Cardiovascular magnetic resonance imaging

Cardiovascular magnetic resonance imaging (CMR), sometimes known as cardiac MRI, is a medical imaging technology for the non-invasive assessment of the function and structure of the cardiovascular system. It is derived from and based on the same basic principles as magnetic resonance imaging (MRI) but with optimisation for use in the cardiovascular system. These optimisations are principally in the use of ECG gating and rapid imaging techniques or sequences. By combining a variety of such techniques into protocols, key functional and morphological features of the cardiovascular system can be assessed.

An example of CMR movies in different orientations of a cardiac tumor - in this case, an atrial myxoma. The full case can be seen here.

History and nomenclature

The phenomenon of nuclear magnetic resonance (NMR) was first described in molecular beams (1938) and bulk matter (1946), work later acknowledged by the award of a joint Nobel prize in 1952. Further investigation laid out the principles of relaxation times leading to nuclear spectroscopy. In 1973, the first simple NMR image was published and the first medical imaging in 1977, entering the clinical arena in the early 1980s. In 1984, NMR medical imaging was renamed MRI. Initial attempts to image the heart were confounded by respiratory and cardiac motion, solved by using cardiac ECG gating, faster scan techniques and breath hold imaging. Increasingly sophisticated techniques were developed including cine imaging and techniques to characterise heart muscle as normal or abnormal (fat infiltration, oedematous, iron loaded, acutely infarcted or fibrosed).

As MRI became more complex and application to cardiovascular imaging became more sophisticated, the Society for Cardiovascular Magnetic Resonance, [1] SCMR was set up (1996) with an academic journal, (JCMR) in 1999, which is going open source in 2008. In a move analogous to the development of ‘echocardiography’ from cardiac ultrasound, the term ‘Cardiovascular Magnetic Resonance’ (CMR) was proposed and has gained acceptance as the name for the field.

Physics

CMR uses the same basic principles as other MRI techniques with the addition of ECG gating. Most CMR uses only ¹H nuclei MR, which are abundant in human tissue. By using magnetic fields and radiofrequency (RF) pulses, the patient's own ¹H nuclei absorb and then emit energy, which can be measured and translated into images, without using ionising radiation.

Techniques

CMR uses several different techniques within a single scan. The combination of these results in a comprehensive assessment of the heart and cardiovascular system. Examples are below:

Visualising heart muscle scar or fat without using a contrast agent

Typically a sequence called spin-echo is used. This causes the blood to appear black. These are high resolution still images which in certain circumstances identify abnormal myocardium through differences in intrinsic contrast.

File:Cardiac magnetic resonance Arrhythmogenic right ventricular dysplasia.gifA short axis view of the heart showing a movie (cine)next to a spin-echo sequence. In this case, the scan demonstrates features of ARVC with fatty infiltration of the left and right ventricles. The full case can be seen here.

Heart function using cine imaging

Images of the heart may be acquired in real-time with CMR, but the image quality is limited. Instead most sequences use ECG gating to acquire images at each stage of the cardiac cycle over several heart beats. This technique forms the basis of functional assessment by CMR. Blood typically appears bright in these sequences due to the contrast properties of blood and its rapid flow. The technique can discriminate very well between blood and myocardium. The current technique typically used for this is called balanced steady state free precession (SSFP), implemented as TrueFISP, b-FFE or Fiesta, depending on scanner manufacturer.

A 4 chamber view of the heart using SSFP cine imaging. Compare the image orientation (4 chamber) with the short axis view of the movie above

Infarct imaging using contrast

Scar is best seen after giving a contrast agent, typically one containing gadolinium bound to DTPA. With a special sequence, Inversion Recovery (IR) normal heart muscle appears dark, whilst areas of infarction appear bright white.

CMR in the 4 chamber view comparing the cine (left) with the late gadolinium image using inversion recovery (right). The subendocardial infarct is clearly seen. Fat around the heart also appears white.

Perfusion

In angina, the heart muscle is starved of oxygen by a coronary artery narrowing, especially during stress. This appears as a transient perfusion defect when a dose of contrast is given into a vein. Knowing whether a perfusion defect is present and where it is helps guide intervention and treatment for coronary artery narrowings.

CMR perfusion. Contrast appears in the right ventricle then left ventricle before blushing into the muscle, which is normal (left) and abnormal (right, an inferior perfusion defect).

Uses

In the investigation of cardiovascular disease the physician has a wide variety of tools available. The key disadvantages of CMR are limited availability, expense, operator dependence and a lack of outcome data. The key advantages are image quality, non-invasiveness, accuracy, versatility and no ionising radiation.

A good overview of the clinical indications for CMR can be found here and here

Training

Training is being increasingly protocolised and is now formal with stages of training and accreditation. A resource for anyone thinking about CMR as a career can be found here

External links

• The Society for Cardiovascular Magnetic Resonance
• An Atlas of normal cardiac structure and function by CMR
• Having a CMR scan
• Cardiac MRI technical primer
• ReviseMRI.com
• Hull physics lecture series
• the basics of MRI
• MRI tutor
• Cardiac MRI technical primer
• A good description of the experience of receiving a CMR can be found here
• For online case examples, see here