Читать книгу Fractures in the Horse - Группа авторов - Страница 127

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.