Magic angle effect: a relevant artifact in MR neurography at 3T?

Abstract

Background and purpose: MRN is an emerging diagnostic method for disorders of peripheral nerves. However, it is unclear whether the influence of the MA on intraneural T2 signal is severe enough to provoke false-positive findings.

Materials and methods: Twenty-five healthy subjects underwent MRN of the sciatic nerve of the proximal thigh at 3T. The T2(app) was calculated from a DE-TSE sequence (TR = 3000 ms, TE1 = 12 ms, TE2 = 69 ms) at 7 angles of the sciatic nerve relative to B0 = 0°, 30°, 35°, 40°, 45°, 50°, and 55°. Precise angle adjustments were performed with a dedicated in-bore positioning aid. Qualitative evaluation of intraneural T2-weighted contrast between this group of healthy subjects and 14 patients with neuropathic lesions was performed by comparing CNRs of a TIRM sequence (TR = 5000 ms, TE = 76 ms, TI = 180 ms).

Results: In healthy subjects, the prolongation of T2(app) from 0° to 55° was from 74.5 ± 13.4 to 104.0 ± 16.9 ms (P < .001). The increase in T2(app) relative to baseline (0°) was 9.6% (30°), 18.4% (35°), 25% (40°), 27.6% (45°), and 37% (55°). Intraneural CNR increased by 1.98 ± 0.69 at 40° and 2.93 ± 0.46 at 55°. Nevertheless, the mean CNR of healthy subjects was substantially lower than that in patients at 40° (P < .0001) and even at the position of maximum MA (55°: 20.6 ± 5.11 versus 52.6 ± 7.12, P < .0001).

Conclusions: Neuropathic lesions are clearly distinguishable from an artificial increase of intraneural T2 by the MA. Even at a maximum MA (55°), the false-positive determination of a neuropathic lesion is unlikely.

Fig 1.

Construction chart of in-bore positioning aid. Distance locks (a–f) precisely define 7 angle positions between the thigh (I) and base board (II) relative to B₀: 0°, 30°, 35°, 40°, 45°, 50°, and 55°. This polyvinyl chloride device was firmly positioned under the gluteal and thigh region of the supine patient and aligned in the z-direction, parallel to the static magnetic field B0. Angulations were achieved by stepwise erection of the thigh board relative to the baseboard. The insert on the upper right displays an oblique top view of the device.

Fig 2.

Parasagittal MR images of the sciatic nerve of 1 representative healthy subject at Θ = 55°. A, T1-weighted TSE image shows that the sciatic nerve (arrows) courses parallel to the femur among the adductor magnus, semitendinosus, and biceps femoris muscles. B, Sagittal T2-weighted TIRM image at the position of maximum occurrence of the MA at Θ = 55° shows hyperintense alteration of the nerve.

Fig 3.

Intraneural variations of the T2_app time constant at all measured angulations of Θ and the calculated theoretic course for both region-of-interest sizes (sROI, lROI). Note that there is a marked increase in T2_app when increasing the angle from Θ = 30° to Θ = 55°. For each angulation above 0°, the difference of T2_app was tested against its baseline value at 0° (asterisk = P < .05, double asterisks = P < .001). The theoretically assumed angular dependency of T2 (compare with equation 3) was fitted to the measured values by using a Levenberg-Marquardt algorithm. R² is 0.94 for sROI and 0.95 for lROI, respectively.

Fig 4.

Bar charts of intraneural contrast variations on T2-weighted TIRM imaging (CNR, y-axis) at all measured angle positions (0°, 40°, 55°) averaged over 25 healthy subjects and 14 patients (x-axis). Given that CNR values reflect mean contrast differences between nerve and adjacent muscle, the mean CNR in patients (dark gray) was significantly higher than in healthy subjects for all measured angle positions (versus 0°, 45°, and 55°: asterisk = P < .0001). Double asterisks indicate that the peripheral nerve orientation in patients was at 12° ± 2.5° relative to B0. Error bars denote standard error of the mean.

Fig 5.

Sciatic nerve of 1 representative healthy subject at the baseline position of 0° relative to B₀ (A and B) at the position of maximum MA occurrence of 55°(C and D) and 1 representative neuropathic lesion in a patient (62-year-old man) aligned with B0 (0°). Transversal T2-weighted TIRM images (left column: A, C, and E) are displayed with corresponding zoomed views focusing on the nerve (right column: B, D, and F). Note the hyperintense intraneural T2-weighted contrast relative to surrounding muscle at 55°(C and D) as opposed to the baseline position at 0° (A and B). A clearly stronger T2-weighted hyperintense contrast is noted in a true neuropathic lesion (E and F; 62-year-old male patient). In addition, fascicles of the affected nerve appear markedly swollen as a typical finding of severe neuropathic injury, confirmed by ENG/EMG as axonal injury.