Sodium MRI of the Lumbar Intervertebral Discs of the Human Spine: An Ex Vivo Study

Abstract

Background: Lower back pain affects 75%-85% of people at some point in their lives. The detection of biochemical changes with sodium (²³Na) MRI has potential to enable an earlier and more accurate diagnosis.

Purpose: To measure ²³Na relaxation times and apparent tissue sodium concentration (aTSC) in ex-vivo intervertebral discs (IVDs), and to investigate the relationship between aTSC and histological Thompson grade.

Study type: Ex-vivo.

Specimen: Thirty IVDs from the lumbar spines of 11 human body donors (4 female, 7 male, mean age 86 ± 8 years).

Field strength/sequence: 3 T; density-adapted 3D radial sequence (DA-3D-RAD).

Assessment: IVD ²³Na longitudinal (T₁), short and long transverse (T_2s* and T_2l*) relaxation times and the proportion of the short transverse relaxation (p_s) were calculated for one IVD per spine sample (11 IVDs). Furthermore, aTSCs were calculated for all IVDs. The degradation of the IVDs was assessed via histological Thompson grading.

Statistical tests: A Kendall Tau correlation (τ) test was performed between the aTSCs and the Thompson grades. The significance level was set to P < 0.05.

Results: Mean ²³Na relaxation parameters of a subset of 11 IVDs were T₁ = 9.8 ± 1.3 msec, T_2s* = 0.7 ± 0.1 msec, T_2l* = 7.3 ± 1.1 msec, and p_s = 32.7 ± 4.0%. A total of 30 IVDs were examined, of which 3 had Thompson grade 1, 4 had grade 2, 5 had grade 3, 5 had grade 4, and 13 had grade 5. The aTSC decreased with increasing degradation, being 274.6 ± 18.9 mM for Thompson grade 1 and 190.5 ± 29.5 mM for Thompson grade 5. The correlation between whole IVD aTSC and Thompson grade was significant and strongly negative (τ = -0.56).

Data conclusion: This study showed a significant correlation between aTSC and degenerative IVD changes. Consequently, aTSC has potential to be useful as an indicator of degenerative spinal changes.

Evidence level: 2 TECHNICAL EFFICACY: Stage 1.

Keywords: 23Na MRI; 23Na relaxation times; IVD; ex vivo; intervertebral disc; sodium MRI.

FIGURE 1

Example region‐of‐interest (ROI) placement illustrated by overlaying the ROI boundaries onto the respective ¹H image. ROI number 1 corresponds to the nucleus pulposus (NP), ROI number 2 encapsulates the annulus fibrosus (AF), ROIs number 3, 4, and 5 are the reference phantoms with the ²³Na concentrations 200 mM, 100 mM, and 50 mM. It is important to note, that ROI number 2 does not include the area of ROI number 1, because the NP is not part of the AF. In the sagittal slices, the innermost 60% of the intervertebral disc (IVD) was defined automatically using a matlab script as NP in elliptical shape, whereas the rest of the IVD was defined as AF.

FIGURE 2

Example fits of the ²³Na relaxation time data of an intervertebral disc (IVD) with a Thompson grade of 2. (a) For the determination of T ₁, a monoexponential fit was performed, resulting in ²³Na T ₁ = 10.7 msec and R ² = 0.998. (b) The data for T ₂* was fitted biexponentially due to the properties of the ²³Na nucleus, resulting in the values ²³Na T _2s* = 0.7 msec, ²³Na T _2l* = 7.0 msec, p _s  = 35.2%, and R ² = 0.999. The parameter p _s indicates the proportion that the short relaxation time T _2s* has in the total transversal relaxation.

FIGURE 3

Scatter plots with linear regressions of calculated aTSCs and their respective Thompson grades for (a) NP, (b) AF, and (c) the whole intervertebral disc (IVD). The Kendall Tau test resulted in a strongly negative correlation for (a) the NP (τ = −0.52) and (c) the whole IVD (τ = −0.56). For (b) the AF the Kendall Tau test resulted in a moderately negative correlation (τ = −0.48).

FIGURE 4

(a, c) Anatomical images for Thompson grading and (b, d) aTSC overlays onto the acquired ¹H MRI images. Figure (a) shows an intervertebral disc (IVD) with a Thompson grade of 2 and a mean aTSC of 371.7 ± 42.4 mM for the NP, 258.7 ± 37.4 mM for the AF, and 298.0 ± 92.7 mM for the whole IVD. Figure (c) shows an IVD with a Thompson grade of 5 and a mean aTSC of 201.1 ± 45.8 mM for the NP, 163.0 ± 46.6 mM for the AF, and 169.2 ± 48.5 mM for the whole IVD. Figures (b) and (d) present the corresponding MR images.