MRI can serve as an adjunct diagnostic tool for CTS when the clinical or neurophysiologic findings are equivocal. The carpal tunnel is a fibro-osseous space with little fat that contains the flexor tendons and the median nerve. The flexor retinaculum composes the volar aspect of the tunnel and normally shows slight palmar bowing. The median nerve courses through the tunnel within its volar and radial aspect, and it can be differentiated from the adjacent tendons because it shows relative higher signal intensity. The carpal tunnel and its contents are best evaluated in the axial plane and should be scrutinized at three standard locations.
- Distal radioulnar joint before the median nerves enter the tunnel.
- Proximal tunnel, at the level of the pisiform.
- Distal tunnel, at the level of the hook of the hamate.
FIGURE 6-13. Normal wrist anatomy as seen on T1-weighted MR images. Axial MR images are at the levels A: of the distal radioulnar joint, B: the proximal and C: the distal carpal tunnel. D: Longitudinal MRI through the median nerve within the carpal tunnel. C, capitate; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis; FR, flexor retinaculum; H, hamate; L, lunate; MN, median nerve; PDN, palmar digital branches of the median nerve; R, radius; T, trapezium; U, ulna; UA, ulnar artery; UN, ulnar nerve. Note fracture through the base of the hook of the hamate (arrow in B).
There are four universal findings of CTS visible by MRI regardless of etiology (21):
- Swelling of the median nerve (i.e., pseudoganglion) in the proximal part of the carpal tunnel at the level of the pisiform. Best evaluated by comparing the size of the median nerve at the level of the distal radioulnar joint with its size at the proximal tunnel.
- Increased signal intensity of the edematous median nerve on T2-weighted images.
- Palmar bowing of the flexor retinaculum, determined by a bowing ratio of more than 15%. The bowing ratio is calculated by drawing a line from the trapezium to the hook of the hamate on the axial plane. The distance from this line to the flexor retinaculum is divided by the previously calculated length.
- Flattening of the median nerve in the distal carpal tunnel at the level of the hamate (Fig. 6-14A–D). MRI also has the potential to establish the cause of CTS. Some of the etiologies visualized by MRI include traumatic tenosynovitis, rheumatoid tenosynovitis, a ganglion cyst of a carpal joint, excessive fat within the carpal tunnel, a hypertrophied adductor pollicis muscle in the floor of the carpal tunnel, and a persistent median artery (23).
FIGURE 6-14. Axial FSE T2-weighted fat suppressed images (A, B, C) and sagittal T1-weighted image in a patient with carpal tunnel syndrome. A, B: There is a normal size to the nerve within the tunnel (arrowheads). C: There is thickening, increased girth and increased signal intensity proximal to the flexor retinaculum (arrowheads). D: Note tapering to the nerve as it approaches the carpal tunnel in the sagittal view (arrowheads).
MRI also provides a means of postoperative evaluation of those patients in whom the symptoms persist, to ensure that the flexor retinaculum has been completely incised and that there are no other complicating postoperative factors producing continuing discomfort. When the flexor retinaculum has been completely incised, the incision site is well documented by MRI and the contents of the carpal tunnel are typically displaced forward (Fig. 6-15A). If the distal part of the flexor retinaculum has been incompletely incised, this can be demonstrated by MRI, and the preoperative MRI findings of CTS will persist (Fig. 6-15B and C).
FIGURE 6-15. Postoperative MR of carpal tunnel syndrome. A: Axial T1-weighted image. There has been release to the flexor retinaculum (short arrow). The median nerve insinuates through the surgical defect. B: Axial T2-weighted fat suppressed image. The median nerve (arrow) has intermediate signal and is better delineated. C: Patient with failed carpal tunnel release. There is a linear area of decreased signal intensity (short arrow) which was found to represent a fibrous band at surgery. The median nerve (long arrow) is flattened underneath the fibrous band. (A and B; courtesy of Zehava Rosenberg, NY. C; courtesy of Mark Kransdorf, FL.) OTHER WRIST ABNORMALITIES
MRI can visualize postincisional neuromas as lobulated masses in the typical location of the palmar cutaneous branches of the median nerve. Other peripheral nerve tumors such as schwanomas (Fig. 6-16) and neurofibromas can be well recognized as well. It can also demonstrate tenosynovitis involving any of the tendons crossing the wrist. MRI also displays marrow abnormalities such as ischemic necrosis of the proximal fragment of a scaphoid fracture and avascular necrosis of the lunate, where the marrow shows reduced signal intensity (24). MRI has the ability to evaluate the integrity of the intrinsic/extrinsic ligaments of the wrist and the triangular fibrocartilage complex (TFCC) (25). The TFCC, scapholunate, and lunotriquetral ligaments are best evaluated with MR arthrography (Fig. 6-17).
FIGURE 6-16. Coronal T2-weighted sequence in a 72-year-old patient with a palpable hypothenar mass. There is a rounded soft tissue mass (arrow) within the ulnar nerve proximal to the retinaculum representing a shwannoma of the ulnar nerve. Courtesy of Dr. Mark Kransdorf, Jacksonville Fla.