Electroanatomical mapping of the stomach with simultaneous biomagnetic measurements

Gastric motility is coordinated by bioelectric slow waves (SWs) and dysrhythmic SW activity has been linked with motility disorders. Magnetogastrography (MGG) is the non-invasive measurement of the biomagnetic fields generated by SWs. Dysrhythmia identification using MGG is currently challenging because source models are not well developed and the impact of anatomical variation is not well understood. A novel method for the quantitative spatial co-registration of serosal SW potentials, MGG, and geometric models of anatomical structures was developed and performed on two anesthetized pigs to verify feasibility. Electrode arrays were localized using electromagnetic transmitting coils. Coil localization error for the volume where the stomach is normally located under the sensor array was assessed in a benchtop experiment, and mean error was 4.2±2.3mm and 3.6±3.3° for a coil orientation parallel to the sensor array and 6.2±5.7mm and 4.5±7.0° for a perpendicular coil orientation. Stomach geometries were reconstructed by fitting a generic stomach to up to 19 localization coils, and SW activation maps were mapped onto the reconstructed geometries using the registered positions of 128 electrodes. Normal proximal-to-distal and ectopic SW propagation patterns were recorded from the serosa and compared against the simultaneous MGG measurements. Correlations between the center-of-gravity of normalized MGG and the mean position of SW activity on the serosa were 0.36 and 0.85 for the ectopic and normal propagation patterns along the proximal-distal stomach axis, respectively. This study presents the first feasible method for the spatial co-registration of MGG, serosal SW measurements, and subject-specific anatomy. This is a significant advancement because these data enable the development and validation of novel non-invasive gastric source characterization methods.

Stomach Geometry Reconstruction Using Serosal Transmitting Coils and Magnetic Source Localization

OBJECTIVE

Bioelectric slow waves (SWs) are a key regulator of gastrointestinal motility, and disordered SW activity has been linked to motility disorders. There is currently a lack of practical options for the acquisition of the 3D stomach geometry during research studies when medical imaging is challenging. Accurately recording the geometry of the stomach and co-registering electrode and sensor positions would provide context for in-vivo studies and aid the development of non-invasive methods of gastric SW assessment.

METHODS

A stomach geometry reconstruction method based on the localization of transmitting coils placed on the gastric serosa was developed. The positions and orientations of the coils, which represented boundary points and surface-normal vectors, were estimated using a magnetic source localization algorithm. Coil localization results were then used to generate surface models. The reconstruction method was evaluated against four 3D-printed anatomically realistic human stomach models and applied in a proof of concept in-vivo pig study.

RESULTS

Over ten repeated reconstructions, average Hausdorff distance and average surface-normal vector error values were 4.7 ±0.2 mm and 18.7 ±0.7° for the whole stomach, and 3.6 ±0.2 mm and 14.6 ±0.6° for the corpus. Furthermore, mean intra-array localization error was 1.4 ±1.1 mm for the benchtop experiment and 1.7 ±1.6 mm in-vivo.

CONCLUSION AND SIGNIFICANCE

Results demonstrated that the proposed reconstruction method is accurate and feasible. The stomach models generated by this method, when co-registered with electrode and sensor positions, could enable the investigation and validation of novel inverse analysis techniques.

Multichannel magnetogastrogram: a clinical marker for pediatric chronic nausea

Chronic nausea is a widespread functional disease in children with numerous comorbidities. High-resolution electrogastrogram (HR-EGG) has shown sufficient sensitivity as a noninvasive clinical marker to objectively detect distinct gastric slow wave properties in children with functional nausea. We hypothesized that the increased precision of magnetogastrogram (MGG) slow wave recordings could provide supplementary information not evident on HR-EGG. We evaluated simultaneous pre- and postprandial MGG and HR-EGG recordings in pediatric patients with chronic nausea and healthy asymptomatic subjects, while also measuring nausea intensity and nausea severity. We found significant reductions in postprandial dominant frequency and normogastric power, and higher levels of postprandial bradygastric power in patients with nausea in both MGG and HR-EGG. MGG also detected significantly lower preprandial normogastric power in patients. A significant difference in the mean preprandial gastric slow wave propagation direction was observed in patients as compared with controls in both MGG (control: 180 ± 61°, patient: 34 ±72°; P < 0.05) and HR-EGG (control: 240 ± 39°, patient: 180 ± 46°; P < 0.05). Patients also showed a significant change in the mean slow wave direction between pre- and postprandial periods in MGG (P < 0.05). No statistical differences were observed in propagation speed between healthy subjects and patients in either MGG or HR-EGG pre/postprandial periods. The use of MGG and/or HR-EGG represents an opportunity to assess noninvasively the effects of chronic nausea on gastric slow wave activity. MGG data may offer the opportunity for further refinement of the more portable and economical HR-EGG in future machine-learning approaches for functional nausea.NEW & NOTEWORTHY Pediatric chronic nausea is a difficult-to-measure subjective complaint that requires objective diagnosis, clinical assessment, and individualized treatment plans. Our study demonstrates that multichannel MGG used in conjunction with custom HR-EGG detects key pathological signatures of functional nausea in children. This quantifiable measure may allow more personalized diagnosis and treatment in addition to minimizing the cost and potential radiation associated with current diagnostic approaches.

Characterizing Spatial Signatures of Gastric Electrical Activity Using Biomagnetic Source Localization

BACKGROUND

The motility patterns in the gastrointestinal tract are regulated, in part, by bioelectrical events known as slow waves (SWs). Understanding temporal and spatial features of gastric SWs can help reveal the underlying causes of functional motility disorders.

OBJECTIVE

This study investigated the ability of source localization techniques to characterize the spatial signatures of SW activity using simulated and experimental magnetogastrography data.

METHODS

Two SW propagation patterns (antegrade and retrograde) with two rhythms (normogastric and bradygastric) were used to simulate magnetic fields using 4 anatomically realistic stomach and torso geometries. Source localization was performed utilizing the equivalent current dipole (ECD) and the equivalent magnetic dipole (EMD) models.

RESULTS

In the normogastric simulations when compared with the SW activity, the EMD model was capable of identifying the SW propagation in the lateral, antero-posterior, and supero-inferior axes with the median correlation coefficients of 0.66, 0.53, and 0.83, respectively, whereas the ECD model produced lower correlation scores (median: 0.52, 0.44, and 0.44). Moreover, the EMD model resulted in distinct and opposite spatial signatures for the antegrade and retrograde propagation. Similarly, when experimental data was used, the EMD model revealed antegrade-like signatures where the propagation was mostly towards the third quadrant in the supero-inferior (preprandial: 49%, postprandial: 35%) and antero-posterior (preprandial: 49%, postprandial: 50%) axes.

CONCLUSION AND SIGNIFICANCE

The EMD model was able to identify and classify the spatial signatures of SW activities, which can help to inform the interpretation of non-invasive recordings of gastric SWs as a biomarker of functional motility disorders.

Anatomically Constrained Gastric Slow Wave Localization using Biomagnetic Data

Detection of dysrhythmic gastric slow wave (SW) activity could have significant clinical utility because dysrhyth-mias have been linked to gastric motility disorders. The elec-trogastrogram (EGG) and magnetogastrogram (MGG) enable the non-invasive assessment of SW activity, but most analysis methods can only resolve frequency and velocity. Improved characterization of dysrhythmic propagation patterns from non-invasive measurements is important for the diagnosis of motility disorders and could allow early treatment stratification. In this study, we demonstrate the use of a penalized linear regression framework to localize SW events on the longitudinal stomach axis using simulated MGG data. Priors relating to spatial sparsity, the organization of wavefronts into complete circumferential rings, and the local distribution of depolar-ization and repolarization phases were used to constrain the inverse solution. This method was applied to MGG computed for a single wavefront case and a multiple wavefront case that were constructed from simulated 3 cycle-per-minute normal SW activity. Propagation patterns along the longitudinal stomach axis were identifiable from reconstructed SW activity for both cases. Localization error was 5.7 ± 0.1 mm and 7.7 ± 0.1 mm for each respective case within the distal stomach when the signal-to-noise ratio was 10 dB. Results indicate that penalized linear regression can successfully localize SW events provided the 3D geometry of the stomach and torso were acquired. Clinical Relevance- This method could help to improve the efficiency and accuracy of diagnosing gastric motility disorders from non-invasive measurements.

Sex Moderates the Relationship Between Nausea Severity and Heart Rate Variability in Adolescents and Young Adults

BACKGROUND

Nausea is a common complaint among children and is particularly prevalent in children with functional abdominal pain (FAP), with nearly half of children with FAP also endorsing nausea. Dysfunction of the autonomic nervous system, which can be indexed by heart rate variability (HRV), leads to abnormalities in gastric electrical activity that are associated with GI symptoms.

AIMS

To evaluate that relationship between nausea severity and HRV in adolescents and young adults with a history of FAP and to assess for sex differences.

METHODS

Participants were pediatric patients with a diagnosis of FAP who were recruited from a pediatric GI clinic between 1993 and 2007 for a prospective study of the course of FAP. Study analyses focused on the cross-sectional relationship between HRV, indexed by standard deviation of the R-R interval (SDRRI) and high-frequency (HF) power, and nausea severity collected during a follow-up visit in late adolescence and young adulthood.

RESULTS

Controlling for age and BMI, a significant nausea by sex interaction emerged for both SDRRI and HF power. Tests of conditional effects of nausea by sex showed that the inverse relation between nausea severity and both SDRRI and HF was significant for females but not for males.

CONCLUSIONS

This is the first study to evaluate the relationship between nausea severity and HRV. Greater nausea severity was associated with lower HRV in females but not in males. Further validation of these results may provide insight into novel treatment approaches for females with nausea that target vagal tone.

Automated Machine Learning Pipeline Framework for Classification of Pediatric Functional Nausea Using High-resolution Electrogastrogram

OBJECTIVE

Pediatric functional nausea is challenging for patients to manage and for clinicians to treat since it lacks objective diagnosis and assessment. A data-driven non-invasive diagnostic screening tool that distinguishes the electro-pathophysiology of pediatric functional nausea from healthy controls would be an invaluable aid to support clinical decision-making in diagnosis and management of patient treatment methodology. The purpose of this paper is to present an innovative approach for objectively classifying pediatric functional nausea using cutaneous high-resolution electrogastrogram data.

METHODS

We present an Automated Electrogastrogram Data Analytics Pipeline framework and demonstrate its use in a 3x8 factorial design to identify an optimal classification model according to a defined objective function. Low-fidelity synthetic high-resolution electrogastrogram data were generated to validate outputs and determine SOBI-ICA noise reduction effectiveness.

RESULTS

A 10 parameter support vector machine binary classifier with a radial basis function was selected as the overall top-performing model from a pool of over 1000 alternatives via maximization of an objective function. This resulted in a 91.6% test ROC AUC score.

CONCLUSION

Using an automated machine learning pipeline approach to process high-resolution electrogastrogram data allows for clinically significant objective classification of pediatric functional nausea.

SIGNIFICANCE

To our knowledge, this is the first study to demonstrate clinically significant performance in the objective classification of pediatric nausea patients from healthy control subjects using experimental high-resolution electrogastrogram data. These results indicate a promising potential for high-resolution electrogastrography to serve as a data-driven screening tool for the objective diagnosis of pediatric functional nausea.

Reconstruction of stomach geometry using magnetic source localization

Routine diagnosis of gastric motility disorders represents a significant problem to current clinical practice. The non-invasive electrogastrogram (EGG) and magnetogastrogram (MGG) enable the assessment of gastric slow wave (SW) dysrhythmias that are associated with motility disorders. However, both modalities lack standardized methods for reliably detecting patterns of SW activity. Subject-specific anatomical information relating to the geometry of the stomach and its position within the torso have the potential to aid the development of relations between SWs and far-fields. In this study, we demonstrated the feasibility of using magnetic source localization to reconstruct the geometry of an anatomically realistic 3D stomach model. The magnetic fields produced by a small (6.35 × 6.35 mm) N35 neodymium magnet sequentially positioned at 64 positions were recorded by an array of 27 magnetometers. Finally, the magnetic dipole approximation and a particle swarm optimizer were used to estimate the position and orientation of the permanent magnet. Median position and orientation errors of 3.8 mm and 7.3° were achieved. The estimated positions were used to construct a surface mesh, and the Hausdorff Distance and Average Hausdorff Distance dissimilarity metrics for the reconstructed and ground-truth models were 11.6 mm and 2.4 mm, respectively. The results indicate that source localization using the magnetic dipole model can successfully reconstruct the geometry of the stomach.

The effect of chronic nausea on gastric slow wave spatiotemporal dynamics in children

BACKGROUND

Chronic nausea in adolescents with functional gastrointestinal disorders is an increasingly reported but poorly understood symptom that negatively affects quality of life. Functional gastrointestinal disorders are known to correlate closely with slow wave rhythm disturbances. The ability to characterize gastric electrophysiologic perturbations in functional nausea patients could provide potential diagnostic and therapeutic tools for nausea patients.

METHODS

We used high-resolution electrogastrograms (HR-EGG) to measure gastric slow wave parameters in pediatric chronic nausea patients and healthy subjects both pre- and postprandial. We computed the dominant frequency, percentage power distribution, gastric slow wave propagation direction, and speed from HR-EGG.

KEY RESULTS

We observed significant differences in the dominant frequency and power distributed in normal and bradyarrhythmia frequency ranges when comparing patients and healthy subjects. Propagation patterns in healthy subjects were predominantly anterograde, while patients exhibited a variety of abnormalities including retrograde, anterograde, and disrupted patterns. There was a significant difference in the preprandial mean slow wave direction between healthy subjects (222° ± 22°) and patients (103° ± 66°; p ˂ 0.01), although the postprandial mean direction between healthy subjects and patients was similar (p = 0.73). No significant difference in slow wave propagation speed was found between patients and healthy subjects in either pre- (p = 0.21) or postprandial periods (p = 0.75).

CONCLUSIONS AND INFERENCES

The spatiotemporal characterization of gastric slow wave activity using HR-EGG distinguishes symptomatic chronic nausea patients from healthy subjects. This characterization may in turn inform and direct clinical decision-making and lead to further insight into its pathophysiology.

Effects of magnetogastrography sensor configurations in tracking slow wave propagation

Magnetogastrography (MGG) is a non-invasive method of assessing gastric slow waves (SWs) by recording the resultant magnetic fields. MGG can capture both SW frequency and propagation, and identify SW dysrhythmias that are associated with motility disorders. However, the impact of the restricted spatial coverage and sensor density on SW propagation tracking performance is unknown. This study simulated MGG using multiple anatomically specific torso geometries and two realistic SW propagation patterns to determine the effect of different sensor configurations on tracking SW propagation. The surface current density mapping and center-of-gravity tracking methods were used to compare four magnetometer array configurations: a reference system currently used in GI research and three hypothetical higher density and coverage arrays. SW propagation patterns identified with two hypothetical arrays (with coverage over at least the anterior of the torso) correlated significantly higher with simulated realistic 3 cycle-per-minute SW activity than the reference array (p = 0.016, p = 0.005). Furthermore, results indicated that most of the magnetic fields that contribute to the performance of SW propagation tracking were located on the anterior of the torso as further increasing the coverage did not significantly increase performance. A 30% decrease in sensor spacing within the same spatial coverage of the reference array also significantly increased correlation values by approximately 0.50 when the signal-to-noise ratio was 5 dB. This study provides evidence that higher density and coverage sensor layouts will improve the utility of MGG. Further work is required to investigate optimum sensor configurations across larger anatomical variations and other SW propagation patterns.

Simulation-based Analysis of Magnetogastrography Sensor Configurations for Characterizing Gastric Slow Wave Dysrhythmias

The routine diagnosis of gastric motility disorders represents a significant problem to current clinical practice. Magnetogastrography (MGG) provides a non-invasive option for assessing gastric slow wave (SW) dysrhythmias that are associated with motility disorders. However, its ability to characterize SW propagation is impaired by the limited spatial coverage of existing superconducting quantum interference devices (SQUIDs). Recently developed optically-pumped magnetometers can potentially substitute SQUIDs and enable subject-specific MGG arrays with greater spatial coverage. This study developed simulations of gastric MGG to determine the distribution of the magnetic fields (MFs) generated by SWs above the torso, and investigated the impact of several realistic dysrhythmic patterns of propagation. The distribution of MFs was found to vary significantly for different patterns of SW propagation, with ectopic dysrhythmia displaying the greatest difference from normal. Notably, some important proportion of the MFs lay outside the coverage of an existing experimental SQUID array used in gastrointestinal research for some simulated SW propagation patterns, such as retrograde activity. Results suggest that MGG measurements should be made over the entire frontal face of the torso to capture all of the strongest MFs generated by SWs.Clinical relevance- This provides a guide for the placement of MGG sensors for the capture of both normal and dysrhythmic gastric slow wave propagation.

Computational Reconstruction of 3D Stomach Geometry using Magnetic Field Source Localization

In this study, we investigated the feasibility of computationally reconstructing the 3D geometry of the stomach by performing source localization of the magnetic field (MF) induced from the stomach surface. Anatomically realistic stomach and torso models of a human participant, reconstructed from the CT images, were used in the computations. First, 128 coils with a radius of 5 mm were positioned on different locations on the stomach model. Next, MF at the sensor positions were computed using Bio-Savart law for the currents of 10 and 100 mA. Then, three noise levels were defined using the biomagnetic data recorded from the same participant and two additional sets of generated white-noise resulting in mean signal to noise ratios (SNR) of 20 and 10 dB. Finally, for each combination of the current and noise level, the magnetic dipole (MDP) approximation was performed to estimate coil positions. The performance of the source localization was assessed by computing the goodness of fit (GOF) values and the distance between the coil and the estimated MDP positions. We obtained GOF values over 98% for all coils and a mean localization error of 0.69±0.08 mm was achieved when 100 mA current was used to induce MF and only biomagnetic data was added. When additional white-noise was added, the GOF values decreased to 95% and the mean localization error increased to around 4 mm. A current of 10 mA was enough to localize the coil positions with a mean error around 8 mm even for the highest noise level we tested but for the few coils furthest from the body surface, the error was around 10 cm. The results indicate that source localization using the MDP approximation can successfully extract spatial information of the stomach.Clinical relevance-Extracting the spatial information of the stomach during the recording of the slow wave activity provides new insights in assessing gastric recordings and relating to disorders.

Source localization for gastric electrical activity using simulated magnetogastrographic data

In this study, the use of magnetic dipole (MDP) approximation to localize the underlying source of magnetogastrographic (MGG) data was investigated. An anatomically realistic torso and a stomach model were used to simulate slow wave (SW) activities and magnetic fields (MFs). SW activity in the stomach was simulated using a grid-based finite element method. The SW activity at each time sample was represented by the dipoles generated for each element and MFs were computed from these dipoles including secondary sources in the torso. Gaussian noise was added to the MFs to represent experimental signal noise. MDP fitting was executed on the time samples of selected 2-second time frames, and goodness of fit (GOF) and the distance from the fitted MDP to the center of gravity (COG) of active dipoles were computed. Then, for each time frame, the spatial changes of COG and MDP positions in x-, y-, and z-directions and correlation scores were computed. Our results showed that MDP fitting was capable of identifying propagation patterns with mean correlation scores of 0.63 ± 0.30, 0.71 ± 0.19, and 0.81 ± 0.24 in x-, y-, and z-directions, respectively. The mean distance from COGs to the identified MDPs was 49±4 mm. The results were similar under the noise conditions as well. Our results suggest that source localization using MDP approximation can be useful to identify the propagation characteristics of SWs using MGG data.

Magnetoenterography for the Detection of Partial Mesenteric Ischemia

BACKGROUND

Acute mesenteric ischemia represents a life-threatening gastrointestinal condition. A noninvasive diagnostic modality that identifies mesenteric ischemia patients early in the disease process will enable early surgical intervention. Previous studies have identified significant changes in the small-bowel electrical slow-wave parameters during intestinal ischemia caused by total occlusion of the superior mesenteric artery. The purpose of this study was to use noninvasive biomagnetic techniques to assess functional physiological changes in intestinal slow waves in response to partial mesenteric ischemia.

METHODS

We induced progressive intestinal ischemia in normal porcine subjects (n = 10) by slowly increasing the occlusion of the superior mesenteric artery at the following percentages of baseline flow: 50%, 75%, 90%, and 100% while simultaneous transabdominal magnetoenterogram (MENG) and serosal electromyogram (EMG) recordings were being obtained.

RESULTS

A statistically significant serosal EMG amplitude decrease was observed at 100% occlusion compared with baseline, whereas no significant change was observed for MENG amplitude at any progressive occlusion levels. MENG recordings showed significant changes in the frequency and percentage of power distributed in bradyenteric and normoenteric frequency ranges at 50%, 75%, 90%, and 100% vessel occlusions. In serosal EMG recordings, a similar percent power distribution (PPD) effect was observed at 75%, 90%, and 100% occlusion levels. Serosal EMG showed a statistically significant increase in tachyenteric PPD at 90% and 100% occlusion. We observed significant increase in tachyenteric PPD only at the 100% occlusion level in MENG recordings.

CONCLUSIONS

Ischemic changes in the intestinal slow wave can be detected early and noninvasively even with partial vascular occlusion. Our results suggest that noninvasive MENG may be useful for clinical diagnosis of partial mesenteric ischemia.

Noninvasive Magnetogastrography Detects Erythromycin-Induced Effects on the Gastric Slow Wave

OBJECTIVE

The prokinetic action of erythromycin is clinically useful under conditions associated with gastrointestinal hypomotility. Although erythromycin is known to affect the electrogastrogram, no studies have examined the effects that erythromycin has on gastric slow wave magnetic fields.

METHODS

In this study, gastric slow wave activity was assessed simultaneously using noninvasive magnetogastrogram (MGG), electrogastrogram, and mucosal electromyogram recordings. Recordings were obtained for 30 min prior to and 60 min after intravenous administration of erythromycin at dosages of 3 and 6 mg/kg.

RESULTS

MGG recordings showed significant changes in the percentage power distribution of gastric signal after infusion of both 3 and 6 mg/kg erythromycin at t = 1-5 min that persisted for t = 30-40 min after infusion. These changes agree with the changes observed in the electromyogram. We did not observe any statistically significant difference in MGG amplitude before or after injection of either 3 or 6 mg/kg erythromycin. Both 3 and 6 mg/kg erythromycin infusion showed retrograde propagation with a statistically significant decrease in slow wave propagation velocity 11-20 min after infusion. Propagation velocity started returning toward baseline values after approximately 21-30 min for the 3 mg/kg dosage and after 31-40 min for a dosage of 6 mg/kg.

CONCLUSION

Our results showed that the magnetic signatures were sensitive to disruptions in normal slow wave activity induced by pharmacological and prokinetic agents such as erythromycin.

SIGNIFICANCE

This study shows that repeatable noninvasive bio-electro-magnetic techniques can objectively characterize gastric dysrhythmias and may quantify treatment efficacy in patients with functional gastric disorders.

Diabetic gastroparesis alters the biomagnetic signature of the gastric slow wave

BACKGROUND

Gastroparesis is characterized by delayed gastric emptying without mechanical obstruction, but remains difficult to diagnose and distinguish from other gastrointestinal (GI) disorders. Gastroparesis affects the gastric slow wave, but non-invasive assessment has been limited to the electrogastrogram (EGG), which reliably characterizes temporal dynamics but does not provide spatial information.

METHODS

We measured gastric slow wave parameters from the EGG and magnetogastrogram (MGG) in patients with gastroparesis and in healthy controls. In addition to dominant frequency (DF) and percentage power distribution (PPD), we measured the propagation velocity from MGG spatiotemporal patterns and the percentage of slow wave coupling (%SWC) from EGG.

KEY RESULTS

No significant difference in DF was found between patients and controls. Gastroparesis patients had lower percentages of normogastric frequencies (60 ± 6% vs 78 ± 4%, p < 0.05), and higher brady (9 ± 2% vs 2 ± 1%, p < 0.05) and tachygastric (31 ± 2% vs 19 ± 1%, p < 0.05) frequency content postprandial, indicative of uncoupling. Propagation patterns were substantially different in patients and longitudinal propagation velocity was retrograde at 4.3 ± 2.9 mm/s vs anterograde at 7.4 ± 1.0 mm/s for controls (p < 0.01). No difference was found in %SWC from EGG.

CONCLUSIONS & INFERENCES

Gastric slow wave parameters obtained from MGG recordings distinguish gastroparesis patients from controls. Assessment of slow wave propagation may prove critical to characterization of underlying disease processes. Future studies should determine pathologic indicators from MGG associated with other functional gastric disorders, and whether multichannel EGG with appropriate signal processing also reveals pathology.

Effects of body mass index on gastric slow wave: a magnetogastrographic study

We measured gastric slow wave activity simultaneously with magnetogastrogram (MGG), mucosal electromyogram (EMG) and electrogastrogram (EGG) in human subjects with varying body mass index (BMI) before and after a meal. In order to investigate the effect of BMI on gastric slow wave parameters, each subject's BMI was calculated and divided into two groups: subjects with BMI ≤ 27 and BMI > 27. Signals were processed with Fourier spectral analysis and second-order blind identification (SOBI) techniques. Our results showed that increased BMI does not affect signal characteristics such as frequency and amplitude of EMG and MGG. Comparison of the postprandial EGG power, on the other hand, showed a statistically significant reduction in subjects with BMI > 27 compared with BMI ≤ 27. In addition to the frequency and amplitude, the use of SOBI-computed propagation maps from MGG data allowed us to visualize the propagating slow wave and compute the propagation velocity in both BMI groups. No significant change in velocity with increasing BMI or meal was observed in our study. In conclusion, multichannel MGG provides an assessment of frequency, amplitude and propagation velocity of the slow wave in subjects with differing BMI categories and was observed to be independent of BMI.

Noninvasive biomagnetic detection of isolated ischemic bowel segments

The slow wave activity was measured in the magnetoenterogram (MENG) of normal porcine subjects (N = 5) with segmental intestinal ischemia. The correlation changes in enteric slow wave activity were determined in MENG and serosal electromyograms (EMG). MENG recordings show significant changes in the frequency and power distribution of enteric slow-wave signals during segmental ischemia, and these changes agree with changes observed in the serosal EMG. There was a high degree of correlation between the frequency of the electrical activity recorded in MENG and in serosal EMG (r = 0.97). The percentage of power distributed in brady- and normoenteric frequency ranges exhibited significant segmental ischemic changes. Our results suggest that noninvasive MENG detects ischemic changes in isolated small bowel segments.

Biomagnetic and bioelectric detection of gastric slow wave activity in normal human subjects--a correlation study

We measured gastric slow wave activity simultaneously with a Superconducting Quantum Interference Device (SQUID) magnetometer, mucosal electrodes and cutaneous electrodes in 18 normal human subjects (11 women and 7 men). We processed signals with Fourier spectral analysis and SOBI blind-source separation techniques. We observed a high waveform correlation between the mucosal electromyogram (EMG) and multichannel SQUID magnetogastrogram (MGG). There was a lower waveform correlation between the mucosal EMG and cutaneous electrogastrogram (EGG), but the correlation improved with the application of SOBI. There was also a high correlation between the frequency of the electrical activity recorded in the MGG and in mucosal electrodes (r = 0.97). We concluded that SQUID magnetometers noninvasively record gastric slow wave activity that is highly correlated with the activity recorded by invasive mucosal electrodes.

Influence of body parameters on gastric bioelectric and biomagnetic fields in a realistic volume conductor

Electrogastrograms (EGG) and magnetogastrograms (MGG) provide two complementary methods for non-invasively recording electric or magnetic fields resulting from gastric electrical slow wave activity. It is known that EGG signals are relatively weak and difficult to reliably record while magnetic fields are, in theory, less attenuated by the low-conductivity fat layers present in the body. In this paper, we quantified the effects of fat thickness and conductivity values on resultant magnetic and electric fields using anatomically realistic torso models and trains of dipole sources reflecting recent experimental results. The results showed that when the fat conductivity was increased, there was minimal change in both potential and magnetic fields. However, when the fat conductivity was reduced, the magnetic fields were largely unchanged, but electric potentials had a significant change in patterns and amplitudes. When the thickness of the fat layer was increased by 30 mm, the amplitude of the magnetic fields decreased 10% more than potentials but magnetic field patterns were changed about four times less than potentials. The ability to localize the underlying sources from the magnetic fields using a surface current density measure was altered by less than 2 mm when the fat layer was increased by 30 mm. In summary, results confirm that MGG provides a favorable method over EGG when there are uncertain levels of fat thickness or conductivity.

Biomagnetic characterization of spatiotemporal parameters of the gastric slow wave

Certain gastric disorders affect spatiotemporal parameters of the gastric slow wave. Whereas the electrogastrogram (EGG) evaluates electric potentials to determine primarily temporal parameters, fundamental physical limitations imposed by the volume conduction properties of the abdomen suggest the evaluation of gastric magnetic fields. We used a multichannel superconducting quantum interference device magnetometer to study the magnetogastrogram (MGG) in 20 normal human subjects before and after a test meal. We computed the frequency and amplitude parameters of the gastric slow wave from MGG. We identified normal gastric slow wave activity with a frequency of 2.6 +/- 0.5 cycles per minute (cpm) preprandial and 2.8 +/- 0.3 cpm postprandial. In addition to frequency and amplitude, the use of surface current density mapping applied to the multichannel MGG allowed us to visualize the propagating slow wave and compute its propagation velocity (6.6 +/- 1.0 mm s(-1) preprandial and 7.4 +/- 0.4 mm s(-1) postprandial). Whereas MGG and EGG signals exhibited strong correlation, there was very little correlation between the MGG and manometry. The MGG not only records frequency dynamics of the gastric slow wave, but also characterizes gastric propagation. The MGG primarily reflects the underlying gastric electrical activity, but not its mechanical activity.

Vector projection of biomagnetic fields

Biomagnetic measurements are increasingly popular as functional imaging techniques for the non-invasive assessment of electrically active tissue. Although most currently available magnetometers utilise only one component of the vector magnetic field, some studies have suggested the possibility of obtaining additional information from recordings of the full magnetic field vector. Three projection techniques were applied to different biomagnetic signals for analysis of the three orthogonal components of the vector magnetic field. Vector magnetic fields obtained from fetal cardiac activity were projected into evenly spaced directions around a unit sphere. The vector magnetic field recorded from multiple intestinal current sources with independent temporal frequencies was then projected. Finally, an external reference signal from an invasive electrode was used to project the recorded vector magnetic fields due to gastric electrical activity. In each case, it was found that the information obtained by examination of the projected magnetic field vectors gave superior clinical insight to that obtained by analysis of any single magnetic field component.

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.

Biomagnetic detection of gastric electrical activity in normal and vagotomized rabbits

We recorded the vector magnetogastrogram (MGG) due to gastric electrical activity (GEA) in normal rabbits using a Superconducting QUantum Interference Device (SQUID) magnetometer and measured the degree of correlation of the MGG with 24 channels of serosal electrodes. The vector magnetometer allows us to non-invasively record three orthogonal magnetic field components and project the recorded magnetic field vector into arbitrary directions. We optimized the magnetic field vector direction to obtain the highest possible correlation with each serosal electrode recording. We performed a vagotomy and examined spatial and temporal changes in the serosal potential and in the transabdominal magnetic field. We obtained spatial information by mapping the recorded signals to the electrode positions in the gastric musculature. Temporal evidence of uncoupling was observed in spectral analyses of both serosal electrode and SQUID magnetometer recordings. We conclude that non-invasive recordings of the vector magnetogastrogram reflect underlying serosal potentials as well as pathophysiological changes following vagotomy.

Changes in intestinal electrical activity during ischaemia correlate to pathology

The gastrointestinal tract possesses an omnipresent electrical slow wave called the basic electrical rhythm (BER). It has been shown that the frequency of the BER falls during intestinal ischaemia. The correlation between changes in the BER and pathology that occur during acute ischaemia are not completely understood. To study this, the electrical activity of the ileum in 14 adult male rabbits was recorded during ischaemia. At baseline, 60, 120 and 210 minutes of ischaemia, segments of bowel were resected for histopathologic evaluation. The BER frequency was determined using the Fast Fourier Transformation (FFT) spectral analysis. The BER amplitude and FFT spectral power were also determined. The results showed significant decrease (p<0.05, Student's T-test) in the BER frequency, amplitude, and spectral power at all time points. Between 60 and 120 minutes, while there was a decrease in BER activity the pathologic grade remained the same (focal loss of surface epithelium). By 210 minutes of ischaemia when the BER could not be recorded, there was diffuse mucosal infarction. The results indicate that changes in the electrical activity of the bowel during acute mesenteric ischaemia occurred prior to the pathologic changes. The presence of electrical activity indicates that there was viable bowel. Thus it should be possible to use recordings of electrical activity to evaluate bowel viability during acute ischaemia.

Histopathologic changes during mesenteric ischaemia and reperfusion

The basic electrical rhythm (BER) of the intestine is known to decrease during mesenteric ischaemia. Some studies have reported the relationship between the BER and the pathologic changes that occur in the bowel during vascular injury. However, these changes have not been completely elucidated. This study describes the histopathologic pattern when the rabbit small intestine was subjected to ischaemia of varying time lengths (30-150 minutes) and subsequent reperfusion for six hours. Intestinal biopsies were taken at baseline, at the end of ischaemia, and at hourly intervals during reperfusion. Microscopic examination of the biopsies revealed evidence of progressive infarction of the mucosa during ischaemia. There was an acute worsening of the pathology during reperfusion, the severity being greater when reperfusion was preceded by longer periods of ischaemia. These changes were statistically significant. The observed pattern in this study shows clearly that reperfusion injury is reflected in the histopathologic response and that this is worse in severity than the response to ischaemia. Studies of longer duration should further clarify the picture during recovery in ischaemia/reperfusion injuries of the bowel.

Spatial filter approach for evaluation of the surface Laplacian of the electroencephalogram and magnetoencephalogram

The surface Laplacian is a technique that has been utilized to improve the spatial resolution of the electroencephalogram (EEG) and the magnetoencephalogram (MEG). We investigate the amount of improvement to the spatial resolution afforded by the surface Laplacian by examining the spatial filters that describe the relationship between cortical current sources and the surface Laplacian. The surface Laplacian spatial filters extend into higher spatial frequencies than do raw signal spatial filters, particularly for EEG Laplacian spatial filters, indicating that substantial improvement in spatial resolution is possible. However, the response of the surface Laplacian operation to the nature and amount of noise in the raw EEG and MEG signals is of paramount importance. Spatially correlated noise, coupled with uncorrelated noise, requires additional regularization of inverse spatial filters resulting in a decrease in spatial resolution. Substantial improvements in spatial resolution may be obtained using the surface Laplacian techniques as long as correlated noise levels are small and raw signals have relatively high signal-to-noise ratios.

Spatial filter approach for comparison of the forward and inverse problems of electroencephalography and magnetoencephalography

We present an analysis of the relative information content of cortical current source reconstructions from electroencephalogram (EEG) and magnetoencephalogram (MEG) forward calculations by examining the spatial filters that relate the internal sources with the externally measured electric potentials and magnetic fields. The forward spatial filters are seen to be low-pass functions of spatial frequency and spatial resolution degrades in external measurements. Inverse spatial filters may be used to reconstruct cortical sources from external data, but since they are high-pass functions of spatial frequency, they must be regularized to avoid instabilities caused by noise at higher spatial frequencies. The regularization process limits the spatial resolution of source reconstructions. EEG forward spatial filters fall off at lower spatial frequencies than MEG filters; hence, there is less information available in higher spatial frequencies resulting in lower spatial resolution in inverse reconstructions. The tangential component of the magnetic field provides even higher spatial resolution than can be obtained using the radial component. An accompanying article examines the surface Laplacian for both the EEG and the MEG.

Volume conductor effects on the spatial resolution of magnetic fields and electric potentials from gastrointestinal electrical activity

An analysis of the relative capabilities of methods for magnetic and electric detection of gastrointestinal electrical activity is presented. The model employed is the first volume conductor model for magnetic fields from GEA to appear in the literature. A mathematical model is introduced for the electric potential and magnetic field from intestinal electrical activity in terms of the spatial filters that relate the bioelectric sources with the external magnetic fields and potentials. The forward spatial filters are low-pass functions of spatial frequency, so more superficial external fields and potentials contain less spatial information than fields and potentials near the source. Inverse spatial filters, which are reciprocals of the forward filters, are high-pass functions and must be regularised by windowing. Because of the conductivity discontinuities introduced by low-conductivity fat layers in the abdomen, the electric potentials recorded outside these layers required more regularisation than the magnetic fields, and thus, the spatial resolution of the magnetic fields from intestinal electrical activity is higher than the spatial resolution of the external potentials. In this study, two smooth muscle sources separated by 5cm were adequately resolved magnetically, but not resolved electrically. Thus, sources are more accurately localized and imaged using magnetic measurements than using measurements of electric potential.

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.

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 +/- 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.

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.

Effects of mesenteric ischemia and reperfusion on small bowel electrical activity

Previous studies involving basic electrical rhythm (BER) have not been carried out far enough to fully characterize the relationship between mesenteric ischemia and BER. The phenomenon of reperfusion injury has also not been correlated with BER activity. The goal of this study was to characterize changes in BER during mesenteric ischemia and reperfusion and to correlate them with changes in pathology.

METHODS

Serosal electrodes were used to record the electrical activity of rabbit jejunum (n = 20) at baseline, during ischemia (90-210 min), and during reperfusion (120-240 min). BER frequency and amplitude were monitored, and biopsies were taken at the end of ischemia and reperfusion. A pathologist blinded to the specimen identity graded the histology on a scale of 0 (no changes) to 6 (transmural necrosis). Paired t test, the Kruskal-Wallis test of non-parametric ranks, and Fisher's r to z test were used for statistical significance where appropriate.

RESULTS

BER frequency and amplitude fell significantly after 15 min of ischemia and became undetectable by 90 min of ischemia in all animals. The likelihood that BER would return during reperfusion was highly correlated with length of ischemia (r = 0.99). Longer periods of reperfusion were associated with increasing pathologic grade.

CONCLUSIONS

BER frequency and amplitude are very sensitive to ischemia and their changes occur well before histopathologic changes. The variation in electrical activity of the small bowel during ischemia and reperfusion is a dynamic process that reflects the metabolic state of the smooth muscle. If electrical activity of the bowel is to be used for assessment of viability, continuous recordings more accurately reflect the metabolic state of the smooth muscle.

Correlation and comparison of magnetic and electric detection of small intestinal electrical activity

The small intestinal basic electrical rhythm (BER) was detected simultaneously with serosal electrodes and a transabdominal superconducting quantum interference device (SQUID) magnetometer in anesthetized rabbits. We induced mesenteric ischemia to correlate serosal electrode recording of changes in BER with the SQUID magnetometer. The BER frequency was obtained by spectral analysis of the data using Fourier and autoregressive techniques. There was a high degree of correlation (r = 0.96) between the BER frequency determined using the serosal electrodes and the BER frequency ascertained from SQUID data. Additionally, the effects of an electrical insulator on the external electric and magnetic fields were studied in the rabbit model. The presence of an insulator profoundly attenuates external electric potentials recorded by cutaneous electrodes but does not significantly affect external magnetic fields or serosal potentials. We conclude that SQUID magnetometers could noninvasively record small intestinal BER that was highly correlated with the activity recorded by invasive serosal electrodes. The advantages of magnetic field measurements have encouraged us to investigate clinical applications.

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 serosal 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.

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.

Noninvasive diagnosis of mesenteric ischemia using a SQUID magnetometer

OBJECTIVE

The authors assessed the ability of a Superconducting Quantum Interference Device (SQUID) magnetometer to noninvasively detect mesenteric ischemia in a rabbit model.

SUMMARY BACKGROUND DATA

Superconducting 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.

METHODS

The 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.

RESULTS

Transabdominal 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.

CONCLUSIONS

A 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.