Prof. Ulrich G. Hofmann Profile

Prof. Ulrich G. Hofmann

Professor for Neuroelectronic Systems at Universitätsklinikum Freiburg

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Proceedings Article | 14 July 2015 Paper

Novel near infrared sensors for hybrid BCI applications

Rand Almajidy, Khang Le, Ulrich Hofmann

Proceedings Volume 9536, 95361H (2015) https://doi.org/10.1117/12.2182066

KEYWORDS: Sensors, Near infrared spectroscopy, Brain-machine interfaces, Electroencephalography, Brain, Prefrontal cortex, Skin, Computing systems, Electrodes, Near infrared

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This study’s goal is to develop a low cost, portable, accurate and comfortable NIRS module that can be used simultaneously with EEG in a dual modality system for brain computer interface (BCI). The sensing modules consist of electroencephalography (EEG) electrodes (at the positions Fp1, Fpz and Fp2 in the international 10-20 system) with eight custom made functional near infrared spectroscopy (fNIRS) channels, positioned on the prefrontal cortex area with two extra channels to measure and eliminate extra-cranial oxygenation. The NIRS sensors were designed to guarantee good sensor-skin contact, without causing subject discomfort, using springs to press them to the skin instead of pressing them by cap fixture. Two open source software packages were modified to carry out dual modality hybrid BCI experiments. The experimental paradigm consisted of a mental task (arithmetic task or text reading) and a resting period. Both oxygenated hemoglobin concentration changes (HbO), and EEG signals showed an increase during the mental task, but the onset, period and amount of that increase depends on each modality’s characteristics. The subject’s degree of attention played an important role especially during online sessions. The sensors can be easily used to acquire brain signals from different cerebral cortex parts. The system serves as a simple technological test bed and will be used for stroke patient rehabilitation purposes.

Proceedings Article | 4 March 2014 Paper

In situ monitoring of brain tissue reaction of chronically implanted electrodes with an optical coherence tomography fiber system

Yijing Xie, Christina Hassler, Thomas Stieglitz, Andreas Seifert, Ulrich Hofmann

Proceedings Volume 8947, 894727 (2014) https://doi.org/10.1117/12.2038913

KEYWORDS: Optical coherence tomography, Brain, Electrodes, Tissues, Signal attenuation, Tissue optics, Neuroimaging, Surgery, Signal processing, Skull

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Neural microelectrodes are well established tools for delivering therapeutic electrical pulses, and recording neural electrophysiological signals. However, long term implanted neural probes often become functionally impaired by tissue encapsulation. At present, analyzing this immune reaction is only feasible with post-mortem histology; currently no means for specific in vivo monitoring exist and most applicable imaging modalities provide no sufficient resolution for a cellular measurement in deep brain regions. Optical coherence tomography (OCT) is a well developed imaging modality, providing cellular resolution and up to 1.2 mm imaging depth in brain tissue. Further more, a fiber based spectral domain OCT was shown to be capable of minimally invasive brain intervention. In the present study, we propose to use a fiber based spectral domain OCT to monitor the the progression of the tissue's immune response and scar encapsulation of microprobes in a rat animal model. We developed an integrated OCT fiber catheter consisting of an implantable ferrule based fiber cannula and a fiber patch cable. The fiber cannula was 18.5 mm long, including a 10.5 mm ceramic ferrule and a 8.0 mm long, 125 μm single mode fiber. A mating sleeve was used to fix and connect the fiber cannula to the OCT fiber cable. Light attenuation between the OCT fiber cable and the fiber cannula through the mating sleeve was measured and minimized. The fiber cannula was implanted in rat brain together with a microelectrode in sight used as a foreign body to induce the brain tissue immune reaction. Preliminary data showed a significant enhancement of the OCT backscattering signal during the brain tissue scarring process, while the OCT signal of the flexible microelectrode was getting weaker consequentially.

Proceedings Article | 3 March 2014 Paper

Estimating the spatial resolution of fNIRS sensors for BCI purposes

Rand Kasim Almajidy, Robert Kirch, Olaf Christ, Ulrich Hofmann

Proceedings Volume 8945, 894504 (2014) https://doi.org/10.1117/12.2037351

KEYWORDS: Sensors, Near infrared spectroscopy, Light emitting diodes, Brain, Tissues, Blood, Star sensors, Palladium, Oxygen, Photodiodes

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Differential near infrared sensors recently sparked a growing interest as a promising measuring modality for brain computer interfacing. In our study we present the design and characterization of novel, differential functional NIRS sensors, intended to record hemodynamic changes of the human motor cortex in the hand-area during motor imagery tasks. We report on the spatial characterization of a portable, multi-channel NIRS system with one module consisting of two central light emitting diodes (LED) (770 nm and 850 nm) and four symmetric pairs of radially aligned photodiodes (PD) resembling a plus symbol. The other sensor module features four similar, differential light paths crossing in the center of a star. Characterization was performed on a concentric, double beaker phantom, featuring a PBS/intralipid/blood mixture (97/1/2%). In extension of previous work, the inner, oxygenated beaker was covered by neoprene sleeves with holes of various sizes, thus giving an estimate on the spatial limits of the NIRS sensor’s measurement volume. The star shaped sensor module formed a diffuse focus of approximately 3 cm in diameter at 1.4 cm depth, whereas the plus shaped arrangement suggested a concentric ring of four separate regions of interest, overall larger than 6 cm. The systems measurement sensitivity could be improved by removing ambient light from the sensing photodiodes by optical filtering. Altogether, we conclude that both our novel fNIRS design as well as its electronics perform well in the double-layered oxygenation phantom and are thus suitable for in-vivo testing.

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