Improving the forward model for electrical impedance tomography of brain function through rapid generation of subject specific finite element models

Electrical Impedance Tomography (EIT) is a non-invasive imaging method which allows internal electrical impedance of any conductive object to be imaged by means of current injection and surface voltage measurements through an array of externally applied electrodes. The successful generation of the image requires the simulation of the current injection patterns on either an analytical or a numerical model of the domain under examination, known as the forward model, and using the resulting voltage data in the inverse solution from which images of conductivity changes can be constructed. Recent research strongly indicates that geometric and anatomical conformance of the forward model to the subject under investigation significantly affects the quality of the images. This thesis focuses mainly on EIT of brain function and describes a novel approach for the rapid generation of patient or subject specific finite element models for use as the forward model. After introduction of the topic, methods of generating accurate finite element (FE) models using commercially available Computer-Aided Design (CAD) tools are described and show that such methods, though effective and successful, are inappropriate for time critical clinical use. The feasibility of warping or morphing a finite element mesh as a means of reducing the lead time for model generation is then presented and demonstrated. This leads on to the description of methods of acquiring and utilising known system geometry, namely the positions of electrodes and registration landmarks, to construct an accurate surface of the subject, the results of which are successfully validated. The outcome of this procedure is then used to specify boundary conditions to a mesh warping algorithm based on elastic deformation using well-established continuum mechanics procedures. The algorithm is applied to a range of source models to empirically establish optimum values for the parameters defining the problem which can successfully generate meshes of acceptable quality in terms of discretization errors and which more accurately define the geometry of the target subject. Further validation of the algorithm is performed by comparison of boundary voltages and image reconstructions from simulated and laboratory data to demonstrate that benefits in terms of image artefact reduction and localisation of conductivity changes can be gained. The processes described in the thesis are evaluated and discussed and topics of further work and application are described

Rapid generation of subject-specific thorax forward models

For real-time monitoring of lung function using accurate patient geometry, shape information needs to be acquired and a forward model generated rapidly. This paper shows that warping a cylindrical model to an acquired shape results in meshes of acceptable mesh quality, in terms of stretch and aspect ratio

Torso shape detection to improve lung monitoring

Two methodologies are proposed to detect the patient-specific boundary of the chest, aiming to produce a more accurate forward model for EIT analysis. Thus, a passive resistive and an inertial prototypes were prepared to characterize and reconstruct the shape of multiple phantoms. Preliminary results show how the passive device generates a minimum scatter between the reconstructed image and the actual shap

Synthesis analysis of the free form curves and surfaces parametrical models

Parametrical models have become an important mathematical tool for free form curves and surface description. they allow the use of state-of-the-art computers to do the various processing and analysis with respect to shape (calculation of the volume and surface area, vibration analysis, NC programs preparation, etc.). Without these models, the current product design and manufacturing would be difficult. The paper presents an analysis study for the most important parametrical models of free form curves and surfaces description (Ferguson, Coons, Bezier, B-Spline and rational models). Firstly, it emphasises external and mathematical properties of each model. Secondly, it gives the most interesting interfaces between models. Thus the process of complex surface design and manufacturing would be more efficient in view this analysis. Finally for validation, the study is ended by an application to the design of a car bonnet.Parametrical models have become an important mathematical tool for free form curves and surface description. they allow the use of state-of-the-art computers to do the various processing and analysis with respect to shape (calculation of the volume and surface area, vibration analysis, NC programs preparation, etc.). Without these models, the current product design and manufacturing would be difficult. The paper presents an analysis study for the most important parametrical models of free form curves and surfaces description (Ferguson, Coons, Bezier, B-Spline and rational models). Firstly, it emphasises external and mathematical properties of each model. Secondly, it gives the most interesting interfaces between models. Thus the process of complex surface design and manufacturing would be more efficient in view this analysis. Finally for validation, the study is ended by an application to the design of a car bonnet

Detecting colorectal cancer using electrical impedance spectroscopy: an ex vivo feasibility study

Objective: Colorectal cancer is the fourth most common cancer worldwide, with a lifetime risk of around 20%. Current solutions do not allow clinicians to objectively assess tissue abnormality during endoscopy and perioperatively. A solution capable of objectively assessing samples in real time could greatly improve the treatment process. A solution that can be integrated in minimally invasive diagnostics and management strategies to provide real-time point-of-care information would be greatly transformative. Electrical impedance spectroscopy (EIS) may provide such a solution. In this paper, we present a feasibility study on using EIS in assessing colorectal tissue. Approach: We performed tetrapolar EIS using ZedScan on excised human colorectal tumour tissue and the matched normal colonic mucosa in 22 freshly resected specimens following elective surgery for colorectal cancer. Histopathological examination was used to confirm the final diagnosis. Statistical significance was assessed with Wilcoxon signed rank test. Main results: Tetrapolar EIS could discriminate cancer with statistically significant results when applying frequencies between 305 Hz – 625 kHz (p < 0.05). 300 Ω was set as the transfer impedance threshold to detect cancer. Thus, the area under the corresponding receiver operating characteristic curve for this threshold was 0.7105. Significance: This feasibility study demonstrates that impedance spectra changes in colorectal cancer tissue are detectable and may be statistically significant, suggesting that EIS has the potential to be the core technology in a novel non-invasive point of care test for detecting colorectal cancer. These results warrant further development and increasing the size of the study with a device specificity designed for colorectal cancer

Towards a thoracic conductive phantom for EIT

Phantom experiments are a crucial step for testing new hardware or imaging algorithms for electrical impedance tomography (EIT) studies. However, constructing an accurate phantom for EIT research remains critical; some studies have attempted to model the skull and breasts, and even fewer, as yet, have considered the thorax. In this study, a critical comparison between the electrical properties (impedance) of three materials is undertaken: a polyurethane foam, a silicone mixture and a thermoplastic polyurethane filament. The latter was identified as the most promising material and adopted for the development of a flexible neonatal torso. The validation is performed by the EIT image reconstruction of the air filled cavities, which mimic the lung regions. The methodology is reproducible for the creation of any phantom that requires a slight flexibility

Cross-sectional chest circumference and shape development in infants

Objective: This study investigates the development of the thoracic cross-section at the nipple line level during the early stages of life. Unlike the descriptive awareness regarding chest development course, there exist no quantitative references concerning shape, circumference and possible dependencies to age, gender or body weight. The proposed mathematical relations are expected to help create guidelines for more realistic modelling and potential detection of abnormalities. One potential application is lung electrical impedance tomography (EIT) monitoring where accurate chest models are crucial in both extracting reliable parameters for regional ventilation function and design of EIT belts. Despite their importance, such reference data is not readily available for the younger age range due to insufficient data amid the regulations of neonatal imaging. Results: Chest circumference shows the highest correlation to body weight following the relation fx=18.3735ln0.0012x+2.1010 where x is the body weight in grams and f(x) is the chest circumference in cm at the nipple line level. No statistically significant difference in chest circumference between genders was detected. However, the shape indicated signs of both age and gender dependencies with on average boys developing a more rectangular shape than girls from the age of 1 years and 9 months

Torso shape detection to improve lung monitoring

Newborns with lung immaturity often require continuous monitoring and treatment of their lung ventilation in intensive care units, especially if born preterm. Recent studies indicate that Electrical Impedance Tomography (EIT) is feasible in newborn&#13; infants and children, and can quantitatively identify changes in regional lung aeration and ventilation following alterations to respiratory conditions. Information on the patient-specic shape of the torso and its role in minimizing the artefacts in the&#13; reconstructed images can improve the accuracy of the clinical parameters obtained from EIT. Currently, only idealized models or those segmented from CT scans are usually adopted. This study presents and compares two methodologies that can&#13; detect the patient-specic torso shape by means of wearable devices based on: (1) previously reported bend sensor technology and (2) a novel approach based on the use of accelerometers. The reconstruction of different phantoms, taking into account&#13; anatomical asymmetries and different sizes, are produced for comparison. As a result, the accelerometers are more versatile than bend sensors, which cannot be used on bigger cross-sections. The computational study estimates the optimal number of&#13; accelerometers required in order to generate an image reconstruction comparable to the use of a CT scan as the forward model. Furthermore, since the patient position is crucial to monitoring lung ventilation, the orientation of the phantoms is automatically&#13; detected by the accelerometer-based method. [Abstract copyright: © 2018 Institute of Physics and Engineering in Medicine.