Magnetic Resonance Imaging
Magnetic resonance imaging combines a powerful magnet, radio waves and advanced computers to provide physicians with a host of detailed information on the location, size and composition of the body tissue being examined. This knowledge can be decisive for a fast, accurate and early diagnosis of diseases.
How MRI works
MRI does not use X-rays. In fact, as the name indicates, the procedure is based on the magnetic properties of hydrogen atoms, which are found in abundance in the human body. The MRI scanner produces a strong magnetic field which causes the nuclei of these hydrogen atoms to align (Fig. 1 A, B). Under the influence of a short pulse of radio waves, nuclei are pushed out of their alignment with the main field (Fig. 1 C). They then drift back to their initial position (Fig. 1 D), thereby emitting electromagnetic signals which can be used to reconstruct an image of the inside of the body (Fig. 1 E). Since protons in different tissues of the body (e.g. fat vs. muscle) realign at different speeds, the different structures of the body can be revealed.
Fig.1: How MRI works
MRI contrast media
Many MRI applications require the use of contrast media, most of which exhibit paramagnetic properties. By causing a change of the electromagnetic behaviour of tissues, they improve the contrast of an MR image. Bayer HealthCare Pharmaceuticals launched the world's first contrast medium for magnetic resonance imaging in 1988, setting a milestone in the development of MR techniques. More new contrast agents have been developed since then that help physicians to better examine specific parts of the body.
Major clinical applications of contrast-enhanced MRI
MRI is particularly useful in imaging the brain, heart, muscles and joints, for detecting cancer and evaluating blood vessels.
Specialized contrast-enhanced MRI techniques include among others
Magnetic resonance angiography (MRA)
MRA is used to produce images of blood vessels (mainly arteries). The technique makes it easier to detect blood-vessel abnormalities such as aneurysms and atherosclerosis (i.e. the plaque that develops in arteries).
MR perfusion techniques
Perfusion MRI is based on the injection of a gadolinium chelate and the rapid acquisition of images, as the bolus of contrast agent passes through the blood vessels in the brain. The contrast agent causes a signal change over time which can be analyzed to measure cerebral hemodynamics. The technique helps physicians, for example, in the diagnosis of strokes, brain tumors and mental disorders.
