Читать книгу Fractures in the Horse - Группа авторов - Страница 128

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 implant. Beam hardening can be difficult to avoid in equine patients. Most CT scanners have beam hardening reduction software that may or may not be available to the operator. Photon starvation is caused by beam hardening between two dense objects. This is of particular importance in horses when two limbs are placed through the gantry at the same time. Even if the operator reduces the field of view to include only one limb, the effect of the pair will be visible in the images.

Motion produces image blurring or mismapping of anatomy. These can have negative impacts on the identification of fragments if the blurring causes margins to become inconspicuous or in fracture evaluation when a hypoattenuating area such as fracture gap can be mismapped to a different region.

Photon starvation is seen in areas of high attenuation, particularly associated with metal implants. Insufficient photons reach the detector, and during reconstruction noise is greatly magnified in these areas creating streaks in the image.