Occasionally, patients with chest pain do not show epicardial obstructive coronary stenosis. Ischemia with no obstructive coronary arteries (INOCA) is defined as evidence of ischemia in patients with no obstructive coronary artery. Previous reports showed that 65% of women and 30% of men undergoing invasive coronary angiography for stable angina have INOCA. [1,2] According to the American College of Cardiology National Cardiovascular Data Registry, approximately 3–4 million patients experience symptoms of ischemia without obstructive coronary disease. [3] INOCA has recently been associated with microvascular dysfunction. Coronary microvascular dysfunction is defined as increased resistance and impaired vasodilatation of the coronary microvessels, which leads to myocardial ischemia due to an inadequate coronary flow increase in response to stress. [4,5]

The 2019 European Society of Cardiology (ESC) guidelines show the importance of angina without obstructive disease in epicardial coronary arteries. [6] The mechanisms of angina have been suggested to involve impaired microcirculatory conductance and arteriolar dysregulation. [6] The diagnosis has been shown in the ESC guidelines as the presence of angina-like chest pain, a positive stress test response, and no significant stenosis of coronary arteries on angiography. [6,7] Coronary microvascular dysfunction is not a benign condition, as it is associated with an increased risk of cardiovascular morbidity and mortality. [5,6,8,9] However, the condition is heterogeneous and difficult to accurately detect and manage.

Papillary muscles (PMs) play an important role in cardiac contractility. Although visualization of the PMs is achievable, it is difficult to visualize them using single-photon emission computed tomography (SPECT). [10,11] However, recent technological improvement has enabled PM visualization using 13N-ammonia positron emission tomography (13NH3 PET). Using 13NH3 PET, Nakao et al. demonstrated that papillary muscle ischemia (PMI) was present in 11.8% of patients with coronary artery disease (CAD) and associated with reduced global myocardial flow reserve (MFR), and its presence was a predictor of major adverse cardiac events (MACEs). [12] We considered that PMI might be useful in assessing microcirculatory impairment and could be used as an indicator of coronary microcirculatory impairment in INOCA. The clinical usefulness of 13NH3 PET in evaluating INOCA remains unclear. Therefore, this study aimed to investigate the noninvasive evaluation of 13NH3 PET and compared patients with INOCA and CAD.