Magnetic resonance imaging (MRI) is a noninvasive procedure with which physicians formulate a three-dimensional image of internal body structures without ionizing radiation. Physicians collect various information by using different MRI protocols, the two most common of which are the T-1 MRI and the T-2 MRI. Both of these use electromagnetic waves to align positively-charged atoms, such as hydrogen, in either a head-toe orientation for a T-1 MRI or a left-right orientation for a T-2 MRI. A radiofrequency pulse then shifts the spin direction of the atoms, and when the pulse is discontinued, the atoms return to their normal alignment, giving off a signal from which the system constructs an echo or image. The T-2 MRI produces images with lower resolution than its counterpart, the T-1-weighted MRI, but it accentuates differences in fluid and cellular content, making the T-2 MRI images the most sensitive to changes in body tissues caused by cancer, inflammation, and blood flow disturbances.
T-2-weighted MRIs selectively use a long interval between pulses (TR), typically between 1500 to 300 milliseconds, and a long interval between each pulse and the analysis (TE), ordinarily between 75 and 250 milliseconds. The T-1 scan, unlike the T-2 scan, has a short TR of only 200 to 700 milliseconds and a short TE time of 20 to 35 milliseconds. In the head, T-1 scans produce appreciable contrast between the gray matter and the white matter of the brain and accent areas of fat. A T-2 MRI highlights blood, tissue swelling, and areas of liquefaction.
The remarkably strong signals on a T-2 MRI from fluid may conceal tissue abnormalities in a surrounding area. For example, T-2 MRIs produce a hyperintense signal from the cerebrospinal in the fluid channels of the brain, the ventricles. Fluid-attenuated inversion recovery (FLAIR) dampens the fluid signal on a T-2 MRI, making this variety of MRI suitable for examining the periventricular white matter of the brain. In multiple sclerosis, plaques develop in the white matter surrounding the ventricles. Identification of plaques in the brain through the use of FLAIR images is critical to making the diagnosis.
Modern MRI machines acquire data in a way that allows the physician to view tissue "slices" in a front-to-back (sagittal), side-to-side (axial), or top-to-bottom (coronal) orientation without the patient needing to change position in the scanner. With these alternatives in orientation, the ordering physician may obtain the best views for displaying the anatomic region of interest. In addition, radiologists may administer to the patient an injectable contrast medium, called gadolinium, which changes the local magnetic field of the tissues. Abnormal tissue reacts differently to gadolinium than normal tissue, providing a way to distinguish clearly any disease processes .