humeral physes is evident, which produces count capture. An area of abnormal IRU is indicated (blue arrow). (b) Postprocessing masking of the physes highlights the abnormal IRU more clearly. (c) Following identification of the abnormal IRU, contemporaneous additional cranial projections were acquired. IRU in the proximal aspect of the first right rib is confirmed and highlighted (blue arrows).
Solar views of the foot can provide further information for distal phalangeal and navicular bone fractures.
In skeletally immature individuals, there is normal intense localization of 99mTc‐MDP in the physes. As this produces count capture, it is important to mask these areas during post‐processing to ensure that areas of abnormal IRU are not obscured (Figure 5.9).
Image Quality
Multiple factors affect the quality of the generated image, including patient preparation, the time between injection and acquisition, uptake of 99mTc‐MDP (intrinsic and extrinsic factors), count density, total counts, motion [94, 99], inherent resolution and sensitivity of the gamma camera and management of urinary tract excretory content. All aspects should be optimized, and it must also be recognized that poor operator technique can significantly affect study quality.
Descriptors
IRU is described by location, pattern (focal or diffuse), shape and intensity expressed as mild (up to 10%), moderate (10–50%) and marked (>50%) in comparison with the opposite and matching anatomical site [75]. The shape, intensity and pattern of uptake are determined by the size, extent and activity of the local remodelling process as well as its blood supply [54].
Quantitative Assessment
Regions of interest (ROIs) can be defined and compared to counts obtained at the same site in the contralateral limb or a separate defined region in the same patient. The relative uptake ratio is calculated by dividing the mean counts per pixel for the target ROI by the mean counts per pixel for a reference ROI on the same image to account for variability in absolute counts. Profile analysis can also be used if ROI analysis is equivocal. It has been reported that subtle differences or abnormal radiopharmaceutical uptake may be more readily identified by using ROI analysis [100]. However, information may be lost from the averaging effect, and subjective assessment has been reported as superior for focal areas of IRU [101].
Qualitative Assessment
Subjective methods of interpretation have been shown to correlate highly with semi‐quantitative techniques [102]. Subjective evaluation for fracture assessment is generally made in greyscale. For thoracic spine assessment, the blue, green and red colour display has been reported to have greater sensitivity for detecting IRU than continuous greyscale [103]. As with all image interpretation, the experience of the interpreter has a significant bearing.
In contrast with human studies [19, 28], qualitative and quantitative analyses of equine tibial stress fractures demonstrated no correlation between grades of IRU, lameness or radiographic findings [104]. There was also no correlation between calculated ratios and lameness grade at presentation or performance outcome [105].
Clinical Indications
Nuclear scintigraphy remains the mainstay for stress fracture identification and risk assessment in horses such as the requirement for keeping a patient cross‐tied and guiding the length of rehabilitation programmes. It is indicated in the evaluation of severely lame horses that are devoid of confident diagnosis (Figure 5.10) and those with clinical signs referable to the axial skeleton including the pelvis. In addition to determining location, fracture displacement can frequently also be identified, e.g. third trochanter, deltoid tuberosity, tuber ischium and tuber coxa fractures.
Limitations
Activity in the distal condyles of the third metacarpal and metatarsal bones requires careful assessment to discriminate a stress‐related response from a potential fracture [98]. It has been suggested that scintigraphy of horses that are lame or performing poorly is not an effective screening technique for prodromal condylar fractures [106]. It would be more accurate to say that nuclear scintigraphy does not predict the likelihood of sustaining a condylar fracture. It is known that bone fatigue associated with condylar fractures may develop rapidly, arise in sound horses and result in a fracture before an osteoblastic response is initiated [98]. This is indeed the risk carried by any horse in training undertaking fast work when prodromal features may not be apparent. However, in the presence of lameness a bone response is likely to have been initiated, and scintigraphy is unlikely to produce a false negative result whether or not the clinical features are related to an impending condylar fracture.
Principles of Interpretation
Results of nuclear scintigraphic examinations have been documented in racing Thoroughbreds [75] and Standardbreds [107], and horses used for showjumping, eventing and hunting [108], reporting the distribution of areas of IRU and their variability between disciplines. Interpretation of the presence of a stress reaction relies on knowledge of injury predilection sites. The spectrum of IRU in stress fractures can vary from a focally marked fusiform area of cortical IRU to an area less intense or well defined which can represent the pathophysiological continuum between fracture and stress reaction. Asymptomatic foci can reflect prodromal change, active remodelling or healing. Alternative imaging may be needed to determine the significance of findings.
Dorsal Cortex of the Third Metacarpal Bone
The stress continuum in the dorsal metacarpus and metatarsus in racing and non‐racing horses has been studied [58, 109], and a grading scheme of one to four [109] suggested. Scintigraphy exhibited excellent sensitivity, but false positives with clinically normal limbs having IRU [109]. Interpretation is further complicated by cross over between dorsal metacarpal disease and cortical stress fractures as one process maybe superimposed on the other [110]. Nuclear scintigraphy has been utilized in Thoroughbreds to differentiate between dorsal metacarpal disease, defined as uniform diffuse IRU in the dorsal cortex relative to the palmar cortex and metaphyses, and cortical stress fractures, defined as focal intense IRU in the dorsal cortex [58]. In this location, the focal nature of the IRU has been considered more significant than intensity [61].
Enostosis‐like Lesions
These lesions are identified scintigraphically by IRU located within the trabecular bone determined on two tangential projections. Although reported to be found close to nutrient foramina [111], this is not consistent. No definitive aetiology has been established, but one proposal is that they are trabecular microfractures caused by cyclical stress [112, 113]. The degree of IRU uptake can vary from mild to marked.
Monitoring Fracture Healing
Nuclear scintigraphy has been used in man to monitor healing in both monotonic and stress fractures [19, 28, 77, 114]. In the first (acute) phase, there is a diffuse area of IRU due to increased blood flow around the fracture site. This is greater than the morphological fracture and persists for two to four weeks after injury. The second (subacute) stage has the most intense well‐defined IRU which corresponds more accurately with the anatomical fracture and lasts for 8–12 weeks (Figure 5.8b). Over the coming weeks and months as callus remodels during the third (reparative) stage, there is a more localized area of IRU with greater separation between normal and abnormal tissues followed by a gradual reduction in activity. The time of scintigraphic normalization is greater than that identified clinically or radiographically due to ongoing bone remodelling. In man, monotonic fractures can take up to 24 months [77] and stress fractures between four to six months [28]. In stress fractures, severity