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Material Parameters Research

Material Parameters Research. of the Human Central Nervous System. Laura Zitella. Material Parameters:. Dura Mater composition. Outermost, elastic membrane Closely adheres to the bones in the cranial regions, but not in the spinal region

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Material Parameters Research

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  1. Material Parameters Research of the Human Central Nervous System Laura Zitella

  2. Material Parameters:

  3. Dura Mater composition • Outermost, elastic membrane • Closely adheres to the bones in the cranial regions, but not in the spinal region • Composed of ~80 laminas that are made of 8-12 thin laminar subunits • The subunits consist of collagen fibers that are 0.1 µm thick. • Elastic fibers, 2 µm thick are vertically oriented at posterior of dural sac, oblique in later portions

  4. Dura Mater Thickness • Along spinal column, 1 mm thick • At lumbar level, thickness varies: • 320 µm in anterior/posterior portions • 80 µm in lateral portions

  5. Dura Mater Stiffness: Technique • Human dorsal lumbar dura mater (ages 15 days-81 years) • 1.5 cm long by 1 cm wide samples • A standard engineering tensile and compression testing instrument was used to stretch samples in longitudinal or transverse direction at 10 cm/min until failure (reversal of the slope of the force-distance curve) • Stiffness values were calculated from the slope of the force-distance curve

  6. Dorsal Lumbar Dura Mater Stiffness: Average human longitudinal stiffness: 196.71 N/cm/cm Average human transverse stiffness: 48.7 N/cm/cm

  7. Dura-Arachnoid Interface • Dural lamina • Subdural space-neurothelial cells and capillaries • Laminar arachnoid layer-compact covering of the dural sac’s internal surface • Reticular lamina-interlaced cells • Barrier lamina-epithelial tissues • Trabecular arachnoid-extends like a spider web around the pia mater • Collagen fibers covered by cells • Subarachnoid space-occupied by CSF

  8. Arachnoid Thickness • Separated from the pia mater by subarachnoid space • Trabecular Arachnoid: Varies between 10-60 µm • Composed of two laminar layers: • Barrier lamina-5 µm • Reticular lamina-10-20 µm

  9. Stiffness of White vs. Gray Brain Matter • Harmonic shear waves (with amplitudes of microns or less) at frequencies of 100Hz • Applied lateral motion to a thermoplastic bite bar that was held in teeth by healthy volunteers • At this frequency, white matter has a higher shear stiffness: avg. 14.2 kPa • Gray matter: avg. 5.3 kPa

  10. Fig. 1. Schematic diagram of magnetic resonance elastography system. Conventional MR imaging gradients and RF pulses that encode spatial positions are shown at the bottom left. The electromechanical driver applies transverse acoustic waves to the object to be imaged via a surface plate (right). The cyclic motion-sensitizing gradients and the acoustic drive are synchronized using trigger pulses provided by the imager. The phase offset (θ) between the two can be varied. As shown by the shaded regions, the motion-sensitizing gradients can be superimposed along any desired axis to detect cyclic motion.

  11. Another method for Stiffness of Brain Tissue Harmonic Magnetic Resononance Elastography • several periods of sinusoidal motion at a single frequency • Developed transient MRE methods • Non-harmonic, non-periodic wave at higher magnitude and frequency • can be used for situations with complex wave patterns such as in the brain

  12. Time of Arrival Method • Shear stiffness is calculated from the wave speed • Tracked the transient wave within the object throughout the time • Inverse speed calculated by one- dimensional gradient • Shear stiffness calculated by: C=wave speed Ρ=density µ=shear stiffness

  13. Transient Direct Inversion Method • To find out if the shear stiffness varies with frequency µ=shear modulus ρ=density Ψ=displacement

  14. Stiffness of Cervical Spinal Cord • Cervical human spinal cords from adults ages 30-84, less than 24 hours postmortem • 3-7 cm length with intact dura mater • Dura mater removed before testing, nerve roots timed to less than 3 mm long • Ends dried and attached to plastic plates using cyanoacrylate adhesive. • Samples strained slowly until 0.5 N then strained to maximum strain at 0.04-0.24 sec-1 and held for 1 minute • Process repeated 4-5 times per sample

  15. Spinal Cord Sample Dimensions and Stiffness Values Average Stiffness: 1.23 MPa

  16. Stiffness of Thoracic Spinal Cord • Thoracic spinal columns from T1-L1 from 15 cadavers; age 43-89; 1-3 days postmortem • Proximal and distal ends fixed to steel rings and held in place with 6 mm diameter pins. • Specimen place on platform of electrohydraulic testing device • Compressive force applied at a rate of 2.5 mm/s until failure • Group I: full spinal cord • Group II: after laminectomy, lamina removed

  17. Specimens used: Average Stiffness: 3.24 ±0.45 kN/cm

  18. Permeability of Brain Tissue • White matter-orderly, parallel nerve fibers and lipid bilayer • more water permeable • Gray matter- • strictly regulates permeability • A model of gray and white brain matter was created • fluid saturated, homogeneous, poroelastic material • Swelling was induced to provide possible material parameters, such as hydraulic permeability.

  19. Proposed parameters

  20. References • Patin et al. “Anatomic and Biomechanical Properties of Human Lumbar Dura Mater.” Regional Anesthesia and Pain Management Anatomy of DuraMater. 76(1993):535-40. • McCracken et al. “Mechanical Transient-Based Magnetic Resonance Elastography” Magnetic Resonance in Medicine. 53(2005):628-639. • Manduca et al. “Magnetic resonance elastography: Non-invasive mapping of tissue elasticity” Medical Image Analysis. 5(2001):237-254. • Bilston et al. “The Mechanical Properties of the Human Cervical Spinal Cord In Vitro” Annals of Biomedical Engineering. 24(1996):67-74. • Yoganandan et al. “Biomechanical Effects of Laminectomy on Thoracic Spine Stability” Neurosurgery. 32.4 (1993): 604-610. • Basser, Peter J. “Interstitial Pressure, Volume, and Flow during Infusion into Brain Tissue” Microvascular Research 44(1992):143-165. • Reina, Miguel Angel, Andres Lopez, Fabiola Maches, Oscar de Leon Casasola, Jose Antonio De Andres. “Electron Microscopy and the explansion of Regional Anesthesia Knowledge.” Techniques in Regional Anesthesia and PainManagement 6.4 (2002):165-171. • Reina et al. “The Origin of the Spinal Subdural Space: Ultrastructure Findings” Regional Anesthesia 94 (2002):991-5. • Reina et al.”The ultrastructure of the spinal arachnoid in humans and its impact on spinal anesthesia, cauda equine syndrome, and transient neurological syndrome” Techniques in Regional Anesthesia and Pain Management.12.3(2008)153-160.

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