The basic principle of nuclear scintigraphy is the detection of gamma rays, emitted from the decay of a radionuclide, by a gamma camera. When the radionuclide (Technetium 99 is typically used) is attached to a specific pharmaceutical compound, a graphical representation of the physiological function, shape, size, and position of the target organ can be made. The clinical information obtained from the images depends on the biochemistry of the pharmaceutical, its interactions with the target organ specifically, and its transport through other tissues and organs.
Equine scintigraphy is typically employed for the study of bone inflammation by using the radiopharmaceutical Technetium 99-MDP (Methylene Diphosphonate). An inflammatory condition of the bone will cause an increased radiopharmaceutical uptake in that region, therefore, the camera sees an increased amount of gamma ray being emitted from the decay process and builds an image that reflects these areas of inflammation.
While scintigraphic images give anatomical information about the target organ, the spatial resolution is poor compared to other imaging modalities such as radiography, magnetic resonance imaging (MRI), and computed tomography (CT).
The main information obtained from the scintigraphic images is based on a physiological process of the target organ. For example, in bone scans the turnover of bone and blood flow to the bone are assessed, and in renal scans the glomerular filtration rate, or effective renal blood flow can be assessed, depending on the radiopharmaceutical used. This is a fundamental principle when interpreting the images formed — although the anatomy of the organ can be appreciated, it is not typically the anatomy of the organ, but rather a physiological process associated with that organ that is being investigated. Therefore, as with all imaging modalities, scintigraphy should never be used in isolation for diagnosis, but in conjunction with a full clinical evaluation and other imaging or diagnostic modalities as required.
It is commonly agreed that full body indiscriminate skeletal scanning in the horse without clinical guidance is of little or no diagnostic value and leads to unnecessary exposure of personnel to radiation.
In the horse, scintigraphy has been used predominantly for the detection of bone pathology, particularly stress fractures and entheseopathies. In the human field the bone scan was considered the “gold standard” for the detection of stress fractures until the advent of MRI.
With equine scintigraphy, the improved ability to diagnose stress fractures has been of great benefit in reducing the mortality rate by enabling quicker, more effective treatment to be administered which will prevent the stress fracture from further developing into a catastrophic fracture.
Scintigraphy is not only more sensitive in the detection of stress fractures than radiography, but it also allows imaging of sites in which radiography has limited capability, such as the pelvis and spine.
The sensitive nature of this imaging modality also assists practitioners in obtaining an accurate diagnosis in lameness cases with multiple affected areas or those that are difficult to understand using traditional techniques.
Gross inflammatory conditions of the appendicular and axial skeletal bone can be assessed. Fractures, Enthesiopathies, Osteoarthritis, and Calcified Soft Tissues are some of the conditions this modality is used for. Sinus and dental disease can be imaged in the skull as well. Non-blockable lameness is also an indication for this modality. Remember, a negative bone scan does not mean the area examined is not the source of the problem, it would simply mean that the bone is not involved in the disease process.
Because of its sensitivity, scintigraphy allows more precise monitoring of the healing process so that horses may be returned to their normal training and exercise routines at a time which is most appropriate to the injury.