Applicative value of T2 mapping in evaluating lumbosacral nerve root injury induced by lumbosacral disc herniation

Abstract

Background

To alleviate the damage caused by nerve root entrapment mediated by lumbosacral disc herniation (LDH), an imaging method that allows quantitative evaluation of the lumbosacral nerve injury is necessary.

Purpose

To investigate the diagnostic value of magnetic resonance (MR) T2 mapping in nerve root injury caused by LDH.

Material and Methods

A total of 70 patients with unilateral sciatic nerve pain and 35 healthy volunteers were divided into three groups: LDH with nerve root entrapment; LDH without nerve root entrapment; and 35 healthy volunteers. All participants underwent 3.0-T MR with T1-weighted (T1W) imaging, T2-weighted (T2W) imaging, and T2-mapping images. T2 was measured and observed with the left and right nerve roots of the L4-S1 segments in healthy volunteers; the differences between the three groups were compared. T2 and the relaxation rate of nerve root injury were analyzed.

Results

T2 showed significant differences among the three groups (F = 89.494; P = 0.000), receiver operating characteristic curve revealed that the T2 relaxation threshold was 79 ms, the area under curve (AUC) area was 0.86, sensitivity was 0.77, and specificity was 0.74; the T2 relaxation rate was 1.06, the AUC area was 0.88, sensitivity was 0.74, and specificity was 0.85.

Conclusion

T2 mapping could quantitatively evaluate the nerve root injury with lumbar disc degeneration. Hence, it can be used for the clinical evaluation of nerve root entrapment caused by LDH.

Keywords

Lumbar intervertebral disc peripheral nerve magnetic resonance imaging T2 mapping

Introduction

Sciatica is a common clinical symptom. It has been estimated that approximately 5% of the population suffers from peripheral nerve diseases, with lumbosacral disc herniation (LDH) being one of the frequently identified disorders (1). Associated with a high incidence rate and fatalities, LDH can cause nerve root entrapment and damage (2), even disability, which together reduce the quality of life of the affected patients. Moreover, patients with LDH are usually diagnosed by auxiliary imaging. Magnetic resonance (MR) neuroimaging has an excellent ability to demonstrate the anatomical position (3). However, conventional MR imaging (MRI) can qualify the morphological changes only, without achieving the quantification of the severity of nerve root entrapment. To alleviate the damage caused by nerve root entrapment mediated by LDH, an imaging method that allows quantitative evaluation of the lumbosacral nerve injury is necessary. The aim of the present study was to evaluate patients with lumbar disc degeneration by T2 mapping imaging so as to investigate and ascertain the diagnostic value and potential of T2 mapping in nerve root injury caused by lumbar disc degeneration.

Material and Methods

Basic information

A total of 70 patients (60 men, 45 women; age range = 20–65 years; mean age = 41.71 ± 4.65 years) with chronic lower back pain and 35 healthy volunteers with no clinical symptoms were enrolled in the present study. The patient inclusion criteria included unilateral sciatica pain and/or unilateral limb radiating pain of different extent, unilateral lower limb muscle weakness, atrophy and tendon reflex attenuation of different extent; previous MRI scanning having confirmed the protrusion of lumbar disc on one side with or without nerve root entrapment. The patient exclusion criteria included thoracolumbar trauma, surgical history, inflammation, tuberculosis, tumors and other related immunological diseases, and MRI contraindications.

Scanning

MRI was performed on a 1.5-T MRI system (Multvia; Philips, Netherlands), and the coils used included the body coil and spine coil. The scanning involved the L3/4-L5/S1 intervertebral disc. The scanning sequence included T2-weighted (T2W) imaging in the sagittal plane, using the following parameters: TR = 2500 ms; TE = 90 ms; field of view (FOV) = 160 × 300 mm; and NSA = 1, with a 4-mm layer thickness and an acquisition time of 175 s. The scanning was conducted in sagittal T1-weighted (T1W) imaging using the following parameters: TR = 847 ms; TE = 100 ms; FOV = 180 × 340 mm; and NSA = 3, with a 4-mm layer thickness and an acquisition time of 105 s. In addition, the scanning was conducted in transverse T1W imaging as follows: TR = 847 ms; TE = 100 ms; FOV = 150 × 150 mm; and NSA = 2, with a 4-mm layer thickness and an acquisition time of 175 s. Eight echo spin-echo sequences were collected in the coronal T2 mapping as follows: TR = 1000 ms; TE = 13–104 ms; and FOV = 250 × 160 mm, and NSA = 1, with a 2.5-mm layer thickness and an acquisition time of 308 s.

LDH identified by MRI

The nucleus pulposus and fibrous ring of the intervertebral disc were intact in the healthy volunteers, without protrusion, while the bilateral sacral_1 nerve roots ran naturally (Fig. 1a). For patients with LDH without nerve root entrapment, the image in the figure show localized protrusion of nucleus pulposus and a subdural space on the protrusion side, without any contact and compression with nerve roots (Fig. 1b). For patients with LDH with nerve root entrapment, the image in the figure shows prominent protrusion of nucleus pulposus and compression and displacement of the nerve root (Fig. 1c).

Image quality and processing

A technician with >8 years of MRI scanning experience was responsible for examining all participants. The pseudo-color map of T2-mapping images was generated by the postprocessing system, as configured by the Philips model. The coronal position of the first echo of T2 mapping and the transverse T2W images were set as the anatomical control diagram (Fig. 1a, b); a physician with >6 years of MRI diagnosis experience operated on the T2-mapping images. Taking the sacral_1 nerve root as an example, the region of interest (ROI) was outlined at the sacral_1 nerve root on one side (lumbar_5-sacral_1 intervertebral foramen plane), with an ROI of 4–8 mm² (Fig. 1c). We attempted to completely cover the nerve root and avoid the lumbar spine, pelvis, and cerebrospinal fluid, followed by automatically measurement of T2. We selected the same plane on the opposite side. To decrease the errors caused by measurement, each nerve root was simultaneously measured by a physician three times, followed by averaging of the obtained values. T2, lumbar_4, and lumbar_5 nerve roots were measured as mentioned earlier.

Statistical analysis

Statistical analyses were conducted using SPSS version 21.0 (IBM Corp., Armonk, NY, USA). The measured and calculated data were expressed as mean ± standard deviation ( $\bar{x}$ ± Sd). One-way analysis of variance (ANOVA) was used to compare the T2 of the healthy volunteers’ L4-S1 segment on one side. The independent sample t test was used to compare the T2 of the healthy volunteers’ L4-S1 segment on the left and right sides. One-way ANOVA was applied to compare the T2 of the healthy volunteers, patients with LDH without nerve root entrapment, and those with LDH with nerve root entrapment. We calculated the T2 relaxation rate and T2 nerve root on entrapment side/healthy volunteers T2. A receiver operating characteristic (ROC) curve was applied to evaluate the accuracy of diagnosis and estimate the threshold of T2 and its relaxation rate. P < 0.05 was considered statistically significant.

Results

Bilateral nerve root T2 of each segment in healthy volunteers

The nerve root T2 of the L4-S1 segments in healthy volunteers were 70 ± 4, 71 ± 2, and 71 ± 5, respectively, without significant differences (F = 0.940; P = 0.396). The bilateral nerve root T₂ of the L4-S1 segments indicated no differences (t = 0.687, P = 0.496; t = −0.501, P = 0.620; and t = 0.582, P = 0.564) (Table 1).

Table 1.

Left and right nerve segments T2 in healthy volunteers.

	T2 (ms)
Nerve root segment	Left	Right	Average	t	P*
L4	71 ± 4	69 ± 3	70 ± 4	0.687	0.496
L5	73 ± 3	70 ± 5	71 ± 2	−0.501	0.620
S1	73 ± 2	71 ± 3	71 ± 5	0.582	0.564
F	0.283	2.03	0.940	/	/
P*	0.755	0.140	0.396	/	/

Values are given as mean ± SD unless otherwise indicated. Independent sample t test and analysis of variance were performed.

*P < 0.05 indicates significant differences.

Nerve root in the groups with and without entrapment

A total of 35 cases in the non-entrapment group walked normally without any nerve root entrapment. In addition, 35 cases in the entrapment group showed unilateral entrapment of nerve roots, including left involvement in 16 cases and right involvement in 19 cases (Figs. 2 and 3) .

T2 and its relaxation rate of different groups

The mean T2 and its relaxation rate of healthy volunteers were 71 ± 3 ms and 0.99, respectively; the mean T2 and its relaxation rate of patients with LDH without nerve root entrapment were 74 ± 5 ms and 1.01, respectively; the mean T2 and its relaxation rate of patients with LDH with nerve root entrapment were 83 ± 4 ms and 1.01, respectively (Table 2). One-way ANOVA results indicated significant differences in T2 and its relaxation rate of the nerve roots in each group (F = 89.494, P = 0.000; F = 55.419, P = 0 .000), with the homogeneous variance (P = 0.274; P = 0.257). The least significant difference method was employed for multiple comparisons between groups, and the results signified no significant differences in T2 and its relaxation rate between the healthy volunteers and patients with LDH without nerve root entrapment (P = 0.611, P = 0.223). Significant differences were noted between patients with LDH with nerve root entrapment and the healthy volunteers and patients with LDH without nerve root entrapment (P = 0.000; P = 0.000) (Figs. 4 and 5).

Table 2.

T2 and its relaxation rate of the nerve among groups.

Group	T2	T2 relaxation rate
Volunteers	71 ± 3	0.99
Non-entrapment group	74 ± 5	1.01
Entrapment group	83 ± 4	1.19
F	89.494	55.419
P*	0.000	0.000

Values are given as mean ± SD unless otherwise indicated. Independent sample t test and analysis of variance were performed.

*P < 0.05 indicate significant differences.

T2 threshold and its relaxation rate in patients with LDH with and without nerve root entrapment

ROC curve analysis demonstrated that, when compared with the non-entrapment group, the T2 relaxation threshold of the nerve roots in the entrapment group was 79 ms, while the area under the ROC curve (AUC) was 0.86, with a sensitivity of 0.77 and a specificity of 0.74. The T2 relaxation rate was 1.06 and the AUC was 0.88, with a sensitivity of 0.74 and a specificity of 0.85 (Fig. 6).

Discussion

T2 mapping could increase T2 in nerve root entrapment caused by LDH. T2 mapping is a quantitative MR technique for measuring the T2 of tissues; it is sensitive to the changes in the water content or water molecules (4), which is a fixed parameter of tissues. This imaging technology conventionally employs multi-echo spin-echo sequences and offers the advantages of fast imaging, which is insensitive to the non-uniformity of field strength and can hence generate high-quality images (5). This imaging method was applied in the present study.

Diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) are considered the most sensitive imaging techniques for the detection of the peripheral nerve injuries (6,7). Nevertheless, DTI is less stable and reliable than T2 as it can be affected by the performance of different MR equipment or the acquisition parameters.

T2 mapping is more accessible and has been widely employed for cartilage or intervertebral disc measurements (8 –11). Past studies have demonstrated that T2 mapping imaging can be applied to peripheral nerve imaging. Riegler et al. (12) reported significant differences in T2 between healthy volunteers and patients with idiopathic carpal tunnel syndrome, while T2 could be used to evaluate median nerve injury. Chen et al. (4) reported that, in rabbits with sciatic nerve pinch injury, a significant difference occurred in T2 between the nerve on the pinch side and that on the control side. The above-mentioned reports indicate that T2 mapping imaging can evaluate peripheral nerve injury.

Riegler et al. (12) reported that the mean T2 of the median nerve was 21.01 ± 0.65 ms among healthy volunteers and 24.27 ± 0.97 ms in patients with carpal tunnel syndrome. In a study carried out on diabetic peripheral neuropathy, Wang et al. (13) found that the mean T2 of tibial neuropathy was 55.06 ± 4.05 ms and the T2 of the tibial nerve was 45.61 ± 1.86 ms among healthy volunteers. In a study on lumbosacral nerve compression and injury caused by lumbar disc degeneration, only a few studies have so far evaluated lumbosacral nerve degeneration (14,15). Sato et al. (14) investigated lumbar spinal stenosis and found that the mean T2 of the nerve root on the compression side was 173.2 ± 39.9 ms and 124.5 ± 31.1 ms on the healthy side. Sollmann et al. (15) found that the mean T2 of preganglionic, ganglion, and postganglionic nerve roots on the compression side in LDH was 77.3 ± 1.9 ms and 74.8 ± 1.4 ms on the healthy side. In the present study, the mean T2 of the bilateral L4-S1 nerve roots of the healthy volunteers was 71 ± 3 ms, the mean T2 of patients with LDH without nerve root entrapment was 74 ± 9 ms, and the mean T2 of patients with LDH with nerve root entrapment was 87 ± 12 ms. The T2 of the healthy volunteers or patients with LDH with nerve root entrapment was different from those reported in the past two studies, which may be attributed to the type of research methods and participants. Sato et al. (14) indicated that the T2 relaxation threshold was 127 ms, while the T2 relaxation rate was 1.07 as per the ROC curve analysis. T2 on the compression side/healthy side was employed to evaluate the relaxation rate. The authors believed that the T2 relaxation rate was more effective than T2 in judging the extent of lumbosacral nerve injury. Currently, there are only research reports on the T2 relaxation threshold and relaxation rate of lumbosacral nerve injury caused by lumbar disc degeneration. According to the ROC curve analyses, the T2 relaxation threshold of lumbosacral nerve injury was 79 ms and the AUC was 0.86, with a sensitivity of 0.77 and a specificity of 0.74. The T2 relaxation rate was 1.06 and the AUC was 0.88, with a sensitivity of 0.74 and a specificity of 0.85. The T2 relaxation rate was 1.06. This study noted a significant difference between the T2 threshold and T2 value. The involved cases included patients without nerve root entrapment, which may have caused deviation in the values of the T2 threshold. In this study, cases without nerve root entrapment were excluded, to improve the accuracy level. However, the T2 relaxation rate was consistent. Therefore, we believe that the T2 relaxation rate can accurately evaluate nerve root injury, but when evaluating injury, both can accurately reflect the degree of nerve injury.

Dimitrios et al. (16) noted that the difference in the T2 value between the L4 ganglion and postganglionic spinal nerve in healthy populations was attributable to the different tissue components of the ganglion and postganglionic nerve. Behr et al. (17) suggested that the increased water content, vascular permeability, myelin renewal, inflammatory mediators, and axon or myelin breakdown products contributed to the increased T2 level in degenerative nerves. In a study on diabetic peripheral neuropathy, Wang et al. (13) found that T2 in patients with tibial neuropathy was higher than that in patients without tibial neuropathy, while T2 in patients with diabetes without tibial neuropathy was higher than that in healthy volunteers. Thus, T2 mapping can reflect the extent of nerve injury and its severity. In this study, T2 in healthy volunteers and that in patients with LDH without nerve root entrapment showed no significant difference, while T2 in patients with LDH with nerve root entrapment showed a significant increase. We thus speculated that lumbosacral nerve degeneration and injury occurred in patients with LDH with nerve root entrapment, which induced neuroedema and inflammatory changes, while the T2 in healthy volunteers and patients with LDH without nerve root entrapment did not change significantly, indicating no injuries in the lumbosacral nerve.

The present study has some limitations First, this is a retrospective study with a small sample size, thus warranting inclusion of a higher number of cases in the future. Second, this study only discusses the T2 of patients with nerve entrapment before the treatment, but did not evaluate the changes in T2 after the treatment, which needs to be investigated in future studies.

In conclusion, our results suggest that T2 can quantitatively analyze lumbosacral nerve injury and hence can be used to evaluate the extent of nerve root entrapment injury caused by LDH in a clinical setting.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article

ORCID iD

Bing Fan

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