To introduce a T2-weighted metal artifact correction imaging technique for the postoperative evaluation of the lumbar spine in patients with metal implantsSpinal fusion surgery is one of the most commonly performed procedures with a rapid growth rate, costing about 9 billion dollars in the U.S. in 2007¹. Unfortunately, about 20% of spinal fusions fail to relieve lower back pain and radicular symptoms², often requiring re-evaluation with MR imaging. However, the severe off-resonance artifacts induced by the metallic implants (spinal fusion devices) frequently prevent the use of MRI in postoperative spine imaging. MAVRIC-SL³ (Multi-Acquisition Variable-Resonance Image Combination) is a promising metal-artifact correction technique, previously allowing proton density-weighted and inversion recovery imaging in patients with metallic implants. However, it has not been extended to support T2-weighted imaging unlike other metal artifact correction techniques^4,5, where T2 weighting is extremely helpful in identifying nerve root impingement in the lumbar spine. Here, we propose a modification of the MAVRIC-SL sequence allowing the acquisition of T2-weighted imaging, improving visualization of soft tissues adjacent to metallic implants in patients after lumbar spinal fusion.

We achieved T2 weighting in MAVRIC-SL with reversed center-out echo ordering as shown in Figure 1. The original MAVRIC-SL is based on a multi-slab 3D fast spin echo sequence where the echoes in the echo-train are ordered in a center-out fashion on the ky-kz space⁶. This echo-ordering is effective for proton-density weighted contrast, or T1-weighted contrast with a short TR, but does not produce T2-weighted contrast. We simply reversed the center-out echo ordering such that the outer k-space is sampled at the beginning of the echo train and the center of the k-space is sampled at the end of the echo train. With an echo train collecting 20 to 32 echoes, and a spacing of 8-10ms, the T2 decay along the echo train produces sufficient T2-weighted contrast in the reconstructed image.

We compared our T2-weighted MAVRIC-SL sequence with a product 2D fast spin echo (FSE) sequence by acquiring sagittal and axial scans in a 50 year old female volunteer with lumbar-fusion hardware on a GE 3T Discovery 750 scanner (GE Healthcare, Milwaukee, U.S.) with a 32 channel torso coil. Sagittal and axial images of the lumbar spine were obtained using TR = 4s, TE = 154ms for MAVRIC-SL, and 131ms for 2D FSE, echo train length = 24, image matrix size = 256 x 230, slice thickness = 4mm, and readout bandwidth = ±125KHz. An FOV of 28.0cm x 25.2cm was used for sagittal imaging and an FOV of 36.0cm x 25.2cm was used for axial imaging.

Figure 1 shows the implemented echo ordering pattern for T2 weighting in ky-kz space with half-Fourier sampling. Echoes with larger echo numbers are sampled in the center of the k-space such that the final image contrast is dominated by signal experiencing sufficient T2 decay along the echo train. Figure 2 compares the conventional 2D FSE and MAVRIC-SL images in a volunteer with anterior and posterior lumbar spinal fusion of L5-S1 with both sequences showing comparable T2 contrast. On the sagittal 2D FSE image (Figure 2A) metal-induced artifact obscures the L5-S1 neural foramen and exiting L5 nerve root. On the axial 2D FSE image (Figure 2C), metal artifact partially obscures the spinal canal and exiting L5 nerve roots bilaterally at L5-S1. Significantly less metal artifacts are seen on the corresponding MAVRIC-SL images (Figure 2B and 2D), allowing improved visualization of the spinal canal and exiting nerve roots.We have developed T2 weighted MAVRIC-SL imaging of the lumbar spine in patients with spinal fusion hardware. The substantial metal artifact correction and T2 contrast improve our diagnostic capabilities in patients with metallic implants presenting with residual or recurrent symptoms of neural compression.