May 22, 2018, 11:38 am IST
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Cardiac MRI for Physicians

Cardiac MRI for Physicians

Dr R Rajesh Kannan, May 12, 2018

Cardiovascular Magnetic Resonance (CMR) imaging has emerged an important clinical technique in the assessment of various cardiac diseases.

It is widely recognised to provide an accurate and reliable means of assessing function and anatomy of the heart and great vessels. Previously, however, the means to obtain CMR had been compromised by extremely long examination times and specialized equipment that was available only at specialized centers. With the recent development of cardiovascular MR scanners, the applications of this technique have become more widely used on a routine basis.

CMR currently provides a comprehensive evaluation of the heart. Multiplanar imaging and excellent spatial resolution give extensive information on both anatomy and function. Precise quantification of flow is possible, and this is important in valvular and congenital heart disease. Importantly, CMR allows characterisation of the myocardium in vivo, avoiding the sampling error and invasive risks of myocardial biopsy. Thus, fat, iron or water accumulation suggesting inflammation can be identified and quantified. The additional use of gadolinium contrast agents detects areas of infarction, fibrosis, or infiltration by extracellular materials.

Gadolinium contrast agents can also be used successfully in myocardial perfusion studies for assessment of ischaemia and in central and peripheral angiograms. First-pass myocardial perfusion is now the most popular CMR method for evaluating myocardial ischaemia, with good results in comparison with SPECT and Positron Emission Tomography (PET). Myocardial infarction can be accurately detected by Late Gadolinium Enhancement (LGE) scan, with close correlation with histological findings. It can accurately distinguish viable from non viable myocardium. With a higher spatial resolution, CMR has been shown to identify small myocardial infarctions even when wall motion and perfusion by SPECT or PET are normal.

LGE can detect the presence of cardiac involvement in different cardiomyopathies, obviating the need for cardiac biopsy, with its inherent invasive risk or potential sampling errors. The use of multiple sequences, coupled with the use of gadolinium, provides information on tissue characteristics, vascularity, necrosis and fibrosis, and can point towards a specific diagnosis in the evaluation of cardiac masses.

In suspected congenital heart diseases, CMR can be useful to assess structures not clearly visualised by the echo (eg, right ventricle, pulmonary artery and veins). Moreover, the Qp/Qs ratio measured by flow mapping can confidently exclude any significant shunt.

Despite pacemakers and implantable electronic devices being regarded as contraindications, CMR can potentially be safely performed when appropriate precautions are taken.

Like Echocardiography, CMR is a safe technique with no proven detrimental effects on the long term, as opposed to other x-ray-based techniques such as X Ray Angiography, Computed Tomography (CT), and Single-Photon Emission Computed Tomography (SPECT), where radiation exposure is a key concern. This is an important issue when multimodality or serial imaging is considered, especially in the young and female population.

Dr R Rajesh Kannan MD, DNB, PDCC

Clinical Associate Professor, Radiology, Centre for Aortic Diseases and Marfan Syndrome

Amrita Institute of Medical Sciences, Kochi, Kerala.

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