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Magnetic Resonance Imaging |  |
| Magnetic resonance imaging (MRI) is the newest form of imaging in general use today. In this imaging modality a powerful magnet, up to 40,000 times as strong as the magnetic field of the earth, is used to transiently align the hydrogen atoms in the body with the magnetic field. All atoms with odd atomic numbers are affected, but the effect on hydrogen overshadows the effect on other natural elements within the body. If the field is abruptly turned off or reversed, the hydrogen
atoms lose their alignment or realign in the opposite direction. The rate at which they do this is restricted by the molecule of which they are a part. During this relaxation or realignment phase, the hydrogen atoms emit radio waves that can be detected by highly sensitive equipment. The frequency of these waves depends on the strength of the magnetic field. By constructing the magnetic field of the scanner in such a way that each small discrete volume (referred to as a voxel) has a
different field strength, each of these volumes can be represented by a frequency. Then, by evaluating the signal strength and duration for each frequency, the chemical composition of each voxel can be estimated. In practice this is done by representing the signal strengths for each volume on a monitor, much as is done with CT. The signal strength from each volume element is very small, so many repetitions or pulses of the magnet field application and relaxation are required to
provide a statistically significant determination of the relative signal strength from the volume elements. Thus, each scanning sequence may require several minutes to perform. Sequential examination of slices through the body is done in the same way that CT examinations are performed. MRI differs from CT in that the data for all the slices in the volume being imaged are acquired simultaneously; however, only 1 set of planes is acquired at a time. Scans are typically acquired in more
than 1 of the 3 orthogonal planes, with different magnet pulsing sequences to highlight different types of tissue. Also, unlike CT, MRI scans are seldom reformatted to project oblique planes, although 3-dimensional rendering is occasionally done. |
| MRI interpretation requires a firm knowledge of sectional anatomy as well as knowledge of the physics of the imaging system. Because this type of imaging is based on chemical composition of the body rather than density, it provides exquisite detail and contrast of body structures. However, the duration of data acquisition limits its use in areas of substantial movement, such as the chest. MRI does not image cortical bone well, and therefore is of limited use in the evaluation of
bony lesions, although it is quite applicable to imaging of bone marrow and cartilage. Like CT, MRI was initially used primarily for neuroimaging and is still the mainstay of imaging in that area. MRI is also now being used frequently for joint and muscle imaging. |
| Most MRI systems are large and expensive to purchase, install, and maintain. The length of time required dicates that studies be performed under general anesthesia. Because powerful magnets are used, ferromagnetic material may not be brought into the room due to safety considerations. For veterinary patients, this means that injectable anesthesia is typically used unless special anesthesia machines are available. These factors, combined with the specialized knowledge required to
perform studies and interpret the images, generally limits these instruments to large private and academic referral specialty practices. |
