What is an MRI?


If your horse has to go to the vets it may have one of a number of types of scan: ultrasound scan, bone scan, CT scan, and MRI scan are the most common. Each type of scan allows the vet to create an image of part of the body, and so see inside the region where they are trying to diagnose a problem. The different types of scan use different physical properties of the body to form the image, and so show different things.
An MRI scan relies on a peculiar quantum property of the hydrogen atom. Hydrogen is a part of everything in the body; in particular it is part of the water molecule (the H in H2O) and of fat. The nucleus of the hydrogen atom has the commonly understood properties of mass and electric charge, and also another called spin. Spin causes the nucleus to wobble when placed in a magnetic field, much like a spinning top will wobble under the influence of gravity.
MRI physicists since the 1950’s have learned how to manipulate these wobbles. By synchronising many different atoms together the effect is made strong enough to detect by radio waves. Using electric coils to change the magnetic field in different places shows where exactly the signal is coming from. And the time it takes for the wobbles to build up and decay away can be used to distinguish fat from water.
The end result, from a single examination of a horse foot, is about 500 different black-and-white images showing slices in different places and different directions, with bright and dark areas relating to different fat and water content.
These images are interpreted by an experienced vet who knows where fat and water would be in the normal animal, and so can detect any abnormalities. The unusual presence of water is particularly significant as swelling and uptake of water is a common response of the body to injury.

The benefits of a standing scan

Because MRI can directly detect water it is a very powerful way to locate lesions. But the signal from the nuclei is tiny, and so has to be collected for a long time to make an image. While an x-ray might take fractions of a second, and ultrasound only a bit longer, an MRI image might take five minutes to collect and a whole examination more than an hour. During this time the animal cannot move, or the image would be smeared and fuzzy. Yet you cannot just ask a horse to stay still in the same way as you can a human patient.

The easy option, and the universal one until the invention of the Hallmarq standing MRI, was to anaesthetise the horse and place it in a modified MRI machine like the ones used in human hospitals. But general anaesthesia of a horse carries some risks, partly because the animal is so heavy it prevents blood flow in the underneath parts squashed down by its own weight, and partly because it may panic on waking up and risk causing another injury.

With the advent in the 1990’s of modern sedative drugs, and computers powerful enough to run motion compensation software, Hallmarq was able to develop a special MRI for standing horses. Today MRI is routinely accepted as the best way to see inside the hoof of a lame animal, and the lessons learned from MRIs of tens of thousands of horses have also helped vets better understand how to interpret all those other scans: ultrasound, CT and nuclear scintigraphy.

Casper Recovers Thanks To MRI Scanning

Event rider Annabelle Farrar’s horse Casper went lame after a competition in May last year.

The eight-year-old gelding showed no external signs of injury, so came to Oakham Veterinary Hospital to investigate the problem.

The vets did nerve blocks that showed the foot was the source of the pain and, when X-rays showed no abnormalities, an MRI scan was performed to investigate the soft tissues.

The MRI scan showed that Casper had a small tear in his deep digital flexor tendon, low down within his hoof.
Casper needed to have keyhole surgery to clean up the tear and an injection of ‘platelet rich plasma’ to help the tendon to heal. This treatment went well.

A repeat scan showed that the tendon had healed well. Casper is now back in full work and hoping to be back out competing this season.

Said Annabelle: “The Hallmarq Standing Equine MRI scanner has been fantastic. It revealed in detail what was wrong with Casper, enabling a full diagnosis. After the treatment it then gave the ability to do a repeat scan to see how well Casper had recovered. Giving me the confidence to push on with him, back into work.

For further information contact Hallmarq Veterinary Imaging on (01483) 877812 or visit www.hallmarq.net

Author: Features Editor

Share This Post On