A phantom study to determine the theoretical accuracy and precision of radial MRI to measure cross‐sectional area differences for the application of coronary endothelial function assessment

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The endothelium is a thin monocellular layer that serves multiple functions and plays an important role in many aspects of cardiovascular health. Among those, the endothelium acts as a semi‐selective barrier to regulate fluid and molecule traffic between blood and tissue, maintains vascular homeostasis, serves as a nonthrombogenic surface, contributes to angiogenesis and tissue wound‐healing, and regulates vascular tone and blood flow 1. Impairment of these normal vascular functions, known as endothelial dysfunction, most often results from an increased oxidative stress 3 and has been linked to pathological inflammatory processes and future adverse cardiovascular events 4. Although measuring any of the endothelium's functions may provide information about the integrity and general health of said endothelium, for practical reasons a frequently investigated aspect in clinical research is the regulation of vascular tone in response to endothelial‐dependent stressors 9.
Assessment of the vasomotor response of the epicardial coronary arteries, using invasive imaging modalities such as coronary angiography 9 and intravascular ultrasound 14, has shown that healthy coronary arteries dilate by about 10% to 25% in response to endothelium‐dependent stressors via the release of nitric oxide 15. Conversely, reduced dilation and even paradoxical vasoconstriction are observed in impaired coronary arteries 9. However, because of their invasive nature, these imaging modalities are restricted to patients with advanced disease and are not clinically justifiable for use in screening or longitudinal studies.
To address these limitations, and to noninvasively discriminate normal from abnormal coronary endothelial function, recent studies have proposed using MRI with isometric handgrip exercise 13 as the endothelial‐dependent stressor. This promising new technique has yielded excellent and reproducible results 18. However, the sensitivity of MRI to measure small changes in cross‐sectional area of the coronary arteries, in response to stress, has yet to be quantitatively examined. Because the spatial resolution of MRI is limited relative to the expected area changes, it is of utmost importance to address this question.
In this study, we have designed a phantom that simulates a physiological range of coronary cross‐sectional areas and used it to measure the sensitivity of MRI for detecting small area differences under both static conditions and mounted on a moving MR‐compatible platform. Radial cine MR images were acquired with different spatial resolutions and different signal‐to‐noise ratios (SNRs) were simulated by adding artificial noise to the images. Cross‐sectional areas were measured with a fully automated procedure and compared to known nominal values.

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