Читать книгу Fractures in the Horse - Группа авторов - Страница 133

Appreciation of artefacts is necessary in order to avoid interpretation errors. Absence of patient motion is important. Many fracture evaluations will employ sMRI, but it is necessary for horses to be sufficiently comfortable to stand square without resting pain. Immobility is essential to avoid phase mismapping and loss of image quality. The team involved in patient handling, sedation and acquisition have a substantial bearing on end image quality.

Phase cancellation or chemical shift artefact is the result of the differing precessional frequencies of protons in water and fat, caused by hydrogen in water being arranged with oxygen and hydrogen in fat being arranged with carbon. When they are in phase their signals add together, and when they are out of phase their signals cancel out. This results in a dark line at the interface of fat and water which is extremely useful in highlighting the presence of intra‐osseous fluid accumulation on T2*W GRE sequences.

Susceptibility artefact is produced by agents that disrupt the local magnetic field due to their ability to become magnetized, e.g. ferromagnetic materials or blood degradation products. This results in dephasing at the agent's interface resulting in signal loss or void and is most prominent on gradient echo sequences as the gradient reversal is unable to compensate for the phase difference. Implants also cause distortion of the magnetic field and can complicate interpretation.

Within each voxel, the signals received are averaged creating the potential for volume averaging artefacts. Increased slice thickness and the poorer resolution of sMRI exacerbate this process [140]. A common example occurs in the metacarpal/metatarsal condyles where the curvature and thin articular cartilage can be susceptible to volume averaging artefacts.