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 hydrogen atoms are subjected to a radio-frequency (RF) pulse, the alignment of these atoms is then flipped to one side or reversed. Once the RF pulse is turned off, the hydrogen atoms realign with the magnetic field. The rate at which they do this is restricted by (and characteristic of) 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 (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 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 may be done either on the scanner's computer or a stand-alone workstation.

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 and upper abdomen. 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. Another major area of MRI usage is in evaluation of blood vessels deep within the body, particularly those of the legs, neck, and head. MRI is also used frequently for joint and muscle imaging where it has become a valuable tool in the assessment of joint integrity because of its unique ability to image cartilage and ligaments.

Contrast enhancement of MRI scans is common when imaging the brain and other soft tissues. Contrast enhancement can permit the radiologist to make a relatively specific diagnosis regarding the etiology of the lesions observed on the scan. In other instances, the contrast images are the only ones that reveal the presence of a lesion. The agents used are specifically designed for use in MRI and are different from those used in CT and radiography.

In the past, MRI systems were large and expensive to purchase, install, and maintain, but many smaller, low-field-strength magnets are now available, including some specifically designed for use in veterinary medicine. Dedicated equine extremity scanners are also available but are still relatively expensive.

The length of time required to complete MRI scans dictates that studies be performed under general anesthesia. For veterinary patients, injectable anesthesia is typically used unless special anesthesia machines, oxygen tanks, and monitoring equipment are available. Injectable anesthesia may not be appropriate for all patients, so facilities dedicated to veterinary patients are well advised to have appropriate anesthetic equipment. Because powerful magnets are used, ferromagnetic material may not be brought into the room due to safety considerations. In addition, MRI scanners should be operated by technologists specially trained in operation of these instruments. This training is not part of either the veterinary curriculum or the veterinary technical curriculum and must be acquired by attending special training sessions or preferably as part of the training program in a school of radiologic technology.

Because of the expense of acquiring and maintaining these instruments, the technical complexity of MRI imaging, and the special training and experience required to interpret the images, MRI scanning systems are generally found in large private and academic referral speciality practices.

Last full review/revision March 2012 by Jimmy C. Latimer, DVM, MS, DACVR, DACVRO