Magnetic resonance imaging (MRI) is the newest form of imaging in general use today. In this imaging modality, a powerful magnet, up to 60,000 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 then subjected to a radiofrequency (RF) pulse of the proper frequency, the alignment of these atoms is then deflected to one side or reversed, which imparts energy to the atom.
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 the energy imparted to them by the RF pulse as radio waves that can be detected by highly sensitive antennae. The frequency of these waves depends on the strength of the large, main magnetic field. By using a second set of magnets referred to as the gradients, the magnetic field of the scanner can be arranged in such a way that each small discrete volume (voxel) has a different field strength. Because the RF emitted by the relaxing hydrogen atoms depends on the strength of the magnetic field in which it is located, each of these volumes can be represented by a unique frequency.
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 recording the signal strengths for each volume in a three-dimensional table, much as is done with CT. This three-dimensional table is then used to display a representation of the signal strength map for each plane in the scan field on a monitor while generating an image data set that is stored permanently on the picture archiving and communication system. Storage of the actual scan data set is not usually done (except in a research setting) because of the much larger size and proprietary nature of these data sets.
The signal strength from each volume element is very small, so many repetitions or pulses of the RF field 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 the same way CT examinations are performed. MRI differs from CT in that the data for all the slices in the volume being imaged are acquired simultaneously; previously only one set of planes was acquired at a time, but recent developments in scanner technology allow acquisition of volume (three-dimensional) data sets. Scans are typically acquired in more than one of the three orthogonal planes, with different magnet pulsing sequences to highlight different types of tissue. Thus, MRI scanning virtually always involves the performance of several different scans, each of which is designed to assess different types of molecules in the body. Unlike CT, MRI scans are seldom reformatted to project oblique planes, although three-dimensional rendering may be done either on the scanner’s computer or a stand-alone workstation.
MRI does not use ionizing radiation and thus has gained rapid and wide use in pediatric imaging in human medicine. Although this is less of a concern in veterinary medicine, the ability to obtain diagnostic images without the use of ionizing radiation is desirable for veterinary personnel.
MRI scanners are extremely sensitive to the presence of certain metals such as iron and cobalt. The presence of such materials within the patient can markedly degrade the quality of the image, even to the point that it is not possible to develop an image at all. Although surgical stainless steel now in use has minimal ferromagnetic properties, it can still distort the image somewhat. Even the small amount of iron present in identity chips can produce significant artifacts on the images. For this reason, animals to be subjected to MRI scanning should have radiographs of the area of interest before being placed in the MRI scanner.
The presence of metallic foreign material in the GI tract or soft tissues can easily result in a nondiagnostic study. An exquisite example of this is the presence of a steel shotgun or BB gun pellets; the presence of even a single such pellet may totally degrade the images. Another potential source of such artifacts could be stainless steel sutures or hemoclips. Depending on their chemical composition, such materials may or may not significantly alter the images.
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, although recent improvement in scanner technology has largely done away with this limitation.
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. Because of its exquisite sensitivity to the changes in tissue organization and composition as well as density, MRI is also used frequently for joint and muscle imaging, where it has become a valuable tool in assessment of joint integrity because of its unique ability to image cartilage and ligaments. This has led to great interest in developing and promoting MRI imaging of equine extremities if the issue of motion can be overcome. Because many lesions of the equine lower limb can only be identified by MRI, such studies are commonly done despite the need for anesthesia to perform them.
Contrast enhancement of MRI scans is common when imaging the brain and other soft tissues. It can frequently permit the radiologist to make a relatively specific diagnosis regarding the etiology of the lesions seen 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 recent years, the use of MRI contrast agents has been implicated in the etiology of chronic renal disease in some human patients, and guidelines for their use have been issued by several sources. To date, this issue has not been well documented in veterinary medicine. The use of such contrast media is still considered safe in animals, although they should be used with caution in patients with preexisting renal disease.
In the past, MRI systems were large and expensive to purchase, install, and maintain, but many smaller, low-field-strength magnets are available, including some specifically designed for use in veterinary medicine. Dedicated equine extremity scanners are available, and although these instruments are relatively expensive, they are within financial reach of many large practices, especially those specializing in imaging and neurology. The use of lower field strengths will reduce the construction requirements of MRI facilities to house these instruments but comes at the price of longer scan times, reduced system flexibility and decreased image resolution.
The length of time required to complete MRI scans and the exquisite sensitivity of MRI to motion dictates that studies be performed under general anesthesia. Because powerful magnets are used, ferromagnetic material may not be brought into the room because of safety considerations. The stronger magnetic fields in large systems can accelerate something like an oxygen bottle to nearly 100 miles per hour before it impacts on the scanner. Any patient or person in the way of such a projectile could be seriously injured or killed. For veterinary patients, injectable anesthesia may be used if special anesthesia machines, oxygen tanks, and monitoring equipment are unavailable. Injectable anesthesia may not be appropriate for all patients, so facilities dedicated to veterinary patients are well advised to have appropriate anesthetic equipment. The cost of such equipment is minor compared with that of developing the MRI facility itself.
Because of their exquisite sensitivity to radiofrequency signals, MRI systems must be shielded from all extraneous signals of this type. This requires installation of specialized shielding material in the walls of the room in which the MRI is located. Further, the larger, more advanced systems with higher field strength typically require liquid helium as a coolant to minimize signal noise from within the machine itself and to maintain a superconductive magnetic field. The construction of an MRI facility must be done under the direction of a qualified architect and engineering firm.
MRI scanners should be operated by technologists specially trained in operation of these instruments. Many factors must be taken into account when preparing an animal for an MRI. 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. Having well-trained technologists to perform these studies will greatly improve the quality of the scans and promote the use of these instruments for a wider variety of imaging applications.
Because of the expense of acquiring and maintaining these instruments (especially those with field strength of 1 Tesla or above), the technical complexity of MRI imaging, and the special training and experience required to interpret the images, MRI scanning systems are generally found only in large, private and academic referral specialty practices.