Fractures in the Horse. Группа авторов

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].

      Technical Considerations

      Time of Evaluation

Osteoblasts have been seen forming callus in experimental fractures within hours of injury [76], and in man scintigraphic uptake has been observed at fracture sites between 6 and 72 hours following the onset of pain [77–79]. A human study concluded that the minimum time for a bone scan to become abnormal following monotonic fracture was influenced by age with younger patients having a quicker detection time [77]. This likely reflects a confounding effect of metabolic bone disease in older patients and should have limited impact on the majority of equine patients. It is likely that most stress fractures will be identifiable scintigraphically when lameness is evident and this has been documented in human and equine patients [27, 28, 31,36–39, 62,79–90]. However, there are two scenarios that may contribute to false negatives. It is possible that very early stress reactions characterized only by cortical tunnelling in the absence of new bone formation may appear as unremarkable cold spots [91]. Secondly, in the equine patient when pelvic fractures are presented in prodromal or per acute phases, a combination of location with muscle and distance attenuation can conceal IRU. This can result in a negative scan with retrospective diagnosis following osseous displacement [92] or in the case of a stress reaction, progression to fracture when the horse returns to training. To avoid false negatives, a delay is recommended between the onset of lameness or trauma and nuclear scintigraphy. Five to seven days have been proposed as a minimum [93]; however, a 10–14 day delay would make the possibility of obtaining a false negative unlikely. Alternatively, if the initial evaluation is negative and a pelvic fracture is still suspected, an additional scintigraphic examination could be performed at a second time point after not less than 10–14 days (Figure 5.8) [91, 92]. The financial and ionizing radiation implications would, in most circumstances, support an initial delay.

      Patient Preparation

      Cold limb syndrome appears as areas of complete or patchy photopenia in the carpus/tarsus and distal limb which can efface areas of IRU. It can occur in any patient, but the incidence increases in cold weather and when the horse cannot be exercised. The majority of suspected fracture patients will be unsafe to exercise in a manner that will enhance distal limb perfusion. In order to try and minimize the incidence of cold limb syndrome and to optimize perfusion, and thus radiopharmaceutical distribution, patients can be stable bandaged and rugged overnight and, prior to injection of the radiopharmaceutical, placed in a stable with radiating heat lamps and a deep shavings bed (for at least one hour) and administered acetylpromazine. Maintaining the patient in a stable with heat lamps for the period between injection and image acquisition has proved the most reliable method for minimizing/eliminating cold limb syndrome.

      Image Acquisition

      Acquisition of images has become increasingly uniform and refined and, in most facilities, follows a set protocol. Images should overlap to ensure that the entirety of the requested areas is evaluated. The field of view of the gamma camera detector will have a bearing on the number of images required to achieve this. In man, at least two orthogonal views of stress fractures are obtained to evaluate the degree of cortical penetration [48].

      Although protocols have been documented [94, 95], each patient should have the study tailored and modified according to the appearance of the images as they are being acquired. Real‐time assessment is therefore optimal. In addition to standard acquisition protocols, the following views can provide additional information;