Shaokoon Cheng

Rheological properties of the tissues of the central nervous system: A review

Knowledge of the biomechanical properties of central nervous system (CNS) tissues is important for understanding mechanisms and thresholds for injury, and aiding development of computer or surrogate models of these tissues. Many investigations have been conducted to estimate the properties of CNS tissues including under shear, compressive and tensile loading, however there is much variability in this body of literature, making it difficult to separate the material properties from effects that result from a given experimental protocol. This review summarises previous studies of brain and spinal cord properties; discussing their main findings and points of difference, and displays the reported data on comparable scales. Additionally, based on the observed effects of methodological choices on reported tissue properties, recommendations for future studies of brain and spinal cord properties are made.

Movement of the tongue during normal breathing in awake healthy humans

Electromyographic (EMG) activity of the airway muscles suggest that genioglossus is the primary upper airway dilator muscle. However, EMG data do not necessarily translate into tissue motion and most imaging modalities are limited to assessment of the surfaces of the upper airway. In this study, we hypothesized that genioglossus moves rhythmically during the respiratory cycle and that the motion within is inhomogeneous. A ‘tagged’ magnetic resonance imaging technique was used to characterize respiratory‐related tissue motions around the human upper airway in quiet breathing. Motion of airway tissues at different segments of the eupnoeic respiratory cycle was imaged in six adult subjects by triggering the scanner at the end of inspiration. Displacements of the ‘tags’ were analysed using the harmonic phase method (HARP). Respiratory timing was monitored by a band around the upper abdomen. The genioglossus moved during the respiratory cycle. During expiration, the genioglossus moved posteriorly and during inspiration, it moved anteriorly. The degree of motion varied between subjects. The maximal anteroposterior movement of a point tracked on the genioglossus was 1.02 ± 0.54 mm (mean ±s.d.). The genioglossus moved over the geniohyoid muscle, with minimal movement in other muscles surrounding the airway at the level of the soft palate. Local deformation of the tongue was analysed using two‐dimensional strain maps. Across the respiratory cycle, positive strains within genioglossus reached peaks of 17.5 ± 9.3% and negative strains reached peaks of −16.3 ± 9.3% relative to end inspiration. The patterns of strains were consistent with elongation and compression within a constant volume structure. Hence, these data suggest that even during respiration, the tongue behaves as a muscular hydrostat.

Combining MR elastography and diffusion tensor imaging for the assessment of anisotropic mechanical properties : a phantom study

To investigate the anisotropic elasticity of soft tissues using MR elastography (MRE) combined with diffusion tensor imaging (DTI).

The effects of preconditioning strain on measured tissue properties

Characterizing the mechanical properties of soft biological tissues presents a formidable challenge. In order to ensure that the structure of the specimens is at a repeatable reference state, preconditioning is commonly performed before the actual test. However, the exact mechanisms of preconditioning remain unclear and more research on this issue needs to be undertaken so that the methods for preconditioning can be refined to reduce experimental variability. Previous studies have suggested that the choice of preconditioning strain may influence the measured properties. In this study, uniaxial tests were performed on three groups of spinal cord tissues. Two groups (Groups 1 and 2) were preconditioned at 5% strain and tested to 5% and 2% each, while the third (Group 3) was preconditioned at 2% strain and tested to 2%. The peak stress measured at 2% strain for group 3 was 0.0054+/-0.0035MPa and this was significantly higher than group 2 (134%; p=0.015) and group 1 (p=0.005). However, the ratio of peak to equilibrium stress and the relaxation time were similar for both preconditioning strains. This study suggests that in future work, the preconditioning strain should be reported and the highest strain being used in the study should be used for preconditioning. This is important to allow meaningful comparison of test data within the study and also with other studies.

Viscoelastic properties of the tongue and soft palate using MR elastography

Biomechanical properties of the human tongue are needed for finite element models of the upper airway and may be important to elucidate the pathophysiology of obstructive sleep apneoa. Tongue viscoelastic properties have not been characterized previously. Magnetic resonance elastography (MRE) is an emerging imaging technique that can measure the viscoelastic properties of soft tissues in-vivo. In this study, MRE was used to measure the viscoelastic properties of the tongue and soft palate in 7 healthy volunteers during quiet breathing. Results show that the storage shear modulus of the tongue and soft palate is 2.67±0.29 and 2.53±0.31 kPa (mean ± SD), respectively. This is the first study to investigate the mechanical properties of the tongue using MRE, and it provides necessary data for future studies of patient groups with altered upper airway function.

Tongue and lateral upper airway movement with mandibular advancement.

STUDY OBJECTIVES To characterize tongue and lateral upper airway movement and to image tongue deformation during mandibular advancement. DESIGN Dynamic imaging study of a wide range of apnea hypopnea index (AHI), body mass index (BMI) subjects. SETTING Not-for-profit research institute. PARTICIPANTS 30 subjects (aged 31-69 y, AHI 0-75 events/h, BMI 17-39 kg/m(2)). INTERVENTIONS Subjects were imaged using dynamic tagged magnetic resonance imaging during mandibular advancement. Tissue displacements were quantified with the harmonic phase technique. MEASUREMENTS AND RESULTS Mean mandibular advancement was 5.6 ± 1.8 mm (mean ± standard deviation). This produced movement through a connection from the ramus of the mandible to the pharyngeal lateral walls in all subjects. In the sagittal plane, 3 patterns of posterior tongue deformation were seen with mandibular advancement-(A) en bloc anterior movement, (B) anterior movement of the oropharyngeal region, and (C) minimal anterior movement. Subjects with lower AHI were more likely to have en bloc movement (P = 0.04) than minimal movement. Antero-posterior elongation of the tongue increased with AHI (R = 0.461, P = 0.01). Mean anterior displacements of the posterior nasopharyngeal and oropharyngeal regions of the tongue were 20% ± 13% and 31% ± 17% of mandibular advancement. The posterior tongue compressed 1.1 ± 2.2 mm supero-inferiorly. CONCLUSIONS Mandibular advancement has two mechanisms of action which increase airway size. In subjects with low AHI, the entire tongue moves forward. Mandibular advancement also produces lateral airway expansion via a direct connection between the lateral walls and the ramus of the mandible. CITATION Brown EC; Cheng S; McKenzie DK; Butler JE; Gandevia SC; Bilston LE. Tongue and lateral upper airway movement with mandibular advancement. SLEEP 2013;36(3):397-404.

The presence of arachnoiditis affects the characteristics of CSF flow in the spinal subarachnoid space: A modelling study

Syringomyelia is a neurological disorder characterised by high pressure fluid-filled cysts within the spinal cord. As syringomyelia is associated with abnormalities of the central nervous system that obstruct cerebrospinal fluid (CSF) flow, it is thought that changes in CSF dynamics play an important role in its pathogenesis. Using three-dimensional computational models of the spinal subarachnoid space (SAS), this study aims to determine SAS obstructions, such as arachnoiditis, change in CSF dynamics in the SAS. The geometry of the SAS was reconstructed from a series of MRI images. CSF is modelled as an incompressible Newtonian fluid with a dynamic viscosity of 1 mPa s. Three computational models simulated CSF flow in either the unobstructed SAS, or with the SAS obstructed by a porous region simulating dorsal or circumferential arachnoiditis. The permeability of this porous obstruction was varied for the model with dorsal arachnoiditis. The results show that arachnoiditis increases flow resistance in the SAS and this is accompanied by a modest increase in magnitude and/or shift in timing (with respect to the cardiac cycle) of the CSF pressure drop across the region of arachnoiditis. This study suggests that syrinx formation may be related to a change in temporal CSF pulse pressure dynamics.

Movement of the human upper airway during inspiration with and without inspiratory resistive loading

The electromyographic (EMG) activity of human upper airway muscles, particularly the genioglossus, has been widely measured, but the relationship between EMG activity and physical movement of the airway muscles remains unclear. We aimed to measure the motion of the soft tissues surrounding the airway during normal and loaded inspiration on the basis of the hypothesis that this motion would be affected by the addition of resistance to breathing during inspiration. Tagged MR imaging of seven healthy subjects was performed in a 3-T scanner. Tagged 8.6-mm-spaced grids were used, and complementary spatial modulation of magnetization images were acquired beginning ∼200 ms before inspiratory airflow. Deformation of tag line intersections was measured. The genioglossus moved anteriorly during normal and loaded inspiration, with less movement during loaded inspiration. The motion of tissues at the anterior border of the upper airway was nonuniform, with larger motions inferiorly. At the level of the soft palate, the lateral dimension of the airway decreased significantly during loaded inspiration (-0.15 ± 0.09 and -0.48 ± 0.09 mm during unloaded and loaded inspiration, respectively, P < 0.05). When resistance to inspiratory flow was added, genioglossus motion and lateral dimensions of the airway at the level of the soft palate decreased. Our results suggest that genioglossus motion begins early to dilate the airway prior to airflow and that inspiratory loading reduces the anterior motion of the genioglossus and increases the collapse of the lateral airway walls at the level of the soft palate.

MR Elastography Can Be Used to Measure Brain Stiffness Changes as a Result of Altered Cranial Venous Drainage During Jugular Compression

The authors evaluated the effect of jugular compression on brain tissue stiffness and CSF flow by evaluating 9 volunteers, with and without jugular compression, with MR elastography and phase-contrast CSF flow imaging. The shear moduli of the brain tissue increased with the percentage of blood draining through the internal jugular veins during venous compression. Subjects who maintain venous drainage through the internal jugular veins during jugular compression have stiffer brains than those who divert venous blood through alternative pathways. BACKGROUND AND PURPOSE: Compressing the internal jugular veins can reverse ventriculomegaly in the syndrome of inappropriately low pressure acute hydrocephalus, and it has been suggested that this works by “stiffening” the brain tissue. Jugular compression may also alter blood and CSF flow in other conditions. We aimed to understand the effect of jugular compression on brain tissue stiffness and CSF flow. MATERIALS AND METHODS: The head and neck of 9 healthy volunteers were studied with and without jugular compression. Brain stiffness (shear modulus) was measured by using MR elastography. Phase-contrast MR imaging was used to measure CSF flow in the cerebral aqueduct and blood flow in the neck. RESULTS: The shear moduli of the brain tissue increased with the percentage of blood draining through the internal jugular veins during venous compression. Peak velocity of caudally directed CSF in the aqueduct increased significantly with jugular compression (P < .001). The mean jugular venous flow rate, amplitude, and vessel area were significantly reduced with jugular compression, while cranial arterial flow parameters were unaffected. CONCLUSIONS: Jugular compression influences cerebral CSF hydrodynamics in healthy subjects and can increase brain tissue stiffness, but the magnitude of the stiffening depends on the percentage of cranial blood draining through the internal jugular veins during compression—that is, subjects who maintain venous drainage through the internal jugular veins during jugular compression have stiffer brains than those who divert venous blood through alternative pathways. These methods may be useful for studying this phenomenon in patients with the syndrome of inappropriately low-pressure acute hydrocephalus and other conditions.

Characterising soft tissues under large amplitude oscillatory shear and combined loading

Characterising soft biological tissues outside the linear viscoelastic regime is challenging due to their complex behaviour. In addition, the viscoelastic properties of tissues have been shown to be sensitive to sample preparation and loading regime resulting in inconsistent data varying by orders magnitude in the literature. This paper presents a novel technique to characterise the non-linear behaviour of tissues which uses Fourier Transformation to decompose the stress output waveform under large amplitude oscillatory shear (LAOS) into harmonic contributions. The effect of varying preload, the compressive strain exerted on a liver tissue specimen prior to shear testing to minimise slip, was also investigated. Results showed that in the linear regime, preload affects the viscoelastic response of liver. Histological analysis indicated that there were structural changes as a result of the preload that may be linked to the differences in observed behaviour. Fourier analysis was used to extract the first and third harmonic components of the shear moduli at large strain. At 50% shear strain, a change in the third harmonic component of the shear moduli was accompanied by a marked change in the micro-structural arrangement of the sinusoids. This paper demonstrates a method of efficiently characterising soft biological tissues under large amplitude oscillatory shear under combined loading.