Computed tomography angiography

Computed tomography angiography

MRI technology evolves as a diagnostic tool; CTA research underway at UC-Davis to study foot injury, lameness
Sep 01, 2004

In a recent paper discussing magnetic resonance imaging (MRI) in live horses, Susan Dyson, DVM, Center for Equine Studies, Animal Health Trust in Newmarket, United Kingdom, notes, "Foot pain is a common cause of equine lameness, and there have been significant limitations of the methods available for the diagnosis of the causes of foot pain (radiography, nuclear scintigraphy and ultrasonography)."

Radiography is limited to assessment of mineralized tissues, and a 40-percent change in bone density is necessary to identify variations, so subtle abnormalities might be missed. Nuclear scintigraphy is highly sensitive, but false positives are fairly common. Ultrasound is limited by the anatomy of the foot, and interpretation might be confounded by the inability of its application.

CTA scans are done in about 30 minutes, while most facilities doing MRI are taking between one and two hours. The CTA technique combines the use of X-rays with computerized analysis of the images.
In her comparison to these other modalities, Dyson concludes that MRI has several advantages over these other procedures: It is adaptable; it provides three-dimensional images; the entire foot is accessible, and it provides physiological information that is anatomically specific. Additionally MRI provides cross-sectional imaging and high sensitivity for alterations of the actual composition of the soft-tissue structures.

According to Tim Mair, DVM, Bell Equine Veterinary Clinic in Kent, United Kingdom, the high soft-tissue contrast afforded by MRI makes it ideal for assessment of articular cartilage, ligaments, tendons, joint capsules, synovium and bone marrow. Their system was the first one in the world performing MRI on standing horses. In addition to the equipment expense, the limitations of MRI for non-standing, recumbent horses, are lengthy scan times and the concerns of general anesthesia. With MRI, positioning the horse for imaging is labor-intensive, though the image acquisition is not. Also, MRI image analysis is time consuming and requires considerable experience.

Optimization of the equine limb MRI scanner is still in progress, but to date, it has proved capable of producing diagnostic images of the foot and fetlock of standing horses.

"We believe this technique could revolutionize the assessment of certain musculoskeletal lesions of the distal limb," Mair suggests.

The ability to perform MRI in the standing equine patient using the open U-shaped magnet is a considerable advantage over conventional closed magnets that require general anesthesia and fairly lengthy scanning times, up to one to two hours. There are several of these standing-horse units recently available and being used in Europe and the United States. However, the open magnet is, by necessity, of lower field strength, which results in a lower signal-to-noise ratio, which in turn results in lower resolution or longer imaging times for the same resolution as a higher-strength magnet. In addition to permitting use in standing horses, this equipment has advantages to the veterinary profession in terms of lower initial cost and lower operating costs.