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MRI is a method of creating images inside our body by utilizing radio frequency waves and a strong magnetic field. This method eliminates the risks of harmful x-ray radiation. Recently, MRI has become increasingly popular since the procedure is noninvasive; currently, there are no known side effects. In addition, the image quality is much improved, providing more clear and detailed images of the internal organs and tissues. MRI technology has become so advanced that we are able now to detect heart disease, strokes and cancer and other conditions.
Let’s begin with the basic structure of the MRI scanner.
A typical MRI scanner is a 7 feet high by 7 feet wide by 10 feet long. It is cylindrical in structure with an automated and protruding bed. The subject is placed on their back on the MRI bed which slides into the bore, the hollow inside of the cylindrical tube.
The most important feature of the MRI is the magnet. The magnets are quite strong (typically 0.5 tesla to 2 tesla) which is why many precautions are taken during MRI scans (See Safety Precautions below). The magnet, however, does not have an effect on the body but rather helps image quality. The typical magnets found in MRI scanners are as follows: resistive magnets, permanent magnets, and super conducting magnets. Resistive magnets are simply coils of wire around the bore. In order to generate a magnetic field, electricity is passed into these coils. Once electricity is passed into these coils, all of the coils turn to produce a north pole and south pole similar to the ends of a typical bar magnet. Different contours of coils are needed to scan different parts of the body; so, cardiac MRI utilizes coils that are different in shape from ones scanning the brain. Permanent magnets are basic magnets that do not require any energy. Like resistive magnets, super conducting magnets also have coiled wires with one major difference: the wire used is immersed in liquid nitrogen or liquid helium at very low temperatures and well-insulated so as not to affect the temperature within the hollow bore.
The magnets align hydrogen protons in the area of interest, such as the heart. A radio frequency pulse is directed to these protons to “energize” them. Once energized, these protons spin. Once the radio frequency pulse is shut off, the protons release energy. The energy released activates the coils and sends information to the computer. The computer, using a mathematical analysis through Fourier transform, translates the information to an image.
The MRI is best for quantitative studies; MRIs can let doctors know the volume of the left ventricle, or the ejection fraction. The ejection fraction is the percent of the blood squeezed out of each ventricle after a contraction (usually around 60%). The MRI can measure myocardial mass, or the rate of blood flow.