Magnetic resonance imaging (MRI) has been used to image the brain and other stationary organs within the body and has become widely available as a diagnostic technique for cardiovascular imaging. This has been possible because of sophisticated ECG gating and respiratory motion suppression methods that facilitate high-quality cross sectional images of the heart. It is now customary to use the term cardiovascular magnetic resonance (CMR) when referring to MRI of the heart and blood vessels.
Among the many techniques employed on MRI systems, three are the mainstays of clinical CMR .
- Spin echo imaging — Spin echo imaging depicts the tissue structures of the heart as bright and the blood pool as dark (black blood approach). The spin echo method is predominantly used for anatomical imaging, and for identifying the fatty infiltration of the right ventricular free wall frequently seen in arrhythmogenic right ventricular cardiomyopathy (ARVC) . (See "Clinical manifestations and diagnosis of arrhythmogenic right ventricular cardiomyopathy".)
- Gradient echo imaging — Gradient echo imaging generates images in which the blood pool appears bright and myocardium dark (bright blood approach). This technique is used to evaluate left and right ventricular cavity sizes and function, ventricular mass, intracardiac shunts, valvular functions, and to detect intracardiac masses. Steady state free precession (SSFP), a related approach, can generate high temporal (less than 30 msec) and spatial (2mm in-plane) resolution cine images in an 8 to 12 second breathhold.
- Flow velocity encoding — Flow velocity encoding (also known as phase contrast) is a technique that is used to directly measure blood flow and is useful for quantifying the severity of valvular regurgitation and stenosis, intracardiac shunt size, and the severity of arterial vascular stenosis .
- Other — Precise assessment of myocardial dynamics can be performed using radiofrequency (RF) tagging . There is a general trend to develop new methods to perform CMR without the use of contrast agents. Magnetic resonance spectroscopy has been studied as a means of evaluating myocardial metabolism.
Although real time CMR methods (acquisition of an entire image in less than 100 msec) are available and sometimes used, this approach suffers from lower temporal and spatial resolution. Thus, cardiac gating is generally used for CMR since data are typically acquired during many cardiac cycles to optimize spatial resolution. Robust ECG gating generally enables good spin-echo and cine image quality during sinus rhythm and even during atrial fibrillation or in the presence of occasional atrial or ventricular premature beats . Although flow velocity encoding imaging can be performed in the presence of atrial fibrillation, image quality may be degraded.
Although most CMR imaging can be performed during breath holds, longer image acquisitions (eg, for high resolution coronary artery imaging) require respiratory gating in addition to cardiac gating. Respiratory gating can be accomplished using either a navigator approach (to track the motion of the diaphragm) or respiratory bellows (using an elastic band wrapped around the thorax) to monitor respiratory motion.