Cardiac MRI

Magnetic Resonance (MR) imaging relies on differences in the way tissues react to a magnetic field, based largely on their water content, to generate an image

• differences in the way the field is applied can produce different types of image and information. Consequently, the image provided by MR is a composite of the data acquired, usually as a series of digital data sets representing only a small portion of the whole heart, and how it is handled (processed) by the computer
• data acquisition requires the presence of the patient, but data processing can take place at a later time
• cardiac MR (CMR) has lagged behind MR imaging of other organs because of artefacts induced by cardiac and respiratory movements that occur during data acquisition. However, ECG and respiratory gating (where data acquisition is synchronized with the cardiac and respiratory cycle) coupled with increasingly rapid data acquisition technologies now allow data sets to be acquired in a single breath hold
• from data acquired over 40-60 minutes CMR now delivers high-resolution images in any imaging plane that can be processed to provide two or three-dimensional static reconstructions or movie loops in almost every patient
• CMR can be considered in terms of MR imaging (MRI) and more specifically MR angiography (MRA)

Uses include:

• aortic disease
  • MRI: investigation of thoracic and abdominal aortic disease; evaluation of complications following aortic surgery.
  • MRA: better than conventional angiography for the demonstration of the full extent of aortic disease; equivalent to transoesophageal echo in diagnosis of aortic dissection (but better at demonstrating infradiaphragmatic complications).
• pericardial disease
  • MRI - can demonstrate pericardial metastases and invasion by mediastinal tumours; allows differentiation of restrictive cardiomyopathy and possible constrictive pericarditis.
• coronary artery disease
  • cine MRI - reproducible evaluation of left and right ventricular function.
  • perfusion MRI to measure the size of myocardial infarctions.
  • MRI - demonstration of complications of an MI eg left ventricular aneurysm, local or global impairment of left ventricular function.
• congenital heart disease
  • MRI - demonstration of pulmonary artery and aortic arch disease; intracardiac anatomy; atrial and ventricular septal defects.
  • MRA - detection and evaluation of sinus venosus defects and partial anomalous pulmonary venous drainage.
• evaluation of coronary artery bypass grafting
  • MRA - shows graft patency (similar accuracy to contrast enhanced CT but without the need for contrast agents)

Notes:

• CMR provides higher resolution and clearer three dimensional images of the heart than echocardiography, but at a substantially greater cost
  • is non-invasive, does not involve exposure to ionizing radiation and, unless contrast agents are used, carries no known risks
  • is highly reproducible and therefore ideal for serial imaging

Reference:

• 1) British Heart Foundation Factfile (January 2010). Cardiac imaging with MRI, CT and Nuclear techniques.