Ischemia Detection using Perfusion MRI

1. Perfusion imaging to detect myocardial ischemia Myocardial ischemia occurs when insufficient blood flow, or perfusion, to the myocardium results in inadequate delivery of oxygen as well as inadequate removal of metabolites. These conditions may occur at rest due to insufficient oxygen supply (supply ischemia) or at stress due to a failure to sufficiently accommodate increased oxygen demand (demand ischemia). Coronary artery stenosis is a common cause of insufficient myocardial perfusion reserve and demand ischemia, with clinical symptoms typically appearing as angina and ST segment deviation on ECG predominantly at stress. Noninvasive imaging of perfusion defects at stress is widely used for the detection of significant coronary stenoses. Numerous imaging modalities can be used to detect myocardial ischemia by perfusion assessment. The most widely used myocardial perfusion modality is SPECT imaging with TI or Tc-labeled agents, which are radiotracers that are rapidly taken up by and slowly cleared from the myocardium after intravenous administration. While SPECT is the standard clinical test for myocardial perfusion imaging (1), limitations of this technique include attenuation artifacts from soft tissue and relatively low spatial resolution. PET imaging with N-labeled ammonia or O-labeled water are more accurate (2;3), however these methods are also more expensive and are not widely available. Myocardial contrast echocardiography (MCE) can also be used for myocardial perfusion imaging (4), but this technique is not yet widely established and can be limited by poor acoustic windows. MRI offers several different methods for imaging myocardial perfusion, including first-pass contrast-enhanced imaging, blood oxygen level dependent (BOLD) imaging, and, in small animal models, arterial spin labeling (ASL). BOLD and ASL have the advantages that they do not require the use of exogenous contrast media, however they are still in the early stages of development and evaluation.