Real-time magnetic resonance imaging (MRI) during active wrist motion--initial observations

Abstract

Background: Non-invasive imaging techniques such as magnetic resonance imaging (MRI) provide the ability to evaluate the complex anatomy of bone and soft tissues of the wrist without the use of ionizing radiation. Dynamic instability of wrist--occurring during joint motion--is a complex condition that has assumed increased importance in musculoskeletal medicine. The objective of this study was to develop an MRI protocol for evaluating the wrist during continuous active motion, to show that dynamic imaging of the wrist is realizable, and to demonstrate that the resulting anatomical images enable the measurement of metrics commonly evaluated for dynamic wrist instability.

Methods: A 3-Tesla "active-MRI" protocol was developed using a bSSFP sequence with 475 ms temporal resolution for continuous imaging of the moving wrist. Fifteen wrists of 10 asymptomatic volunteers were scanned during active supination/pronation, radial/ulnar deviation, "clenched-fist", and volarflexion/dorsiflexion maneuvers. Two physicians evaluated distal radioulnar joint (DRUJ) congruity, extensor carpi ulnaris (ECU) tendon translation, the scapholunate (SL) interval, and the SL, radiolunate (RL) and capitolunate (CL) angles from the resulting images.

Results: The mean DRUJ subluxation ratio was 0.04 in supination, 0.10 in neutral, and 0.14 in pronation. The ECU tendon was subluxated or translated out of its groove in 3 wrists in pronation, 9 wrists in neutral, and 11 wrists in supination. The mean SL interval was 1.43 mm for neutral, ulnar deviation, radial deviation positions, and increased to 1.64 mm during the clenched-fist maneuver. Measurement of SL, RL and CL angles in neutral and dorsiflexion was also accomplished.

Conclusion: This study demonstrates the initial performance of active-MRI, which may be useful in the investigation of dynamic wrist instability in vivo.

Figure 1. The forearm of a healthy volunteer in the wrist harness.

The harness has been slid outwards for better visualization of both the harness and the MR coil.

Figure 2. Metrics associated with wrist instability that were derived from standardized measurements on active-MR images.

(a) DRUJ subluxation ratio – the subluxation ratio for the DRUJ was determined on axial images by connecting the palmar and dorsal aspects of the sigmoid notch and drawing perpendicular lines. The subluxation ratio was computed as a ratio between CD and AB; (b) SL interval and ulnar variance – the measurement of SL interval (two-sided arrow) and ulnar variance (distance between the dots with white centers on the solid red lines) was performed on the coronal images. The solid red lines represent the most distal aspect of the distal ulna and most proximal aspect of the distal radius articular surfaces; (c) SL, RL and CL angles – the axis of the scaphoid (S) was drawn by connecting the proximal and distal poles along the volar cortex (dotted line) on sagittal images. Similarly, the axis of the lunate was determined by connecting the distal dorsal and distal palmar corners and creating a perpendicular line. The SL angle (SLA) was determined by measuring the angle between the scaphoid and the lunate (L) axes. Similarly, the axes of the capitate and the radial shaft were determined and the CL angle was measured as the angle between the capitate and the lunate axes (not shown). The RL angle was measured as the angle between the long axis of the radial shaft and the lunate (not shown).

Figure 3. Extensor carpi ulnaris (ECU) tendon translation during wrist rotation.

The relationship of the ECU tendon (arrow) to its groove as the forearm is rotated from (a) pronation, through (b) neutral to (c) supination – on axial images of the DRUJ using the active-MRI scan. In this volunteer, the ECU tendon was located within its groove in pronation, while in the neutral position, the tendon is subluxated eccentrically at the margin of the ulnar groove. In supination, the tendon is dislocated. Also visualized is the trajectory of the ulnar styloid process (white star) during the supination/pronation maneuver. Lister's tubercle (white triangle) at the dorsal aspect of the radius is shown as an anatomical reference point.

Figure 4. Active-MRI of the wrist during active ulnar-radial deviation.

Snapshots of the coronal images of the wrist in the ulnar deviation (a), neutral (b) and radial deviation (c) positions during the continuous radial-ulnar deviation maneuver. SL interval and ulnar variance (see Figure 2 for these definitions) were measured from the resulting images.