ECG signal

Dr Diana R Holdright
MD, FRCP, FESC, FACC, MBBS, DA, BSc

Consultant Cardiologist

Dr Diana Holdright
 
 

Cardiac Investigations - Cardiac MRI (CMR) Scan

Magnetic resonance imaging (MRI) is a highly sophisticated technique with numerous medical applications. However, only recently has it proved especially useful for investigating cardiac problems, so-called cardiac magnetic resonance (CMR). Unlike conventional X-rays, CT scans and coronary angiograms, there is no ionising radiation involved in the process; instead MRI uses a very powerful magnetic field to produce images of the heart.  CMR can evaluate multiple aspects of the heart, including structure, function and blood supply to the heart.

For the scan the patient must change into a gown and remove all jewellery and any magnetic items, including hearing aids; a screening questionnaire will be completed to check that there are no magnetic implants that would preclude the scan. Most metal implants are MRI-compatible but there are contraindications, such as most pacemakers and implantable cardioverter-defibrillators (ICDs), cochlear implants and clips for aneurysms in the brain; in part it will depend on the type and position of metal implant, whether it is magnetic, and the strength of the magnet used in the scanner. All coronary stents and metal heart valve replacements are MRI-compatible; implantable loop recorders (ILRs) should have their data downloaded prior to the scan.

The patient lies on a moveable examination table, which then slides inside the scanner.  Compared with a CT scanner, the scanner tube is longer and also narrower; some patients feel claustrophobic although most cope well with the test. As the images are acquired they generate considerable noise and headphones with music are usually provided to minimise this. The scan takes between 20 and 45 minutes, during which the patient must lie still. Some imaging sequences require breath holds for a few seconds whilst images are being taken, and for some scans an injection of a substance called gadolinium is required which, unlike many other contrast agents, does not contain iodine, to which some people are allergic. Patients with reduced kidney function, which is assessed with a simple blood test, may not be able to receive gadolinium. If the blood supply to the heart is also being assessed an infusion of a drug, such as adenosine, will be administered. After the scan the patient is free to return to normal daily activities.

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Related page: News - December 2011 - MRI heart scans - latest study shows benefit over older techniques

Cardiac Investigations - Cardiac MRI (CMR) Scan