Equine Diagnostic Imaging in Three Advanced Modalities

Dr. Mathieu Spriet, Assistant Professor of Clinical Diagnostic Imaging at the UC Davis School of Veterinary Medicine, specializes in magnetic resonance imaging in horses. Here, he presents three recent case studies, spotlighting three different imaging techniques: scintigraphy, computerized tomography (CT) and magnetic resonance imaging (MRI).

Advanced Imaging in the Horse

Radiography and ultrasound are commonly used in the horse. However, in more complex cases, these imaging modalities may not be sufficient to obtain a precise diagnosis or gain all the information needed to plan the best treatment for the patient. Advanced imaging modalities, such as scintigraphy, computed tomography and magnetic resonance imaging are now available for horses in some referral hospitals.

Positioning such a large animal for imaging procedures can pose a challenge. During scintigraphy and MRI, the horse may remain standing, however, CT procedures require general anesthesia. For CT, horses are anesthetized and lifted onto the table with a crane.

Scintigraphy (Nuclear Medicine)

Scintigraphy is performed by intravenous injection of a radioactive isotope, most commonly Technetium, attached to a ligand specific for some lesions. In horses, scintigraphy is most commonly performed to detect bone lesions by using a bisphosphonate ligand. This procedure is referred to as a “bone scan.”

Bone scans are used in horses for lameness of unknown origin, or when the area where the lesion is suspected cannot be adequately imaged with radiographs. Figure 1 shows a bone scan of the pelvis of a 4-year-old racehorse with right hind limb lameness. The bright area marked with an arrow represents a stress fracture of the right ilium. Following this diagnosis, the mare was rested for 6 months to allow fracture healing before returning to work on the track.

Computerized Tomography

Computed tomography is available for imaging of the head and distal limbs of the horse. Due to size limitations, imaging of the proximal limbs, thorax and abdomen is not possible. General anesthesia is required for CT examination in horses. Figure 2 is a CT image of the head of a 23-year-old Appaloosa mare presented for nasal discharge of one-year duration.

An endotracheal tube is visible in the oral cavity as the horse is under anesthesia. A large structure of fluid opacity is present with the left paranasal sinuses and nasal passages (right side of the image). There is no evidence of bone destruction but the infraorbital canal is displaced by the mass. These findings are characteristic of a paranasal sinus cyst, which was confirmed with biopsy. The CT also allowed determination of the exact extent of the cyst for surgical planning. The cyst was surgically removed and the mare recovered uneventfully.

Magnetic Resonance Imaging

Magnetic resonance imaging is the modality of choice for assessment of musculoskeletal injury, thanks to the ability to image both bone and soft tissue in any desired plane. Similarly to CT, MRI is available for extremities only, due to size limitation of the available machines. Depending on the configuration of the system used, MRI is performed either under general anesthesia or with the patient standing under sedation.

Figure 3 are transverse and sagittal MR images of the foot of an 8-year-old Warmblood gelding presented for a left front foot lameness. The arrows show a large tear in the deep digital flexor tendon, the main tendon in the horse foot. This unfortunately represents a severe lesion that requires 2 months of rest with stall confinement and progressive return to work. Horses with such lesions usually do not return to full work before 12 months. (Photo of MRI scan seen above.)