177 Quantitative Perfusion CMR in ANOCA Visual, semi-quantitative and fully quantitative assessment of perfusion CMR Visual assessment of first-pass contrast-enhanced imaging relies on identifying perfusion defect based on the signal variations within the imaged section of the heart.6 In our study we identified two presumable defects and one definite perfusion defect when interpreted blindly. However, upon reviewing the clinical final report of these three participants, no perfusion defects were described. Instead, these so-called perfusion defects were labeled as two dark-rim artifacts and one unspecified artifact. Remarkably, two of these cases were reported by the same observer. This shows the difficulties of visual assessment, as encountered in daily practice. Visual assessment is primarily aimed at identifying segmental perfusion defects. Therefore, visual assessment may be inaccurate when MBF is globally reduced, as assumed in CMD, potentially resulting in perfusion appearing falsely normal.22 Furthermore, in ANOCA endocardial perfusion abnormalities are described as appropriate, especially during stress, since the subendocardial layer is most vulnerable to ischemia.23, 24 Visual assessment of subtle endocardial perfusion defects is hampered by the high presence of dark-rim artifacts, which should be discriminated from true perfusion defects. This discrimination can be made by assessing the number of RR intervals of hypo-perfusion, the number of pixels involved and timing of the defect, since a true perfusion defect should be persistent for more than 5 RR-intervals over ≥ 2 more pixels, while a dark-rim artifact is typically < 5 RR intervals and only one pixel-wide appearing before myocardial enhancement occurs.25 However, the discrimination between true endocardial perfusion and dark-rim defects can be challenging, making visual assessment of first-pass contrast-enhanced imaging highly dependent on expertise and subjective. Semi-quantitative and fully quantitative perfusion CMR analyses rely on signal intensity curves, making use of the AIF to overcome the saturation of T1 signal intensity, to determine the linear relationship between the contrast agent and the MR signal intensity.26, 27 Fully quantitative analysis enables the measurement of MBF in units of milliliters of blood per minute per gram of tissue, facilitating a non-invasive method to measure the vasodilatory capacity accurately. Also, by comparison of quantitative stress MBF a distinction can be made between dark-rim artifacts from remote myocardium and true perfusion.28 Our study found a low prevalence of perfusion defects by visual assessment, while a diminished MPR was present in half of the patients. This is in line with previous studies, showing a relatively low sensitivity of visual assessment to diagnose ANOCA.18 This implicates that visual assessment of first-pass contrast-enhanced CMR imaging is insufficient as a diagnostic tool in ANOCA, and quantitative assessment of perfusion should be considered as a non-invasive method. ANOCA, particularly CMD, is commonly understood as a condition that affects all myocardial tissue, resulting in a consistent reduction in the distribution of coronary flow throughout the myocardium. On the contrary, during CFT measurements, coronary flow and resistance are obtained only in the LAD. However, previous literature has described 8
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