MRI, CT, Bone Scan: What’s The Difference And How Do They Help Horses?

Modern technology has given equine medicine a menu of diagnostic options — digital X-ray, nuclear scintigraphy, magnetic resonance imaging (MRI), thermography, ultrasonography, and most recently computed tomography (CT). In Part One of this two-part series, each of these modalities will be explained; in Part Two, Dr. Larry Bramlage, orthopedic surgeon at Rood & Riddle Equine Hospital in Lexington, will offer his advice to horsemen who want to take best advantage of this technology.

Digital X-rays

Digital X-rays show cracks, fractures, or other abnormalities in bone. Analogous to film versus digital photography, this computerized form of X-ray enables the veterinarian to manipulate the image to enlarge, sharpen, and lighten areas that are difficult to read. This reduces the likelihood of missing subtle changes that might not be discernible on film X-rays. Special software helps the veterinarian interpret the results.

Nuclear Scintigraphy

Nuclear scintigraphy (commonly called “bone scan”) detects abnormal bone-remodeling activity by showing the level of blood flow to areas of the skeleton. The horse is injected with an intravenous radioactive isotope that attaches to molecules in the blood. A special nuclear camera scans the horse to show areas of increased blood flow and bone activity as red, while areas of diminished blood flow and bone deterioration are black. A whole-body scintigraphy scan takes about two hours, during which the horse must be heavily sedated.

MRI

MRI uses the body’s natural magnetic properties to produce detailed images of soft tissue and bone. Each pulse of the magnetic field elicits a unique response from individual tissues, based on density and water content, and captures a “slice” view of the structure being imaged. Software interprets the series of slices to produce an image. For lameness, MRI is limited to knees, hocks, and the lower limb. The modality also is capable of “seeing” inside the hoof capsule, making it especially useful for diagnosing foot ailments. A scan of a specific area, such as a fetlock, takes about an hour.

MRI units are available in two configurations: the standing or open MRI, which requires the horse to be sedated so it will stand quietly in the unit, and the more powerful closed-magnet unit, for which a horse must undergo general anesthesia and be conveyed into the unit for the imaging procedure. The strength of MRI units is expressed in Teslas, with the highest number indicating the most powerful-field unit.

Thermography

Thermography uses a thermal-imaging camera to present a pictorial representation of the skin-surface temperature of the horse to indicate alterations in circulation in deeper tissues. Thermography is ideal for diagnosing certain lameness conditions of the upper hind limb, back, and neck. In his extensive work with thermography, Dr. Tracy Turner, a board-certified surgeon at Anoka Equine Veterinary Services in Elk River, Minn., claimed he was able to uncover a problem two weeks before clinical signs appeared. Sedation to get the horse to stand quietly may be necessary to obtain good images, which are as quick as the click of the shutter.

Ultrasonography

Commonly called ultrasound, this modality is used principally to identify the cause of heat or filling associated with soft-tissue problems such as muscle, tendon, and ligament injuries. It also can be used to locate abscess pockets and to identify drainage tracts from infection or foreign objects. More recently, ultrasound has been used to determine if joint fluid is clear or cloudy, which indicates bleeding or infection, or to characterize thickening of the joint membrane.

Ultrasound uses high-frequency sound waves to generate a computer image of the underlying structures. The veterinarian moves the ultrasound wand over the area being investigated while viewing the image on the display screen. If the horse is fractious, sedation may be necessary to obtain a good image.

Computed Tomography (CT)

Sagittal (A), dorsal (B) and transverse (C, D) fused PET/CT images of the right carpus of a 2-year-old Thoroughbred racehorse. There is marked increased radiopharmaceutical uptake at the distal medial aspect of the radial carpal bone and proximal medial aspect of the third carpal bone.

CT is the most recent addition to the diagnostic armamentarium for horses and one that promises exciting applications. Presently, CT is used primarily to detect changes in bone before they are visible on X-rays. As the technology advances, developers expect to be able to use CT to image soft tissue and organs.

The CT scan produces “slices” of the anatomy that can be manipulated to view the target area from different views and even a three-dimensional image. Traditional CT requires the horse to undergo general anesthesia and lie in the unit.

Standing CT, the type introduced at the University of Pennsylvania’s New Bolton Center, requires only mild sedation while robotic arms are maneuvered around the standing horse to image the target area. A scan takes less then one minute. Motion-correction software adjusts for the typical sway of a sedated horse so it doesn’t produce artifacts, referred to as “fog.” The combination of a fast scan and the motion-correction software reduces errors. Ultimately, the goal is to develop the capability to scan CT images of a running horse on a treadmill.

To learn Dr. Bramlage’s advice for horsemen who want to use high-tech modalities to detect potential breakdowns, look for Part Two of this feature tomorrow.

 

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