Biomedical subjects

A J Pullan

Publications and source records attributed to A J Pullan.

12 recordsLinked to original sources

PubMed2006

Computational simulations of the human magneto- and electroenterogram.

Many functional pathologies of the small intestine are difficult to diagnose clinically without an invasive surgical intervention. Often such conditions are associated with a disruption of the normal electrical activity occurring within the musculature of the small intestine. The far field electrical signals on the torso surface arising from the electrical activity within the small intestine cannot be reliably measured. However, it has been shown that abnormal electrical activity in the small intestine can be distinguished by recording the magnetic fields of intestinal origin immediately outside the torso surface. We have developed an anatomically-based computational model to simulate slow wave propagation in the small intestine, the resulting cutaneous electrical field and the magnetic field outside the torso. Using both a one-dimensional and a three-dimensional model of the duodenum we investigate the degree of detail that is required to realistically simulate this far field activity. Our results indicate that some of the qualitative behavior in the far field activity can be replicated using a one-dimensional model, although there are clear situations where the greater level modeling detail is required.

Action Potentials↗

PubMed2006

Multiscale modelling of human gastric electric activity: can the electrogastrogram detect functional electrical uncoupling?

During recent years there has been a growing interest in the assessment of gastric electrical health through cutaneous abdominal recordings. The analysis of such recordings is largely limited to an inspection of frequency dynamics, and this has raised doubts as to whether functional gastric electrical uncoupling can be detected using this technique. We describe here a computational approach to the problem in which the equations governing the underlying physics of the problem have been solved over an anatomically detailed human torso geometry. Cellular electrical activity was embedded within a stomach tissue model, and this was coupled to the torso using an equivalent current source approach. Simulations were performed in which normal and functionally uncoupled (through the introduction of an ectopic antral pacemaker) gastric slow wave activity was present, and corresponding cutaneous electrogastrograms were produced. These were subsequently analysed using the currently recommended techniques, and it was found that the functionally uncoupled situation was indistinguishable from normal slow wave activity using this approach.

Computer Simulation↗

PubMed2005

Rapid construction of a patient-specific torso model from 3D ultrasound for non-invasive imaging of cardiac electrophysiology.

One of the main limitations in using inverse methods for non-invasively imaging cardiac electrical activity in a clinical setting is the difficulty in readily obtaining high-quality data sets to reconstruct accurately a patient-specific geometric model of the heart and torso. This issue was addressed by investigation into the feasibility of using a pseudo-3D ultrasound system and a hand-held laser scanner to reconstruct such a model. This information was collected in under 20 min prior to a catheter ablation or pacemaker study in the electrophysiology laboratory. Using the models created from these data, different activation field maps were computed using several different inverse methods. These were independently validated by comparison of the earliest site of activation with the physical location of the pacing electrodes, as determined from orthogonal fluoroscopy images. With an estimated average geometric error of approximately 8 mm, it was also possible to reconstruct the site of initial activation to within 17.3 mm and obtain a quantitatively realistic activation sequence. The study demonstrates that it is possible rapidly to construct a geometric model that can then be used non-invasively to reconstruct an activation field map of the heart.

Cardiac Pacing, Artificial↗

PubMed2005

Anatomically based modelling of the human skull and jaw.

We present here an anatomically based model of the human masticatory system that provides a framework for simulating the complex chewing process. The initial motivation for creating this model was the desire to have a computational model of the human jaw that can be used to simulate the action of simple bites, and to calculate the stresses and forces on the teeth that are involved. The model created also provides a platform that can be used to investigate other features of the masticatory system. To construct this global model, individual models of the bones of the skull and jaw were created from generic data sets. Geometric models of the muscles of mastication were also created and attached to the appropriate bones. To complete this initial model, representations of the crowns of the teeth were created and a basic model of the temporomandibular joint (TMJ) was included. The finite element method was used to solve for the stresses and strains created by the loading conditions during a clenching simulation involving the mandible bone. The model presented here is also discussed in relation to a model of the entire musculo-skeletal system being developed as part of the Physiome Project.

Computer Simulation↗

PubMed2004

An anatomical model of the gastric system for producing bioelectric and biomagnetic fields.

Between 60 and 70 million people in the United States are affected by gastrointestinal disorders. Many of these conditions are difficult to assess without surgical intervention and accurate noninvasive techniques to aid in clinical assessment are needed. Through the use of a superconducting quantum interference device (SQUID) gradiometer, the weak magnetic field generated as a result of muscular activity in the digestive system can be measured. However, the interpretation of these magnetic recordings remains a significant challenge. We have created an anatomically realistic biophysically based mathematical model of the human digestive system and using this model normal gastric electrical control activity (ECA) has been simulated. The external magnetic fields associated with this gastric ECA have also been computed and are shown to be in qualitative agreement with recordings taken from normal individuals. The model framework thus provides a rational basis from which to begin interpreting magnetic recordings from normal and diseased individuals.

Digestive System Physiological Phenomena↗

PubMed2001

From cell to body surface: a fully coupled approach.

Different types of equivalent cardiac sources are often used in the forward problem of electrocardiology. These may not be adequate to accurately reproduce the body surface potentials that are the result of cellular electrical activity, because they do not guarantee current conservation across the myocardial surfaces. Presented here are the outlines of 2 new coupling techniques that seek to overcome this problem by creating a continuous electrical pathway from the cellular level through to the body surface. The first technique directly couples the extracellular cardiac bidomain region to the surrounding passive torso regions creating a single system of equations without the need for matrix inversions. The second technique uses a fixed point iteration across each myocardial surface that has the goal of matching the potential fields and current flows across the cardiac boundaries into the neighbouring tissues. These techniques can be combined, directly coupling some regions and iterating across other region boundaries. Simulations on a transverse two-dimensional slice through a human male torso are presented, showing the convergence of these methods.

Body Surface Potential Mapping↗

PubMed2001

The computational performance of a high-order coupled FEM/BEM procedure in electropotential problems.

This paper presents a thorough analysis of the computational performance of a coupled cubic Hermite boundary element/finite element procedure. This C1 (i.e., value and derivative continous) method has been developed specifically for electropotential problems, and has been previously applied to torso and skull problems. Here, the behavior of this new procedure is quantified by solving a number of dipole in spheres problems. A detailed set of results generated with a wide range of the various input parameters (such as dipole orientation, location, conductivity, and solution method used in each spherical shell [either finite element or boundary elements]) is presented. The new cubic Hermite boundary element procedure shows significantly better accuracy and convergence properties and a significant reduction in CPU time than a traditional boundary element procedure which uses linear or constant elements. Results using the high-order method are also compared with other computational methods which have had quantitative results published for electropotential problems. In all cases, the high-order method offered a significant improvement in computational efficiency by increasing the solution accuracy for the same, or fewer, solution degrees of freedom.

Biomedical Engineering↗

PubMed2001

Noninvasive electrical imaging of the heart: theory and model development.

The aim of this work is to begin quantifying the performance of a recently developed activation imaging algorithm of Huiskamp and Greensite [IEEE Trans. Biomed. Eng. 44:433-446]. We present here the modeling and computational issues associated with this process. First, we present a practical construction of the appropriate transfer matrix relating an activation sequence to body surface potentials from a general boundary value problem point of view. This approach makes explicit the role of different Green's functions and elucidates features (such as the anisotropic versus isotropic distinction) not readily apparent from alternative formulations. A new analytic solution is then developed to test the numerical implementation associated with the transfer matrix formulation presented here and convergence results for both potentials and normal currents are given. Next, details of the construction of a generic porcine model using a nontraditional data-fitting procedure are presented. The computational performance of this model is carefully examined to obtain a mesh of an appropriate resolution to use in inverse calculations. Finally, as a test of the entire approach, we illustrate the activation inverse procedure by reconstructing a known activation sequence from simulated data. For the example presented, which involved two ectopic focii with large amounts of Gaussian noise (100 microV rms) present in the torso signals, the reconstructed activation sequence had a similarity index of 0.880 when compared to the input source.

Algorithms↗

PubMed2000

Generation of an anatomically based three-dimensional model of the conducting airways.

An anatomically accurate model of the conducting airways is essential for adequately simulating gas mixing, particle deposition, heat and water transfer, and fluid distribution. We have extended a two-dimensional tree-growing algorithm to three dimensions for generation of a host-shape dependent three-dimensional conducting airway model. Terminal branches in the model are both length limited and volume-supplied limited. A limit is imposed on the maximum possible branch angle between a daughter and parent branch. Comparison of the resulting model with morphometric data shows that the algorithm produces branching and length ratios, path lengths, numbers of branches, and branching angles very close to those from the experimental data. The correlation between statistics from the generated model and those from morphometric studies suggests that the conducting airway structure can be described adequately using a "supply and demand" algorithm. The resulting model is a computational mesh that can be used for simulating transport phenomena.

Algorithms↗

PubMed2000

Effects of material properties and geometry on electrocardiographic forward simulations.

This paper investigates the effect on torso potentials of changes in the material properties of the torso volume conductor and changes in the relative geometry of the heart and torso. The investigations are performed using a number of forward simulations with a high-order coupled finite element/boundary element torso model. This torso model contains descriptions of the epicardium, lungs, skeletal muscle (with a continuously varying fiber field) and subcutaneous fat. The number of nodes, elements and solution degrees-of-freedom used in the computational torso model are considerably smaller than previous torso models of similar complexity. The successful forward simulations in this paper hence demonstrate the use of the high-order coupled approach with realistic problems. The results of the torso simulations show that the torso inhomogeneities do affect the torso potentials but do not affect the distribution or pattern of the torso potentials. The inhomogeneities considered are found to have a varying, but important, effect on the torso potentials. The effect of the subcutaneous fat is found to be more important and the effect of the skeletal muscle is found to be less important than previous reports in the literature. The results also show that the relative geometry of the heart and torso is very important in determining the torso potential magnitudes and distributions.

Abdomen↗

PubMed2000

Generation of an anatomically based geometric coronary model.

A discrete anatomically accurate finite element model of the largest six generations of the coronary arterial network is developed. Using a previously developed anatomically accurate model of ventricular geometry the boundaries of the coronary mesh are defined from measured epicardial coronaries. Network topology is then generated stochastically from published anatomical data. Spatial information is added to this topological data using an avoidance algorithm accounting for global network geometry and optimal local branch angle properties. The generated vessel lengths, radii and connectivity are consistent with the published studies and a relativity even spatial distribution of vessels within the ventricular mesh is achieved. The local finite element coordinates of the coronary nodes within the ventricular mesh are calculated such that the coronary geometry can be recalculated within a deformed ventricular mesh.

Biomedical Engineering↗

PubMedDate not supplied

Geometric modeling of the human torso using cubic hermite elements.

We discuss the advantages and problems associated with fitting geometric data of the human torso obtained from magnetic resonance imaging, with high-order (bicubic Hermite) surface elements. These elements preserve derivative (C1) continuity across element boundaries and permit smooth anatomically accurate surfaces to be obtained with relatively few elements. These elements are fitted to the data with a new nonlinear fitting procedure that minimizes the error in the fit while maintaining C1 continuity with nonlinear constraints. Nonlinear Sobelov smoothing is also incorporated into this fitting scheme. The structures fitted along with their corresponding root mean-squared error, number of elements used, and number of degrees of freedom (df) per variable are: epicardium (0.91 mm, 40 elements, 142 df), left lung (1.66 mm, 80 elements, 309 df), right lung (1.69 mm, 80 elements. 309 df), skeletal muscle surface (1.67 mm, 264 elements, 1,010 df), fat layer (1.79 mm, 264 elements, 1.010 df), and the skin layer (1.43 mm, 264 elements, 1,010 df). The fitted surfaces are assembled into a combined finite element/boundary element model of the torso in which the exterior surfaces of the heart and lungs are modeled with two-dimensional boundary elements and the layers of the skeletal muscle, fat, and skin are modeled with finite elements. The skeletal muscle and fat layers are modeled with bicubic Hermite linear elements and are obtained by joining the adjacent surface elements for each layer. Applications for the torso model include the forward and inverse problems of electrocardiography, defibrillation studies, radiation dosage studies, and heat transfer studies.

Abdomen↗