Human dural thickness measured by ultrasonographic method: reflection of intracranial pressure

Investigation of direction- and age-dependent prestretch in mouse cranial dura mater

Cranial dura mater is a dense interwoven vascularized connective tissue that helps regulate neurocranial remodeling by responding to strains from the growing brain. Previous ex vivo experimentation has failed to account for the role of prestretch in the mechanical behavior of the dura. Here we aim to estimate the prestretch in mouse cranial dura mater and determine its dependency on direction and age. We performed transverse and longitudinal incisions in parietal dura excised from newborn (day ∼ 4) and mature (12 weeks) mice and calculated the ex vivo normalized incision opening (measured width over length). Then, similar incisions were simulated under isotropic stretching within Abaqus/Standard. Finally, prestretch was estimated by comparing the ex vivo and in silico normalized openings. There were no significant differences between the neonatal and adult mice when comparing cuts in the same direction, but adult mice were found to have significantly greater stretch in the anterior-posterior direction than in the medial-lateral direction, while neonatal dura was essentially isotropic. Additionally, our simulations show that increasing curvature impacts the incision opening, indicating that flat in silico models may overestimate prestretch.

An adaptive h-refinement method for the boundary element fast multipole method for quasi-static electromagnetic modeling

Objective.In our recent work pertinent to modeling of brain stimulation and neurophysiological recordings, substantial modeling errors in the computed electric field and potential have sometimes been observed for standard multi-compartment head models. The goal of this study is to quantify those errors and, further, eliminate them through an adaptive mesh refinement (AMR) algorithm. The study concentrates on transcranial magnetic stimulation (TMS), transcranial electrical stimulation (TES), and electroencephalography (EEG) forward problems.Approach.We propose, describe, and systematically investigate an AMR method using the boundary element method with fast multipole acceleration (BEM-FMM) as the base numerical solver. The goal is to efficiently allocate additional unknowns to critical areas of the model, where they will best improve solution accuracy. The implemented AMR method's accuracy improvement is measured on head models constructed from 16 Human Connectome Project subjects under problem classes of TES, TMS, and EEG. Errors are computed between three solutions: an initial non-adaptive solution, a solution found after applying AMR with a conservative refinement rate, and a 'silver-standard' solution found by subsequent 4:1 global refinement of the adaptively-refined model.Main results.Excellent agreement is shown between the adaptively-refined and silver-standard solutions for standard head models. AMR is found to be vital for accurate modeling of TES and EEG forward problems for standard models: an increase of less than 25% (on average) in number of mesh elements for these problems, efficiently allocated by AMR, exposes electric field/potential errors exceeding 60% (on average) in the solution for the unrefined models.Significance.This error has especially important implications for TES dosing prediction-where the stimulation strength plays a central role-and for EEG lead fields. Though the specific form of the AMR method described here is implemented for the BEM-FMM, we expect that AMR is applicable and even required for accurate electromagnetic simulations by other numerical modeling packages as well.

An Adaptive H-Refinement Method for the Boundary Element Fast Multipole Method for Quasi-static Electromagnetic Modeling

Objective

In our recent work pertinent to modeling of brain stimulation and neurophysiological recordings, substantial modeling errors in the computed electric field and potential have sometimes been observed for standard multi-compartment head models. The goal of this study is to quantify those errors and, further, eliminate them through an adaptive mesh refinement (AMR) algorithm. The study concentrates on transcranial magnetic stimulation (TMS), transcranial electrical stimulation (TES), and electroencephalography (EEG) forward problems.

Approach

We propose, describe, and systematically investigate an AMR method using the Boundary Element Method with Fast Multipole Acceleration (BEM-FMM) as the base numerical solver. The goal is to efficiently allocate additional unknowns to critical areas of the model, where they will best improve solution accuracy.The implemented AMR method's accuracy improvement is measured on head models constructed from 16 Human Connectome Project subjects under problem classes of TES, TMS, and EEG. Errors are computed between three solutions: an initial non-adaptive solution, a solution found after applying AMR with a conservative refinement rate, and a "silver-standard" solution found by subsequent 4:1 global refinement of the adaptively-refined model.

Main Results

Excellent agreement is shown between the adaptively-refined and silver-standard solutions for standard head models. AMR is found to be vital for accurate modeling of TES and EEG forward problems for standard models: an increase of less than 25% (on average) in number of mesh elements for these problems, efficiently allocated by AMR, exposes electric field/potential errors exceeding 60% (on average) in the solution for the unrefined models.

Significance

This error has especially important implications for TES dosing prediction - where the stimulation strength plays a central role - and for EEG lead fields. Though the specific form of the AMR method described here is implemented for the BEM-FMM, we expect that AMR is applicable and even required for accurate electromagnetic simulations by other numerical modeling packages as well.

Power Budget of a Skull Unit in a Fully-Implantable Brain-Computer Interface: Bio-Heat Model

The aim of this study is to estimate the maximum power consumption that guarantees the thermal safety of a skull unit (SU). The SU is part of a fully-implantable bi-directional brain computer-interface (BD-BCI) system that aims to restore walking and leg sensation to those with spinal cord injury (SCI). To estimate the SU power budget, we created a bio-heat model using the finite element method (FEM) implemented in COMSOL. To ensure that our predictions were robust against the natural variation of the model's parameters, we also performed a sensitivity analysis. Based on our simulations, we estimated that the SU can nominally consume up to 70 mW of power without raising the surrounding tissues' temperature above the thermal safety threshold of 1°C. When considering the natural variation of the model's parameters, we estimated that the power budget could range between 47 and 81 mW. This power budget should be sufficient to power the basic operations of the SU, including amplification, serialization and A/D conversion of the neural signals, as well as control of cortical stimulation. Determining the power budget is an important specification for the design of the SU and, in turn, the design of a fully-implantable BD-BCI system.

The effect of meninges on the electric fields in TES and TMS. Numerical modeling with adaptive mesh refinement

BACKGROUND

When modeling transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS) in the brain, the meninges - dura, arachnoid, and pia mater - are often neglected due to high computational costs.

OBJECTIVE

We investigate the impact of the meningeal layers on the cortical electric field in TES and TMS while considering the headreco segmentation as the base model.

METHOD

We use T1/T2 MRI data from 16 subjects and apply the boundary element fast multipole method with adaptive mesh refinement, which enables us to accurately solve this problem and establish method convergence at reasonable computational cost. We compare electric fields in the presence and absence of various meninges for two brain areas (M1_HAND and DLPFC) and for several distinct TES and TMS setups.

RESULTS

Maximum electric fields in the cortex for focal TES consistently increase by approximately 30% on average when the meninges are present in the CSF volume. Their effect on the maximum field can be emulated by reducing the CSF conductivity from 1.65 S/m to approximately 0.85 S/m. In stark contrast to that, the TMS electric fields in the cortex are only weakly affected by the meningeal layers and slightly (∼6%) decrease on average when the meninges are included.

CONCLUSION

Our results quantify the influence of the meninges on the cortical TES and TMS electric fields. Both focal TES and TMS results are very consistent. The focal TES results are also in a good agreement with a prior relevant study. The solver and the mesh generator for the meningeal layers (compatible with SimNIBS) are available online.

Comparison of the Morphologic and Mechanical Features of Human Cranial Dura and Other Graft Materials Used for Duraplasty

OBJECTIVE

This study aimed to compare the thickness and mechanical properties of the frontal; parietal; temporal; occipital human dura; autogenous grafts (facia lata, temporal fascia, galea aponeurotica); and artificial dura.

METHODS

Sagittal and transverse dura samples were obtained from standard regions of the cranial dura from 30 autopsies for histologic and mechanical property measurements. Identical measurements were made for the autogenous grafts artificial dura, and the results were statistically analyzed.

RESULTS

The thickness of the temporal (0.35 ± 0.11 mm), parietal (0.44 ± 0.13 mm), frontal (0.38 ± 0.12 mm), and occipital (0.46 ± 0.18 mm) dura showed regional variations. The parietal and occipital dura were significantly thicker than the temporal dura. The occipital dura was considerably thicker than the frontal dura. The frontal and temporal dura of males were significantly thicker than females. The sagittal maximum tensile force measurements were significantly greater than transverse, for the frontal, temporal, and occipital dura. The stiffness measurements in sagittal direction were greater than the measurements in transverse direction for the frontal dura. The mechanical properties and thickness of the autogenous and artificial dura were not similar to the human dura.

CONCLUSIONS

The thickness and mechanical properties of the regional cranial dura should be taken into consideration for a better cure and fewer complications. The mechanical properties of sagittal and transverse dura should be kept in mind for the preference of dura material. The present study's data can pave the way to produce artificial regional dura by mimicking the thickness and mechanical properties of the human dura.

Novel Paradigm of Subdural Cortical Stimulation Does Not Cause Thermal Damage in Brain Tissue: A Simulation-Based Study

The thermal effect of a novel effective electrical stimulation mapping (ESM) technique using an Ojemann's stimulation electrode in open craniotomy areas causes a nondestructive local increase in temperature. Another type of stimulating electrode is a subdural strip, routinely used in intraoperative electrocorticography (ECoG), which applies ESM in a covered subdural area over the motor cortex. ECoG electrode geometry produces a different electrical field, causing a different Joule heat distribution in tissue, one that is impossible to measure in subdural space. Therefore, the previous safety control study of the novel ESM technique needed to be extended to include an assessment of the thermal effect of ECoG strip electrodes. We adapted a previously well-validated numerical model and performed coupled complex electro-thermal transient simulations for short-time (28.4 ms) high-frequency (500 Hz) and hyperintense (peak 100 mA) ESM paradigm. The risk of heat-induced cellular damage was assessed by applying the Arrhenius equation integral on the computed time-dependent spatial distribution of temperature in the brain tissue during ESM stimulation and during the cooldown period. The results showed increases in temperature in the proximity around ECoG electrode discs in a safe range without destructive effects. As opposed to open craniotomy, subdural space is not cooled by the air; hence a higher - but still safe - induced temperature was observed. The presented simulation agrees with the previously published histopathological examination of the stimulated brain tissue, and confirms the safety of the novel ESM technique when applied using ECoG strip electrodes.

Enhanced tES and tDCS computational models by meninges emulation

OBJECTIVE

Understanding how current reaches the brain during transcranial electrical stimulation (tES) underpins efforts to rationalize outcomes and optimize interventions. To this end, computational models of current flow relate applied dose to brain electric field. Conventional tES modeling considers distinct tissues like scalp, skull, cerebrospinal fluid (CSF), gray matter and white matter. The properties of highly conductive CSF are especially important. However, modeling the space between skull and brain as entirely CSF is not an accurate representation of anatomy. The space conventionally modeled as CSF is approximately half meninges (dura, arachnoid, and pia) with lower conductivity. However, the resolution required to describe individual meningeal layers is computationally restrictive in an MRI-derived head model. Emulating the effect of meninges through CSF conductivity modification could improve accuracy with minimal cost.

APPROACH

Models with meningeal layers were developed in a concentric sphere head model. Then, in a model with only CSF between skull and brain, CSF conductivity was optimized to emulate the effect of meningeal layers on cortical electric field for multiple electrode positions. This emulated conductivity was applied to MRI-derived models.

MAIN RESULTS

Compared to a model with conventional CSF conductivity (1.65 S m-1), emulated CSF conductivity (0.85 S m-1) produced voltage fields better correlated with intracranial recordings from epilepsy patients.

SIGNIFICANCE

Conventional tES models have been validated using intracranial recording. Residual errors may nonetheless impact model utility. Because CSF is so conductive to current flow, misrepresentation of the skull-brain interface as entirely CSF is not realistic for tES modeling. Updating the conventional model with a CSF conductivity emulating the effect of the meninges enhances modeling accuracy without increasing model complexity. This allows existing modeling pipelines to be leveraged with a simple conductivity change. Using 0.85 S m-1 emulated CSF conductivity is recommended as the new standard in non-invasive brain stimulation modeling.

Skull Base Dural Thickness and Relationship to Demographic Features: A Postmortem Study and Literature Review

Dural membrane is an important anatomic structure that surrounds and protects the entire central nervous system. Physical properties of the dura have many pathophysiological and therapeutic implications in cranial surgery, especially skull base disorders. The aim of this study is to investigate variation in skull base dural thickness and correlation with different demographic parameters. At the time of autopsy, the petrous apex dura with the underlying bone of 20 cadavers was harvested. Dural thickness was independently measured by two pathologists at the thinnest and thickest segments in the specimen. Correlational analyses were then performed to compare dural thickness with gender, age, neck circumference, height, weight, and body mass index (BMI). Mean, minimum, and maximum skull base dural thickness in our study was 0.36, 0.27, and 0.46 mm, respectively. Age demonstrated a negative correlation with dural thickness with significantly thinner dura in the older subjects, p  = 0.01. There was a trend toward thinner dura in females that approached statistical significance, p  = 0.06. No strong correlation could be found with body weight, height, neck circumference, or BMI. Our findings show a considerable intersubject and intrasubject variability in skull base dural thickness. Some demographic parameters also seem to impact dural thickness. Additional histological studies are needed for better understanding of the pathophysiological mechanisms pertaining to the tensile properties of the dural membrane.

Intrathecal recombinant human 4-sulfatase reduces accumulation of glycosaminoglycans in dura of mucopolysaccharidosis VI cats

INTRODUCTION

Mucopolysaccharidosis VI (MPS-VI) is caused by a deficiency in N-acetylgalactosamine-4-sulfatase activity, resulting in lysosomal accumulation of partially degraded glycosaminoglycans (GAGs). Compressive myelopathy in early-onset MPS-VI patients has been partly attributed to thickening of the dura mater following engorgement with GAG. In this study, we therefore tested whether the dural abnormalities could be prevented in a feline model of the disorder.

RESULTS

All intrathecal injections (IT-INJs) were well tolerated. MPS-VI cats treated with IT-INJ of recombinant human N-acetylgalactosamine-4-sulfatase (rhASB) exhibited reduced vacuolation in the dural fibroblasts, diminished levels of sulfated-N-acetylhexosamine (HNAc(+S)) in the cerebrospinal fluid (CSF) and no hind-limb paresis. Serum anti-rhASB antibodies remained low in MPS-VI cats treated with intravenous enzyme replacement therapy (IV-ERT) and increased slightly in normal cats treated with IT-INJ of rhASB alone. Anti-rhASB antibodies in CSF remained undetectable.

DISCUSSION

These data indicate that repeated IT-INJ of rhASB can safely prevent GAG storage in MPS-VI dura.

METHODS

Cats were assigned to three groups: (i) receiving weekly IV-ERT of rhASB from birth plus six monthly IT-INJs of rhASB from age 2 months; (ii) receiving six monthly IT-INJs of vehicle; or (iii) untreated. Additional normal cats received five fortnightly IT-INJs of rhASB or vehicle alone.

Scalp thickness in the temporal region: its relevance to the development of cochlear implants

The thickness of the scalp in the temporal region was measured at two representative points: point 1, over the most laterally prominent part of the mastoid process: and point 2, at the horizontal level of the upper border of the orbit, vertically in line with point 1. Measurements were made directly at autopsy in four patients, then in 50 live patients by magnetic resonance imaging (MRI). The relevance of the methodology and of these measurements to cochlear implant development is discussed. As the term 'scalp' usually includes the periosteum of the skull, the authors have used the term 'scalp' to include the range from the surface of the scalp to the bone shadow as seen on MRI, and the term 'skin' where, at autopsy, the skin is lifted leaving the periosteum intact.

Innovative non-invasive method for absolute intracranial pressure measurement without calibration

A new absolute ICP (aICP) measurement method was designed which does not need calibration. In this study we compared a new method with invasive aICP method in ICU on the patients with closed severe traumatic brain injury. A new method is based on two-depth TCD technique for aICP and external absolute pressure aPe comparison using the eye artery (EA) as natural "balance". The intracranial segment of EA is compressed by aICP and the extracranial segment is compressed by aPe applied to the tissues surrounding the eye. The blood flow parameters in both EA segments are approximately the same when aPe = aICP. Two-depth TCD device is used as an indicator of balance aPe = aICP when the pulsatility index of blood flow velocity waveform in intracranial and extracranial segments are the same. Fifty seven simultaneous invasive and non-invasive aICP measurements were performed in aICP range from 3.0 to 37.0 mmHg. Bland Altman plot of the differences between simultaneous invasive and non-invasive aICP measurements shows the negligible mean difference (mean = 0.94 mmHg) with a standard deviation of 6.18 mmHg. This validation study shows that it is possible to measure aICP non-invasively without calibration of the system with 95% confidence interval of 12 mmHg.