Paraspinal Muscle in Chronic Low Back Pain: Comparison Between Standard Parameters and Chemical Shift Encoding-Based Water-Fat MRI

Abstract

Background: Paraspinal musculature (PSM) is increasingly recognized as a contributor to low back pain (LBP), but with conventional MRI sequences, assessment is limited. Chemical shift encoding-based water-fat MRI (CSE-MRI) enables the measurement of PSM fat fraction (FF), which may assist investigations of chronic LBP.

Purpose: To investigate associations between PSM parameters from conventional MRI and CSE-MRI and between PSM parameters and pain.

Study type: Prospective, cross-sectional.

Population: Eighty-four adults with chronic LBP (44.6 ± 13.4 years; 48 males).

Field strength/sequence: 3-T, T1-weighted fast spin-echo and iterative decomposition of water and fat with echo asymmetry and least squares estimation sequences.

Assessment: T1-weighted images for Goutallier classification (GC), muscle volume, lumbar indentation value, and muscle-fat index, CSE-MRI for FF extraction (L1/2-L5/S1). Pain was self-reported using a visual analogue scale (VAS). Intra- and/or interreader agreement was assessed for MRI-derived parameters.

Statistical tests: Mixed-effects and linear regression models to 1) assess relationships between PSM parameters (entire cohort and subgroup with GC grades 0 and 1; statistical significance α = 0.0025) and 2) evaluate associations of PSM parameters with pain (α = 0.05). Intraclass correlation coefficients (ICCs) for intra- and/or interreader agreement.

Results: The FF showed excellent intra- and interreader agreement (ICC range: 0.97-0.99) and was significantly associated with GC at all spinal levels. Subgroup analysis suggested that early/subtle changes in PSM are detectable with FF but not with GC, given the absence of significant associations between FF and GC (P-value range: 0.036 at L5/S1 to 0.784 at L2/L3). Averaged over all spinal levels, FF and GC were significantly associated with VAS scores.

Data conclusion: In the absence of FF, GC may be the best surrogate for PSM quality. Given the ability of CSE-MRI to detect muscle alterations at early stages of PSM degeneration, this technique may have potential for further investigations of the role of PSM in chronic LBP.

Level of evidence: 2 TECHNICAL EFFICACY STAGE: 2.

Keywords: Goutallier classification; chemical shift; fat fraction; low back pain; paraspinal musculature.

FIGURE 1:

Goutallier classification (GC) for paraspinal musculature (PSM). Representative axial T1-weighted fast spin-echo (FSE) images for the different GC scores: 0—normal muscle (a); 1—some fatty streaks (b); 2—less than 50% fatty muscle atrophy (c); 3—50% fatty muscle atrophy (d); and 4—greater than 50% fatty muscle atrophy (e).

FIGURE 2:

Determination of the lumbar indentation value (LIV). Representative axial T1-weighted fast spin-echo (FSE) image demonstrating the measurement of the LIV at the level of L3/L4, which amounted to 13.22 mm. The LIV is measured as the distance equal to the length of the bulge of the muscle to the attachment of the spinous process.

FIGURE 3:

Segmentation of paraspinal musculature (PSM) and subcutaneous fat. Manual segmentations of the multifidus muscles (1) and erector spinae muscles (2) of both sides are shown at level L3/L4, together with a circular region of interest (ROI) placed in the subcutaneous fat (3). The segmentations of the multifidus and erector spinae muscles were used for the extraction of the level-wise fat fraction (FF) and muscle volume of PSM (based on the axial slices of an iterative decomposition of water and fat with echo asymmetry and least-squares estimation [IDEAL] sequence). Furthermore, segmentations of these muscles together with the ROIs in subcutaneous fat were used to calculate the muscle-fat index (MFI) as the mean signal intensity (SI) of PSM divided by the mean SI of homogenous subcutaneous fat (based on the axial slices of a T1-weighted fast spin-echo [FSE] sequence).

FIGURE 4:

Anatomical variation of the fat fraction (FF) of paraspinal musculature (PSM). This graph shows the mean with the standard error of the mean (horizontal lines) for the level-wise FF of PSM (in %) as derived from the segmentations of the multifidus and erector spinae muscles on levels L1/L2, L2/L3, L3/L4, L4/L5, and L5/S1. The difference of the FF of PSM of all levels was significant (α = 0.05) in relation to L5/S1 as the level with the highest FF of PSM.

FIGURE 5:

Fat fraction (FF) of paraspinal musculature (PSM) in relation to the Goutallier classification (GC). This graph shows the mean with the standard error of the mean (horizontal lines) for the FF of PSM (in %) as derived from the segmentations of the multifidus and erector spinae muscles in relation to the semiquantitative scoring according to the GC (0—normal muscle; 1—some fatty streaks; 2—less than 50% fatty muscle atrophy; 3—50% fatty muscle atrophy; and 4—greater than 50% fatty muscle atrophy).