MRI physics and its applications

Magnetic resonance imaging (MRI) results from the effect of magnetic properties of atoms found in biologic tissues. The magnetic moment of the nuclei within these atoms interacts with the external magnetic field, the most common of which is hydrogen. Hydrogen is abundant in all tissues and has a relatively large magnetic moment, making it the optimal choice for magnetic resonance imaging.

When an animal is placed in the bore of a magnet, all the available hydrogen atoms that are normally randomly oriented align with the external magnetic field. At this time, a short-duration radiofrequency pulse is applied to the area of interest, and the hydrogen protons are disrupted. This results in energy release that is captured as a signal, called an echo. Images are obtained from the multitude of collected signals. A wide range of pulse sequences can be created to highlight specific tissues, and there is no limit to the number of different slice planes that can be requested. Magnetic resonance imaging allows the collection of specific, thin cross-sectional, gray-scale images of any area of interest that is within the bore of the magnet, and it is sensitive enough to provide information at the molecular level, not just at the anatomical level.

The high-field system of MRI can acquire images 1 mm thick, providing an in-depth view of the anatomy, even for small structures. The field strength of the magnet is in units of Tesla. In the equine patient, MRI images can be obtained at the carpus and tarsus, and distally. The high-resolution images permit detection of structural and physiologic alterations within bone and soft tissues before they can be detected by most other available imaging modalities.

"Structural injuries may develop more frequently in horses than was previously suspected; however, these injuries may go unrecognized on radiographic and ultrasound evaluation," says Chad Zubrod, DVM, MS, Dipl. ACVS, Oakridge Equine Hospital, Edmond Okla.

High-field strength versus standing unit

Unfortunately not everyone has the capability to use the high-field or down magnet (down because the horse has to be anesthetized). The lower-field strength (0.3 Tesla) standing unit can give you a similar diagnosis, but it requires a little bit of effort to get a good image or sequence. It requires a good MRI technician, training and perhaps the hardest part, a still animal. There is some image correction software and other technical innovations soon to be available to improve its capability. There are several people with the low-field units that use consultants to read their images.

"It's not that easy to read an MRI study, and it requires definite time commitment to accurately evaluate and interpret the scans. It is very important to read the images properly for the final report, but applying this information to the horse should be done in conjunction with the clinical examination and the results of any other imaging tools that might have been involved," says Rich Redding, DVM, MS, Dipl. ACVS, North Carolina State University College of Veterinary Medicine. "I think the general practitioner who refers a horse for MRI should be comfortable that the facility has a certain level of experience reading MRI exams. They need someone with considerable education in MRI and experience reading the images. We want to make sure that MRI does not get a bad name because of a missed or inaccurate diagnosis, first and foremost."

Standing MRI units can be a great screening tool to pick up fairly obvious lesions, but they may miss some of the subtle lesions.

"My greatest concern is that the lower-field systems might not be able to perceive some of the changes that a high-field strength system will be able to pick up," Redding says. "It would be nice to know what you are able to see with the standing magnet, what are its limitations, what lesions may potentially be missed and how certain lesions may appear while the horse is standing versus off-weighted."

But clinicians are learning how to use leading MRI technologies with increased efficacy and efficiency. Some facilities are going so far as to anesthetize horses with the standing unit to eliminate the motion artifact.

"But the main advantage of the standing unit over the down unit is that you don't have to anesthetize them," Redding says. "It might not be realistic depending on the availability of a high-field strength unit, but if you're going to anesthetize them, you might as well put them in a high-field magnet."