The median nerve and ulnar nerve are best seen on axial MR images; both display intermediate signal on T1-weighted images and mild increased signal on fluid-sensitive images. The median nerve is round to oval proximal to the carpal tunnel and flat within the tunnel. The division of the nerve into its branches is usually identified at the level of the metacarpals.
Because of its smaller size, variations in the size and signal of the ulnar nerve can be difficult to assess. The nerve is highlighted by fat within Guyon's canal. It abuts the bones bordering the tunnel, a position that can aid in distinguishing it from its accompanying vessels. The ulnar nerve's position varies with wrist motion: it is located ulnar to the hook of hamate in wrist flexion and radial to the hook in wrist extension.
Numerous anatomic variants have been found at both the carpal tunnel (41%) and Guyon's canal (21%) [25,34]. Median nerve variations include alterations in the course of the thenar branch, accessory branches proximal to or at the distal carpal tunnel, bifid median nerve, and high division of the distal median nerve (Fig. 6) . Furthermore, the motor branch may arise in the forearm or may be split by a persistent median artery or an aberrant muscle . Many anomalous muscles have also been described in the carpal tunnel region, such as the reversed palmaris longus muscle, the accessory palmaris profundus, the accessory flexor digitorum superficialis, and aberrant origins of the thenar and lumbrical muscles [35-37].
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