Standing Equine MRI
Standing equine MRI (magnetic resonance imaging) is a recently introduced diagnostic modality in equine diagnostic imaging. MRI allows simultaneous examination of both bone and soft tissue structures in three dimensions. Standing equine MRI has revolutionised the diagnosis of causes of foot lameness in the horse. When a horse undergoes standing MRI either a forelimb or hind limb is placed into a magnetic field and pulses of radiofrequency are applied to the area which results in a signal within the tissues (also in the form of radio waves) that is received and analysed by a computer which generates an image.
Since 2009 the Equine Referral Hospital has been equipped with a low-field (0.27 Tesla) standing equine MRI scanner that allows examination of areas within the distal limb (from the foot to the carpus or hock) in the standing sedated horse. Before the introduction of standing equine MRI scanners, MRI examination of horses was only possible in a few selected referral centres around the world that were using conventional human MRI scanners. Standing equine MRI examination using open magnets has several advantages over the use of conventional human scanners which would require placing the horse under general anaesthesia. As far as currently known, standing MRI examination is safe in horses and it does not involve exposure of patients and personnel to ionizing radiation (such as radiography, computed tomography (CT) and nuclear scintigraphy). The shoes need to be removed before the limb is placed in the magnetic field as metal interferes with image quality. A standing equine MRI scan can take between 1 to 3 hours, depending on the area being examined and the temperament of the horse.
Standing equine MRI examination is not suitable for examination of large areas on the horse's limb. Standing equine MRI has specific indications for differentiation of different types of lameness in the horse. The area of the limb to be examined therefore usually needs to be identified with a thorough lameness examination (usually by means of nerve blocks) prior to scanning. One of the most useful indications for standing equine MRI is the examination of the foot. Previous work has shown that many horses with chronic forelimb lameness originating from the feet can have soft tissue injuries that cannot be accurately demonstrated with other imaging modalities such as radiography, ultrasonography or nuclear scintigraphy (bone scan).
Examples for such lesions that can be identified with standing equine MRI include tears in the distal part of the deep digital flexor tendon or lesions in one of the ligaments in the foot. Standing equine MRI is also a very sensitive imaging modality to identify early pathological processes in bone that would not be visualised using other techniques.
T1-weighted 3D sagittal MRI scan of the foot of a horse: There is a moderately well-defined area of increased signal intensity within the dorsal aspect of the distal articular surface of the middle phalanx (orange circle). These findings are consistent with a subchondral bone cyst (SBC).
Motion insensitive transverse (left) and sagittal (right) MRI scans of the left distal tarsus of a horse. The transverse section through the central tarsal bone demonstrates a comminuted fracture with three distinct fragments in the dorsal aspect of the bone (red circle). The sagittal scan reveals a fracture line through the central tarsal bone extending from the proximal to the distal intertarsal joint (red circle).
The Benefits of Standing Equine MRI
- Standing equine MRI provides high quality images of bone and soft tissue
- Standing equine MRI is the only imaging technique that distinguishes between many causes of foot pain
- Standing equine MRI does not require general anaesthesia
- Standing equine MRI facilitates accurate diagnosis, prognosis and treatment
What is the procedure for Standing Equine MRI?
- Before the standing equine MRI examination the horses shoes are removed
- Standing equine MRI is performed under light sedation to ensure that the horse stands still during image acquisition
- The MRI scanner is carefully aligned to the area of injury
- Many images are acquired including comparative images of the opposite limb
- The MRI examination is completed and the horse recovers from the sedation
- A detailed MRI report is compiled and sent to the Referring Veterinary Surgeon and to the owner.
| See Guidance Notes for Owners: Standing equine MRI |