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Case 84

Wrap around artefact

MRI of a man with headaches - a curious lesion

This middle aged individual complained of recurrent headaches. An MRI was requested to investigate this further.

Findings

Note that the left edge of the image is missing, and projected over the right side of the image. The same effect is noted in the opposite direction. The structure arrowed is thus the pinna of the ear from the other side, projected over the brain.

Diagnosis

Wrap around artefact

Discussion

This artefact occurs in the phase encoding direction of an MRI image when the field of view selected is not wide enough. Structures outside the field of view are therefore assumed by the computer to be on the other side of the image.

This can be corrected by widening the field of view, or changing the phase encoding direction. The latter would be inappropriate here as it could project the neck and chest over the brain.

Changing the phase encoding direction is also useful when motion artefact obscures the image, such as due to vascular motion. For instance when imaging the knee in the axial plane, the artefacts from the popliteal vessels will run either in the sagittal plane, across the knee joint, or coronally, behind it, depending on the phase encoding direction chosen. The latter is obviously preferable.

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