Читать книгу Fractures in the Horse - Группа авторов - Страница 112

Nuclear scintigraphy provides both physiological and metabolic activity information [31, 48, 49], aids in the diagnosis of occult and stress fractures which can precede identifiable structural bone changes and can be used to monitor healing [22, 34, 36, 37,50–62]. In man, sensitivity is better than radiography for detection of both traumatic and stress fractures with few false positives or negatives [28, 51]. In contrast to radiographs that rely on a significant decrease in mineral content of bone, nuclear scintigraphy is relatively independent of calcium homeostasis [63]. Following a negative radiographic examination in human patients, and provided it is not contraindicated [17], stress fracture diagnosis has now moved to MRI regardless of location [64]. However, in the equine patient, nuclear scintigraphy remains the ‘gold standard’ for identification of fractures that have not been localized by other techniques. The objectives are to locate lesions, evaluate their extent and phase of evolution and determine the presence of multiple lesions rather than define cause [49, 63] (Figure 5.7).

Figure 5.6 Abaxial fracture (arrows) of a left hind medial proximal sesamoid bone. (a) Dorsolateral–plantaromedial oblique radiograph. (b) Longitudinal ultrasound image of the medial suspensory ligament branch (proximal to the left). An abaxial avulsion fracture is evident with fragment displacement and resultant loss of tension in the associated ligament. The proximodistal length of the injury and degree of compromise of the suspensory ligament branch can be assessed. (c) Transverse ultrasound image (dorsal to left) enables the dorsoplantar location of the fracture to be assessed and thus directs the surgical approach/technique.

The physical decay characteristics of technetium 99m (^99mTc) make this currently the radiopharmaceutical of choice for equine diagnostic imaging. For the purposes of bone evaluation, it is linked to a tracer phosphorous complex whose biodistribution favours localization in the skeleton [65]. In man, methylene diphosphonate (MDP) initially became the tracer of choice due to high skeletal uptake and fast blood clearance [32, 63, 66, 67]. MDP, disodium oxidronate (HDP) and methylene hydroxydiphosphonate (MHDP) have all been used in equine scintigraphy for their selective localization in bones. MDP historically has been used most and will be referred to in this chapter. Technetium 99m‐MDP (^99mTc‐MDP) is administered intravenously, is rapidly distributed throughout the extracellular fluid and accumulates in the skeleton by simulating the movement of one or more of the inorganic components of bone, principally the hydroxyapatite crystal [53, 63, 68, 69]. Accumulation is thought to be by both chemical adsorption onto the surface and incorporation into the crystalline structure of hydroxyapatite [70, 71] and is greatest where the body is depositing calcium phosphate. Blood flow, bone metabolic activity, capillary permeability and local extracellular volume govern this exchange process [63, 65, 68]. At normal and subnormal rates of blood flow to healthy bone, uptake appears proportional to blood flow; at higher rates, uptake is determined by the available crystal area [72, 73]. A low pH is also reported to be a factor [71]. Increased osteocyte activity in an area of bone trauma/fracture exposes more of the mineral face of the hydroxyapatite crystals, leading to increased adsorption. At a cellular level, locally increased deposition of ^99mTc‐MDP correlates histologically with the presence of osteoid in early stages of mineralization [32, 66, 67].

Figure 5.7 Forelimb scintigram of a two‐year‐old Thoroughbred racehorse with reported loss of action. Visible physes are active and symmetrical. Note multiple abnormal areas of increased radiopharmaceutical uptake in the radial carpal bones, third carpal bones and dorsodistal aspect of the third metacarpal bones. To enable complete assessment, the physes should be masked during post‐processing to eliminate the effects of count capture.

Skeletal uptake of ^99mTc‐MDP starts immediately after administration, reaches approximately 50% by one hour [48] and is effectively complete within two hours of administration [65]. Most imaging is delayed until between two and three hours post injection, depending on patient size, to allow ^99mTc‐MDP not localized in the bone to be excreted in urine. This reduces non‐skeletal activity and improves osseous image quality. The timing of acquisition is therefore called the delayed or bone phase. However, 2–4% of the dose is retained in the renal parenchyma that images the kidneys [63] and may obscure rib and thoracolumbar lesions.

Assessment is made with a gamma camera that utilizes the gamma photon sensitivity of sodium iodide crystals. The ^99mTc decay emissions from the patient cause the crystals to produce scintillation light. This is detected by photomultiplier tubes, transmitted to an electronic circuit and then displayed on a computer monitor [69]. It is planar (two‐dimensional) imaging. Normal skeletal uptake is symmetric [63], so active bone formation causes increased tracer deposition and increased radiopharmaceutical uptake (IRU).

Osteogenic aberrations identified by ^99mTc‐MDP uptake represent a non‐specific response of osteoblasts to activation. Once an area of abnormal uptake is identified, alternative imaging is necessary if structural information is required.

It has been demonstrated consistently that different patterns and locations of ^99mTc‐MDP uptake can be predictive of certain pathological findings. Both humeral and tibial stress reaction and stress fractures can be identified more readily on nuclear scintigraphy than radiography [15, 74, 75].