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Nuclear medicine in equine practice

The demand for advanced diagnostic imaging procedures such as nuclear scintigraphy and ultrasonography has increased dramatically over the last ten years. Veterinarians, seeking to improve their diagnos- tic capabilities, and clients willing to pursue "best medicine" have driven this demand, resulting in installation of facilities at most academic and private referral practices. Knowledge of the potential benefits of scintigraphy allows the referring veterinarian to optimize their use of nuclear medicine services.

Skeletal scintigraphy is the most commonly performed equine nuclear medicine procedure. Skeletal scintigraphy (or bone scanning) offers high sensitivity for detecting early disease, and the ease of evaluation of the entire skeleton (or a region), making it an ideal tool for screening cases of obscure or occult lameness. Scintigraphy also offers an easily quantified method for determining the activity of a bone lesion, thus allowing assessment of the significance of radio- graphically identified lesions of questionable activity, and monitoring of lesions in response to therapy.

Skeletal scintigraphy involves the intravenous injection of a radiolabeled phosphonate compound, and subsequent imaging of the distribution of radioactivity within the patient. The most commonly used radiopharmaceutical is technetium-99m labeled methylene diphosphonate (99mTc-MDP). The distribution of 99mTc-MDP is dependent upon delivery (blood flow) and uptake (osteoblastic activ- ity). Imaging at different time periods following injection allows evaluation of different physiologic processes. The initial vascular phase follows the course of the radiopharmaceutical through vessels. After distribution in the blood pool, the 99mTc-MDP rapidly distributes into the extracellular fluid space giving a soft tissue phase. The 99mTc- MDP is taken up by metabolically active bone over the next 20 minutes or so. That 99mTc MDP which is not taken up by the skeleton is cleared in the kidneys. Bone phase images are made two to three hours after injection to allow an optimal bone to soft tissue (target to background) ratio.

Vascular phase images made immediately after injection are dependent primarily on blood flow to a region. They are a sensitive method of identifying hyperemia, ischemia and abnormal patterns of flow. Unlike other methods of evaluation of perfusion, the radionuclide readily penetrates the (normal or abnormal) superficial soft tissues of the limb allowing evaluation of deep and superficial vascular supply. De- creased flow may be seen following frostbite, degloving injuries, or following wrapping of objects around the limb. Increased vascular flow may be seen in acute, active lesions with hyperemia or neovascularization.

Soft tissue phase images made 2 to 10 minutes after injection represent the distribution in the extracellular fluid which is determined by delivery to the region (blood flow), flux in and out of the local ECF space, and the overall amount of ECF in a region. Decreased activity to a region can occur with ischemia. Increased activity is commonly seen at sites of active inflammation due to increase blood flow and capillary permeability. While these changes are nonspecific, they can be a sensitive indicator of the site of a soft tissue injury such as synovitis, tendinitis, cellulitis or navicular bursitis.

Volume 17, Number 8, 1997

Bone phase images are dependent on blood flow, ECF distribution, and bone metabolic activity. While all normal bone is constantly remodeling, areas of bone injury often demonstrate increased rates of bone activity, which subsequently result in increased uptake of radioactivity visible as "hot spots" on the scan. The degree of uptake of the 99mTc-MDP is determined by the activity at the lesion. Areas of mild increased turnover occur with mild degenerative joint disease. More intense uptake occurs at sites of high bone activity such as fractures, tumors, or infection. Bone lesions typically will exhibit increased activity within 24 to 72 hours following injury, while radiographs may not demonstrate lesions for up to two weeks. Bone phase images have proven to be extremely sensitive and accurate in assessing occult lameness due to stress fractures of the tibia, humerus, radius and metacarpals/metatarsals, or in non- radiographically apparent fractures of the third phalanx. Since bone phase images characterize the metabolic activity of bone, it can provide a more sensitive indicator of navicular bone degeneration versus radiographs which are limited to demonstrating anatomic changes. Scintigraphy provides a definitive diagnosis for avulsion of the suspensory ligament origin which often remains radiographically inapparent. In addition to finding lesions which are inapparent, scintigraphy can character- ize the activity at known sites of disease or radiographic abnormality. Scintigra- phy has been used to determine the level of activity of degenerative changes at joints and the navicular bone, confirm bone involvement in sites of adjacent soft tissue infection, and monitor healing of bone injuries.

From The North American Veterinary Conference James J. Hoskinson, DVM, Diplomate ACVR College of Veterinary Medicine Kansas State University, Manhattan, Kansas

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