Reduced-distortion diffusion MRI of the craniovertebral junction

Abstract

Background and purpose: CVJ lesion suffers from a high sensitivity to susceptibility and distortion artifacts, which sometimes makes diffusion image difficult to interpret. Our purpose was to evaluate the potential for diffusion MR imaging using RS-EPI compared with SS-EPI in the assessment of the CVJ.

Materials and methods: RS-EPI and SS-EPI DTI images were acquired from 10 healthy volunteers using 3T MRI with a 32-channel head coil. For both sequences, the following parameters were used: 1-mm(2) in-plane resolution; 3-mm section thickness; TR = 5200 ms; 1 acquisition at b = 0 and 12 different encoding directions at b = 1000 seconds/mm(2). The RS-EPI sequence scan time was 9.44 minutes (1 average). The SS-EPI sequence was 9.37 minutes (8 averages). Diffusion tensor calculation and image analysis were performed using DTIStudio software. Diffusion trace images and color-coded fiber orientation maps were evaluated by 2 independent readers for distortion and delineation of fine structure using a semiquantitative scale in selected landmark locations. The absolute distances between the temporal base and the cerebellar contour between the T2-weighted images and the diffusion trace images obtained with RS-EPI and SS-EPI were also compared.

Results: The contours of the temporal lobe and cerebellum were better delineated and distortion artifacts were clearly reduced with the RS-EPI sequence. More fine structures were also visible in the brain stem and cerebellum with the RS-EPI sequence. The amount of distortion was significantly reduced with RS-EPI compared with SS-EPI (P < .01).

Conclusions: The RS-EPI DTI sequence was less prone to geometric distortion than the SS-EPI sequence and allowed a better delineation of CVJ internal structure. Although the acquisition time is still relatively long, the RS-EPI appears as a promising approach to perform DTI studies in CVJ lesions, such as brain stem ischemia, neurodegenerative diseases, brain and skull base tumors, or inflammation.

Fig 1.

40-year-old healthy male volunteer. Coronal MR images of brain obtained at the same level: A, T2-weighted image, (B) RS-EPI diffusion trace image, (C) RS-EPI color-coded fiber orientation map, (D) the fusion image of RS-EPI and T2-weighted image of left temporal lobe, (E) SS-EPI diffusion trace image, (F) SS-EPI color-coded fiber orientation map, (G) the fusion image of SS-EPI and T2-weighted image of left temporal lobe. The natural lateral contour of the left temporal lobe in the T2-weighted image (A) becomes prominently distorted in the SS-EPI image (E), but the convex shape manages to be preserved in the RS-EPI image (B). The decussation of the internal arcuate fibers is seen at the medulla level on the RS-EPI image (C, arrow), more clearly compared with the SS-EPI image (F, arrow). The distance between the contours of the left temporal base is 5.58 mm for RS-EPI (D), and 9.36 mm for SS-EPI (G).

Fig 2.

A, Distortion measurements at the level of the temporal base. B, Distortion measurements at the level of the cerebellum. The absolute distances between the contours of the temporal base (A) and the cerebellum (B), visible on the diffusion trace images for RS-EPI and SS-EPI and the corresponding T2-weighted images, are shown. The amount of distortion is significantly reduced with RS-EPI compared with SS-EPI (P <.01).