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Biomedical subjects

S B Baumann

Publications and source records attributed to S B Baumann.

16 recordsLinked to original sources
PubMed2000

Intraoperative SSEP detection of ulnar nerve compression or ischemia in an obese patient: a unique complication associated with a specialized spinal retraction system.

OBJECTIVE: To report a case of peripheral nerve compression caused by a specialized spinal retraction system, the Thompson-Farley retractor system, that most likely would not have been detected without intraoperative monitoring of the ulnar nerve. DESIGN: Bilateral median and peroneal nerve somatosensory evoked potentials (SSEPs) were monitored continuously during a C5 corpectomy, as was core body temperature. RESULTS: Within minutes after cervical soft-tissue retraction, the left ulnar nerve SSEP began to decline in amplitude. Peroneal nerve SSEPs were normal throughout the surgery; core body temperature remained at 36 degrees +/- 0.2 degrees C. After much effort to reposition the patient, the SSEPs returned to baseline and the Thompson-Farley system was replaced by a self-retracting system. CONCLUSIONS: To our knowledge, this is the first report of peripheral nerve compression caused by the Thompson-Farley retractor system. Even with careful positioning on the operating table, obese patients may be particularly at risk for upper arm compression. Continuous monitoring of SSEPs is suggested to prevent postoperative morbidity.

Diskectomy↗
PubMed1999

A modified electrode cap for EEG recordings in MRI scanners.

A stretchable electrode cap containing 64 electrodes was modified to make it compatible for functional magnetic resonance imaging (fMRI). Metallic components were individually tested for magnetic susceptibility, and those that perturbed a free-swinging magnet or moved in a strong magnetic field were replaced with non-ferromagnetic components. Studies with a phantom indicate that placement of the cables carrying signals from the cap to the amplifiers can significantly affect MR image quality. Anatomical and functional images obtained with the modified electrode cap show modest signal loss, but not enough to substantially interfere with the low-noise images required for fMRI. The cap enables faster application of large arrays of electrodes in conjunction with MRI studies, and thus makes combined EEG/fMRI studies more practical, especially those with EEG source localization as one of the goals.

Brain↗
PubMed1998

Effect of conductivity uncertainties and modeling errors on EEG source localization using a 2-D model.

This paper presents a sensitivity study of electroencephalography-based source localization due to errors in the head-tissue conductivities and to errors in modeling the conductivity variation inside the brain and scalp. The study is conducted using a two-dimensional (2-D) finite element model obtained from a magnetic resonance imaging (MRI) scan of a head cross section. The effect of uncertainty in the following tissues is studied: white matter, gray matter, cerebrospinal fluid (CSF), skull, and fat. The distribution of source location errors, assuming a single-dipole source model, is examined in detail for different dipole locations over the entire brain region. We also present a detailed analysis of the effect of conductivity on source localization for a four-layer cylinder model and a four-layer sphere model. These two simple models provide insight into how the effect of conductivity on boundary potential translates into source location errors, and also how errors in a 2-D model compare to errors in a three-dimensional model. Results presented in this paper clearly point to the following conclusion: unless the conductivities of the head tissues and the distribution of these tissues throughout the head are modeled accurately, the goal of achieving localization accuracy to within a few millimeters is unattainable.

Brain↗
PubMed1997

The electrical conductivity of human cerebrospinal fluid at body temperature.

The electrical conductivity of human cerebrospinal fluid (CSF) from seven patients was measured at both room temperature (25 degrees C) and body temperature (37 degrees C). Across the frequency range of 10 Hz-10 kHz, room temperature conductivity was 1.45 S/m, but body temperature conductivity was 1.79 S/m, approximately 23% higher. Modelers of electrical sources in the human brain have underestimated human CSF conductivity by as much as 44% for nearly two decades, and this should be corrected to increase the accuracy of source localization models.

Body Temperature↗
PubMed1997

Computational aspects of finite element modeling in EEG source localization.

A comparison is made of two different implementations of the finite element method (FEM) for calculating the potential due to dipole sources in electroencephalography (EEG). In one formulation (the direct method) the total potential is the unknown that is solved for and the dipole source is directly incorporated into the model. In the second formulation (the subtraction method) the unknown is the difference between the total potential and the potential due to the same dipole in an infinite region of homogeneous conductivity, corresponding to the region where the dipole is located. Both methods have the same FEM system matrix. However, the subtraction method requires an additional calculation of flux integrations along the edges of the elements in the computation of the right-hand side (RHS) vector. It is shown that the subtraction method is usually more accurate in the forward modeling, provided the flux integrations are computed accurately. Errors in calculating the flux integrations may result in large errors in the forward solution due to the ill-conditioned nature of the FEM system matrix caused by the Neumann boundary condition. To minimize the errors, closed-form expressions for the flux integrations are used for both linear and quadratic triangular elements. It is also found that FEM forward modeling errors may cause false extrema in the least-square objective function obtained from the boundary potential, near boundaries between media of differing conductivity. Multiple initial guesses help eliminate the possibility of the solution getting trapped in these false extrema.

Brain↗
PubMed1996

Preoperative cortical localization with functional MRI for use in stereotactic radiosurgery.

Accurate localization of the lesion with respect to functionally significant brain is essential to safe stereotactic radiosurgical dose planning. We report the use of functional MR imaging in 3 patients to identify critical areas of surrounding brain and to provide assistance with dose planning, especially with regard to shaping the peripheral isodose around the lesion. We used a functional MRI system employing a conventional 1.5-tesla MRI unit that can detect decreases in deoxyhemoglobin concentration occurring with performance of specific tasks. Two of the patients had supratentorial arteriovenous malformations and 1 patient had a recurrent parasagittal meningioma. Functional MRI provided information on the location of speech, motor, and sensory cortex in these patients. Radiosurgical dose plans were constructed that kept these cortical areas outside of the 30% isodose curves. We believe that the safety of supratentorial parenchymal radiosurgery will be enhanced by the localization of critical brain regions around the target.

Adult↗
PubMed1995

Comparison of functional magnetic resonance imaging with positron emission tomography and magnetoencephalography to identify the motor cortex in a patient with an arteriovenous malformation.

Alterations in gyral contour made it difficult to identify the motor cortex thought to be near an arteriovenous malformation (AVM) in a 24-year-old man considered for stereotactic radiosurgery. Functional imaging in three modalities was performed preoperatively to compare the reliability of localization using functional magnetic resonance imaging (fMRI) on a conventional scanner with positron emission tomography (PET) and magnetoencephalography (MEG). Similar tasks were used for each imaging modality in an attempt to activate and identify the sensory and motor cortex. Data from all three modalities converged for the sensory task, and fMRI and PET data converged for the motor task. The right hemisphere motor strip was localized adjacent and anterior to the AVM. These data were used in planning the radiosurgery isodose configuration to the AVM in order to reduce the irradiation of motor cortex parenchyma. A postoperative fMRI study was also performed using newer techniques to reduce head motion artifact and to improve signal-to-noise ratio. The data confirmed the conclusions derived from the preoperative evaluations. This study demonstrates how conventional MRI scanners can be used for functional studies of use in surgical planning.

Adult↗
PubMed1993

Comparison of single current dipole and magnetic field tomography analyses of the cortical response to auditory stimuli.

Measurements of the magnetic field elicited by a 50 ms long auditory stimulus, from three normal subjects and one head injured subject, are used to estimate the three dimensional distribution of generators in the brain. The resulting images are compared with point source solutions obtained with the usual single current dipole fitting procedures, over a latency range which includes the extrema in the (average) measured signal. In all cases considered, 100 or so epochs time-locked to the stimulus were magnetically recorded. These were averaged, and then analyzed using two techniques; a new distributed current model known as Magnetic Field Tomography (MFT), and the standard single current dipole (SCD) model. Both methods provide estimates of the current generators in the brain. In two of the normal subjects, the MFT solutions are super-imposed onto Magnetic Resonance Images (MRI) of the relevant cortical area. The results show that when the SCD model provides a reasonable description of the data, the MFT estimate shows one dominant localized region in agreement with the current dipole position. In the MFT sequence of solutions the activity evolves smoothly; multiple areas of activity often arise as the focal activity in one region declines while focal activity in another region grows. In contrast the SCD solutions during these intermediate periods fit the data poorly, and may move erratically from one locale to another. We conclude that MFT seems to provide a reasonable description of the activity through cortical and subcortical regions. The evolution of activity, as derived from the average signal, can be traced continuously from the onset of the stimulus, not just at the peaks.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1992

Late magnetic fields and positive evoked potentials following infrequent and unpredictable omissions of visual stimuli.

Randomized and infrequent omissions during presentation of a steady train of visual stimulation produced distinctive wave forms of both the magnetic fields and electrical potentials. Electrical potentials at Pz showed a positive peak in response to the omitted stimuli which occurred on the average 445 msec after the time when a stimulus was anticipated. Analyses of the magnetic wave forms indicated that at least two separate sources appear to be active coincident with the electrical positive peak. One source localized in the occipital lobes in the vicinity of the visual cortex while the other source was located in the medial aspects of the temporal lobe or even deeper in the lateral thalamus. Judging from the calculated direction of current flow it appeared that the deep source would contribute greater potentials in the frontal areas of the scalp while the source in the occipital area would contribute to more posterior placement of electrodes, especially at Pz.

Brain↗
PubMed1991

Localization of the P3 sources using magnetoencephalography and magnetic resonance imaging.

In this study, two related issues were addressed: first, whether the P3 component of auditory evoked responses, obtained in the context of an oddball paradigm, and its magnetoencephalographically recorded counterpart (P3m) are generated by the same intracranial sources; and, second, whether these sources, modeled as equivalent current dipoles, can be localized in particular brain structures using magnetic resonance imaging. The study involving 8 normal adult subjects resulted in the following findings. (1) Both the similarities and differences in wave form characteristics of the simultaneously recorded P3 and P3m can be best accounted for by common intracranial sources. (2) Several successively activated single-dipolar sources, rather than a single source, account for the entire evolution of the P3m component. (3) Most of these sources were localized in the vicinity of the auditory cortex in all subjects, although some sources appeared to be in deeper structures, possibly the lateral thalamus. (4) The successive activation of sources followed an orderly medial-to-lateral course. These results suggest that activity responsible for the surface-recorded P3 (and P3m) component may be initiated in deep structures, but it quickly spreads over and is sustained in areas near the auditory cortex.

Adult↗
PubMed1991

Magnetoencephalographic localization of interictal spike sources. Case report.

The reliability of localization of interictal spike sources using magnetoencephalography (MEG) was examined by repeated measurements in a patient with temporal lobe epilepsy. During two preoperative recording sessions, the estimated sources, projected onto magnetic resonance images of the patient's brain, were found to lie less than 1 cm apart within the area subsequently resected. The MEG localization was in close agreement with intraoperative cortical recordings.

Adult↗
PubMed1990

Intersession replicability of dipole parameters from three components of the auditory evoked magnetic field.

The replicability of dipole localizations between sessions in an unselected group of subjects was studied. Auditory evoked magnetic fields (AEMFs) in response to contralaterally and ipsilaterally presented 1 kHz tone bursts were recorded from the right hemisphere of 12 subjects with normal hearing in two replicate sessions several days apart. Three long-latency components of the AEMF were studied, occurring at latencies near 50 msec (P1m), near 100 msec (N1m) and near 165 msec (P2m). A spherical model of the head was used to fit equivalent-current dipoles to the data. Statistical analysis of dipole parameters revealed virtually no differences between the two testing sessions. The variability between sessions had a mean absolute difference of 3 to 10 mm for the spatial parameters. Comparison of dipole parameters between components showed that there was a replicable, but nonsignificant, trend for a difference in the location of the N1m from contralateral vs. ipsilateral stimulation, and a statistically significant confirmation that the P2m is located anterior to the N1m for contralateral stimulation. Magnetic resonance images from each subject were used to locate the dipoles near the primary auditory cortex in the Sylvian fissure.

Acoustic Stimulation↗
PubMedDate not supplied

Neuromagnetic evidence of a dynamic excitation pattern generating the N100 auditory response.

Evoked magnetic field recordings were used to localize multiple sources of the negative component of cortical responses to auditory stimuli. The negative cortical component of the auditory evoked response, often called the N100, has traditionally been of interest due to its sensitivity to both stimulation parameters and cognitive variables. Results indicate that this component appears to reflect spreading activation of adjacent cortical columns within the primary projection area of the temporal lobe, extending anteriorly for about 1 cm following the downward slope of the superior surface of the lobe.

Acoustic Stimulation↗
PubMedDate not supplied

Gender differences in source location for the N100 auditory evoked magnetic field.

Auditory evoked magnetic fields were recorded in response to contralateral stimulation over the right hemisphere in 6 adult males and 6 adult females. The data were fit to a model of a current-dipole source in a homogeneous sphere and 5 parameters of the dipole were computed--3 spatial coordinates, orientation, and strength. When average values for the dipole parameters were compared between sexes, it was found that the current source for the N100m is located more than 1 cm posterior in females and is oriented pointing more downward. These findings were replicated in separate measurement sessions. Viewing of individual magnetic resonance images did not reveal a corresponding anatomical disparity in the location of the primary auditory cortex which is assumed to produce the N100m. Therefore, functional organization of the auditory cortex may be different for the sexes.

Adult↗