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Finite Element and Neuroimaging Techniques toImprove Decision-Making in Clinical Neuroscience
KTH, School of Technology and Health (STH), Neuronic Engineering.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Our brain, perhaps the most sophisticated and mysterious part of the human body, to some extent, determines who we are. However, it’s a vulnerable organ. When subjected to an impact, such as a traffic accident or sport, it may lead to traumatic brain injury (TBI) which can have devastating effects for those who suffer the injury. Despite lots of efforts have been put into primary injury prevention, the number of TBIs is still on an unacceptable high level in a global perspective.

Brain edema is a major neurological complication of moderate and severe TBI, which consists of an abnormal accumulation of fluid within the brain parenchyma. Clinically, local and minor edema may be treated conservatively only by observation, where the treatment of choice usually follows evidence-based practice. In the first study, the gravitational force is suggested to have a significant impact on the pressure of the edema zone in the brain tissue. Thus, the objective of the study was to investigate the significance of head position on edema at the posterior part of the brain using a Finite Element (FE) model. The model revealed that water content (WC) increment at the edema zone remained nearly identical for both supine and prone positions. However, the interstitial fluid pressure (IFP) inside the edema zone decreased around 15% by having the head in a prone position compared with a supine position. The decrease of IFP inside the edema zone by changing patient position from supine to prone has the potential to alleviate the damage to axonal fibers of the central nervous system. These observations suggest that considering the patient’s head position during intensive care and at rehabilitation should be of importance to the treatment of edematous regions in TBI patients.

In TBI patients with diffuse brain edema, for most severe cases with refractory intracranial hypertension, decompressive craniotomy (DC) is performed as an ultimate therapy. However, a complete consensus on its effectiveness has not been achieved due to the high levels of severe disability and persistent vegetative state found in the patients treated with DC. DC allows expansion of the swollen brain outside the skull, thereby having the potential in reducing the Intracranial Pressure (ICP). However, the treatment causes stretching of the axons and may contribute to the unfavorable outcome of the patients. The second study aimed at quantifying the stretching and WC in the brain tissue due to the neurosurgical intervention to provide more insight into the effects upon such a treatment. A nonlinear registration method was used to quantify the strain. Our analysis showed a substantial increase of the strain level in the brain tissue close to the treated side of DC compared to before the treatment. Also, the WC was related to specific gravity (SG), which in turn was related to the Hounsfield unit (HU) value in the Computerized Tomography (CT) images by a photoelectric correction according to the chemical composition of the brain tissue. The overall WC of brain tissue presented a significant increase after the treatment compared to the condition seen before the treatment. It is suggested that a quantitative model, which characterizes the stretching and WC of the brain tissue both before as well as after DC, may clarify some of the potential problems with such a treatment.

Diffusion Weighted (DW) Imaging technology provides a noninvasive way to extract axonal fiber tracts in the brain. The aim of the third study, as an extension to the second study was to assess and quantify the axonal deformation (i.e. stretching and shearing)at both the pre- and post-craniotomy periods in order to provide more insight into the mechanical effects on the axonal fibers due to DC.

Subarachnoid injection of artificial cerebrospinal fluid (CSF) into the CSF system is widely used in neurological practice to gain information on CSF dynamics. Mathematical models are important for a better understanding of the underlying mechanisms. Despite the critical importance of the parameters for accurate modeling, there is a substantial variation in the poroelastic constants used in the literature due to the difficulties in determining material properties of brain tissue. In the fourth study, we developed a Finite Element (FE) model including the whole brain-CSF-skull system to study the CSF dynamics during constant-rate infusion. We investigated the capacity of the current model to predict the steady state of the mean ICP. For transient analysis, rather than accurately fit the infusion curve to the experimental data, we placed more emphasis on studying the influences of each of the poroelastic parameters due to the aforementioned inconsistency in the poroelastic constants for brain tissue. It was found that the value of the specific storage term S_epsilon is the dominant factor that influences the infusion curve, and the drained Young’s modulus E was identified as the dominant parameter second to S_epsilon. Based on the simulated infusion curves from the FE model, Artificial Neural Network (ANN) was used to find an optimized parameter set that best fit the experimental curve. The infusion curves from both the FE simulations and using ANN confirmed the limitation of linear poroelasticity in modeling the transient constant-rate infusion.

To summarize, the work done in this thesis is to introduce FE Modeling and imaging technologiesincluding CT, DW imaging, and image registration method as a complementarytechnique for clinical diagnosis and treatment of TBI patients. Hopefully, the result mayto some extent improve the understanding of these clinical problems and improve theirmedical treatments.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , x, 76 p.
Series
Trita-STH : report, ISSN 1653-3836 ; 2012:1
Keyword [en]
Traumatic brain injury, Intracranial Pressure, Brain edema, Gravitational force, Finite Element Model, Poroelastic parameter, Decompressive craniotomy, Image registration, Water content, Strain level, Diffusion Weighted Imaging
National Category
Medical Engineering Applied Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-72345ISBN: 978-91-7501-240-7 (print)OAI: oai:DiVA.org:kth-72345DiVA: diva2:487687
Public defence
2012-02-09, 3-221, Alfred Nobels Alle 10, Huddinge, Sweden, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120201Available from: 2012-02-01 Created: 2012-01-31 Last updated: 2012-02-01Bibliographically approved
List of papers
1. Influence of gravity for optimal head positions in the treatment of head injury patients
Open this publication in new window or tab >>Influence of gravity for optimal head positions in the treatment of head injury patients
2011 (English)In: Acta Neurochirurgica, ISSN 0001-6268, Vol. 153, no 10, 2057-2064 p.Article in journal (Refereed) Published
Abstract [en]

BACKGROUND:

Brain edema is a major neurological complication of traumatic brain injury (TBI), commonly including a pathologically increased intracranial pressure (ICP) associated with poor outcome. In this study, gravitational force is suggested to have a significant impact on the pressure of the edema zone in the brain tissue and the objective of the study was to investigate the significance of head position on edema at the posterior part of the brain using a finite element (FE) model.

METHODS:

A detailed FE model including the meninges, brain tissue and a fully connected cerebrospinal fluid (CSF) system was used in this study. Brain tissue was modelled as a poroelastic material consisting of an elastic solid skeleton composed of neurons and neuroglia, permeated by interstitial fluid. The effect of head positions (supine and prone position) due to gravity was investigated for a localized brain edema at the posterior part of the brain.

RESULTS:

The water content increment at the edema zone remained nearly identical for both positions. However, the interstitial fluid pressure (IFP) inside the edema zone decreased around 15% by having the head in a prone position compared with a supine position.

CONCLUSIONS:

The decrease of IFP inside the edema zone by changing patient position from supine to prone has the potential to alleviate the damage to central nervous system nerves. These observations indicate that considering the patient's head position during intensive care and at rehabilitation might be of importance to the treatment of edematous regions in TBI patients.

National Category
Medical Engineering Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-72443 (URN)10.1007/s00701-011-1078-2 (DOI)000294787400021 ()21739174 (PubMedID)2-s2.0-80054110038 (Scopus ID)
Funder
Swedish Research Council, 621-2008-3400
Note
QC 20120201Available from: 2012-01-31 Created: 2012-01-31 Last updated: 2012-02-01Bibliographically approved
2. Increased strain levels and water content in brain tissue after decompressive craniotomy
Open this publication in new window or tab >>Increased strain levels and water content in brain tissue after decompressive craniotomy
2012 (English)In: Acta Neurochirurgica, ISSN 0001-6268, E-ISSN 0942-0940, Vol. 154, no 9, 1583-1593 p.Article in journal (Refereed) Published
Abstract [en]

At present there is a debate on the effectiveness of the decompressive craniotomy (DC). Stretching of axons was speculated to contribute to the unfavourable outcome for the patients. The quantification of strain level could provide more insight into the potential damage to the axons. The aim of the present study was to evaluate the strain level and water content (WC) of the brain tissue for both the pre- and post-craniotomy period. The stretching of brain tissue was quantified retrospectively based on the computerised tomography (CT) images of six patients before and after DC by a non-linear image registration method. WC was related to specific gravity (SG), which in turn was related to the Hounsfield unit (HU) value in the CT images by a photoelectric correction according to the chemical composition of brain tissue. For all the six patients, the strain level showed a substantial increase in the brain tissue close to the treated side of DC compared with that found at the pre-craniotomy period and ranged from 24 to 55 % at the post-craniotomy period. Increase of strain level was also observed at the brain tissue opposite to the treated side, however, to a much lesser extent. The mean area of craniotomy was found to be 91.1 +/- 12.7 cm(2). The brain tissue volume increased from 27 to 127 ml, corresponding to 1.65 % and 8.13 % after DC in all six patients. Also, the increased volume seemed to correlate with increased strain level. Specifically, the overall WC of brain tissue for two patients evaluated presented a significant increase after the treatment compared with the condition seen before the treatment. Furthermore, the Glasgow Coma Scale (GCS) improved in four patients after the craniotomy, while two patients died. The GCS did not seem to correlate with the strain level. We present a new numerical method to quantify the stretching or strain level of brain tissue and WC following DC. The significant increase in strain level and WC in the post-craniotomy period may cause electrophysiological changes in the axons, resulting in loss of neuronal function. Hence, this new numerical method provides more insight of the consequences following DC and may be used to better define the most optimal size and area of the craniotomy in reducing the strain level development.

Keyword
Traumatic brain injury, Stroke, Decompressive craniotomy, Strain level, Water content
National Category
Medical Engineering
Identifiers
urn:nbn:se:kth:diva-72457 (URN)10.1007/s00701-012-1393-2 (DOI)000307957500007 ()2-s2.0-84865829083 (Scopus ID)
Funder
Swedish Research Council, 621-2008-3400
Note

QC 20120921. Updated from accepted to published.

Available from: 2012-01-31 Created: 2012-01-31 Last updated: 2017-12-08Bibliographically approved
3. Decompressive craniotomy causes significant strain increase in axonal fiber tracts
Open this publication in new window or tab >>Decompressive craniotomy causes significant strain increase in axonal fiber tracts
2012 (English)In: Journal of clinical neuroscience, ISSN 0967-5868, E-ISSN 1532-2653, Vol. 20, no 4, 509-513 p.Article in journal (Refereed) Published
Abstract [en]

Background

Decompressive craniotomy allows expansion of the swollen brain outside the skull, resulting in axonal stretch, which might lead to neural injury and consequently cause unfavorable outcome for the patients. The aim of this study was to assess and quantify the axonal deformation at both pre- and post-craniotomy period in order to provide more insight into the mechanical effects on the axonal fibers upon such a treatment.

Methods

Displacement fields representing the structural changes in whole brain were obtained by a nonlinear image registration method based on the three-dimensional CT imaging data sets of a patient both before and after decompressive craniotomy. Axonal fiber tracts together with their orientations were extracted from diffusion weighted (DW) images from a healthy brain and adapted to the patient’s brain by image registration. The deformation of the brain tissue in the form of Lagrangian finite strain tensor for the entire brain was then calculated from the displacement field. Based on the obtained brain tissue strain tensor and the axonal fiber tracts, 1st principal strain was extracted at axonal fibers. Furthermore, other axonal deformation measures, i.e., axonal strain, and axonal effective shear strain were also quantified.

Results

Greatest axonal fiber displacement (up to 12 mm) was found predominantly located in the treated part of the craniotomy, accompanied by a large axonal deformation, e.g., 1st principal strain up to 0.49. This indicated the extent of axonal fiber stretching due to the neurosurgical intervention. Other strain measures, such as axonal strain and axonal effective shear strain also showed an increased level at the treated part for post-craniotomy compared to that found in the pre-craniotomy period.

Conclusions

The distortion (stretching or shearing) of axonal fibers at the treated part of the craniotomy may influence the axonal fibers in such a way that the neurochemical events are jeopardized. It is suggested that such a quantitative model may clarify some of the potential problems with such a treatment. Also, by further development of the technology it is quite possible to judge the outcome of strain levels already before the decompressive craniotomy is performed. This may have the possibility to optimize the size as well as the area of craniotomy.

National Category
Medical Engineering
Identifiers
urn:nbn:se:kth:diva-72466 (URN)10.1016/j.jocn.2012.04.019 (DOI)000317632500004 ()2-s2.0-84875539223 (Scopus ID)
Note

QC 20160818

Available from: 2012-01-31 Created: 2012-01-31 Last updated: 2017-12-08Bibliographically approved
4. Influences of brain tissue poroelastic constants on intracranial pressure (ICP) during constant-rate infusion
Open this publication in new window or tab >>Influences of brain tissue poroelastic constants on intracranial pressure (ICP) during constant-rate infusion
2013 (English)In: Computer Methods in Biomechanics and Biomedical Engineering, ISSN 1025-5842, E-ISSN 1476-8259, Vol. 16, no 12, 1330-1343 p.Article in journal (Refereed) Published
Abstract [en]

A 3D finite element (FE) model has been developed to study the mean intracranial pressure (ICP) response during constant-rate infusion using linear poroelasticity. Due to the uncertainties in the poroelastic constants for brain tissue, the influence of each of the main parameters on the transient ICP infusion curve was studied. As a prerequisite for transient analysis, steady-state simulations were performed first. The simulated steady-state pressure distribution in the brain tissue for a normal cerebrospinal fluid (CSF) circulation system showed good correlation with experiments from the literature. Furthermore, steady-state ICP closely followed the infusion experiments at different infusion rates. The verified steady-state models then served as a baseline for the subsequent transient models. For transient analysis, the simulated ICP shows a similar tendency to that found in the experiments, however, different values of the poroelastic constants have a significant effect on the infusion curve. The influence of the main poroelastic parameters including the Biot coefficient alpha, Skempton coefficient B, drained Young's modulus E, Poisson's ratio nu, permeability kappa, CSF absorption conductance C-b and external venous pressure p(b) was studied to investigate the influence on the pressure response. It was found that the value of the specific storage term S-epsilon is the dominant factor that influences the infusion curve, and the drained Young's modulus E was identified as the dominant parameter second to S-epsilon. Based on the simulated infusion curves from the FE model, artificial neural network (ANN) was used to find an optimised parameter set that best fit the experimental curve. The infusion curves from both the FE simulation and using ANN confirmed the limitation of linear poroelasticity in modelling the transient constant-rate infusion.

Keyword
finite element model, CSF dynamics, parametric study, specific storage term S-epsilon, artificial neural network
National Category
Medical Engineering
Identifiers
urn:nbn:se:kth:diva-72472 (URN)10.1080/10255842.2012.670853 (DOI)000326351000009 ()2-s2.0-84887053036 (Scopus ID)
Funder
Swedish Research Council, 621-2008-3400Vinnova
Note

QC 20131125. Updated from accepted to published.

Available from: 2012-01-31 Created: 2012-01-31 Last updated: 2017-12-08Bibliographically approved

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