in sarcoidosis is being diagnosed more than in the past, and this is due to MRI and PET imaging emerging as the new gold standard for diagnosis, replacing invasive endomyocardial biopsy. On F-18 FDG PET, the inflammatory granulomas appear as either focal or focal on diffuse (patchy) uptake of radiotracer (see Figure 1). As sarcoidosis may involve multiple organs, it is not rare that other organs show avid uptake of tracer on the scan other than the heart (see Figure 2). Additionally, in cardiac involvement, there may be more than one granuloma present and therefore many areas of avid focal uptake may be seen (see Figures 3 and 4).
B. Bacterial acute endocarditis is an infection of the endocardium. This disease occurs mostly in patients with anatomic abnormalities (e.g. valvular abnormalities) when bacteria (most commonly Staphylococcus aureus) enter the bloodstream. Risk factors include cardiac devices, surgery, IV drug use and dental procedures. On PET, this results in avid focal F-18 FDG uptake in the heart.
C. Prosthetic valve endocarditis is the most likely diagnosis in this situation, due to both the imaging results and the clinical presentation. Prosthetic valve endocarditis occurs due to formation of thrombi made of platelets and fibrin. These thrombi are formed due to sutures and endothelial damage. Bacteria adhere to these thrombi and colonize. Staphylococcus epidermidis is most commonly associated with this sub-type of endocarditis due to its ability to form a biofilm on harsh surfaces. The inflammation recruits immune cells, which have heightened glucose metabolism. Therefore, on F-18 FDG PET, the area of the prosthetic valve will show avid focal radiotracer uptake. There may also be FDG-avid bacterial emboli localized at the joints (e.g. knee).
D. Rheumatic fever is an autoimmune disease caused by molecular mimicry that causes inflammation of the heart and the joints. It is brought on by group A Streptococcus infections, as cross reactivity occurs between the streptococcal M proteins and host proteins (e.g. myosin, troponin), causing immune cells to react with and attack the host isotopes. On PET, this affliction demonstrates diffuse myocardial uptake of radiotracer in the left ventricle (see Figure 5).
Q1. Answer: C.
Discussion:
As mentioned above, F-18 FDG PET studies require that patients follow a low carbohydrate diet for 24 hours prior to imaging. In cardiovascular imaging, this helps switch cardiac myocyte metabolism from glucose to fatty-acid metabolism and therefore reduces physiologic FDG uptake by the myocardium.
For endocarditis, F-18 FDG PET is more sensitive in detection of prosthetic valve endocarditis rather than native valve. This is because sites of infection differ between the two scenarios; prosthetic valve infections are usually located at non-mobile areas such as the anchor points in the prosthtic valves, while native valve infections are usually located on mobile vegetations. Moving objects are not well visualized by PET.
Normal physiological uptake of F-18 FDG is described in Figures 6, 7 and 8. As demonstrated, the brain, palatine and lingual tonsils, myocard, bone marrow, liver, stomach, spleen, kidneys and bladder all show physiological uptake. Normal uptake is also seen in testes in men and in breasts and ovaries in women. In children, normal uptake is seen in the thymus and the epiphyseal plates.
Physiologic uptake in the heart may cause a false positive reading, Other causes for false positives may be post-surgery inflammation, sterile inflammation due to surgical adhesive or other cardiac pathologies such as heart tumors.
Figure 1 – Patterns of myocardial F-18 FDG uptake