Evaluation of image quality of diffusion weighted readout segmentation of long variable echo-trains MR pulse sequence for lumbosacral nerve imaging at 3T

Abstract

Background: Limited magnetic resonance (MR) pulse sequences facilitate lumbosacral nerve imaging with acceptable image quality. This study aimed to evaluate the impact of parameter modification for Diffusion Weighted Image (DWI) using Readout Segmentation of Long Variable Echo-trains (RESOLVE) sequence with opportunities for improving the visibility of lumbosacral nerves and image quality.

Methods: Following ethical approval and acquisition of informed consent, imaging of an MR phantom and twenty healthy volunteers (n=20) was prospectively performed with 3T MRI scanner. Acquired sequences included standard two-dimensional (2D) turbo spin echo sequences and readout-segmented echo-planar imaging (EPI) DWI-RESOLVE using three different b-values b-50, b-500 and b-800 s/mm². Signal-to-noise ratio (SNR), apparent diffusion coefficient (ADC) and nerve size were measured. Two musculoskeletal radiologists evaluated anatomical structure visualisation and image quality. Quantitative and qualitative findings for healthy volunteers were investigated for differences using Wilcoxon signed-rank and Friedman tests, respectively. Inter and intra-observer agreement was determined with κ statistics.

Results: Phantom images revealed higher SNR for images with low b-values with 206.1 (±10.9), 125.1 (±45.2) and 59.2 (±17.8) for DWI-RESOLVE images acquired at b50, b500 and b800, respectively. Comparable results were found for SNR, ADC and nerve size across normal right and left sided for healthy volunteer images. The SNR findings for b-50 images were higher than b-500 and b-800 images for healthy volunteer images. The qualitative findings ranked images acquired using b-50 and b-500 images significantly higher than corresponding b-800 images (P<0.05). Inter and intra-observer agreements for evaluation across all b-values ranged from 0.59 to 0.81 and 0.83 to 0.92, respectively.

Conclusions: The modified DWI-RESOLVE images facilitated visualization of the normal lumbosacral nerves with acceptable image quality, which support the clinical applicability of this sequence.

Keywords: Magnetic resonance imaging; diffusion weighted imaging; spine.

Figure 1

Image of the copper sulphate-based MR image quality phantom. The inner cylinder was filled with deionized water doped with copper sulphate and sodium chloride. The outer ring was filled with corn oil. MR, magnetic resonance.

Figure 2

The green circles in diffusion weighted readout segmentation of long variable echo-trains (DW-RESOLVE) images (A) b=50, (B) b=500, (C) b=800 demonstrate examples of regions of interest (ROI) placements for S1 nerves. Image (D) b=50, the green dimensional calipers for size measurements.

Figure 3

The relationship between ADC values and lumbosacral nerves sides for both ADC 500 and ADC 800. Circle = potential outlier, defined as value more than 1.5 interquartile above third quartile or below first quartile. Star = extreme value, more than 3 interquartiles above 3^rd quartile or below 1^st quartile. ADC, apparent diffusion coefficient.

Figure 4

Axial DW-RESOLVE images for a 51-year-old female healthy volunteer highlighting the visualization performance for the lumbosacral nerves. While L5 nerves (arrows) were well visualized with b-50 images (A), L5 nerves (arrow heads) were poorly visualized with b-800 images (B). The signal intensity and therefore the conspicuity of the sacral nerves is slightly reduced in (B). DW-RESOLVE, diffusion weighted readout segmentation of long variable echo-trains.

Figure 5

Axial DW-RESOLVE images of lumbosacral region in a healthy volunteer (A) b=50 and (B) b=800 demonstrating the effect of susceptibility artifact. The susceptibility effect was minor and slight signal loss was evident in the right side compared to the left (A). The image demonstrates moderate loss of signal on the right side of the body compared to the left (B). The right sciatic nerve (arrow head) is hypointense relative to the sciatic nerve (arrow) in the left side. The normal left sciatic nerve (arrow) appears asymmetric due to this spurious artifact. DW-RESOLVE, diffusion weighted readout segmentation of long variable echo-trains.

Figure 6

Axial DW-RESOLVE images for a 32-year-old female healthy volunteer demonstrating the contrast between lumbosacral nerves and blood vessels. The contrast between right L5 nerve root (arrows) and adjacent blood vessels (arrow heads) was poor with DWI-RESOLVE b-50 image (A). The contrast between L5 (arrows) and blood vessels (arrow heads) was excellent with DW-RESOLVE b-800 image as flowing blood vessel generated hypointense signal compared to the L5 nerves (B). DW-RESOLVE, diffusion weighted readout segmentation of long variable echo-trains.