MRI

Taking A Deeper Look : Magnetic Resonance Imaging
At The UMN Veterinary Medical center
Dr. Annette McCoy, Intern, Large Animal Medicine & Surgery, UMN
Dr. Travis Saveraid, Assistant Clinical Professor, Medical Imaging, UMN

One of the UMN’s upcoming changes is the opening of a newly-expanded imaging center in Fall 2007, to be located on the west end of the current large animal hospital.  In addition to computed radiography, ultrasound, nuclear scintigraphy (“bone scanning”), and the existing computed tomography (CT) unit, the expanded imaging center will include a state-of-the-art Magnetic Resonance Imaging (MRI) machine. 

What is MRI?
As the name implies, MRI uses a strong magnetic field to image parts of the body, usually limbs and heads in the case of horses.  It takes advantage of the natural magnetic properties of hydrogen atoms in the body to create an extremely detailed image or “picture” that can differentiate among soft tissues, bone structures, and body fluids. 
The magnetic field strength of the system determines the types of images that can be acquired, the clarity and detail (resolution) of the images, and how quickly the images can be acquired.  Magnetic field strength is measured in a unit called the Tesla (T).  1.0 T, 1.5 T and 3.0 T systems are all called “high-field” magnets.  Systems with magnetic field strength below 1.0 T, such as the 0.3 T “standing” magnets found in a number of equine clinics, are called “low-field” magnets.  Simplistically, a 3.0 T system has twice the speed and resolution of a 1.5 T system. 

All MRI systems are not the same
A common belief is that all MRI systems make the same quality images.  In reality, just like your backyard Shetland pony can’t compete in a race against a Thoroughbred, not all MRI systems are the same.  Currently, there are both “low-field” and “high-field” MRI systems in use.  High-field magnets are far superior to low-field magnets in both overall image quality and in speed of acquisition.   Standing MRI systems are usually low-field magnets, with associated limitations in image resolution.  High-field systems are more complex and require general anesthesia, but have superior resolution.  The UMN-VMC system will be a high-field system that will provide very high quality images.

Why do we need MRI?
MRI can provide information that cannot be obtained by any other imaging technique currently available at the UMN-VMC.  Its image clarity and detail are unparalleled.  Also, unlike radiography and CT scans, MRI does not use potentially harmful radiation.  Using MRI will allow doctors at the UMN-VMC to make diagnoses in horses that were previously difficult or impossible to confirm, enhancing their ability to recommend appropriate treatment options for their patients.  For example, in the field of equine lameness, MRI has begun to sort out many poorly understood syndromes, such as those grouped as heel pain or navicular syndrome.

What kind of cases may benefit from MRI?
There are many kinds of cases for which MRI is an excellent diagnostic tool.  It is the best imaging tool for evaluating injuries to all joint structures and soft tissue structures such as tendons and ligaments.  Chronic lameness that has been localized to a general area, but for which an underlying cause cannot be determined using traditional methods, can often be diagnosed using MRI.  MRI can also be used to diagnose diseases of the brain, nasal sinuses, and guttural pouch.  It has been invaluable for evaluating pain in the foot previously hidden from diagnostic imaging by the hoof wall.

MRI images of a horse's foot. These are part of a series of dozens of images. The image on the left is a slice through the foot parallel to the ground while the image on the right is perpendicular to the ground. The MRI images can be altered to emphasize or deemphasize different types of structures in the foot (fluid, fat, etc.) to enhance contrast and better identify lesions.

When may MRI not be the best choice as a diagnostic tool?
Like any diagnostic tool, MRI is not an appropriate choice for every case.  The unit is not designed to image the chest or abdomen of a full-sized horse, limiting its usefulness for diagnosing disease in these body cavities.  Likewise, MRI may not be necessary in cases where the problem is immediately obvious on radiographs (x-rays). An MRI exam is best performed to identify clearly the exact source of an injury after the injury has already been localized to a given region, such as the foot.

Are there any risks in performing a MRI?
There are no known side-effects of MRI in horses.  Like all high-field MRI systems, however, the MRI unit that will be installed in the UMN-VMC will require that patients are lying down, which means that horses will need to be placed under general anesthesia for the procedure.  As with any procedure requiring general anesthesia, there are risks associated with the drugs and with the recovery period.  However, VMC board-certified anesthesiologists have extensive experience with equine anesthesia and do everything possible to minimize the risks.  The anesthetic experience is similar to, and often shorter than, the anesthesia period required to perform surgeries. 
Low-field MRI systems do exist that allow images to be taken in the heavily sedated, standing horse  -  hence, the term “standing MRI”.  More recently, however, some hospitals with the low-field systems are performing even these exams with the horses under general anesthesia in an attempt to improve image quality and shorten the time of the scan.

How much does MRI cost?
Cost for a high-field MRI exam at the UMN-VMC will depend largely on the body part(s) being imaged.  The total price, including anesthesia, will likely be close to $2000.  By comparison, taking a full set of traditional radiographs of a single joint costs approximately $220, and a full lameness exam for a lameness of unknown origin with nerve blocks, radiographs, and ultrasound can cost upwards of $1200.  The use of MRI represents a modest investment, and the diagnostic value of the results cannot be matched by other techniques.