Cardiac magnetic resonance imaging (CMR – Cardiac MRI)
Cardiac magnetic resonance imaging (CMR – Cardiac MRI) uses special software in magnetic resonance imaging equipment, optimized for imaging the heart which moves at a fast rate than most other structures which are stationary or moves slowly. Imaging of the heart is usually done with ECG gating so that images can be timed to contractions and relaxation of the heart. Rapid sequence imaging is required to capture the heart which contracts and relaxes at 60-100 beats per minute in a normal individual. Three dimensional reconstructions and cine imaging can be done using CMR techniques. Scars in the heart muscle (after a heart attack) can be identified by injection of gadolinium based contrast material into the veins. Normal heart muscle appear as dark and scars appear as white as they take up gadolinium.
Which are the special situations in which CMR is used?
CMR can be used to detect whether a part of the heart muscle which has been damaged due to a block in the blood vessel is viable. If it is viable, there is a role for opening up the blocked vessel by angioplasty.
In inflammation of the heart (myocarditis), CMR is useful in diagnosis as well as to assess the extent of damage to heart muscle.
Scars in the heart muscle after a heart attack can be detected by CMR. Scarred regions can be the future foci for development of heart rhythm disorders (cardiac arrhythmia).
Certain disorders of heart rhythm like arrhythmogenic right ventricular dysplasia has certain diagnostic features on CMR, which is difficult to make out on other imaging modalities like echocardiogram (ultrasound study of the heart).
Congenital heart disease (birth defects of the heart) is another situation in which CMR comes handy. Detailed assessment of chambers of the heart, blood vessels and defects in them can be made by CMR, which will guide the surgeon in accurate treatment.
What are the challenges in CMR?
CMR requires special software, which is not available in every MRI scanner. Good deal of expertise is also needed for interpretation. Patient should be very co-operative as movement artefacts will make interpretation very difficult. Heart rate should be stable and in a reasonable range for ECG gating to work well. In small children, anaesthesia will be required as they are unlikely to stay quiet in the MRI suite which even adults find it difficult. Requirement for anaesthesia increases the need for monitoring equipment and all of them have to be MRI compatible and naturally more expensive.