by Dr. Justin High, DVM
Technology touches every part of our lives. It cools our houses, helps us run faster, gets us better fuel mileage, and ensures there is always a song we like on the radio. When technology is applied to horses, the best application in a long time has been the use of MRI in the diagnosis and treatment of lameness.
Sure, there are many other advancements in recent years that are indispensible. Digital X-rays, ultrasound, and the use of micro-cameras in endoscopes and arthroscopy are great examples. X-rays are ideal for bone imaging, but are very limited in soft tissue. Ultrasound is best used in soft tissue, but cannot penetrate bone. Scopes the size of 18-gauge needles can look into joints, but not inside bones.
This is why MRI is at the top of the "Diagnostic Food Chain." For all intents and purposes, nothing can hide from an MRI if it can fit inside the machine. So far, only the lower legs (knees and hocks down) and the head/first few cervical vertebrae will fit inside the aperture of the magnet. Exceptions would be some newborn foals/miniature horses. But, since the lower limbs account for a large part of lameness cases in horses, MRI is becoming much more common in equine practice.
When your veterinarian can isolate with nerve blocks a certain part of your horses foot or lower leg as a source of lameness, but other conventional technologies we've just mentioned are unable to image the lesion/cause of the lameness, the next step in the process is an MRI. In other words, you know there is something there, but you cannot definitively demonstrate what is affected. Unfortunately, this is more common than you might think.
So, how does an MRI work? How can it look into bones, see through soft tissue and still provide 360-degree views of the bone? Well, the mathematics goes over my head in a hurry. That's why I'm a vet and not a physicist. So, here is a functional, but basic look at what makes MRI different than other imaging technologies.
As you may know from others, or firsthand experience, magnetic resonance imaging has been around for some time now. The primary magnet, gradient magnets, and a coil compromise machines currently used. MRI uses the properties of hydrogen atoms to distinguish between the various types of tissue in the body such as bone, muscle, fat, tendons, etc. Atoms within the body have a certain spin to them. Basically, one direction or the other. This spin orientation is also called a precess. When a horse has a limb in the MRI machine and the magnet is turned on, a part of the machine called the coil emits a radiofrequency that causes the atoms to change their spin direction.
As the molecules transition from one spin direction to the other, the energy that is released is what creates the signal to create an MRI image. The exchange of energy between spins is called resonance, which is created by a magnetic field; therefore, magnetic resonance imaging. Important components of the machine refine and specifically target segments of anatomy being imaged, so as the computer interprets the data, it creates images that depict the difference between tissue types with varying shades of gray. Diseased bone tissue looks different than healthy bone tissue. Tears in tendons and ligaments appear differently than normal tissue fibers. Even bone that appears normal to the eye and X-ray will show bruising or swelling on an MRI that would otherwise be unrecognizable.
Hopefully, you will not need an MRI for your horse, but it's good to know it's there if you need it. As time and technology progress, the cost will likely come down as well, but for now it is a detailed and highly specialized undertaking that is well worth the time and money invested if it gets the answers you're looking for. I'm just waiting till they figure out how to look at stifles. Then you'll really see some advancements.