Hannes Nowak

Bio: Hannes Nowak is an academic researcher from University of Jena. The author has contributed to research in topics: QRS complex & Magnetoencephalography. The author has an hindex of 22, co-authored 81 publications receiving 1524 citations. Previous affiliations of Hannes Nowak include Schiller International University.

Influence of tissue resistivities on neuromagnetic fields and electric potentials studied with a finite element model of the head

Abstract: Modeling in magnetoencephalography (MEG) and electroencephalography (EEG) requires knowledge of the in vivo tissue resistivities of the head. The aim of this paper is to examine the influence of tissue resistivity changes on the neuromagnetic field and the electric scalp potential. A high-resolution finite element method (FEM) model (452162 elements, 2-mm resolution) of the human head with 13 different tissue types is employed for this purpose. Our main finding was that the magnetic fields are sensitive to changes in the tissue resistivity in the vicinity of the source. In comparison, the electric surface potentials are sensitive to changes in the tissue resistivity in the vicinity of the source and in the vicinity of the position of the electrodes. The magnitude (strength) of magnetic fields and electric surface potentials is strongly influenced by tissue resistivity changes, while the topography is not as strongly influenced. Therefore, an accurate modeling of magnetic field and electric potential strength requires accurate knowledge of tissue resistivities, while for source localization procedures this knowledge might not be a necessity.

Neuronavigation in surgery of intracranial and spinal tumors.

Abstract: Purpose: To demonstrate the new possibilities and advantages of neuronavigation in the surgery of intracranial and spinal tumors, based on patient populations treated in our hospital. Materials and methods: An infrared navigation system with integrated microscope guidance was used for frameless intracranial neuronavigation. The biopsies of intracranial tumors were carried out using a frame-based stereotactic technique. Intracranial navigation was, in part, combined with the use of an intraoperative CT scanner and a three-dimensional ultrasound system for data acquisition, correction of brain shifts, and intraoperative quality control. The navigation was also supported by presurgical brain mapping with magnetic source imaging. Navigation in spinal surgery was exclusively performed using an infrared navigation system in combination with an intraoperative CT scanner. Results: The stereotactic tumor biopsies (n=57) were carried out with an accuracy of 91.4% as compared with the histological diagnosis. The work flow of stereotactic procedures could be increased by using the intraoperative CT scanner. Fifty-seven patients with intracranial tumors were treated with the aid of neuronavigation between July 1997 and December 1999. These patients showed an improvement from 80% to 86% on the Karnofsky index 8 weeks postoperatively. The majority of intracranial cases were primary brain tumors (n=30) and metastases (n=13) in functionally important areas of the brain. In four patients, a significant brain shift was observed during neuronavigation, and could be corrected by an image update using either the intraoperative CT scanner (n=2) or the three-dimensional ultrasound system (n=2). The presurgical brain mapping with magnetoencephalography was shown to be reliable in the sensory cortex (n=25). Eleven patients with a thoracic or lumbar tumor were treated by open surgery or stabilization, using a combination of spinal neuronavigation and the intraoperative CT scanner. Two patients with spinal tumors underwent navigated biopsies. Neither of them showed a reduction in the clinical stage, but the Karnofsky index improved from 63% up to 72% 8 weeks postoperatively. Conclusion: Neuronavigation allows very precise intracranial and spinal surgery. The problem of brain shift during the navigation procedures has been solved by intraoperative image acquisition. The use of neuronavigation was shown to improve the postoperative quality of life of patients suffering from brain and spinal tumors.

Signal analysis of auditory evoked cortical fields in fetal magnetoencephalography.

Abstract: Magnetoencephalography (MEG) using auditory evoked cortical fields (AEF) is an absolutely non-invasive method of passive measurement which utilizes magnetic fields caused by specific cortical activity. By applying the exceptionally sensitive SQUID technology to record these fields of dipolar configuration produced by the fetal brain, MEG as an investigational tool could provide new insights into the development of the human brain in utero. The major constraint to this application is a very low signal-to-noise ratio (SNR) that has to be attributed to a variety of factors including the magnetic signals generated by the fetal and maternal hearts which inevitably obscure a straightforward signal analysis. By applying a new algorithm of specific heart artefact reduction based on the relative regularity of the heart signals, we were able to increase the chance of extracting a fetal AEF from the raw data by the means of averaging techniques and principle component analysis. Results from 27 pregnant, healthy women (third trimester of their uncomplicated pregnancy) indicate an improved detection rate and the reproducibility of the fetal MEG. We evaluate and discuss a-priori criteria for signal analyses which will enable us to systematically analyze additional limiting factors, to further enhance the efficiency of this method and to promote the assessment of its possible clinical value in the future.

Meta-Analysis of Regional Brain Volumes in Schizophrenia

Abstract: Objective: The authors’ goal was to determine whether patients with schizophrenia differ from comparison subjects in regional brain volumes and whether these differences are similar in male and female subjects. Method: They conducted a systematic search for structural magnetic resonance imaging (MRI) studies of patients with schizophrenia that reported volume measurements of selected cortical, subcortical, and ventricular regions in relation to comparison groups. They carried out a meta-analysis of the volumes of these regions in the patients with schizophrenia and the comparison subjects using a random effects model; they also used random effects regression analysis to examine the influence of gender on effect sizes. Results: Fifty-eight studies were identified as suitable for analysis; these studies included 1,588 independent patients with schizophrenia. Assuming a volume of 100% in the comparison group, they found that the mean cerebral volume of the subjects with schizophrenia was smaller (98%), but the mean total ventricular volume of the subjects with schizophrenia was greater (126%). Relative to the cerebral volume differences, the regional volumes of the subjects with schizophrenia were 94% in the left and right amygdala, 94% in the left and 95% in the right hippocampus/amygdala, and 93% in the left and 95% in the right parahippocampus. Relative to the global ventricular system differences, the largest differences in ventricular subdivisions were in the right and left body of the lateral ventricle, where the volumes of schizophrenic subjects were 116% and 116%, respectively. For most regions, effect size was not significantly related to gender. Conclusions: Regional structural differences in patients with schizophrenia include bilaterally reduced volume of medial temporal lobe structures. There is a need for greater integration of results from structural MRI studies to avoid redundant research activity. (Am J Psychiatry 2000; 157:16‐25)

Dysconnection in Schizophrenia: From Abnormal Synaptic Plasticity to Failures of Self-monitoring

Abstract: Over the last 2 decades, a large number of neurophysiological and neuroimaging studies of patients with schizophrenia have furnished in vivo evidence for dysconnectivity, ie, abnormal functional integration of brain processes. While the evidence for dysconnectivity in schizophrenia is strong, its etiology, pathophysiological mechanisms, and significance for clinical symptoms are unclear. First, dysconnectivity could result from aberrant wiring of connections during development, from aberrant synaptic plasticity, or from both. Second, it is not clear how schizophrenic symptoms can be understood mechanistically as a consequence of dysconnectivity. Third, if dysconnectivity is the primary pathophysiology, and not just an epiphenomenon, then it should provide a mechanistic explanation for known empirical facts about schizophrenia. This article addresses these 3 issues in the framework of the dysconnection hypothesis. This theory postulates that the core pathology in schizophrenia resides in aberrant N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity due to abnormal regulation of NMDARs by neuromodulatory transmitters like dopamine, serotonin, or acetylcholine. We argue that this neurobiological mechanism can explain failures of self-monitoring, leading to a mechanistic explanation for first-rank symptoms as pathognomonic features of schizophrenia, and may provide a basis for future diagnostic classifications with physiologically defined patient subgroups. Finally, we test the explanatory power of our theory against a list of empirical facts about schizophrenia.