A number of definitions have been proposed.
In 2005, a molecular imaging summit, sponsored by the RSNA and SNM, recommended the following definition. Molecular imaging techniques directly or indirectly monitor and record the spatiotemporal distribution of molecular or cellular processes for biochemical, biologic, diagnostic, or therapeutic applications (Thakur ML, Lentle BC; J Nucl Med. 2005 46(9):11N-13N, 42N).
In 2007, the SNM molecular imaging center of excellence recommended an expanded definition. Molecular imaging is the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in humans and other living systems. Molecular imaging typically consists of 2- or 3-dimensional imaging as well as quantification over time. The techniques used include radiotracer imaging/nuclear medicine, MR imaging, MR spectroscopy, optical imaging, ultrasound, and others (Mankoff DA; JNM 48(6)18N, 2007).
Exactly what constitutes molecular imaging can be confusing. For example, measurement of myocardial blood flow with a radioactive tracer such as Tc-99m sestamibi or tetrofosmin would not be a molecular imaging technique because measurement of blood flow is not a molecular or cellular process. Blood flow can be most accurately measured with labeled microspheres that are trapped in capillary beds. No metabolic or cellular processes are necessary for this trapping to occur.
Another example of a physiologic measurement that does not fit the definition of molecular imaging is measurement of left ventricular ejection fraction (LVEF). Although left ventricular function ultimately is the end product of cellular and biological processes, the measurement of LVEF is based on geometric assumptions or count based techniques.
Molecular imaging has enormous relevance for patient care: it reveals the clinical biology of the disease process; it has the potential to personalize a patient’s care. For example, rather than assuming that an individual patient with cancer is the average patient that has the average cancer that will have the average response to the average treatment, the patients care is personalized based on biologic characteristics of the patient, the tumor and the early response to treatment.
Molecular imaging offers unique insights that allow a more personalized approach to evaluation and management of cardiovascular disease conditions, such as: ischemic injury, heart failure, and left ventricular remodeling; thrombosis, atherosclerosis, and vulnerable plaque; and angiogenesis, transplant rejection, and arrhythmic substrates.
1: Thakur ML, Lentle BC; SNM; Radiological Society of North America (RSNA). Joint SNM/RSNA Molecular Imaging Summit Statement.J Nucl Med. 2005 Sep;46(9):11N-13N, 42N.
2: Mankoff DA. A definition of molecular imaging. J Nucl Med. 2007 Jun;48(6):18N, 21N.