Figure 5.10 Four‐year‐old Thoroughbred racehorse with acute severe right hindlimb lameness. (a) Caudocranial radiograph of the right tibia on the day of presentation. No abnormalities detected. (b) Lateral and caudal scintigrams of the right tibia. Linear IRU is present in the distal tibial metaphysis and diaphysis compatible with a propagating tibial fracture. (c) Radiographs taken at two, four and eight weeks post injury. Progressive osseous resorption permits identification of sharply marginated radiolucent fracture lines (black arrows). Areas of increased radiopacity are consistent with formation of trabecular and cortical callus (white arrows), which gradually bridges the fracture. Note also the distal lateral fracture line eight weeks post injury that is slow to become radiographically apparent.

      It has been suggested that horses with evidence of stress fracture undergo scintigraphic review before they return to work [106]. This is not routinely practised in the UK where financial constraints and well‐accepted stress fracture management regimes have precluded longitudinal studies. Horses in training that have undergone nuclear scintigraphy in subsequent seasons have demonstrated subtle uptake at previous fracture sites. The degree and distribution of the ^99mTc‐MDP uptake is usually mild, ill‐defined and compatible with bone remodelling.

      In a study of equine distal phalangeal fractures, activity was reported to persist for >25 months. This was ascribed to a fibrocartilaginous union, fracture instability, osteolysis and osteoid formation [115].

      A study of dorsal cortical fractures of the third metacarpal bone reported correlation between persistence of a radiographically evident fracture line with less intense scintigraphic uptake and individuals who did not heal and required surgical intervention [58]. The supposition made was that the degree of ^99mTc‐MDP uptake was directly correlated with osteogenesis and rate of repair, thus diminished uptake in the absence of radiographic resolution indicated either a delayed or non‐union.

      Sequential evaluations in the days and weeks following surgery were reported in three horses (four year old, yearling and foal) that had sustained a variety of traumatic fractures to the third metacarpal or metatarsal bones. Two cases developed photopenic regions less than six days post‐operatively, one was described as extensive and at necropsy this correlated with osteomyelitis and sequestration [116].

Computed Tomography

      General Principles

      CT is a high‐resolution, X‐ray based, quantitative, cross‐sectional imaging technique. It has for some time been integral to fracture diagnosis and management in man, and application in horses has recently evolved rapidly. Like radiography, it measures tissue attenuation of penetrating photons; however, the X‐ray source rotates around the patient. Multidetector row CT affords excellent spatial resolution and thin and overlapping slices, which approach isotropic, allow for multi‐planar reformatted (MPR) images that can be reconstructed in any chosen plane. The MPR reconstruction and thin slices both optimize fracture identification. Articular surfaces can be assessed [117, 118], and the superior bone detail produced by CT enhances identification and mapping of fissures, subchondral bone fractures, unicortical fractures and other articular fractures. Three‐dimensional surface rendering details the topographical aspects of the fracture configuration and with segmentation permits selective removal of overlying tissues in order to visualize the complexity of a fracture.

      Cone beam CT (CB‐CT) has recently been introduced to equine use. It requires markedly different image reconstruction, does not provide quantitative information about tissue density and hosts a new complement of imaging artefacts that can detract from diagnostic accuracy.

      Technical Considerations

      CT requires precise and relatively rapid movement of the patient relative to the photon source and detectors (gantry). Moving gantry CT scanners allow the horse to be supported by a surgery table, and the gantry itself is responsible for movement accuracy. Equipment for CT in the standing horse is now possible using both conventional and CB‐CT scanners for the head, cervical spine and distal limbs.

      CT provides quantitative imaging information with high spatial resolution. Each pixel is assigned a value described as a CT or Hounsfield unit (HU). This is a measure of each pixel's density with respect to pure water which is arbitrarily designated a value of zero HU. Pixel size is determined by the field of view (set at the time of image acquisition or reconstruction) and the pixel matrix of the image; it is often sub‐millimetre size. HUs are based on X‐ray attenuation in tissue. Gas is generally −1000 HU, fat is approximately −120 HU, soft tissues 100–200 HU, cancellous bone 400–600 HU and cortical bone in the range of 1500–2000 HU; dental enamel is higher than cortical bone. Slice thickness can be varied in some machines to sub‐millimetre size, resulting in high‐resolution images even when reformatted. CB‐CT is not quantitative and does not produce a measurement of HU.

Image processing occurs through mathematical manipulation of the density data and has a profound impact on the appearance and clinical utility of an image. Unprocessed or raw CT data are typically not used in diagnostic imaging and may not even be stored by the acquisition device or picture archiving and communication system (PACS). Most CT scanners have several processing algorithms that allow the operator to choose the degree and type of processing at the time of acquisition. The methods of processing evolve but in general will include bone, sharp or edge‐enhanced algorithms along with soft tissue or smoothing algorithms. Complete examination of an anatomic region should include both so that all tissues can be evaluated. A sharpening algorithm will produce very pleasing diagnostic images of bone and fractures but will enhance artefacts such as high‐density edge gradient artefacts causing streaking through regional soft tissues.

      Image display is flexible. The end user is able to selectively manipulate the image to emphasize structures of different density. Window width refers to the range of HU over which the greyscale is applied, and window level refers to the centre point of the window. In order to fully evaluate a region, both window level and width require manipulation.

      CT produces excellent bone images due to the inherent high subject contrast when using tissue density/X‐ray attenuation (400–2000 HU). It is particularly good for imaging fractures due to the combination of high inherent contrast between intact and disrupted bone and high spatial resolution that permits identification of very small areas of disruption. In principle, soft tissues have less inherent contrast and are imaged less well. Modern scanners, capable of high tube output, produce very good soft tissue image quality, although when immediately adjacent to a high‐density tissue, such as cortical bone, this can be more problematic.

      Artefacts

      CT, like all imaging modalities, has its own complement of artefacts. These are defined as a discrepancy between the CT number or HU in the reconstructed image and the actual attenuation coefficient of the object. Non‐conventional use of CT technologies, such as standing CT, results in an additional gamut of artefacts that must be understood and evaluated for what they are.

      Partial volume averaging results in the incorrect assignment of an HU value when the values of two structures are averaged in one voxel. This is problematic in fracture identification if the fracture is non‐ or minimally displaced and/or running obliquely through the scan plane but can be mitigated by reformatting the images into multiple different planes.

      High‐density edge gradient or beam hardening occurs when a very dense subject is present in the scan plane, attenuating the low‐energy portion of the polychromatic photon beam and resulting in a preponderance of higher energy X‐rays. This results in dark bands or streaks either between two high‐density structures (e.g. petrous temporal bone) or around the margins of a high‐density structure such as a metallic