Fluorine-18 fluorodeoxyglucose positron emission tomography for cardiac sarcoidosis—is it time to consider a new radiotracer?

Cardiac fluorine-18 fluorodeoxyglucose (F-FDG) positron emission tomography (PET) is increasingly used for detecting cardiac involvement and assessing the presence and severity of myocardial inflammation in sarcoidosis [1]. Numerous reports have highlighted the usefulness of F-FDG PET in improving the ability to identify and treat patients with this disease, and several societies now incorporate cardiac F-FDG PET findings as a diagnostic criterion for cardiac sarcoidosis [1]. Despite its increasing use, cardiac F-FDG PET has several limitations in identifying cardiac sarcoidosis. The main drawback relates to physiologic myocardial F-FDG uptake, which may occur under normal resting conditions in unaffected myocardium and poses challenges for the assessment of myocardial inflammation by F-FDG PET. Several approaches to suppress physiologic myocardial glucose uptake have been proposed. The majority advocate dietary preparations which include restricting carbohydrate intake while promoting high-fat consumption, prolonged fasting, heparin administration, or a combination of these approaches. However, a number of studies have demonstrated that, even with these approaches, physiologic uptake may not be completely suppressed. The reasons are multifold. First, lowcarbohydrate, high-fat diets are very difficult to follow, and ensuring compliance is particularly problematic [2–4]. Second, it is virtually impossible to completely eliminate carbohydrate intake from the diet. Third, even in the absence of carbohydrate ingestion, glucose metabolism is active in tissues with high concentrations of glycogen, such as the myocardium. With respect to heparin administration in an effort to increase free fatty acid availability and suppress glucose metabolism [5], the majority of the studies demonstrate suppression of physiologic uptake with heparin use [5–9], although two studies suggest the converse [10, 11]. Heparin administration is also associated with increased bleeding risk and may not be appropriate in some patients. Another limiting factor to the use of F-FDG for assessment of cardiac sarcoidosis is that both inflammation and myocardial ischemia can increase glucose utilization and can further reduce the specificity of myocardial F-FDG uptake. Thus, an alternative tracer that does not require complicated dietary or fasting preparations and that has less nonspecific myocardial uptake would be highly useful for cardiac sarcoidosis. 3′-deoxy-3′-F-fluorothymidine (F-FLT), a promising PET tracer for evaluating tumor proliferative activity, has potential in this regard but has not yet been investigated in a systematic manner. In contrast to F-FDG, F-FLT uptake in normal myocardium is low even without prolonged fasting and/or a special diet prior to imaging. In this issue of the journal, Norikane et al. [12] attempt to investigate the diagnostic accuracy of F-FLT in 20 patients with newly diagnosed cardiac or extracardiac sarcoidosis. The authors report that F-FLT was successful in detecting nearly all lesions (10 of 11 focal lesions) that were detected with F-FDG. This is the first study in the literature to directly compare the two tracers in patients with new untreated sarcoidosis. * Correspondence: [email protected] Department of Cardiovascular Medicine, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA Full list of author information is available at the end of the article