Cardiac Magnetic Resonance Imaging
Cardiovascular magnetic resonance imaging (CMR), sometimes known as cardiac MRI, is a medical imaging technology for the non-invasive assessment of the function and structure of the cardiovascular system. It is derived from and based on the same basic principles as magnetic resonance imaging (MRI) but with optimization for use in the cardiovascular system. These optimizations are principally in the use of ECG gating and rapid imaging techniques or sequences. By combining a variety of such techniques into protocols, key functional and morphological features of the cardiovascular system can be assessed.
CMR uses several different techniques within a single scan. The combination of these results in a comprehensive assessment of the heart and cardiovascular system.
Visualising heart muscle scar or fat without using a contrast agent
Typically a sequence called spin echo is used. This causes the blood to appear black. These are high resolution still images which in certain circumstances identify abnormal myocardium through differences in intrinsic contrast.
Heart function using cine imaging
Images of the heart may be acquired in real-time with CMR, but the image quality is limited. Instead most sequences use ECG gating to acquire images at each stage of the cardiac cycle over several heart beats. This technique forms the basis of functional assessment by CMR. Blood typically appears bright in these sequences due to the contrast properties of blood and its rapid flow. The technique can discriminate very well between blood and myocardium. The current technique typically used for this is called balanced steady state free precession (bSSFP), implemented as TrueFISP, b-FFE or Fiesta, depending on scanner manufacturer.
Infarct imaging using contrast
Scar is best seen after giving a contrast agent, typically one containing gadolinium bound to DTPA. With a special sequence, Inversion Recovery (IR) normal heart muscle appears dark, whilst areas of infarction appear bright white.
In angina, the heart muscle is starved of oxygen by a coronary artery narrowing, especially during stress. This appears as a transient perfusion defect when a dose of contrast is given into a vein. Knowing whether a perfusion defect is present and where it is helps guide intervention and treatment for coronary artery narrowings.