Chapter 26

### INTRODUCTION

Magnetic resonance imaging (MRI) is a non-invasive method that allows the anatomic and functional evaluation for practically every organ system of the human body. MRI is performed in a strong homogenous magnetic field (1.0–3.0 T), which is 20,000–60,000 times stronger than the earth's magnetic field. Radiofrequency pulses in the FM range are used to excite the tissues, transferring energy to the hydrogen nuclei and causing resonance of their precession. Soon after the application of the radiofrequency pulses, the hydrogen nuclei return to their former energy state, releasing electromagnetic energy that can be detected with specially designed antennas (coils). The signal that is received is then used to create an image through a series of complex mathematical transformations that convert the phase and frequency of the electromagnetic waves to a spatially localized gray-scale pixel intensity. Depending on the technical characteristics of the imaging sequence, the same tissue may have different appearance. In general, cardiac imaging is performed with "black blood" or "bright blood" sequences in which the flowing blood appears as dark or bright, respectively. The physics principles behind MRI are quite complicated, but their in-depth knowledge is not necessary for the understanding of the clinical utility of the method. This chapter will only discuss the current applications of MRI for imaging of the heart.

MRI offers several advantages that make it a unique imaging tool: it is non-invasive and the exposure to the high magnetic field has no known immediate or late side effects. Neither the patient nor the medical staff are exposed to ionizing radiation and there is no need for potentially nephrotoxic contrast media. The contrast agents that are frequently used for cardiac MRI examinations are gadolinium products that, in patients who do not suffer from end-stage renal disease, have negligible complications and bear no structural, cross-allergy, or other similarity to the iodinated contrast media used for conventional radiographic imaging. MRI allows for three-dimensional acquisitions in any orientation, plane, and angulation with exquisitely high resolution. It offers high contrast among different tissues (e.g., flowing blood, myocardium, fat, etc.). Furthermore, the same tissue may have different imaging characteristics depending on its pathophysiologic state (e.g., normal myocardium vs. infarcted vs. edematous from acute ischemic injury or inflammation, etc.). Information about tissue composition may be important for the initial diagnosis and subsequent follow-up for many cardiac diseases. Finally, because of the ability to offer combined structural and functional imaging, cardiac MRI has the potential to non-invasively provide a comprehensive cardiac evaluation in a single examination.

### SAFETY ISSUES

In general, MRI is a very safe imaging modality. However, the exposure to high magnetic fields and rapidly applied gradients is contraindicated for certain patients, such as those with implanted pacemakers, defibrillators, or retained pacing wires. Fatalities have been reported when patients with pacemakers inadvertently had an MRI examination, presumably due to erratic sensing or pacing, local heating at ...

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