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;
Dorsal and oblique images of the spine help to differentiate IRU in laminar arches and spinous processes.
Lateral (costal fovea to costochondral junction) and dorsal images of the ribs will confirm IRU within ribs rather than superimposed structures. Cranial images of the thoracic inlet (Figure 5.9c) and a modified lateral image with the forelimb closest to the detector pulled backwards [96] permit assessment of cranial rib fractures.
Oblique images of the cranial [97] and caudal pelvis reduce superimposition together with soft tissue and distance attenuation and can better image ilial wing, ilial shaft, ischial and pelvic floor fractures. They also help differentiate proximal ilial wing, tuber sacrale and sacral fractures as these areas are superimposed in dorsal images. It can also differentiate lesions when there is a question over possible superimposed urine pooling: if the IRU is within the skeleton it will maintain a constant relationship with the bone irrespective of gamma camera position (Figure 5.8).Figure 5.8 Adult warmblood showjumper that went acutely lame in its left hindlimb while jumping. (a) Initial nuclear scintigraphy study 48 hours post lameness. Note activity from excreted 99Tc‐MDP in the urinary bladder superimposed over the cranial left ilial shaft (dashed blue circle) and how the presence of both the urinary bladder and motion artefact degrades the dorsal pelvis image quality. (b) Second study nine weeks post lameness. Diffuse area of marked IRU involving the caudal left ilial wing and cranial ilial shaft (arrows) consistent with a fracture.
In addition to the standard view of the tuber ischii (detector positioned at 45° to vertical with the tail lifted to one side to avoid overlay and effacement of the axial ischium and symphysis), positioning the detector at 90° (again with the tail lifted to the side) can give further information regarding fractures of the ischium and tuber ischium.
Proximal tibial stress fractures can occasionally be present caudomedially and have the potential to be overlooked on the lateral view if IRU is mild. Additional caudal views of the stifle are recommended.
Mid‐diaphyseal tibial fractures can be missed if there is inadequate overlap between lateral hock and lateral stifle views, especially if the detector field of view is small: a lateral image of the entire tibia is useful.
A combination of dorsal and lateral views of the scapula can differentiate stress fractures of the scapula and vertebral lesions [39].
Cranial views of the shoulder and proximal humerus aid identification of deltoid tuberosity fractures.
A cranial view of the elbow and distal humerus can highlight subtle IRU in the distal medial humerus (stress fracture) or in the medial humeral subchondral bone (compression fracture): on lateral projections alone both can be obscured by attenuation.
A flexed dorsal view of the carpus separates the carpal bones and helps in identification and localization of lesions.
Flexed lateral views of the fetlocks can help separate the metacarpal/metatarsal condyles from the proximal sesamoid bones and change the orientation of the condyle with the proximal phalanx.
Flexed dorsal views of the fetlocks can differentiate parasagittal IRU from condylar IRU [98].Figure 5.9 Scintigrams of the proximal forelimb of a two‐year‐old Thoroughbred racehorse with acute onset right forelimb lameness. (a) Lateral scintigram centred on the scapulae. Normal symmetrical metabolic