Daniel J. Staton

Magnetoenterography (MENG): noninvasive measurement of bioelectric activity in human small intestine.

The basic electrical rhythm (BER) of the gastrointestinal tract creates minute magnetic fields that have been measured in animals using a Superconducting QUantum Interference Device (SQUID) gradiometer. The aim of this study was to measure noninvasively the biomagnetic fields of human stomach and small intestine. Twenty-one human volunteers were studied using a 37-channel SQUID gradiometer positioned over the epigastrium and umbilicus. In one volunteer additional biomagnetic recordings were performed in order to map the spatial variation of the biomagnetic fields. Cyclical waveforms consistent with gastric BER [3.0 ± 0.5 cycles per minute (cpm)] and small intestine BER (10.26 ± 1.74 cpm) were seen in the epigastrium and umbilicus, respectively. The mapping study identified the expected frequency gradient (12.0 cpm in duodenum, 11.3 cpm in jejunum, to 9.7 cpm in ileum) within the small intestine. Noninvasive recordings of human gastric and small intestinal BER can be obtained using a SQUID gradiometer.

The human vector magnetogastrogram and magnetoenterogram

Electrical activity in the gastrointestinal system produces magnetic fields that may be measured with superconducting quantum interference device magnetometers. Although typical magnetometers have detection coils that measure a single component of the magnetic field, gastric and intestinal magnetic fields are vector quantities. We recorded gastric and intestinal magnetic fields from nine abdominal sections in nine normal human volunteers using a vector magnetometer that measures all three Cartesian components of the magnetic field vector. A vector projection technique was utilized to separate the magnetic field vectors corresponding to gastric and intestinal activity. The gastric magnetic field vector was oriented in a cephalad direction, consistent with previously observed data, and displayed oscillatory characteristics of gastric electrical activity (f=3.03/spl plusmn/0.18 cycles/min). Although the small bowel magnetic field vector showed no consistent orientation, the characteristic frequency gradient of the small bowel electrical activity was observed. Gastric and intestinal magnetic field vectors were oriented in different directions and were thus distinguished by the vector projection technique. The observed difference in direction of gastric and intestinal magnetic field vectors indicates that vector recordings dramatically increase the ability to separate physiological signal components from nonphysiological components and to distinguish between different physiological components.

Instrumentation and techniques for high-resolution magnetic imaging

Abstract not available.

Noninvasive diagnosis of mesenteric ischemia using a SQUID magnetometer.

ObjectiveThe authors assessed the ability of a Superconducting Quantum Interference Device (SQUID) magnetometer to noninvasively detect mesenteric ischemia in a rabbit model. Summary Background DataSuperconducting Quantum Interference Device magnetometers have been used to detect magnetic fields created by the basic electrical rhythm (BER) and to detect changes in BER of exteriorized bowel of anesthetized rabbits during mesenteric ischemia. MethodsThe BER of rabbit ileum was noninvasively measured transabdominally using a SQUID magnetometer and compared with the electrical activity recorded with surgically implanted serosal electrodes before, during, and after snare occlusion of the superior mesenteric artery. ResultsTransabdominal SQUID recording of BER frequency was highly correlated to the measurements obtained with electrodes (R = 0.91). Basic electrical rhythm frequency decreased from 16.4 ± 0.8 to 8.3 ± 0.3 cpm (p < 0.001) after 25 minutes of ischemia. Reperfusion of ischemic bowel resulted in recovery of BER frequency to 14.3 ± 0.4 cpm 10 minutes after blood flow was restored. ConclusionsA SQUID magnetometer is capable of noninvasively detecting mesenteric ischemia reliably and at an early stage by detecting a significant drop in BER frequency. These positive findings have encouraged the authors to continue development of clinically useful, noninvasive, detection of intestinal magnetic fields using SQUID magnetometers.

An improved method for magnetic identification and localization of cracks in conductors

A SQUID magnetometer can be used to measure the magnetic field produced by flaws in a two-dimensional, conducting plate carrying a current. Identification of the flaw-induced magnetic field is difficult because of the large magnetic field associated with the edges of the plate and the current in the leads that connect the plate to the power supply. We have developed a technique by which the wire and edge fields can be cancelled prior to mapping the magnetic field. In this technique, a similar unflawed conducting sheet is placed adjacent to the flawed plate, with a connection between the sheet and the plate at one edge, and with the opposite edges of the sheet and of the plate connected to the two conductors of a coaxial cable. Thus, an applied current will flow along one conductor of the cable, across the cancelling sheet, cross into the flawed plate, return along the plate, and then return to the power supply along the other conductor of the coaxial cable. As a result of this geometry, there is no magnetic field from the lead-in wires because they are coaxial, and the magnetic field due to the edges of the plate is cancelled by the opposing magnetic field of the edges in the adjacent sheet. The extent of cancellation is determined primarily by the separation between the plate and the cancelling sheet, by the thickness of the plate, and by macroscopic inhomogeneities in their electrical conductivities.

High-resolution SQUID imaging of octupolar currents in anisotropic cardiac tissue

A monopolar stimulus electrode triggered depolarization in a 1-mm-thick slice of canine ventricular myocardium, maintained in a thermally regulated, oxygenated chamber. Five hundred milliseconds of data at 2 kHz were recorded with a MicroSQUID (superconducting quantum interference device) magnetometer at 1-mm intervals over a 23-mm*23-mm area, centered on the stimulus site. Without averaging, a signal-to-noise ratio of better than 100:1 was achieved, and the field maps provided evidence of a propagating wavefront of activity. Application of an inverse Fourier filter yielded current density images that consisted of four expanding circular current paths, in agreement with predictions of the bidomain model. The ability to image action currents in a DC to 2-kHz bandwidth should prove useful for understanding the complex anisotropy of cardiac tissue and how it is altered by pharmacological interventions.<<ETX>>

A mathematical analysis of the magnetic field produced by flaws in two-dimensional current-carrying conductors

We examine the magnetic field produced by small flaws in a two-dimensional, conducting plate carrying an otherwise-uniform current. We use a conjugate function approach to calculate the current and voltage distributions about the circular and elliptical flaws in the conducting plate, and examine the dependence of the normal component of the magnetic field upon distance, hole size, elliptical eccentricity, and elliptical orientation. We show that when the field is calculated, far from the hole, the field falls off as 1/z3, wherez is the distance above the plate, and as 1/r2, wherer is the distance from the center of the hole to the observation point. We also show that for circular and elliptical flaws, the normal component of the magnetic field in the far-field region is linearly related to the area of the flaw.

High-resolution magnetic mapping using a SQUID magnetometer array

A four-channel, high-resolution superconducting quantum interference device (SQUID) magnetometer array was used to map magnetic fields from various samples. Each SQUID has a 3-mm-diameter pickup coil located 4.4 mm from the adjacent channel. The spacing between the cryogenic array and the room-temperature sample is adjustable from 1.5 mm to 4.0 mm. The authors mapped the field from a 350- mu m-diameter hole in an 11 cm*15 cm*60 mu m copper sheet that was carrying a current of 100 mA. Field shape and strength were compared with predictions from analytical and finite-element models, which indicate that this technique should be able to detect an order of magnitude smaller flaws in flat plates. The ability is demonstrated to detect magnetic contamination in a 230- mu m-deep by 1.1-mm-long slot that was electric-discharge-machined into a nonmagnetic tube, and to determine the orientation of the slot with respect to the tube axis. Slices of pyroclastic rock of thickness as low as 30- mu m-thick have also been mapped.

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.

Superconducting magnetometry: a possible technique for aircraft NDE

Superconducting quantum interference device (SQUID) magnetometers offer promise as multi- mode instruments capable of obtaining high resolution images of extremely low frequency injected currents or eddy currents, and they can be configured to image the magnetic susceptibility of titanium, aluminum, and nonmetallic composites. While high resolution SQUID magnetometers will generally be noisier than conventional SQUIDs, the small coils and reduced coil-to-source spacing more than compensate to provide low-noise, high- resolution images. To explore SQUID NDE, we have developed research facilities that include the high-resolution MicroSQUID magnetometer, a magnetic shield, a scanning stage, and a computer-based control and data acquisition system. Using this instrumentation, we have imaged magnetic fields produced by varied sources. In support of the experimental studies, we have developed analytical and numerical models for the simulation of flaws with several geometries inside thick and thin current-carrying plates and thin-walled tubes, and have demonstrated that two-dimensional magnetic images can be deconvolved into images of current or magnetization by filtering techniques, finite element models, lead field analyses, and maximum entropy methods.