|Fig. 1: Illustration depicting coronary artery disease (Source: Wikimedia Commons).|
Coronary artery disease (CAD) is the worlds leading cause of death. It is a complex disease that causes reduced blood flow in one or more of the arteries that supply the heart. CAD, also known as atherosclerosis, is a result of a buildup of plaque inside the coronary arteries, which supply oxygen-rich blood to the heart (Fig. 1). In developed nations, CAD occurs more often in male populations between the ages of 63.4 to 68 years.  Major risk factors for CAD include raised blood glucose, raised blood pressure, obesity, and smoking.  Because the initial diagnosis of CAD typically only occurs after a coronary event, early detection of CAD is imperative to reduce morbidity and mortality rates and decrease the medical and economic costs associated with CAD.
CAD is diagnosed through various tests including coronary angiography, echocardiogram (ECHO), electrocardiogram (ECG or EKG), stress echocardiogram, and the stress thallium test. To date, the gold standard for the diagnosis of anatomic CAD has been invasive coronary angiography (ICA), which utilizes X-ray visualization of a radiopaque dye to detect the percentage of occlusion of a coronary vessel (Fig. 2).  While ICA is the standard diagnostic tools, it is expensive and subjects patients to procedural risks. Due to its limitations, new methods for diagnosing CAD - such as nuclear imaging - are being studied.
|Fig. 2: Coronary Angiography (Source: Wikimedia Commons).|
Nuclear medicine is a branch of medical imaging that creates images by utilizing radiation emitted from within the body using injectable radionuclides.  Nuclear medicine uses small amounts of radioactive materials called radiotracers that are typically injected into the bloodstream, inhaled or swallowed. The radiotracer makes contact an electron in the body and creates an annihilation event resulting in the emission of gamma rays. These rays are detectable by a special camera that creates anatomical images of the inside of the body.
Positron emission tomography (PET) is an imaging modality for visualizing and measuring a pathophysiological processes in vivo (Fig. 3). Due to its high temporal and spatial resolutions and high reproducibility, PET can be used to assess regional myocardial blood flow (MBF), allowing for a novel form of non-invasive CAD imaging.
|Fig. 3: Image of a typical positron emission tomography (PET) facility (Source: Wikimedia Commons)|
PET totes a higher positive predictive value than ICA - 86% for PET versus 81% for ICA.  PET has also shown to be more cost-effective in diagnosing CAD, despite higher single scan costs, due to its higher specificity. In addition, the radiation burden of MBF imaging with PET is fairly low (averaging 4.1 mSv in the brain).  Overall, the assessment of functional coronary artery abnormalities with PET may help in classifying the early functional and progressive stages of CAD before structural alteration within the arterial wall (which would be detected by ICA) is magnified. 
PET has main limitations are due to the difficulty of quantitative analysis and limited tracer availability. Currently, PET tracers are restricted to North America and Europe, causing accessibility issues for more widespread use. Furthermore, kinetic analysis, which is required for quantification of blood flow, can only be performed with specialized software.
© Gigi Nwagbo. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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