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 activity in the proximal 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
