Diagnostic value of water-fat-separated images and CT-like susceptibility-weighted images extracted from a single ultrashort echo time sequence for the evaluation of vertebral fractures and degenerative changes of the spine

Abstract

Objectives: To evaluate the performance of single-echo Dixon water-fat imaging and computed tomography (CT)-like imaging based on a single ultrashort echo time (sUTE) MR sequence for imaging of vertebral fractures as well as degenerative bone changes of the spine in comparison to conventional CT and MR sequences.

Methods: Thirty patients with suspected acute vertebral fractures were examined using a 3-T MRI, including an sUTE sequence as well as short-tau inversion recovery (STIR) and T1-weighted sequences. During postprocessing, water-fat separation was performed by solving the smoothness-constrained inverse water-fat problem based on a single-complex UTE image. By removing the unwanted low-frequency phase terms, additional MR-based susceptibility-weighted-like (SW-like) images with CT-like contrast were created. Two radiologists evaluated semi-quantitative and quantitative features of fractures and degenerative changes independently and separately on CT and MR images.

Results: In total, all 58 fractures were accurately detected of whom 24 were correctly classified as acute fractures with an edema detected on the water-fat-separated UTE images, using STIR and T1w sequences as standard of reference. For the morphological assessment of fractures and degenerative changes, the overall agreement between SW-like images and CT was substantial to excellent (e.g., Genant: κ 0.90 (95% confidence interval 0.54-1.00); AO/Magerl: κ 0.75 (95% confidence interval 0.43-1.00)). Overall inter-reader agreement for water-fat-separated UTE images and SW-like images was substantial to almost perfect.

Conclusion: Detection and assessment of vertebral fractures and degenerative bone changes of the spine were feasible and accurate using water-fat-separated images as well as SW-like images, both derived from the same sUTE-Dixon sequence.

Key points: • The detection of acute vertebral fractures was feasible using water-fat-separated images and CT-like images reconstructed from one sUTE sequence. • Assessment of the vertebral fractures using SW-like images with CT-like contrast was found to be comparable to conventional CT. • sUTE imaging of the spine can help reduce examination times and radiation exposure.

Keywords: Bone marrow edema; Magnetic resonance imaging (MRI); Ultrashort echo time (UTE) imaging; Vertebral fractures.

Fig. 1

Standard short-tau inversion recovery sequence image (STIR) showing a bone marrow edema of an acute fracture of the second lumbar vertebra (A). UTE-derived water-separated image showing an edema equivalent to the STIR sequence (B). Standard T1-weighted sequence image (C) and correlating to UTE-derived fat-separated image (D) both showing a signal reduction within the fractured vertebra

Fig. 2

UTE-derived SW-like images with CT-like contrast and corresponding water- and fat-separated images (A–G) showing an acute compression fracture in L2. For postprocessing of SW-like images, the input UTE phase was corrected and unwanted phase terms were removed. Afterwards phase masks were applied to the UTE magnitude image to increase the contrast of areas with negative phase. Note the increasing contrast of the vertebrae with the increasing number of phase masks applied (A S′₁ with phase mask applied once and E S′₅ with phase mask applied 5 times), in comparison to the conventional CT scan (H). F and G show the sUTE-Dixon water- and fat-separated images, respectively

Fig. 3

STIR images of acute vertebral fractures showing hyperintense edema-like signal in L1 (A), L5 (B), and L3 (C). Corresponding water-separated sUTE images showing an equivalent bone marrow edema within the fractured vertebrae (D–F)

Fig. 4

UTE-derived SW-like images with CT-like contrast of an acute compression fracture of the 1st lumbar vertebra (A). The fracture can be evaluated equivalent compared to the conventional CT scan (B)