HomeExpert AdviceArticleGetting to the bottom of MRI

Magnetic resonance imaging, or MRI, is an incredibly useful diagnostic tool that, following many years of successful use in human medicine, is also available to vets for use in equine medicine. Find out how MRI could help your horse should the worst happen…

Hallmark equine MRI

Image credit: Image Courtesy of Oklahoma Equine Hospital, USA

We all dread that feeling – a drop in performance or a traumatic incident that means you need to call out your vet to look over your now-lame horse. It’s every owner’s worst nightmare. Some cases simply require a bit of time off – a little well-deserved rest and relaxation and you’re ready to bring your horse back into work again. Unfortunately, however, it’s not always so simple.

Equine anatomy is incredibly complicated. When it’s working well the lower limb performs exceptionally, allowing your horse to operate at speed, jump and perform complex dressage movements. In order for him to carry out this wide range of movements, his lower limbs and feet are made up of a large number of bones to enable maximum flexibility and range of motion, ligaments that connect bone to bone and tendons that connect bone to muscle. However, the complicated nature of how all these integral areas of anatomy function within the leg means an almost limitless number of problems can occur, some more common than others.

When investigating a lame horse, the first steps your vet will perform include a clinical examination and watching him move. Once the affected limb, level of lameness and any obvious signs of trauma have been assessed, it’s common for your vet to perform further diagnostic tests. These can include flexion of the affected limb and nerve blocking – injecting local anaesthetic into the joint or soft tissues around certain nerves to target specific areas. This helps pinpoint the area causing pain or lameness and allows further investigation. Commonly, diagnostic imaging is then used to give a diagnosis or determine the cause of the lameness. Similar to human imaging, diagnostic imaging for horses may include the use of X-ray or ultrasound and advanced imaging may include computed tomography (CT) or MRI.

MRI is a type of advanced diagnostic imaging. It’s not appropriate for every lame horse, but where the vet’s investigation identifies the source of pain, but no abnormalities can be seen on X-ray or ultrasound, or where the source of pain is within the foot, MRI can be very useful. MRI uses a magnetic field and radiofrequency pulses to create multiple images of a horse’s leg. One MRI exam can produce in excess of 100 pictures or images. Research tells us that, in many cases, horses who undergo MRI when the lameness is in the early stages, rather than once it’s become chronic, are more likely to make a quick recovery. This is most likely because an accurate diagnosis is reached faster, and appropriate treatment started sooner.

Most horses tolerate standing MRI well, though the equipment needs to be operated in a specialist environment so your horse will have to travel to a clinic with MRI equipment. Shoes are removed so they don’t interfere with the magnet while it’s working to create the images and the whole process takes between 1–4 hours. The horse needs to stand still, so some sedation is commonly given, though the whole process is completely painless, and the horse can normally travel home the same day.

Horse's leg in MRI magnet

Image credit: Image Courtesy of Oklahoma Equine Hospital, USA

MRI can give us information about bones, soft tissues and fluid all in one image. It can detect subtle lesions that might not show up on other types of imaging and is more sensitive to bony changes, making diagnosis faster. X-ray is commonly used to assess bony problems but isn’t as useful for helping your vet assess soft tissues such as tendons. Bony changes often need to have developed for several weeks before they’re visible on X-ray. Ultrasound can be more useful for examining soft tissues including tendons, but it’s unable to give much information about bony structures. X-ray and ultrasound only allow the vet to view two-dimensional images of the horse. There’s no method to add images together or use a computer to create a three-dimensional image in the same way as more advanced imaging is able to. In addition, if the vet wants to assess the ligaments or tendons within the foot, ultrasound can’t be used because it can’t see through the hoof.

Although both these diagnostic methods are incredibly useful, they do have limitations. This is where MRI is invaluable. MRI gives us the ability to assess all structures within the limb and, most importantly, within the foot. MRI can be used to look at anatomy from lots of different angles, giving the vet the best chance of finding even the smallest of problems – it’s often injuries affecting a small area of the leg that cause some of the most significant lamenesses. For example, tearing of the deep digital flexor tendon occurring in the section of tendon within the foot can be identified, even if the tear is only a few millimetres in size.

Image of horse leg sagittal view and labelled view

Image credit: ©Hallmarq Veterinary Imaging Ltd 2020 

Case study

A 10-year-old event horse sustained a traumatic injury to a forelimb by clipping a jump while cross-country schooling. He was mildly lame immediately after schooling, but markedly lame within 48 hours and swelling developed over the front of his pastern.

Initially, the horse was rested, and cold therapy and anti-inflammatories given. However, even after the swelling had reduced, lameness persisted. The horse improved when a nerve block was performed to numb the area where the swelling had originated from. X-ray showed no problem that could be the cause of the lameness, so MRI imaging was performed. The MRI showed severe bone bruising, or oedema, correlating to the area where the swelling was initially seen.

With the additional information that only MRI was able to provide, the vet was confident to advise a prolonged period of box rest before bringing the horse back to work, which resulted in a full recovery. This case is a great example of the value of MRI in diagnosing a condition that can’t be identified using routine imaging methods.

Image of Bone bruise (oedema) and image of normal leg for comparison

Image credit: ©Hallmarq Veterinary Imaging Ltd 2020 

To learn more about equine MRI, visit hallmarq.net

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