Magnetic resonance imaging works using the principle of nuclear magnetic resonance. That is, in the presence of a strong magnetic field (typically 0.5 – 3.0 Tesla (T) for clinical applications) atoms in the body (typically hydrogen) are stimulated to emit radio waves. These radio waves are detected by an antenna (coil) placed around, or over, the body part of interest allowing an image of the body to be reconstructed. Extra magnetic fields (gradients) are used to constantly change the magnetic field to allow images of the body to be reconstructed. These fields are created by gradient coils which make the familiar banging sounds of an MRI scan. Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants and cardiac pacemakers are prevented from having an MRI due to effect of the powerful magnetic field.

Contrast (or difference in brightness) in the MR image is primarily due to the inherent magnetic relaxation times within the tissue structure known as the longitudinal relaxation time (T1) and the transverse relaxation time (T2). These two time constants are dependent on the type, and structure, of atoms in the tissue of interest. By altering the timing and strength of the gradient fields during imaging, or through the use of contrast agents, differences in T1 and T2 values between differing tissues can be exploited to produce images that highlight a specific tissue of interest, such as a tumor, stroke, or scar tissue.

Cardiac MR has unique aspects due to the fact that the heart is continually moving. While very rapid MR imaging techniques can generate an image of the heart in a fraction of a heartbeat (thus allowing for real-time imaging) most clinical images requiring high spatial resolution or good tissue contrast, require several heartbeats to generate an image. Consequently, most CMR scans are timed (or gated) to the patient's ECG such that a small portion of the image is captured per heartbeat, at the same time during the cardiac cycle. The result is a clear image of the heart without any distortion or blurring from cardiac motion.