Biomedical subjects

Martin L Buist

Publications and source records attributed to Martin L Buist.

5 recordsLinked to original sources

PubMed2006

An integrative software package for gastrointestinal biomagnetic data acquisition and analysis using SQUID magnetometers.

The study of bioelectric and biomagnetic activity in the human gastrointestinal (GI) tract is of great interest in clinical research due to the proven possibility to detect pathological conditions thereof from electric and magnetic field recordings. The magnetogastrogram (MGG) and magnetoenterogram (MENG) can be recorded using superconducting quantum interference device (SQUID) magnetometers, which are the most sensitive magnetic flux-to-voltage converters currently available. To address the urgent need for powerful acquisition and analysis software tools faced by many researchers and clinicians in this important area of investigation, an integrative and modular computer program was developed for the acquisition, processing and analysis of GI SQUID signals. In addition to a robust hardware implementation for efficient data acquisition, a number of signal processing and analysis modules were developed to serve in a variety of both clinical procedures and scientific investigations. Implemented software features include data processing and visualization, waterfall plots of signal frequency spectra as well as spatial maps of GI signal frequencies. Moreover, a software tool providing powerful 3D visualizations of GI signals was created using realistic models of the human torso and internal organs.

Gastrointestinal Diseases↗

PubMed2006

Modelling slow wave activity in the small intestine.

We have developed an anatomically based model to simulate slow wave activity in the small intestine. Geometric data for the human small intestine were obtained from the Visible Human project. These data were used to create a one-dimensional finite element mesh of the entire small intestine using an iterative fitting procedure. The electrically active components of the intestinal walls were modelled using a modified Fitzhugh-Nagumo cell model embedded within a longitudinal smooth muscle layer and a layer containing Interstitial Cells of Cajal. Within these layers, the monodomain equation was used to describe slow wave propagation. To solve the monodomain equation, a high-resolution finite difference grid, with an average spatial resolution of 0.95 mm, was embedded within each finite element. The resulting simulations of intestinal activity agree with the experimental observation that slow wave frequency gradually declines from 12 cycles per minute (cpm) in the duodenum to 8 cpm at the terminal ileum. Furthermore, the simulations demonstrated a decrease in conduction velocity with distance along the small intestine (10.7 cm/s in the duodenum, 5.1cm/s in the jejunum and 1.4 cm/s in the ileum), matching experimental recordings from the canine small intestine. We conclude that the framework presented here is capable of qualitatively simulating normal slow wave activity in an anatomical model of the small intestine.

Finite Element Analysis↗

PubMed2003

Cardiac electrical activity--from heart to body surface and back again.

We report here on our latest developments in the forward and inverse problems of electrocardiology. In the forward problem, a coupled cellular model of cardiac excitation-contraction is embedded within an anatomically realistic model of the cardiac ventricles, which is itself embedded within a torso model. This continuum modelling framework allows the effects of cellular-level activity on the surface electrocardiogram (ECG) to be carefully examined. Furthermore, the contributions of contraction and local ischemia on body surface recordings can also be elucidated. Such information can provide theoretical limits to the sensitivity and ultimately the detection capability of body surface ECG recordings. Despite being very useful, such detailed forward modelling is not directly applicable when seeking to use densely sampled ECG information to assess a patient in a clinical environment (the inverse problem). In such a situation patient specific models must be constructed and, due to the nature of the inverse problem, the level of detail that can be reliably reproduced is limited. Extensive simulation studies have shown that the accuracy with which the heart is localised within the torso is the primary limiting factor. To further identify the practical performance capabilities of the current inverse algorithms, high quality experimental data is urgently needed. We have been working towards such an objective with a number of groups, including our local hospital in Auckland. At that hospital, in patients undergoing catheter ablation surgery, up to 256 simultaneous body surface signals were recorded by using various catheter pacing protocols. The geometric information required to customize the heart and torso model was obtained using a combination of ultrasound and laser scanning technologies. Our initial results indicate that such geometric imaging modalities are sufficient to produce promising inversely-constructed activation profiles.

Algorithms↗

PubMed2003

Altered T wave dynamics in a contracting cardiac model.

INTRODUCTION: The implications of mechanical deformation on calculated body surface potentials are investigated using a coupled biophysically based model. METHODS AND RESULTS: A cellular model of cardiac excitation-contraction is embedded in an anatomically accurate two-dimensional transverse cross-section of the cardiac ventricles and human torso. Waves of activation and contraction are induced by the application of physiologically realistic boundary conditions and solving the bidomain and finite deformation equations. Body surface potentials are calculated from these activation profiles by solving Laplace's equation in the passive surrounding tissues. The effect of cardiac deformation on electrical activity, induced by contraction, is demonstrated in both single-cell and tissue models. Action potential duration is reduced by 7 msec when the single cell model is subjected to a 10% contraction ramp applied over 400 msec. In the coupled electromechanical tissue model, the T wave of the ECG is shown to occur 18 msec earlier compared to an uncoupled excitation model. To assess the relative effects of myocardial deformation on the ECG, the activation sequence and tissue deformation are separated. The coupled and uncoupled activation sequences are mapped onto the undeforming and deforming meshes, respectively. ECGs are calculated for both mappings. CONCLUSION: Adding mechanical contraction to a mathematical model of the heart has been shown to shift the T wave on the ECG to the left. Although deformation of the myocardium resulting from contraction reduces the T wave amplitude, cell stretch producing altered cell membrane kinetics is the major component of this temporal shift.

Action Potentials↗

PubMed2003

The effect of torso impedance on epicardial and body surface potentials: a modeling study.

Experimental results have been published that report marked changes in measured epicardial potentials when the conductivity of the material surrounding the heart is altered. These reports raise a question as to the validity of the traditional two step, equivalent cardiac source approach to modeling the forward problem of electrocardiology as the equivalent source calculation occurs in what is effectively an isolated cardiac region. In the physical situation the heart is surrounded by a torso that contains many different tissue types with different conductivities and is certainly not isolated. Here, a fully coupled model of the problem is employed where the electrical pathways are continuous from a cellular level through to the body surface. This model is used to investigate the effects that torso inhomogeneities have on epicardial and body surface potentials, including comparisons with a traditional two step approach. In particular, it is shown that adding lungs changes the epicardial potentials by 17%, which is consistent with the reported experimental results. In none of the tested situations did the equivalent source approach completely reproduce the fully coupled results, supporting the notion that a fully coupled approach is required to properly solve the forward problem of electrocardiology.

Abdomen↗