Equine scintigraphy: basic principles and interpretation
Abstract
Bone scintigraphy is the only imaging technique able to demonstrate organ function rather than anatomical details. It involves intravenous injection of a radiopharmaceutical that binds to the bone structures and display alteration in osteoblastic activity. It is commonly used in horses presented for lameness investigation or poor performances, although the sensitivity of the technique in identifying the lesion causing lameness in poor performing sport horses was demonstrated to be low. Conversely, scintigraphy shows high sensitivity in detecting remodelling related to stress-related bone injury and stress fractures, which often occur in racehorses. It must be remembered, however, that the presence of increased radiopharmaceutical uptake does not necessarily indicate a lesion causing pain, and lameness and scintigraphy should be always used in conjunction with in-depth lameness investigation and, when possible, diagnostic analgesia.
Scintigraphy is a procedure that involves the use of a radioactive material labelled with a tissue-seeking pharmaceutical. Based on the pharmaceutical used, the radioactive material is carried to different organs and will display the activity of the target organ. The radioactive material decays with production of gamma radiation that is captured by a gamma camera and processed to obtained a visible image. Scintigraphy is the only imaging technique that is capable of evaluating organ function/physiology rather than anatomy. However, it has poor spatial resolution, and therefore specific anatomical details can be lost.
For bone scintigraphy, commonly known as a ‘bone scan’, the radioactive material used is 99mtechnetium; this is attached to diphosphonate salts which selectively carry it to the bone. Pharmaceuticals used are mytilene diphosphonate (MDP) and hydroxymethylene diphosphonate (HDP). The diphosphonate salts bind to hydroxyapatite in the bone and display areas with increased osteoblastic activity.
The radiopharmaceutical is injected intravenously before image acquisition, usually through a catheter placed in the jugular vein. It has been demonstrated that exercising the horse before injection of the radiopharmaceutical increases the blood flow to the distal limb, improving the uptake in this region (Dyson et al, 2001) and therefore the horse is usually lunged before injection, unless clinically contraindicated. A dose of 1 MBq per 100 kg is recommended to obtain diagnostic images, and the horse should remain confined for 24hours after injection until most of the radioactive material has been eliminated through the urine or decayed.
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