Myocardial imaging with radiolabeled free fatty acids: applications and limitations

The assessment of myocardial fatty acid metabolism using radiolabeled substrates has recently become a new diagnostic modality in noninvasive cardiology. The development of metabolic tracers has been made possible largely due to a combined increase in the understanding of myocardial biochemistry and in nuclear-medicine technology. Initially, imaging and the exploration of myocardial metabolism appeared to be the exclusive domain of positron-emission tomography. However, investigators have been successful in applying radioiodine-labeled fatty acids that can be monitored using conventional gamma cameras. These metabolic substrates can be used not only for imaging purposes, but also for the evaluation of regional metabolic clearance rates, which may serve as a parameter for myocardial fatty acid metabolism. Although the initial results have been promising, the analysis and interpretation of clearance curves appears to be rather complicated and may produce a lot of unanswered questions. A great deal remains to be done due to the complex biological behavior of the tracers employed and the difficulties encountered in quantitatively delineating the distribution of radioactivity in the beating heart in vivo. Therefore, closer integration of myocardial biochemistry and the metabolic imaging technique seems to be necessary for enhancing our knowledge of myocardial fatty acid metabolism and to make metabolic imaging clinically useful.