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Monitoring Intracranial Pressure

Final Report Summary - MICP (Monitoring Intracranial Pressure)

Increased intracranial pressure (ICP) can arise for a variety of reasons such as trauma to the head and brain tumors. Prolonged intracranial hypertension, a common pathway in the presentation of traumatic head injury, can lead to brain damage or even death. Therefore it is essential that accurate monitoring of this pressure is undertaken clinically. ICP is the pressure inside the skull and therefore involves the tissue of the brain and cerebrospinal fluid (CSF). The current state of the art techniques for measuring ICP are all invasive, which means that devices must be inserted into patients, such as ventricular, extradural, subarachnoid, epidural and lumbar CSF monitoring methods. These methods have many disadvantages including being invasive, having low-accuracy and cross-infection. Although many efforts have recently been made to improve the mini-traumatic or non-invasive methods used, currently there are no non-invasive methods available. A novel non-invasive technique for measuring the pressure was developed using the engineering techniques based on the deformed cranium as the ICP fluctuates.
The 3D visualization of the human skull represents an advance in the interpretation of images, because it makes possible the volumetric analysis of the anatomical structures and it evidences more clarity in its space configurations and its relations with other organs. Indeed, research is focusing more and more on the acquisition of models and simulation parameters from real people rather than procedurally simulating their appearance and movements. For the purpose of our analysis, we adopted the model of hollow sphere. We presented the development and validation of a 3D finite element (FE) model intended to better understand the deformation mechanisms of human skull corresponding to the ICP change. Based on the established knowledge that cranial cavity is importantly composed of skull and dura mater, a thin-walled structure was simulated by the composite shell elements of the finite element software. Reconstructions of the finite element model of cranial cavity are mostly through the multi-CT or MRI scanning technology.

Prof. Duncan and others proposed that it might be a key point that the relationships between the microstructural skeleton, tissue fluid and deformation properties of cranial cavity for the viscoelasticity of compact and cancellous skull bone. Here Scanning Electron Microscopy (SEM) was used to determine the microstructure of the porcine skull bone. The results were used to create a microscopic model of the bone subjected to a fluid-solid interaction (FSI) analysis. Then the stresses and strains of a compact and cancellous skull bone could be predicted within a one-way fluid flow regime with the changing ICP. By remodelling compact and cancellous bone at the microscopic scale with finite element, it for the first time showed that the interstitial-fluid flow affected the deformation of skull bone in the channel region as well. It has been shown that the dynamic pressure significantly influences the bone-fluid flow and its associated streaming potentials, which can be controlled quantitatively. The tissue fluid has a large influence on the microstructure of compact and cancellous skull bone when characterizing the deformation of cranial cavity. The pressure loading and bone-fluid flow can initiate and control bone morphology at the tissue level. This work has just been submitted for publications in four high-ranked peer-reviewed journals.

The project of Dr. Xianfang aimed to investigate the material properties of skull bone and brain to demonstrate the viscoelasticity of cranial tissues, which can be utilized effectively for the real-time monitoring system for ICP. The function of human skull is mainly protecting the brain – the most important organ in the human body. Generally, each cranial bone is the skull skeleton contains two forms of tissue: external compact bone and inside cancellous bone. Compact bone is thickest and relatively compact, and the cancellous bone is the spongy construction with the biphasic of viscous fluid and elastic solid materials. Skull fractures are the break in the cranial bone, also known as the skull, and injury, tumor invasion, or infection can cause the cranial defects. An injury to the brain can also accompany the fracture. The possible types of injuries are more likely to depend on if the dynamic material properties of skull bone are identified. The main material properties include elastic modulus, ultimate stress, and ultimate strain, which dictate the response and failure of skull bone to loading. It will not only provide insight into the reaction of cranial cavity to loading, but also improve the accuracy of skull fracture impacts to a head determining the material properties of skull bone. Meanwhile, the cranial implants are reliable with time-to-delivery an average of 4 weeks, in which the saved-time procedure will make it simpler to reconstruct the large cranial defects. There has been extensive characterization of the dynamic material properties of skull and brain tissue, but little has been reported for the dynamic properties of skull bone and brain in response to ICP-induced mechanical loading with the time. Prof. Duncan and Dr. Daniel proposed to design an experimental approach in a porcine model to perform a tension with equal distance distribution to simulate the ICP fluctuation in the head and test the material properties of cranial cavity with the time-to-delivery. The ultimate stress and elastic modulus of porcine cranial cavity had a tendency to increase at the higher time-to-delivery tested. The elastic modulus of whole skull wasn’t just the sum of that of every composite layer, such as external compact, inside cancellous and internal compact bone. The time-to-delivery was found to significantly increase elastic modulus and ultimate stress in samples. And the elastic modulus of monolayer compact bone was about 2.5 times as much as that of cancellous bone. This study has shown that the importance of testing material properties with the time-to-delivery as the applicable environment, and provides insight into the role of time-dependent material properties in understanding the mechanisms of cranial injury in the human population.

Another aspect of Dr. Xianfang’s work was to investigate the material properties of brain using the DMA testing device and system (ElectroForce 3200 Instrument with WinTest DMA software) in Prof. Duncan’s bio-medical laboratory. The soft tissue represents a significant challenge for analysis because of its lack of structural integrity. Frequency sweeps were performed at very low levels on fresh calf liver in order to determine the Dynamic Modulus under different strain conditions. For this test and other applications to soft tissue, it is necessary to have very fine dynamic control at low levels of excitation. And this device need continuously be tuned for particular material – brain because the system is normally tuned for bone materials. But the internal structure of fresh brain is very soft, and the surface of brain will be broken after the brain is cut into small samples. Then the DMA testing will be more difficult even at low levels of excitation. Thus we can test the whole brain tissue with the 3200 DMA device, or a new testing method ‘Dynamic Torsion Test for the Mechanical Characterization of Soft Biological Tissues’ can be substituted.

Furthermore as part of her fellowship, Dr. Xianfang has been involved in a collaborative FP7-People-2013-IIF funded project “Monitoring Intracranial Pressure - MICP”, where Prof. Duncan serves as work package leader. This has provided her with a unique opportunity to acquire advanced FE and DMA skills and apply them to the simulation and experiment data. As this project is still ongoing, Dr Xianfang will still be involved in this study work after the end of her fellowship and several publications from this project are anticipated upon final analysis by the project end in 2017.

Alongside her main projects, Dr. Xianfang has started research into quantifying the relationship between the deformation of cranial cavity and ICP and investigating the structure of human head as the brain-FSI-skull remodeling and DMA testing of porcine brain. These relationships and data were used to provide a novel, real-time and precise means of monitoring ICP and could further help to setup the mechanisms underlying the therapeutic effects of neuromodulation in neurological and neuropsychiatric conditions using modelling and functional imaging techniques. Meeting this aim optimizes current therapies in existing patient populations with intracranial interventions, implements new approaches and techniques, explores new targets, and defines new disease populations; thus delivering clinical and financial benefits.