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Regional myocardial blood flow can now be measured non-invasively with contrast echocardiography, CT, MRI, and positron emission tomography (PET). Measurements with PET have advanced furthest; their accuracy and reproducibility are well established, they can be and are employed in the clinical setting, and they are useful for probing the coronary circulation in humans. The ability to quantify myocardial blood flow rather than to only qualitatively evaluate its relative distribution expands current diagnostic capabilities. Flow measurements offer a means for functionally delineating more accurately the extent and severity of coronary artery disease and, importantly, for probing and measuring microcirculatory reactivity as a component of coronary circulatory function that thus far has escaped non-invasive examinations. This article briefly addresses methodological aspects of PET measurements of myocardial blood flow, and then examines how observations made with this new measurement tool relate to coronary circulatory function and how these measurements can uncover disease related alterations.
Methodological aspects of flow measurements
Several positron emitting radiotracers are available for PET measurements of myocardial blood flow. Oxygen-15 labelled water (O-15 water) is considered the ‘ideal flow tracer’ because its uptake in myocardium increases linearly with flow. Yet, its physical properties limit its clinical use: the 122 s physical half-life necessitates close coordination of clinical studies with the cyclotron production of the radiotracer. Further, the freely diffusible radiolabelled water equilibrates with the water space of the myocardium but also with those of blood and adjacent anatomical structures. The resulting low myocardium-to-blood activity ratio images require corrections for blood pool activity, either through additional labelled blood pool imaging or by mathematically separating the myocardial from blood pool radiotracer activity. In the clinical setting, most laboratories therefore prefer nitrogen-13 labelled ammonia (N-13 ammonia) or rubidium-82 chloride (Rb-82). Both are administered intravenously and promptly accumulate in myocardium while rapidly clearing from blood. The resulting high signal-to-noise myocardial …