Unveiling coronary microvascular dysfunction with ¹³N-NH₃ PET - Myocardial flow reserve: the emerging frontier for symptomatic non-obstructive coronary artery disease patients

Nearly half of the symptomatic patients undergoing invasive coronary angiography do not have obstructive coronary artery disease (CAD). Such patients usually have functional disorder of the epicardial vessels or microvasculature or both. 2/3rd of non-obstructive CAD (NOCAD) patients has coronary microvascular dysfunction (CMD) and are at an increased risk of developing major adverse cardiac events. This prospective observational study, aimed to quantify MBF and MFR using ¹³N-NH₃ stress/ rest cardiac PET, in patients with suspected CMD. 30 symptomatic (angina or angina equivalents), therapy naive patients with NOCAD in invasive coronary angiography, were prospectively enrolled in this study between January 2021 and January 2023. Absolute quantification of MBF in ml/min/ gm and MFR were obtained, using a dynamic rest and stress protocol with intravenous adenosine (140 mcg/kg/min, 6-minute protocol) as a hyperemic agent and 400 MBq of ¹³N-NH₃ as PET perfusion tracer. The remaining 9 patients with perfusion abnormality on PET perfusion imaging had normal MFR and adequate contractility. With normal coronaries, the presence or absence of perfusion defects alone does not rule in or rule out CMD and these defects should be interpreted in light of dynamic PET parameters such as MFR