Cardiac magnetic resonance imaging

Cardiovascular magnetic resonance imaging (CMR), also known as cardiac MRI, is a medical imaging technology for non-invasive assessment of the function and structure of the cardiovascular system. Conventional MRI sequences are adapted for cardiac imaging by using ECG gating and high temporal resolution protocols. The development of CMR is an active field of research and continues to see a rapid expansion of new and emerging techniques. Cardiovascular magnetic resonance imaging (CMR), also known as cardiac MRI, is a medical imaging technology for non-invasive assessment of the function and structure of the cardiovascular system. Conventional MRI sequences are adapted for cardiac imaging by using ECG gating and high temporal resolution protocols. The development of CMR is an active field of research and continues to see a rapid expansion of new and emerging techniques. Cardiovascular MRI is complementary to other imaging techniques, such as echocardiography, cardiac CT, and nuclear medicine. The technique has a key role in evidence-based diagnostic and therapeutic pathways in cardiovascular disease. Its applications include assessment of myocardial ischemia and viability, cardiomyopathies, myocarditis, iron overload, vascular diseases, and congenital heart disease. It is the reference standard for the assessment of cardiac structure and function, and is valuable for diagnosis and surgical planning in complex congenital heart disease. Combined with vasodilator stress it has a role in detecting and characterizing myocardial ischemia due to disease affecting the epicardial vessels and microvasculature. Late gadolinium enhancement (LGE) and T1 mapping allow infarction and fibrosis to be identified for characterizing cardiomyopathy and assessing viability. Magnetic resonance angiography may be performed with or without contrast medium and is used to assess congenital or acquired abnormalities of the coronary arteries and great vessels. Obstacles to its wider application include limited access to suitably-equipped scanners, lack of technologists and clinicians with the necessary skills to run a service, relatively high costs, and competing diagnostic modalities. Cardiac MRI does not pose any specific risks compared to other indications for imaging and is considered a safe technique that avoids ionizing radiation. Gadolinium based contrast medium is frequently used in CMR and has been associated with nephrogenic systemic fibrosis, predominantly using linear compounds in patients with renal disease. More recently evidence of intra-cranial deposition of gadolinium has been shown - although no neurological effects have been reported. Genotoxic effects of cardiac MRI have been reported in vivo and in vitro, but these findings have not been replicated by more recent studies, and are unlikely to produce the complex DNA damage associated with ionizing radiation. CMR uses the same basic principles of image acquisition and reconstruction as other MRI techniques. Imaging of the cardiovascular system is usually performed with cardiac gating using an adaptation of conventional ECG techniques. Cine sequences of the heart are acquired using balanced steady state free precession (bSSFP) which has good temporal resolution and intrinsic image contrast. T1-weighted sequences are used to visualize anatomy and detect the presence of intra-myocardial fat. T1 mapping has also been developed to quantify diffuse myocardial fibrosis. T2-weighted imaging is mainly used to detect myocardial edema which may develop in acute myocarditis or infarction. Phase-contrast imaging uses bipolar gradients to encode velocity in a given direction and is used to assess valve disease and quantify shunts. A CMR study typically comprises a set of sequences in a protocol tailored to the specific indication for the exam. A study begins with localisers to assist with image planning, and then a set of retrospectively-gated cine sequences to assess biventricular function in standard orientations. Contrast medium is given intravenously to assess myocardial perfusion and LGE. Phase contrast imaging may be used to quantify valvular regurgitant fraction and shunt volume. Additional sequences may include T1 and T2-weighted imaging and MR angiography. Examples are below: Functional and structural information is acquired using bSSFP cine sequences. These are usually retrospectively-gated and have intrinsically high contrast in cardiac imaging due to the relatively high T2:T1 ratio of blood compared to myocardium. Images are typically planned sequentially to achieve the standard cardiac planes used for assessment. Turbulent flow causes dephasing and signal loss allowing valvular disease to be qualitatively appreciated. The left ventricular short axis cines are acquired from base to apex and are used for quantifying end-diastolic and end-systolic volumes, as well as myocardial mass. Tagging sequences excite a grid pattern that deforms with cardiac contraction allowing strain to be assessed. Gadolinium-based contrast agents are administered intravenously and delayed imaging is performed at least 10 minutes later to achieve optimum contrast between normal and infarcted myocardium. An inversion recovery (IR) sequence is used to null the signal from normal myocardium. Myocardial viability can be assessed by the degree of transmural enhancement. Cardiomyopathic, inflammatory and infiltrative diseases may also have distinctive patterns of non-ischemic LGE.