L. Alan Bradshaw

Falling-edge, variable threshold (FEVT) method for the automated detection of gastric slow wave events in high-resolution serosal electrode recordings.

High resolution (HR) multi-electrode mapping is increasingly being used to evaluate gastrointestinal slow wave behaviors. To create the HR activation time (AT) maps from gastric serosal electrode recordings that quantify slow wave propagation, it is first necessary to identify the AT of each individual slow wave event. Identifying these ATs has been a time consuming task, because there has previously been no reliable automated detection method. We have developed an automated AT detection method termed falling-edge, variable threshold (FEVT) detection. It computes a detection signal transform to accentuate the high ‘energy’ content of the falling edges in the serosal recording, and uses a running median estimator of the noise to set the time-varying detection threshold. The FEVT method was optimized, validated, and compared to other potential algorithms using in vivo HR recordings from a porcine model. FEVT properly detects ATs in a wide range of waveforms, making its performance substantially superior to the other methods, especially for low signal-to-noise ratio (SNR) recordings. The algorithm offered a substantial time savings (>100 times) over manual-marking whilst achieving a highly satisfactory sensitivity (0.92) and positive-prediction value (0.89).

Noninvasive detection of ischemic bowel

PURPOSE Acute mesenteric arterial occlusion is an abdominal catastrophe that carries high morbidity and mortality rates. Current diagnostic methods, however, lack sensitivity and specificity and do not provide information about the viability of the affected bowel. Early diagnosis and intervention would improve patient outcomes and survival rates. The basic electrical rhythm (BER) is the omnipresent electrical slow wave of the gastrointestinal tract that characterizes the underlying electrical activity of the bowel. BER frequency is known to fall with ischemia. Superconducting quantum interference devices (SQUIDs) can detect BER by measuring the magnetic fields generated by the electrical activity of the smooth muscle of the small bowel. The purpose of this study was to determine the ability of a SQUID to detect mesenteric ischemia in a free-lying section of small bowel in an animal model of acute superior mesenteric artery occlusion. METHODS Seven adult male rabbits (six experimental and one control) were studied with transabdominal SQUID and electrode recordings during baseline and after the induction of mesenteric ischemia with balloon occlusion of the superior mesenteric artery. Continuous recordings were taken for 120 minutes of ischemia and analyzed with autoregressive spectral analysis to determine the BER frequency during specific time points of the study. Two independent investigators blinded to the experimental preparation examined the results to determine whether there was decreased BER frequency and thus ischemia. The results are expressed as mean +/- SEM, and paired t tests were used to determine statistical significance. RESULTS BER was detected in all seven animals and fell from 10.7 +/- 0.5 cpm to 7.0 +/- 1.8 cpm after 30 minutes of ischemia in the magnetic channels (P <.05, with t test). The fall in BER was detected by the SQUID in all six experimental animals. The blinded observers correctly identified healthy and ischemic magnetic data recording, with a sensitivity of 94% and specificity of 100%. CONCLUSION SQUIDs can noninvasively detect bowel ischemia early in a free-lying segment of small bowel in this animal model with a high degree of sensitivity and specificity.

Intestinal tachyarrhythmias during small bowel ischemia

The electrical control activity (ECA) of the bowel is the omnipresent slow electrical wave of the intestinal tract. Characterization of small bowel electrical activity during ischemia may be used as a measure of intestinal viability. With the use of an animal model of mesenteric ischemia, serosal electrodes and a digital recording apparatus utilizing autoregressive spectral analysis were used to monitor the ECA of 20 New Zealand White rabbits during various lengths of ischemia. ECA frequency fell from 18.2 +/- 0.5 cycles per minute (cpm) at baseline to 12.2 +/- 0.9 cpm (P < 0.05) after 30 min of ischemia and was undetectable by 90 min of ischemia in all animals. Tachyarrhythmias of the ECA were recorded in 55% of the animals as early as 25 min after ischemia was induced and lasted from 1 to 48 min. Frequencies ranged from 25 to 50 cpm. These tachyarrhythmias were seen only during ischemia, suggesting that they are pathognomonic for intestinal ischemia. The use of the detection of ECA changes during intestinal ischemia may allow earlier diagnosis of mesenteric ischemia.

Artifact reduction in magnetogastrography using fast independent component analysis

The analysis of magnetogastrographic (MGG) signals has been limited to epochs of data with limited interference from extraneous signal components that are often present and may even dominate MGG data. Such artifacts can be of both biological (cardiac, intestinal and muscular activities, motion artifacts, etc) and non-biological (environmental noise) origin. Conventional methods-such as Butterworth and Tchebyshev filters-can be of great use, but there are many disadvantages associated with them as well as with other typical filtering methods because a large amount of useful biological information can be lost, and there are many trade-offs between various filtering methods. Moreover, conventional filtering cannot always fully address the physicality of the signal-processing problem in terms of extracting specific signals due to particular biological sources of interest such as the stomach, heart and bowel. In this paper, we demonstrate the use of fast independent component analysis (FICA) for the removal of both biological and non-biological artifacts from multi-channel MGG recordings acquired using a superconducting quantum intereference device (SQUID) magnetometer. Specifically, we show that the signal of gastric electrical control activity (ECA) can be isolated from SQUID data as an independent component even in the presence of severe motion, cardiac and respiratory artifacts. The accuracy of the method is analyzed by comparing FICA-extracted versus electrode-measured respiratory signals. It is concluded that, with this method, reliable results may be obtained for a wide array of magnetic recording scenarios.

Ellipsoidal electrogastrographic forward modelling.

The theoretical and computational study of the electromagnetic forward and inverse problems in ellipsoidal geometry is important in electrogastrography because the geometry of the human stomach can be well approximated using this idealized body. Moreover, the anisotropies inherent to this organ can be highlighted by the characteristics of the electric potential associated with current dipoles in an ellipsoid. In this paper, we present a forward simulation for the stomach using an analytic expression of the gastric electric potential that employs a truncated expansion of ellipsoidal harmonics; we then demonstrate that an activation front of dipoles propagating along the body of an ellipsoid can simulate gastric electrical activity. In addition to the usefulness of our model, we also discuss its limitations and accuracy.

Magnetogastrographic detection of gastric electrical response activity in humans

The detection and characterization of gastric electrical activity has important clinical applications, including the early diagnosis of gastric diseases in humans. In mammals, this phenomenon has two important features: an electrical control activity (ECA) that manifests itself as an electric slow wave (with a frequency of 3 cycles per minute in humans) and an electrical response activity (ERA) that is characterized by spiking potentials during the plateau phase of the ECA. Whereas the ECA has been recorded in humans both invasively and non-invasively (magnetogastrography-MGG), the ERA has never been detected non-invasively in humans before. In this paper, we report on our progress towards the non-invasive detection of ERA from the human stomach using a procedure that involves the application of principal component analysis to MGG recordings, which were acquired in our case from ten normal human patients using a Superconducting QUantum Interference Device (SQUID) magnetometer. Both pre- and post-prandial recordings were acquired for each patient and 20 min of recordings (10 min of pre-prandial and 10 min of post-prandial data) were analysed for each patient. The mean percentage of ECA slow waves that were found to exhibit spikes of suspected ERA origin was 41% and 61% for pre- and post-prandial recordings, respectively, implying a 47% ERA increase post-prandially (P < 0.0001 at a 95% confidence level). The detection of ERA in humans is highly encouraging and points to the possible use of non-invasive ERA recordings as a valuable tool for the study of human gastric disorders.

Superconducting quantum interference device magnetometer for diagnosis of ischemia caused by mesenteric venous thrombosis

Although mesenteric venous thrombosis carries a better prognosis than arterial thrombosis, mortality and morbidity are still high. Previous studies have shown that the basic electrical rhythm (BER) of the bowel decreases early after induction of arterial ischemia. Furthermore, our studies have shown that these changes occur prior to pathologic changes and that they can be recorded noninvasively using a superconducting quantum interference device (SQUID). SQUIDs measure magnetic fields that are created by the electrical activity of the gastrointestinal smooth muscle and have been used to measure the BER of the small intestine in human volunteers. This study was conducted to determine if a SQUID could be used for early noninvasive detection of mesenteric venous ischemia in an animal model. Simultaneous recordings from serosai electrodes and a SQUID outside the abdomen were taken from anesthetized New Zealand rabbits. Recordings were made for 15 minutes before and 90 minutes after injection of thrombin into the superior mesenteric vein. The basic electrical rhythm of the small bowel dropped from 16.42 ± 0.69 to 8.80 ± 0.74 cycles per minute at 30 minutes and to 6.82 ± 0.722 after 90 minutes (p < 0.0001, paired t-test). The correlation coefficient between the SQUID and electrical recordings was 0.954 (p < 0.0001). These data suggest that the ischemia caused by mesenteric venous thrombosis results in changes in the bioelectrical activity, which can be noninvasively detected using a SQUID.RésuméLe pronostic de la thrombose veineuse mésentérique est meilleur que celui de la thrombose artérielle mais la morbidité et la mortalité restent élevées. Des études antérieures ont montré que le Basic Electrical Rhythm (BER) de l’intestin diminue lorsque l’on provoque une ischémie artérielle. Nos études indiquent que ces modifications précèdent les changements anatomopathologiques et qu’elles peuvent être enregistrécs de façon non-invasive en utilisant l’appareil SQUID (Superconduction QUantum Interference Device). Le SQUID mesure le champs magnétique créé par l’activité électrique du muscle lisse de l’intestin grêle et a déjà été utilisé pour mesurer le BER de l’intestin grêle chez des volontaires humains. Cette étude a eu pour but de déterminer si le SQUID peut être utilisé pour la détection non-invasive de l’ischémie veineuse mésentérique dans un modèle animal. Des enregistrements simultanés à partir d’électrodes sur la séreuse et d’un SQUID placé en dehors de l’abdomen ont été réalisés chez le lapin New Zealand. On a enregistré l’activité 15 minutes avant et 90 minutes après l’injection de thrombine dans la veine mésentérique supérieure. L’activité électrique de base a chuté de 16.42 ± 0.69 à 8.80 ± 0.74 cycles/minute après 30 minutes et à 6.82 ± 0.722 après 90 minutes (p < 0.0001, test de Student apparié). Le coefficient de corrélation entre le SQUID et les enregistrements électriques étaient de 0.954 (p < 0.0001). Ces données indiquent que l’ischémie en rapport avec la thrombose mésentérique veineuse est responsable d’une modification d’activité bioélectrique détectable de façon non-invasive par le SQUID.ResumenAunque la trombosis venosa mesentérica conlleva un mejor pronóstico que la trombosis arterial, su mortalidad y morbilidad todavía son altas. Estudios previos han demostrado que el Ritmo Eléctrico Básico (REB) del intestino se disminuye luego de la inducción de isquemia arterial. Además, nuestros propios estudios han señalado que tales cambios se presentan con anterioridad a las alteraciones patológicas y que pueden ser registrados mediante una técnica no invasora que utiliza un SQUID (Superconducting Quantum Interference Device). Los SQUIDs miden los campos magnéticos que se crean por la actividad eléctrica de la musculatura lisa gastrointestinal y se han utilizado para medir la REB del intestino delgado en voluntarios humanos. El presente estudio fue realizado para determinar si se podría utilizar un SQUID para la detección temprana y no invasora de isquemia venosa mesentérica en un modelo animal. Registros simultáneos a partir de electrodos serosos y de un SQUID por fuera del abdomen fueron tomados en conejos anestesiados. Se tomaron registros a los 15 minutos antes y a los 90 minutos después de la inyección de trombina en la vena mesentérica superior. El REB de intestino delgado disminuyó de 16.42 B1 0.69 a 8.8 B1 0.74 ciclos por minuto a los 30 minutos y a 6.82 B1 0.722 a los 90 minutos (p< 0.0001, prueba apareada). El coeficiente de correlación entre los registros del SQUID y los eléctricos fue 0.954 (p< 0.0001). Estos datos sugieren que la isquemia causada por trombosis venosa mesentérica resulta en cambios en la actividad bioeléctrica que puedan ser detectados de manera no invasora utilizando un SQUID.

Separation of gastric electrical control activity from simultaneous MGG/EGG recordings using independent component analysis

Spatiotemporal parameters of gastric electrical control activity such as its amplitude, direction and propagation velocity are physiological parameters of distinctive clinical interest due to their potential use for differentiating between the healthy and diseased states of the human stomach. Whereas their time evolution is relatively well behaved in the case of healthy subjects, significant deviations from normal have been observed in patients suffering from a number of gastric diseases such as gastroparesis and gastropathy. For this reason, monitoring ECA parameters noninvasively may offer a useful test for the presence of such diseases whose diagnosis remains problematic. Here, we describe a method for computing ECA direction and orientation from simultaneous, noninvasive magnetogastrographic (MGG) and electrogastrographic (EGG) recordings. We demonstrate how independent component analysis and standard frequency analysis methods can be used to predict the locations and orientations of gastric current dipoles from MGG/EGG data. We compare our MGG-based dipole parameters to analogous ones obtained from simultaneous EGG recordings within the experimental framework of a human model. We find that magnetic recordings are superior in their ability to portray the underlying physiology of the stomach

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.

Characterization of Electrophysiological Propagation by Multichannel Sensors

Objective: The propagation of electrophysiological activity measured by multichannel devices could have significant clinical implications. Gastric slow waves normally propagate along longitudinal paths that are evident in recordings of serosal potentials and transcutaneous magnetic fields. We employed a realistic model of gastric slow wave activity to simulate the transabdominal magnetogastrogram (MGG) recorded in a multichannel biomagnetometer and to determine characteristics of electrophysiological propagation from MGG measurements. Methods: Using MGG simulations of slow wave sources in a realistic abdomen (both superficial and deep sources) and in a horizontally-layered volume conductor, we compared two analytic methods (second-order blind identification, SOBI and surface current density, SCD) that allow quantitative characterization of slow wave propagation. We also evaluated the performance of the methods with simulated experimental noise. The methods were also validated in an experimental animal model. Results: Mean square errors in position estimates were within 2 cm of the correct position, and average propagation velocities within 2 mm/s of the actual velocities. SOBI propagation analysis outperformed the SCD method for dipoles in the superficial and horizontal layer models with and without additive noise. The SCD method gave better estimates for deep sources, but did not handle additive noise as well as SOBI. Conclusion: SOBI-MGG and SCD-MGG were used to quantify slow wave propagation in a realistic abdomen model of gastric electrical activity. Significance: These methods could be generalized to any propagating electrophysiological activity detected by multichannel sensor arrays.