Cardiovascular clinical applications of PET/MRI

Cardiac PET imaging is well established as the reference modality for quantitative evaluation of myocardial perfusion and for the detection of ischemic alterations in coronary blood flow using radiolabeled PET tracers such as ¹³N-ammonia, ⁸²Rb and ¹⁵O-labeled water. While these techniques have served as the gold standard for clinical research and advanced evaluation of ischemic disease, the emergence of ¹⁸F-labeled perfusion tracers may, in the future, bring cardiac PET imaging into the mainstream of clinical imaging for assessment of coronary artery disease (CAD). FDG PET imaging has also served as a standard for objective assessment of the extent of myocardial viability after myocardial infarction and remains the most accurate technique for predicting recovery of myocardial function after reperfusion and for selecting patients who may benefit from revascularization procedures. Cardiac MRI has concurrently evolved as a technique that provides high-definition dynamic images of the heart and great vessels, surpassing echocardiography for accurate evaluation of cardiac function. Recent developments have also shown its ability to identify areas of reduced myocardial perfusion after gadolinium injection at rest and during pharmacological stress, thus promoting stress cardiac MRI as an alternative modality for detection of CAD. Furthermore, delayed accumulation of contrast media in scar and fibrotic tissue allows for differentiation of viable and non-viable tissue after myocardial infarction. PET and MRI, relying on different biological and physiological mechanisms, are truly complementary in their ability to detect stress-induced myocardial ischemia and tissue viability. This complementary ability of the two modalities opens up new perspectives in the assessment of cardiovascular disease. It is therefore foreseeable that the emergence of hybrid devices combining PET and MR modalities will lead to the development of new diagnostic protocols offering better diagnostic accuracy and allowing more objective assessment of cardiovascular disease. These applications may well extend beyond the scope of CAD and allow the technique to enter other areas: the evaluation of vascular plaques, neurotransmitter alterations and mechanisms of angiogenesis, the differentiation of different gene expression phenotypes, and the follow-up of stem cell therapy.