High-Contrast Lumbar Spinal Bone Imaging Using a 3D Slab-Selective UTE Sequence

Abstract

Ultra-short echo time (UTE) MRI with post-processing is a promising technique in bone imaging that produces a similar contrast to computed tomography (CT). Here, we propose a 3D slab-selective ultrashort echo time (UTE) sequence together with image post-processing to image bone structures in the lumbar spine. We also explore the intermodality agreement between the UTE and CT images. The lumbar spines of two healthy volunteers were imaged with 3D UTE using five different resolutions to determine the best imaging protocol. Then, four patients with low back pain were imaged with both the 3D UTE sequence and CT to investigate agreement between the imaging methods. Two other patients with low back pain were then imaged with the 3D UTE sequence and clinical conventional T₁-weighted and T₂-weighted fast spin-echo (FSE) MRI sequences for qualitative comparison. The 3D UTE sequence together with post-processing showed high contrast images of bone and high intermodality agreement with CT images. In conclusion, post-processed slab-selective UTE imaging is a feasible approach for highlighting bone structures in the lumbar spine and demonstrates significant anatomical correlation with CT images.

Keywords: CT-like contrast; UTE MRI; ZTE MRI; bone imaging; lumbar spine; slab selective.

Figure 1

3D slab-selective UTE sequences. The 3D UTE sequence enables slab selection by using a half, soft pulse for excitation together with a slice-selective gradient. After excitation, the spatial encoding gradient is turned on and simultaneous data acquisition begins.

Figure 2

UTE images with five different resolutions (0.9, 1.0, 1.2, 1.6, and 2 mm³) of a healthy volunteer (29-year-old female). The corresponding UTE zoomed-in images of L1 (blue boxes in second row) are shown for better comparison. The resolution with a voxel size of 1.2 mm³ shows a good balance between image SNR and bone structure visualization.

Figure 3

Bland-Altman plots of differences between 3D CT and 3D UTE MRI for vertebral body height (A) and vertebral body length (B) measured on the mid-sagittal plane from the in vivo study. A solid line represents the mean of all differences (bias), while a dashed line indicates the 95% interval confidence of agreement. All measurements were within the 95% confidence interval limits.

Figure 4

Correlation between CT and the 3D slab-selective UTE MRI sequence in a 72-year-old male with low back pain (T11-L5). Compression fractures are seen in T12 and L3 on the CT image (arrow and arrowhead in (A), respectively), showing excellent anatomic correlation with the 3D slab-selective UTE sequence (arrow and arrowhead in (B), respectively). Coronal (C, D) and axial (E–H) images also showed excellent anatomic correlation of the fractures between the CT scan and the 3D slab-selective UTE sequence (arrows in (E–H). Note the retropulsion of a bone fragment in T12 narrowing the spinal canal as well as its proximity to the spinal cord which can be seen on the UTE sequence (yellow arrowheads in (B, F).

Figure 5

Correlation between sagittal and coronal CT images and the 3D slab-selective UTE MRI sequence obtained three months apart in a 55-year-old male with low back pain. A compression fracture is seen in L1 on both the CT images (white arrowheads in A, D) and clinical MR image (C) associated with retropulsion of the posterior vertebral body cortex (yellow arrowheads in A). The 3D slab-selective UTE sequence shows the progression of the fracture with further loss of vertebral body height and fragmentation of the upper endplate (white arrowheads in B, E) in comparison to CT images. Note also the retropulsion of the posterior vertebral body cortex (yellow arrowheads in B).

Figure 6

Anatomical correlation between coronal CT and the 3D slab-selective UTE MRI sequence in a 75-year-old male with low back pain. Note the bilateral facet joint osteoarthritis with subchondral irregularities and cysts (arrows).

Figure 7

Sagittal T₂w-FS and T₁w-FSE images (A, B) and sagittal (C), coronal (D), and axial (E, F) 3D slab-selective UTE images from a 42-year-old male patient with low back pain (T12-L5). Note the Schmorl’s node in the superior endplate of L3 (arrows in (A, B) which demonstrates excellent anatomic correlation with the 3D slab-selective UTE sequence on the sagittal (C), coronal (D), and axial (E, F) images (arrows). Another Schmorl’s node can be seen in the inferior endplate of L2 [arrowheads in (D, E)].