13 General introduction Figure 2. Currently, all endotypes of ANOCA can only simultaneously be assessed invasively. However, these invasive assessments burden patients with subsequent risks for complications, hospitalization and high costs. Non-invasive methods to detect coronary vasospasm are limited, with proposed tests including the cold pressor and hyperventilation test, but literature on these methods is scare.24 The majority of non-invasive approaches only allow for the assessment of CMD as a possible cause of ANOCA. Myocardial perfusion (i.e. MBF and CFR) can be most accurately and most completely assessed with positron emission tomography (PET) using oxygen-15-labeled water ([15O]H2O-PET). [15O]H2O freely diffuses within the myocardium, is extracted from the arterial blood pool, and is metabolically inert, making [15O]H2O an ideal tracer to quantify MBF in ml/min/g.25-27 However, while [15O]H2O-PET provides excellent information about myocardial perfusion, it only enables noninvasive diagnosis of CMD, offers limited anatomical details, is expensive compared to other imaging modalities and is not widely available. Therefore, in selected centers cardiovascular magnetic resonance (CMR) imaging and transthoracic Doppler echocardiography are available to non-invasively assess CMD, both using adenosine to evaluate coronary flow. Transthoracic Doppler echocardiography enables visualization of the coronary flow in the LAD by measuring pulse-wave Doppler as a flow signal in the mid-distal LAD towards the transducer during diastole.28 Flow measurements are obtained 1
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