Magnetic resonance imaging (MRI) is the newest form of imaging in general use. 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; however, 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 element (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 3-dimensional table, as is done with CT. This 3-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 dataset that is stored permanently on the picture archiving and communication system (PACS). The actual scan dataset is not usually stored (except in a research setting) because of the much larger size and proprietary nature of these datasets.
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 can 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; historically, only one set of planes was acquired at a time, but modern scanner technology allows acquisition of volumetric (3-dimensional) datasets.
Scans are typically acquired in more than 1 of the 3 orthogonal planes, with different pulse 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 3-dimensional rendering can 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 for veterinary patients, 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 ferromagnetic material 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 microchips 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.
Pearls & Pitfalls
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The presence of metallic foreign material in the GI tract or soft tissues can easily result in a nondiagnostic study. An example of this is the presence of a steel shotgun or BB gun pellets; the presence of even a single pellet can totally degrade the images. Another potential source of such artifacts could be stainless steel sutures or hemoclips. Depending on their chemical composition, such materials might or might not substantially alter the images.
Interpretation of MRI, like the previously discussed imaging modalities, requires a firm knowledge of 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 improvement in scanner technology has essentially eliminated this limitation.
MRI does not image cortical bone as well as CT, although it is quite useful for 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 in small animals.
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 unique 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 the equine distal limb.
Because many lesions of the equine lower limb can only be identified by MRI, these studies are commonly performed (see equine midsagittal MRI and transverse MRI). Both standing systems of a lower magnetic field strength and recumbent systems, of both low and high magnetic field strengths, are used. Standing MRIs can be done with sedation, while recumbent MRIs require the use of general anesthesia. Image quality is significantly improved with the patient under general anesthesia due to the minimal movement compared to standing systems.
Some hospitals are able to adequately image the equine brain with MRI.
Courtesy of Dr. Jimmy Lattimer.
Courtesy of Dr. Jimmy Lattimer.
Contrast enhancement of MRI scans is common when imaging the brain and other soft tissues, particularly in small animals (see midbrain mass image). It can frequently permit the radiologist to make a relatively specific diagnosis regarding the etiology of the lesions evident 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.
Courtesy of Dr. Jimmy Lattimer.
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.
Historically, MRI systems were large and expensive to purchase, install, and maintain; however, 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. Lower field strengths will decrease the construction requirements of MRI facilities to house these instruments but come at the price of longer scan times, decreased system flexibility, and decreased image resolution.
The length of time required to complete MRI scans and the extreme sensitivity of MRI to motion dictate that small animal studies be performed under general anesthesia. Powerful magnets are used, and therefore ferromagnetic material cannot 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 can 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 inherent 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 the typical veterinary or veterinary technical curriculum and often must be acquired by attending special training sessions or preferably as part of the training program in a school of radiological 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 or academic referral specialty practices.
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Also see pet health content regarding magnetic resonance imaging.
