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Vet's Corner

Nuclear Scintigraphy - An Invaluable Tool

Nuclear Scintigraphy, commonly known as bone scans, has become an invaluable diagnostic tool in helping to detect types of lameness that have been hard to diagnose. Standard radiographs often miss such things as small stress cracks in bones and microfractures or can't be used readily in areas like the upper hind-end region (pelvis area). Bone scans, while not yielding a definitive diagnosis, will decisively pinpoint problem spots (called hot spots) that need a closer look. Advanced technological procedures, such as xeroradiography, cat scans or MRI, can than be used to provide greater detail of that area.

Nuclear Scintigraphy uses a radioactive isotope (technetium) that is administered intravenously to act as a tracer. This tracer attaches to certain molecules in the horse's blood (phosphate molecules), which are then distributed throughout the body via the circulatory system. In a normal horse with no lameness problems, these radioactive labeled molecules are distributed evenly, and no so called "hot spots" show up on the computer screen. A hot spot refers to a concentration of the radioactive tracers, which occurs where there is active bone remodeling or inflammatory disease present. A vascular phase (technetium distributed through blood vessels) and a soft tissue phase (muscles, tendons and ligaments) occur soon after IV injection, and about two hours later, a bone phase occurs with adherence or absorption of technetium to the bones. All of these phases can be examined at the right time post injection.

A large gamma camera, sensitive to gamma ray emissions of the radioactive isotope tracer is used to capture the images of a horse's whole body or what ever part is desired, which is then interpreted by a nuclear computer. The computer produces what is called a scintigram, which shows the hot spots, areas of increased activity, as red areas on the film. "Cold spots", or areas of decreased activity, are also investigated.

Even subtle changes in bone can be detected by comparing scintigrams of opposite legs and then noting asymmetrical differences in tracer distribution between the two legs. Ideally, veterinarians would like to refer to a bone scan after having localized a lameness to a region, but were unable to come up with a definitive diagnosis. Standard radiographs, ultrasound, nerve blocks, thermography and other modalities have already been used. More often than not, in cases like this, the bone scan findings will correlate with previous history and clinical findings and yield the final piece of the puzzle. Sometimes in practice however, bone scans are frequently used to find a totally vague or non-isolated lameness, and cases like these often end up with non-applicable results.

There are some nuclear scintigraphy cases that yield hot spots, but further diagnostic evaluation with radiographs, etc, still yields no distinguishable findings. In these cases, the diagnosis is made from the scintigrams, as we sometimes see in such problems as small stress cracks in the coffin bone. These fractures often do show up much later down the road, when follow up radiographs are taken. Another beneficial use of scintigraphy is for fractures that occur in the small bones of the hock joint, which invariably rarely show up on standard radiographs.

Occasionally we will find a lesion that show poor radioactivity, but this is not very common. Interpretation by a skilled radiologist is obviously very important.

Another application for bone scans, which is not frequently used, is the presentation of a clinically sound horse that a perspective buyer just wants to know about before he buys it. This screening tool is not one that is recommended by most veterinarians. Unblockable or multiple limb problems are often best figured out using bone scans. Pelvic injuries are often very difficult to diagnose because of difficulty in radiographing, and nuclear scintigraphy can be a very viable alternative in these cases.

Nuclear scintigraphy exposes horses to less radioactivity than a series of radiographs, with the technetium passing completely out of the horse's system in 24-36 hours. Scintigraphy has no residual side effects, no reported allergic reactions, nor does it affect present or future fertility. It is however not recommended for pregnant mares or exposure to a fetus.

Bone scans are expensive technology ($250,000 and up) that is mostly found in academic institutions like the Atlantic Veterinary College and usually require an overnight stay so the horse becomes non-radioactive. The cost ranges from about $240 for hind or front limbs, and about $400 for a total body scan. Not that bad a price for all this technology when answers are necessary.