Magnetic resonance imaging (MRI) is a medical imaging technique that uses strong magnetic fields and radio waves to produce detailed images of the body’s internal structures. Hydrogen atoms in the body’s water molecules align with the magnetic field; radio wave pulses briefly disturb this alignment, and the energy released as the atoms return to alignment is detected and converted into images. Unlike X-rays and CT scans, MRI uses no ionising radiation, making it suitable for repeated use and for detailed imaging of soft tissues.
In cardiac medicine, cardiac MRI (CMR, or cardiac magnetic resonance) has become the gold standard for assessing heart structure and function. It provides precise measurements of ventricular volumes, mass, and ejection fraction; characterises heart muscle with high resolution including scar tissue identification using late gadolinium enhancement (LGE); and is particularly valuable for diagnosing cardiomyopathies, myocarditis, and pericardial disease. Unlike echocardiography, cardiac MRI does not depend on acoustic windows and produces highly reproducible, operator-independent measurements.
In the context of cardiac arrest investigation, cardiac MRI helps characterise the underlying cause. LGE patterns distinguish ischaemic scar (from coronary artery disease) from non-ischaemic cardiomyopathy patterns (from conditions such as myocarditis, ARVC, or hypertrophic cardiomyopathy), guiding both diagnosis and risk stratification. Brain MRI is also used after cardiac arrest to assess the extent of hypoxic-ischaemic brain injury and to aid neurological prognostication.
MRI is contraindicated with certain metallic implants, particularly non-MR-conditional pacemakers and ICDs. Most modern devices are now manufactured with MR-conditional specifications, but compatibility must always be verified with the implanting centre before MRI is performed. See also: MR Conditional/Unsafe.
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