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MR Image quality and artifacts

Magic angle

por Denis Hoa

Origin

This artifact is produced by the particular physical properties of fibrillary tissues and their interaction with the static magnetic field. Water molecules in contact with structured collagen fibers (tendons, ligaments, nerves, menisci…) present dipolar interactions that reduce their T2 relaxation time. The extent of these interactions varies according to the angle of the fibers in relation to the axis of field B0. They are maximal at angles of 0° and 90° and minimal at an angle of 55°.
The basic short T2 relaxation time of tendons and ligaments associated with dipolar interactions is responsible for the usual hyposignal of these structures. However T2 relaxation time is lengthened and maximal when these fibrillary structures are at a 55° angle to B0, giving a hypersignal that varies in intensity.

Remedies

The intensity of signal variation induced by the magic angle will vary according to TE: it is maximal for relatively short TE (of the order of T2) and regresses when TE is lengthened. Variation in the relative hypersignal will depend on the angle, gradually evolving between 0° and 55°.
The magic angle has virtually no effect on T1 relaxation time. Therefore T1-weighted sequences are less affected.

Usage

The magic angle can be used in certain conditions to visualize tendons and ligaments in positive contrast, allowing classic analysis with T1 and T2-weighting as well as studies of enhancement after the injection of a contrast agent or with magnetization transfer.