Article ENTWICKLUNG UND ERPROBUNG EINES NICHTMAGNETISCHEN, DIGITALEN WINKEL-ENCODERS FÜR BIOMAGNETOMETER was published on January 1, 2001 in the journal Biomedical Engineering / Biomedizinische Technik (volume 46, issue s1).

Electroencephalography (EEG) is a non-invasive method to analyze the temporal and spatial extent of neuronal activity in the brain. Skull discontinuities, such as fontanelles of the neonate skull or post-surgical skull conditions, affect EEG diagnostics and EEG-based source reconstruction. However, experimental analyses are rare. The aim of this study is to develop an experimental setup to characterize and quantify the effect of skull discontinuities on the EEG. We constructed an artificial current source (dipole) and a highresolution EEG array suitable for a rodent animal model. The results clearly demonstrate alterations of the EEG caused by a hole in the skull which extend beyond the hole itself. The presence of a return current path through the skull modifies the electric field topography which is in line with theoretic expectations. We conclude that discontinuities in the skull should be accounted for in volume conductor models used in the reconstruction of electric sources in the brain, such as finite element models.

The aim of our work was to quantify the influence of white matter anisotropic conductivity information on electroencephalography (EEG) source reconstruction. We performed this quantification in a rabbit head using both simulations and source localization based on invasive measurements. In vivo anisotropic (tensorial) conductivity information was obtained from magnetic resonance diffusion tensor imaging and included into a high-resolution finite-element model. When neglecting anisotropy in the simulations, we found a shift in source location of up to 1.3 mm with a mean value of 0.3 mm. The averaged orientational deviation was 10 degree and the mean magnitude error of the dipole was 29%. Source localization of the first cortical components after median and tibial nerve stimulation resulted in anatomically verified dipole positions with no significant anisotropy effect. Our results indicate that the expected average source localization error due to anisotropic white matter conductivity is within the principal accuracy limits of current inverse procedures. However, larger localization errors might occur in certain cases. In contrast, dipole orientation and dipole strength are influenced significantly by the anisotropy. We conclude that the inclusion of tissue anisotropy information improves source estimation procedures.

There is an indication that central processing in the auditory system is changed in schizophrenics with auditory hallucinations. Otoacoustic emissions generated from cochlear hair cells are a valuable indicator for the functioning of efferent and afferent pathways in the auditory system. They can be recorded easily without strain for the test persons or patients. To test whether auditory information processing is delayed or otherwise changed in schizophrenia, auditory evoked magnetic fields (AEF) were evoked with tone bursts (1 kHz, duration 50 ms, rise time 5 ms, intensity 70 dB SPL; series per 200 single bursts with random interstimulus intervals) in schizophrenic patients with auditory hallucinations (n=32) and in healthy controls (n=40). Two recording sessions were performed for each person. Tone bursts were applied via an acoustically tested tube and funnel system to the contralateral ear of the test person. AEF were recorded with the Philips 2×31 channel double Dewar device (spherical sensor plate with a diameter of 140mm). The cryostat was placed over the temporal cortex contralaterally to the stimulated ear. AEF were evaluated with the CURRY program. For better recognition of the relevant signals, raw data were filtered digitally with a Fourier filter. The patients had suffered for more than 2 years from schizophrenia and were receiving neuroleptic treatment. Most of the patients had no hallucinations during the measurements, only a subgroup of 6 reported on hallucinations during the test. In healthy controls, AEF typically consisted of 4 prominent components, P50m, N100m, P160m, and P200m with latencies resembling the numbering of the components. Significant changes of the AEF, however, were found in schizophrenics: the latency of the AEF component N100m was prolonged and typical components (especially the P160m and P200m) were often missing, although there were no hallucinations during the trials. The data imply that auditory information processing is influenced during auditory hallucinations in schizophrenics. Tests will be repeated in each patient in a hallucination-free period to prove if the changes in AEF persist after ending neuroleptic treatment.

Biomagnetic signals are up to six orders of magnitude smaller than the usual level of urban magnetic interference and noise. SQUID gradiometers and magnetically shielded rooms (MSRs) are therefore commonly used to reduce the disturbances and to record magnetic physiological signals. MSRs are very expensive and are inconvenient for many patients due to their narrow dimensions. We have developed an active; magnetic shield with the aim of compensating for the urban magnetic field and its first-order spatial field gradient to allow biomagnetic measurements outside an MSR. Two separate feedback circuits measure the magnetic field and the field gradient and feed two sets of compensation coils. We tested two field gradient compensation configurations which made use of different gradiometer designs and different separations between the reference and measurement sensors. The two types of shield were found to achieve interference suppressions of 43 and 26 dB, respectively. We were able to measure a human magnetocardiogram (MCG) signal in an open laboratory, using the less effective shielding configuration.

Spreading depression (SD) propagates in cortical regions that are different in their morphological and functional characteristics. We tested whether the propagation pattern of spreading depression was different between parts of the cortex. In six adult rats, we recorded the ECoG by a 4×4 electrode array that covered parts of the frontal, parietal cortex and the cingulate cortex. Simultaneously a 16-channel magnetoencephalogram was recorded to characterize the development and direction of intracortical ion movements accompanying this phenomenon. Spreading depression was initiated by occipital application of 0.3 molar KCl solution. Depolarization was observed, at first, at lateral cortical regions and then at medial cortical regions. Thereafter, the propagation velocity increased in medial cortical regions and was faster than in lateral regions. Negative potential shifts were detected by all electrodes, but the depolarization reached a maximum over lateral and caudal cortical regions. The recorded magnetic fields indicated the same orientation of currents underlying these fields, which was perpendicular to the wave front and points away from the depolarization region. Overall, the data indicated that propagation patterns of spreading depression differed between parts of the cortex and, thus, propagation was inhomogeneous. This propagation was accompanied by strong currents parallel to the cortical surface.

The knowledge about the origin and the spatio-temporal pattern of propagation of spreading depolarization (SD) after focal ischemic brain infarction is limited. Using the information of the simultaneously recorded ECoG and MEG it seems possible to describe the localization of underlying neurophysiological processes. We investigated in 8 rats 32 periinfarct depolarizations by simultaneously recorded ECoG and MEG. The ECoG was recorded by a grid of 4×4 electrodes with a spatial distance of 1.25mm between adjacent electrodes. The MEG was recorded by a 16-channel Micro-SQUID system built at the Biomagnetic Center (1st order asymmetric gradiometers, 6.7mm pick-up-coil diameter, 30mm baselength, covering an area of 3.2×3.2cm2). Infarction was initiated via occlusion of the right middle cerebral artery. From the ECoG the moment and location of the first detected negative deflection, the sequence of the involvement of cortical regions and the spatial distribution of the amplitude of depolarization were determined. From MEG the temporal pattern of main intracortical current was estimated. The frequency of SD was 5/hr (=12±6min). Negative deflection of electric potential could be detected at first over rostro-medial regions. The location of the maximal depolarization amplitude varied intra- and interindividually. In most cases SD waves propagated from rostro-medial to caudal regions. In 26 of 32 electric SD, magnetic field changes were detected. Often (n=12) long-lasting magnetic field changes started before electric changes. The mean duration of these changes varied considerably (205±163s). We suppose that the origin of periinfarct depolarization is frequently located in frontal cortical regions and that subcortical depolarization may contribute to the MEG signal.

Since the high costs of common large array SQUID system may hinder widespread application of fetal magnetoencephalography (fMEG) and magnetocardiography (fMCG), we intended to investigate a small non-commercial 3-channel SQUID system. The system comprises 3 axial first order gradiometers with 7 cm base length, 2 cm diameter and 2x2 windings of niobium wire, dc-SQUIDs (UJ-111), and current locked mode SQUID electronics that form an equal length triangle (22.5 mm). The system is mounted in a Cryostat BFH-7 model 16 with 5 mm "warm"-"cold" distance. System noise is about 10 fT/Hz1/2. The fMEG and fMCG were recorded between 29 - 40 weeks of gestation after sonographic localization of the fetal head and heart using a 31-channel biomagnetometer (Philips) and the 3-channel-system, both in the same magnetically shielded room. The fMEG was recorded continuously over 500 sec (500 auditory stimuli, 100 dB SPL, 500 Hz, 50 ms, ISI 0.8-1.2/1.6-2.4 sec, trigger channel, maternal ECG lead, sampling rate 1 kHz). The fMCG was recorded over a period of 5 minutes after dewar readjustment. The detection rates of cortical auditory evoked responses (CAER) reached 100 % for both systems. Cross confirmation of the components was difficult and may have uncovered false positive component detection. The fMCG was characterized by a systematic increase in SNR under application of the smaller device. The small size array provides a profitable alternative for the fetal applications.

Magnetic field sensors are used in various fields of technology. In the past few years a large variety of magnetic field sensors has been established and the performance of these sensors has been improved enormously. In this review article all recent developments in the area of sensitive magnetic field sensory analysis (resolution better than 1 nT) are presented and examined regarding their parameters. This is mainly done under the aspect of application fields in biomedical engineering. A comparison of all commercial and available sensitive magnetic field sensors shows current and prospective ranges of application.

Magnetic field sensors are used in various fields of technology. In the past few years a large variety of magnetic field sensors has been established and the performance of these sensors has been improved enormously. In this review article all recent developments in the area of sensitive magnetic field sensory analysis (resolution better than 1 nT) are presented and examined regarding their parameters. This is mainly done under the aspect of application fields in biomedical engineering. A comparison of all commercial and available sensitive magnetic field sensors shows current and prospective ranges of application.

Biomagnetic fields - in particular in the low-frequency range - are subject to environmental interference, which cannot be adequately reduced by most passive shielding methods. However, the signal-to-noise ratio can be increased by active compensation. For this purpose, the interference is detected by reference sensors and fed back through integrated compensation coils. To establish deviation of normal directions between reference sensors and compensation coils, an angle encoder was developed.The rotation of the reference sensors about two axes at right angles to each other, is converted into voltage pulses by means of codewheels and photoelectric beams. The pulses are counted by incremental encoders, and represent a measure of the angles, A cardanic suspension and a plumb-line act as a reference system. The pulses counted are converted into binary angle values, which are used for coordinate transformation of the interfering fields. The angle encoder can determine the tilt of the reference sensors with an accuracy of 1 degree within a range between -45 and +45 degrees. The noise level of the system remains unaffected during a biomagnetic measurement. Magnetic signals of up to 5 pT arising during the oscillation of the plumb-line can be neglected because of the static nature of the angular measurement.

Biomagnetic fields - in particular in the low-frequency range - are subject to environmental interference, which cannot be adequately reduced by most passive shielding methods. However, the signal-to-noise ratio can be increased by active compensation. For this purpose, the interference is detected by reference sensors and fed back through integrated compensation coils. To establish deviation of normal directions between reference sensors and compensation coils, an angle encoder was developed. The rotation of the reference sensors about two axes at right angles to each other, is converted into voltage pulses by means of codewheels and photoelectric beams. The pulses are counted by incremental encoders, and represent a measure of the angles. A cardanic suspension and a plumb-line act as a reference system. The pulses counted are converted into binary angle values, which are used for coordinate transformation of the interfering fields. The angle encoder can determine the tilt of the reference sensors with an accuracy of 1 degree within a range between -45 and +45 degrees. The noise level of the system remains unaffected during a biomagnetic measurement. Magnetic signals of up to 5 pT arising during the oscillation of the plumb-line can be neglected because of the static nature of the angular measurement.

Source localization in the human brain based on EEG and MEG data becomes more and more important in the field of clinical research and routine. Thus, information about source localization accuracy is indispensable. Source localization accuracy has been tested with various artificial source and volume conductor models. However, there are a number of limitations inherent to these artificial models. Thus, we address source localization accuracy with an animal model. Furthermore, most investigations have been performed with artificial and physiological sources of a tangential orientation. However, 20 percent of the sources of the human brain have a radial orientation. Hence, an animal model with a lissencephalic cerebrum (flat cortical surface) with radial oriented physiological sources would take this into account.

Article Animal Experimentation Study of Atrial Activity Propagation Using a Multi-channcl SQUID System was published on January 1, 2001 in the journal Biomedical Engineering / Biomedizinische Technik (volume 46, issue s2).

Measurements of DC near biomagnetic fields are disturbed by low frequency noise, that is not reduced sufficiently by most magnetically shielded rooms or gradiometers. An effective SQUID based active shielding system has been developed and installed at the magnetically shielded rooms in the Biomagnetic Center of the University Jena to reduce external low frequency disturbances. A reduction of the magnetic noise of about 23 dB could be achieved at 50 mHz.

The M100 wave representing a robust component of cortical Auditory Evoked Magnetic Fields (AEFs) is used to localize neuronal activation in the primary auditory cortex for a long time in neurophysiological research and with increasing importance also in clinical research [1-4]. Changes of location, orientation and strenght of equivalent current dipoles (ECD) are considered being correlated to changing physical parameters of the stimulus as well as specific pathological disorders [2,3]. However, the influence of defined types of periphere hearing loss was investigated not as intensive. The presented study is performed to evaluate wether partial hearing loss of different genesis leads to considerable aberrations in the dipole parameters, especially of the M100 component, possibly reflecting adaptive central reorganization processes.

To evaluate possible prespike field synchronizations, its relation to both interictal discharges and postspike return to baseline, penicillin-induced cortical interictal discharges were recorded in anaesthetized rabbits by magnetoencephalography (MEG) and electrocorticography (EcoG). Statistical parameters of spatial (global field power (GFP)) and temporal properties (Z-parameter) of field synchronization were calculated. In our previous report, three types of prespike field synchronization were found before the onset of interictal spike. We report here that the continuous and fluctuating, but not the abrupt prespike increases, were more often associated with a spike and wave pattern of interictal discharge than with a spike alone. Furthermore, the postspike return of these statistical parameters shows the same three patterns as the prespike field synchronizations, but in the inverse time sequence. More often than not pre- and postspike pattern were of the same type. The results suggest an influence of prespike field synchronization upon interictal discharge and subsequent field return dynamics.

In many experiments we observed differences between simultaneously measured ECoG and MEG signals concerning the temporal relationship of characteristic features like maxima or minima. This was not surprising despite MEG and extracellular electric potential are functionally related. They are caused by different bioelectric processes. The MEG is mainly caused by intracellular currents of neuronal cell populations. The extracellular electric potential is caused by currents through the cellular membrane. These currents can be elucidated by current source density analysis (CSD).

The aim of the present study was to evaluate results of the distorted central auditory process. Localization of human cortical responses to acoustical stimulation with tones and speech signals began with magnetoencephalographic (MEG) studies on primary auditory cortex performed by R. HARI and her working group [1],[2]. The MEG studies in connection with MRT-picture have demonstrated that the sources of the magnetic fields (peak-latencies 100 ms) are in the primary auditory cortex.