In-vivo characterization of the lumbar annulus fibrosus in adults with ultrasonography and shear wave elastography

In vivo characterization of intervertebral disc (IVD) mechanical properties and microstructure could give an insight into the onset and progression of disc pathologies. Ultrasound shearwave elastography provided promising results in children, but feasibility in adult lumbar discs, which are deep in the abdomen, was never proved. The aim of this work was to determine the feasibility and reliability of ultrasound assessment of lumbar IVD in adults. Thirty asymptomatic adults were included (22 to 67 years old). Subjects were lying supine, and the annulus fibrosus of the L3-L4 IVD was imaged by conventional ultrasonography and shearwave elastography. Shear wave speed (SWS) and lamellar thickness were measured. Reliability was determined through repeated measurements acquired by three operators. Average SWS in AF at the L3L4 level was 4.0 ± 0.9 m/s, with an inter-operator uncertainty of 8.7%, while lamellar thickness was 255 ± 27 µm with an uncertainty of 9.6%. Measurement was not feasible in one out of four subjects with BMI > 24 kg/m² (overweight). Ultrasound assessment of annulus fibrosus revealed feasible, within certain limitations, and reproducible. This method gives an insight into disc microstructure and mechanical properties, and it could be applied for the early detection or follow-up of disc pathologies.

Ultrasound Shear Wave Elastography of the Intervertebral Disc and Idiopathic Scoliosis: A Systematic Review

Ultrasound shear wave elastography is a radiation-free and low-cost technique for evaluating the mechanical properties of different tissues. This study systematically reviewed all relevant literature on shear wave elastography of the intervertebral disc. The purpose was twofold: first, to determine the validity of the elastography method, that is, the correlation between elastographically measured shear wave speed and disc mechanical properties, and inter-/intra-operator reliability; and second, to explore if disc elastography is potentially useful in identifying children at risk for idiopathic scoliosis. This systematic review was performed according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analyses) guidelines. A comprehensive search was performed in PubMed and Embase, and study quality was assessed using the AXIS (Appraisal Tool for Cross-sectional Studies) critical appraisal instrument. Seven articles were included. Three animal ex vivo studies reported moderate-to-good correlations between shear wave speed and disc mechanical properties (r = 0.45-0.81). Three studies reported high intra-operator repeatability (intra-class correlation coefficient [ICC] 0.94-0.99) and inter-operator reproducibility (ICC 0.97-0.98). Four clinical studies measured shear wave speed in asymptomatic children. Two studies reported significantly higher shear wave speeds in scoliosis patients compared with healthy controls, measured in discs both inside and outside the scoliotic curve. In conclusion, shear wave elastography appears reliable in assessing intervertebral disc mechanical characteristics. Despite its promising capabilities to distinguish patients with asymptomatic from those with pathological discs, the exact correlation between disc mechanical properties and shear wave speed remains unclear.

Sonographic Assessment of Human Lumbar Intervertebral Disks: A Cadaveric Study

Objectives:

The intervertebral disk has traditionally been imaged by magnetic resonance imaging (MRI); however, advances in sonography mean it can now be visualized with this modality. The objectives of this human cadaveric study were to visualize the internal structure of the lumbar intervertebral disks and map any defects. Shear wave sonography was explored as a method for assessing the disks.

Materials and Methods:

In a human cadaver, L4-L5 and L5-S1 disks were imaged with sonography through the anterior abdominal wall and directly through the anterior longitudinal ligament. Gray-scale images and shear wave elastography velocities were obtained. An MRI was performed for image comparison.

Results:

Defects in the disks were clearly seen with sonography, imaging through the anterior abdominal wall and also directly through the anterior longitudinal ligament. The defects identified on sonography were less well visualized on MRI. Shear wave velocities could only be obtained from the anterior aspect of the disk and were unreliable, primarily owing to the stiffness of the tissues.

Conclusion:

Sonography can provide an accurate map of defects within the disk, corresponding with MRI. Shear wave elastography should be used with caution in the human cadaveric intervertebral disk, acknowledging the many confounding factors influencing the measurements.

Large-scale functional ultrasound imaging of the spinal cord reveals in-depth spatiotemporal responses of spinal nociceptive circuits in both normal and inflammatory states

Despite a century of research on the physiology/pathophysiology of the spinal cord in chronic pain condition, the properties of the spinal cord were rarely studied at the large-scale level from a neurovascular point of view. This is mostly due to the limited spatial and/or temporal resolution of the available techniques. Functional ultrasound imaging (fUS) is an emerging neuroimaging approach that allows, through the measurement of cerebral blood volume, the study of brain functional connectivity or functional activations with excellent spatial (100 μm) and temporal (1 msec) resolutions and a high sensitivity. The aim of this study was to increase our understanding of the spinal cord physiology through the study of the properties of spinal hemodynamic response to the natural or electrical stimulation of afferent fibers. Using a combination of fUS and ultrasound localization microscopy, the first step of this study was the fine description of the vascular structures in the rat spinal cord. Then, using either natural or electrical stimulations of different categories of afferent fibers (Aβ, Aδ, and C fibers), we could define the characteristics of the typical hemodynamic response of the rat spinal cord experimentally. We showed that the responses are fiber-specific, located ipsilaterally in the dorsal horn, and that they follow the somatotopy of afferent fiber entries in the dorsal horn and that the C-fiber response is an N-methyl-D-aspartate receptor-dependent mechanism. Finally, fUS imaging of the mesoscopic hemodynamic response induced by natural tactile stimulations revealed a potentiated response in inflammatory condition, suggesting an enhanced response to allodynic stimulations.

Region- and degeneration dependent stiffness distribution in intervertebral discs derived by shear wave elastography

Information on the local stiffness characteristics of the intervertebral disc (IVD) is crucial for the understanding of its structure-function properties in health and disease and may improve numerical modeling. Previous studies have attempted to map local tissue stiffness by sectioning the disc and performing mechanical testing on these discrete tissue units, which is technically challenging and may bias the results. Shear wave elastography (SWE) represents a nondestructive alternative that can provide spatially continuous elasticity estimates. We investigated the feasibility of SWE for human intervertebral disc elasticity mapping in a laboratory setting. To this end, global spinal segment mechanical behavior was determined in 6 loading directions and served as ground truth data for the validation of the approach. Subsequently, the cranial spinal vertebra was removed and shear wave elastographic scans of the IVD were acquired. SWE-measurements were reconstructed into three-dimensional elastographic maps, discretized into distinct IVD regions and correlated with global segment mechanical parameters. SWE-derived Young's modulus estimates were compared among different regions and as a function of their state of degeneration. We found annulus shear wave speed to be moderately correlated with segment mechanical behavior irrespective of the loading direction whereas shear wave speed in the nucleus pulposus showed a very weak association (mean (SD) absolute Pearson correlation coefficients: 0.51 (0.14) and 0.17 (0.12), respectively). Young's modulus mapping of the intervertebral disc revealed stiffness to be highest in the ventral annulus with a stiffness decrease both circumferentially towards the dorsal aspect as well as towards the center of the disc. SWE hence provides a valid alternative to disc sectioning and piecewise mechanical testing.

Is a simplified Finite Element model of the gluteus region able to capture the mechanical response of the internal soft tissues under compression?

BACKGROUND

Internal soft tissue strains have been shown to be one of the main factors responsible for the onset of Pressure Ulcers and to be representative of its risk of development. However, the estimation of this parameter using Finite Element (FE) analysis in clinical setups is currently hindered by costly acquisition, reconstruction and computation times. Ultrasound (US) imaging is a promising candidate for the clinical assessment of both morphological and material parameters.

METHOD

The aim of this study was to investigate the ability of a local FE model of the region beneath the ischium with a limited number of parameters to capture the internal response of the gluteus region predicted by a complete 3D FE model. 26 local FE models were developed, and their predictions were compared to those of the patient-specific reference FE models in sitting position.

FINDINGS

A high correlation was observed (R = 0.90, p-value < 0.01). A sensitivity analysis showed that the most influent parameters were the mechanical behaviour of the muscle tissues, the ischium morphology and the external mechanical loading.

INTERPRETATION

Given the progress of US for capturing both morphological and material parameters, these results are promising because they open up the possibility to use personalised simplified FE models for risk estimation in daily clinical routine.

Shear wave elastography of lumbar annulus fibrosus in adolescent idiopathic scoliosis before and after surgical intervention

OBJECTIVES

To determine lumbar intervertebral disc properties with shear wave elastography in adolescent idiopathic scoliotic (AIS) patients before and after surgery, and compare them with asymptomatic controls.

METHODS

Twenty-five severe AIS patients with an indication for fusion surgery (15 ± 1.5 years old, the Cobb angle ranging between 40 and 93°) and fifty-nine asymptomatic adolescents (13 ± 2 years old) were included prospectively. Shear wave speed (SWS) was measured in the annulus fibrosus of L3-L4, L4-L5, and L5-S1 discs of each subject. In AIS patients, measurements took place before surgery, and 3 months (N = 13) or 1 year after (N = 12).

RESULTS

No difference was observed between disc levels in any group. When pooling disc levels, SWS was significantly higher in preop AIS patients (4.0 ± 0.5 m/s) than in asymptomatic subjects (3.1 ± 0.5 m/s, p < 0.0001). SWS decreased 3 months postop (3.5 ± 0.3 m/s), and it decreased further towards normal values 1 year after (3.3 ± 0.4 m/s). SWS in preop AIS patients tended to decrease with the Cobb angle (Spearman's rho = - 0.4, p = 0.05).

CONCLUSION

Shear wave elastography measurements showed that discs in AIS patients were altered relative to asymptomatic ones, and this alteration tended to normalize 1 year post fusion surgery. Further studies should aim at determining if bracing of mild scoliosis has an effect on disc properties.

KEY POINTS

• Shear wave elastography shows alteration of annulus fibrosus in adolescent idiopathic scoliosis. • Disc elastography tends to normalize 1 year after surgery.

Microstructural characterization of annulus fibrosus by ultrasonography: a feasibility study with an in vivo and in vitro approach

The main function of the intervertebral disc is biomechanical function, since it must resist repetitive high loadings, while giving the spine its flexibility and protecting the spinal cord from over-straining. It partially owes its mechanical characteristics to the lamellar architecture of its outer layer, the annulus fibrosus. Today, no non-invasive means exist to characterize annulus lamellar structure in vivo. The aim of this work was to test the feasibility of imaging annulus fibrosus microstructure in vivo with ultrasonography. Twenty-nine healthy adolescents were included. Ultrasonographies of L3-L4 disc were acquired with a frontal approach. Annulus fibrosus was segmented in the images to measure the thickness of the lamellae. To validate lamellar appearance in ultrasonographies, multimodality images of two cow tail discs were compared: ultrasonography, magnetic resonance and optical microscopy. In vivo average lamellar thickness was 229.7 ± 91.5 μm, and it correlated with patient body mass index and age. Lamellar appearance in the three imaging modalities in vitro was consistent. Lamellar measurement uncertainty was 7%, with good agreement between two operators. Feasibility of ultrasonography for the analysis of lumbar annulus fibrosus structure was confirmed. Further work should aim at validating measurement reliability, and to assess the relevance of the method to characterize annulus alterations, for instance in disc degeneration or scoliosis.

ELASTICITY MEASUREMENTS BY SHEAR WAVE ELASTOGRAPHY: COMPARISON AND SELECTION OF SHEAR WAVE, RAYLEIGH WAVE AND LAMB WAVE THEORY

Shear wave elastography (SWE) is a powerful method for the diagnosis of tissue disorders or degeneration based on tissue elasticity. In SWE application, it was recognized that the wave speed depends not only on the tissue elasticity but also on the structural shape, leading to different theoretical models. For liver, skin and myocardium, the appropriate theoretical model is known to be shear wave, Rayleigh wave and Lamb wave theory, respectively. Therefore, appropriate theoretical model should be adopted for the proper application of SWE. In this study, we verify these theoretical models in gelatin samples of different thicknesses, using experimental and numerical SWE tests. The results indicate that the wave speed was influenced by the ratio of the wavelength and sample thickness and the measurement region. Based on these results, the selection of theoretical model could be divided into three cases, and the appropriate theoretical model can be selected accordingly.

SPEED OF SOUND MEASUREMENT IN PORCINE INTERVERTEBRAL DISCS: AN IN VITRO STUDY

Disc degeneration is associated with premature ageing of intervertebral discs (IVD) and a gradual degradation of the nucleus pulposus (NP) biomechanical properties. The objective of this study is to investigate whether quantitative ultrasound (QUS) technique can be used to determine the speed of sound (SOS) in the NP and to correlate SOS with histological measurements. The ultrasonic measurements are realized with a 3.5[Formula: see text]MHz focused monoelement transducer used in echographic mode. The value of the interspecimen variability of SOS is significantly superior than the reproducibility of the measurements, which indicates that the technique is sensitive to variations of the material properties of the NP. A significant correlation between SOS values and the percentage of physaliphorous cells ratios is obtained ([Formula: see text]) when considering all samples. QUS can be useful to assess the biomechanical properties of the IVD, which may be useful in the context of tissue engineering applications.

Non-invasive assessment of human multifidus muscle stiffness using ultrasound shear wave elastography: A feasibility study

There is a lack of numeric data for the mechanical characterization of spine muscles, especially in vivo data. The multifidus muscle is a major muscle for the stabilization of the spine and may be involved in the pathogenesis of chronic low back pain (LBP). Supersonic shear wave elastography (SWE) has not yet been used on back muscles. The purpose of this prospective study is to assess the feasibility of ultrasound SWE to measure the elastic modulus of lumbar multifidus muscle in a passive stretching posture and at rest with a repeatable and reproducible method. A total of 10 asymptotic subjects (aged 25.5 ± 2.2 years) participated, 4 females and 6 males. Three operators performed 6 measurements for each of the 2 postures on the right multifidus muscle at vertebral levels L2-L3 and L4-L5. Repeatability and reproducibility have been assessed according to ISO 5725 standard. Intra-class correlation coefficients (ICC) for intra- and inter-observer reliability were rated as both excellent [ICC=0.99 and ICC=0.95, respectively]. Reproducibility was 11% at L2-L3 level and 19% at L4-L5. In the passive stretching posture, shear modulus was significantly higher than at rest ( µ < 0.05). This preliminary work enabled to validate the feasibility of measuring the shear modulus of the multifidus muscle with SWE. This kind of measurement could be easily introduces into clinical routine like for the medical follow-up of chronic LBP or scoliosis treatments.

Lumbar annulus fibrosus biomechanical characterization in healthy children by ultrasound shear wave elastography

OBJECTIVES

Intervertebral disc (IVD) is key to spine biomechanics, and it is often involved in the cascade leading to spinal deformities such as idiopathic scoliosis, especially during the growth spurt. Recent progress in elastography techniques allows access to non-invasive measurement of cervical IVD in adults; the aim of this study was to determine the feasibility and reliability of shear wave elastography in healthy children lumbar IVD.

METHODS

Elastography measurements were performed in 31 healthy children (6-17 years old), in the annulus fibrosus and in the transverse plane of L5-S1 or L4-L5 IVD. Reliability was determined by three experienced operators repeating measurements.

RESULTS

Average shear wave speed in IVD was 2.9 ± 0.5 m/s; no significant correlations were observed with sex, age or body morphology. Intra-operator repeatability was 5.0 % while inter-operator reproducibility was 6.2 %. Intraclass correlation coefficient was higher than 0.9 for each operator.

CONCLUSIONS

Feasibility and reliability of IVD shear wave elastography were demonstrated. The measurement protocol is compatible with clinical routine and the results show the method's potential to give an insight into spine deformity progression and early detection.

KEY POINTS

• Intervertebral disc mechanical properties are key to spine biomechanics • Feasibility of shear wave elastography in children lumbar disc was assessed • Measurement was fast and reliable • Elastography could represent a novel biomarker for spine pathologies.

Non-invasive biomechanical characterization of intervertebral discs by shear wave ultrasound elastography: a feasibility study

OBJECTIVES

Although magnetic resonance is widely spread to assess qualitatively disc morphology, a simple method to determine reliably intervertebral disc status is still lacking. Shear wave elastography is a novel technique that allows quantitative evaluation of soft-tissues' mechanical properties. The aim of this study was to assess preliminary the feasibility and reliability of mechanical characterization of cervical intervertebral discs by elastography and to provide first reference values for asymptomatic subjects.

METHODS

Elastographic measurements were performed to determine shear wave speed (SWS) in C6-C7 or C7-T1 disc of 47 subjects; repeatability and inter-operator reproducibility were assessed.

RESULTS

Global average shear wave speed (SWS) was 3.0 ± 0.4 m/s; measurement repeatability and inter-user reproducibility were 7 and 10%, respectively. SWS was correlated with both subject's age (p = 1.3 × 10(-5)) and body mass index (p = 0.008).

CONCLUSIONS

Shear wave elastography in intervertebral discs proved reliable and allowed stratification of subjects according to age and BMI. Applications could be relevant, for instance, in early detection of disc degeneration or in follow-up after trauma; these results open the way to larger cohort studies to define the place of this technique in routine intervertebral disc assessment.

KEY POINTS

A simple method to obtain objectively intervertebral disc status is still lacking. Shear wave elastography was applied in vivo to assess intervertebral discs. Elastography showed promising results in biomechanical disc evaluation. Elastography could be relevant in clinical routine for intervertebral disc assessment.