Cardiovascular Magnetic Resonance
Cardiovascular Magnetic Resonance (CMR), sometimes know 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
MRIbut 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 [http://www.scmr.org/caseoftheweek/case06-01.cfm here]
History of CMR 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 prizein 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, [http://www.scmr.org] SCMRwas 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 of CMR
CMR uses the same basic principles as other
MRItechniques 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 patients own 1H nuclei absorb and then emit energy, which can be measured and translated into images, without using ionising radiation. For further information, see MRI, earlier versions of this document, or the following outside links:
[http://www.hull.ac.uk/mri/lectures/gpl_page.html Hull physics lecture series]
[http://www.cis.rit.edu/htbooks/mri/ the basics of MRI]
[http://www.mritutor.org/mritutor/ MRI tutor]
The different techniques in CMR
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 image which in certain circumstances identify abnormal myocardium through differences in intrinsic contrast.
A short axis view of the heart showing a movie (cine)next to a spin-echo sequence. In this case, the scan demonstrates features of
ARVCwith fatty infiltration of the left and right ventricles. The full case can be seen [http://www.scmr.org/caseoftheweek/case07-14.cfm 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 cycleover 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 discrimate 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.
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).
Current applications of CMR
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 list of current CMR indications can be found [http://www.scmr.org/documents/2004pennell_ehjcmr_indications.pdf here]
Case examples of the use of CMR
For online case examples, see [http://www.scmr.org/caseoftheweek/archive.cfm here]
pecific clinical uses of CMR
A good overview of the clinical indications for CMR can be found [http://www.scmr.org/documents/2004pennell_ehjcmr_indications.pdf here] and [http://www.acc.org/qualityandscience/clinical/pdfs/CCT.CMR.pdf here]
Centres performing CMR
Training in CMR
Training is being increasingly protocolised and is now formal with stages of training and accreditation. Internationally approved training guidelines can be found [http://www.scmr.org/documents/TF12CMR.pdf here] A resource for anyone thinking about CMR as a career can be found [http://www.scmr.org/starting.cfm here]
What it is like having a CMR scan
A good description of the experience of receieving a CMR can be found [http://www.radiologyinfo.org/en/info.cfm?pg=cardiacmr&bhcp=1 here]
Manufacturers of CMR scanners
[http://www.gehealthcare.com/usen/mr/index.html GE Healthcare]
[http://www.medical.philips.com/main/clinicalsegments/cardiovascular/portfolio/mri/index.asp Philips medical systems]
[http://www.siemensmedical.com/ Siemens medical solutions, Inc]
* [http://www.scmr.org The Society for Cardiovascular Magnetic Resonance]
* [http://atlas.scmr.org/ An Atlas of normal cardiac structure and function by CMR]
* [http://www.radiologyinfo.org/en/info.cfm?pg=cardiacmr&bhcp=1 Having a CMR scan]
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