Visualization of lumbar nerves using reduced field of view diffusion tensor imaging in healthy volunteers and patients with degenerative lumbar disorders

Abstract

Objective: We investigated high resolution diffusion tensor imaging (DTI) of lumbar nerves with reduced field of view (rFOV) using 3 T MRI.

Methods: DTI measured with rFOV was compared with conventional FOV (cFOV) 3.0 T MRI in 5 healthy volunteers and 10 patients with degenerative lumbar disorders. The intracanal, foramina and extraforamina of the L5 nerve were established as the regions of interest and fractional anisotropy (FA) values and apparent diffusion coefficient (ADC) values were measured. Image quality for tractography and FA maps and ADC maps, interindividual and intraindividual reliability of FA and ADC, and signal-to-noise (SNR) were studied.

Results: Both of image qualities with tractography, FA map and ADC map showed that lumbar nerves were more clearly imaged with the rFOV. Intraindividual reliability was higher with rFOV compared with the conventional method for ADC values, while interindividual reliability was higher for both FA values and ADC values with the rFOV method over the conventional method (p < 0.05). Significantly higher SNR was obtained with rFOV compared with cFOV in the spinal canal (p < 0.05).

Conclusion: rFOV enabled clearer imaging of the lumbar nerve, allowing for more accurate measurement of FA and ADC values. Significantly higher SNR was obtained with rFOV compared with cFOV in the spinal canal. To our knowledge, this research showed for the first time the usefulness of rFOV in patients with degenerative lumbar disorders. High resolution DTI using rFOV may become useful in clinical applications because visualization of nerve entrapments and quantification of DTI parameters may allow more accurate diagnoses of lumbar nerve dysfunction. Advances in knowledge: Compared with traditional methods, rFOV allows for clear imaging of the lumbar nerve and enables accurate measurements of the FA and ADC values. High-resolution DTI with rFOV may be used to visualize nerve entrapments and allow for more accurate diagnosis of DTI parameter quantification with opportunities for clinical applications.

Figure 1.

SNR values were measured usingb=0 images. ROIs were placed in the right psoas muscle (a), spinal canal (b) and left psoas muscle (c) in the rFOV images at the L4-5 level. ROIs were placed on the right psoas muscle (d), spinal canal (e) and left psoas muscle (f) in the cFOV images at the L4-5 level. T₂ weighted cube images which is isotropic volume imaging at same L4/5 level (g).

Figure 2.

Tractography of the L5 and S1 nerves in a healthy volunteer. (a) rFOV and (b) cFOV. Expanded images of the left L5 nerve at the same magnification. (c) rFOV and (d) cFOV. Fiber counts are higher with rFOV (a, c) vs cFOV (b, d) allowing for clearer imaging of the lumbar nerve.

Figure 3.

Axial FA map (a, b) and ADC map (c, d) at L4/5 level in a healthy volunteer (a, c) rFOV (b, d) cFOV FA map and ADC map quality is higher with rFOV allowing for clearer imaging of the nerve root (arrow head) and spinal canal (arrow).

Figure 4.

SNR values from the cFOV (red bar) and rFOV (blue bar). In the spinal canal, SNR values were 4.866 ± 3.703 for rFOV and 1.840 ± 0.777 for cFOV, showing significantly higher SNR with a rFOV compared with cFOV. In the left iliopsoas muscle, SNR values were 4.203 ± 1.800 for rFOV and 4.584 ± 1.537 for cFOV. In the right iliopsoas muscle, SNR values were 4.429 ± 1.843 for rFOV and 5.166 ± 0.913 for cFOV. No significant difference was found between SNR values of rFOV and cFOV in the iliopsoas muscles. cFOV,conventional field of view; rFOV, reduced field of view; SNR, signal-to-noiseratio.

Figure 5.

T₂ weighted MR images of the patient with left L5 lumbar foraminal stenosis. (a) Central sagittal image and (b) parasagittal image. The white arrow indicates left L5 foraminal stenosis.

Figure 6.

Tractography (a–d: using Functool software, e: using TrackVis) of the patient with left L5 lumbar foraminal stenosis. (a) rFOV image, (b) cFOV image, (c) expanded images of rFOV and (d) expanded images of cFOV. The rFOV images (a, c) clearly show signs of nerve pinching at the L5 foramen (white arrow). rFOV image (e) allowed higher tract fiber counts and clearer image of L5 lumbar nerve compared with cFOV image (f) using TrackVis. (g, e) No injury (FA value normal: white) to injury (FA value decreased: orange) and FA values are expressed in colours to show the compressed region in orange, indicating decreased FA (white arrowhead).