A modular multichannel SQUID-system, in which every single channel can be optimized or replaced individually, is presented. The DC-SQUIDs based on the materials NbN/MgO are prepared by thin film technology and show noise values below 10μΦ0/√Hz. A simplified way of coupling the modulation and feedback current directly to the coupling coil is realized The complete SQUID module including the superconducting shield was miniaturized down to a diameter of 5mm. The gradiometers are wire wound and an as made balancing better than 10−3 is achieved. The cryogenic system was optimized with respect to low vibrations and low helium boil off rate. Simple conductive paint with precisely adjusted surface resistivity is used for RF-shielding. The complete SQUID-electronic of one channel has been realized on one single board and uses a new bias modulation scheme to completely suppress intrinsic 1/f noise. The noise level of the complete system is below 10fT/√Hz. Biomagnetic measurements of the human heart and brain are presented. Single current dipole reconstructions and current density imaging techniques can be used to find the underlying sources. Using a special coil positioning system an overlay of the functional current images with morphological MR-images can be carried out.
Current sources in the human body can be localized by measuring the biomagnetic fields with multichannel SQUID systems. Important system aspects are the noise level, the ambient field suppression, the dynamic range, the reliability, the number of channels, and the arrangement of gradiometers. From the users point of view the most important quality factor is the accuracy with which a current dipole can be localized. A test procedure is proposed to determine the localization power of the system. A 31-channel-SQUID system is presented together with the results of the test. The crucial parts of the system determining the accuracy are pointed out.
M. Fuchs, W. H. Kullmann, and O. Dössel. Functional imaging of neuronal brain activities. Overlay of distributed neuromagnetic current density images and morphological MR images. In European Radiology, vol. 3(1) , pp. 41-43, 1993
Neuromagnetic imaging is a relatively new diagnostic tool for examination of electrical activities in the nervous system. It is based on the non-invasive detection of extremely weak magnetic fields around the human body with superconducting quantum interference device (SQUID) detectors. Often the equivalent current dipole model is used to describe the centre of the electrical activity. New current density reconstruction methods enable the imaging of the spatial extent and structure of neuronal activities. For practical use in medical diagnosis a combination of the abstract neuromagnetic images with MR or CT images is required in order to match the functional activity with anatomy and morphology. The neuromagnetic images can be overlaid onto three-dimensional morphological images with spatially arbitrarily selectable slices. The matching of both imaging modalities is discussed. On the basis of the detection of auditory evoked magnetic fields, neuromagnetic images are reconstructed with linear estimation theory algorithms. The MR images are used as a priori information of the volume conductor geometry and allow an attachment of functional and morphological properties.
G. Gauglitz, A. Brecht, G. Kraus, and W. Nahm. Chemical and biochemical sensors based on interferometry at thin (multi-) layers. In Sensors and Actuators B: Chemical, vol. 11(1) , pp. 21-27, 1993
Spectral interferometry is presented as a tool to monitor the swelling of polymers caused by organic gases or hydrocarbons in waste water as well as the adsorption and interaction of antigens and antibodies in immunoreactions. Modern diode-array technology allows the consequent observation of changes in optical pathlength on a fractional nanometer scale with subsecond repetition times. The theory of multiple-reflection principles in white-light interferometry determines the possibilities and limitations of this method. The optical set-up and some applications in gas sensing and label-free immunosensing are discussed with respect to the sensitivity, selectivity and limits of detection at present.
We have fabricated Y1Ba2Cu3O7-x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions...
Oligomers with a dimethylsiloxane backbone coated as thin films on different substrate surfaces were thermally as well as photochemically cross-linked. The structure and the degree of cross-linking were examined spectroscopically. Diffusion of different gases in the thin polymer films was measured by time resolved infrared ATR-spectroscopy. The process of diffusion is almost immediately followed by a swelling of the polymer proportional to gas concentration. Therefore diffusion may also be measured by spectral interferometry, giving a very sensitive device for optical sensing of hydrocarbons. Furthermore, diffusion in polymers may be measured very accurately by spatially resolved UV/Vis-spectroscopy. Diffusion coefficients may also be determined indirectly from the equilibrium of monomers and excimers indicated by the fluorescence intensities. This method allows the in situ observation of the cross-linking process.
A modular multichannel superconducting quantum interference device (SQUID) system, in which every channel can be optimized or replaced individually, was further improved. The number of channels was increased to 31. The noise level is better than 10 fT/√Hz. A novel way of RF shielding using conductive paint avoids degradation of the SQUID characteristics due to RF interference without introducing significant extra noise, so that the system works without any Faraday cage. A simplified way of coupling the modulation and feedback signal directly to the SQUID was developed and tested successfully. The SQUID module with superconducting connections to the gradiometer and its superconducting shield was miniaturized to an outer diameter of 5 mm, so that it can be placed near the gradiometer without introducing significant unbalance. Tests have demonstrated that the accuracy of the system with respect to the localization of a single current dipole is better than 2 mm
W. Nahm, H. Gehring, E. Konecny, and P. Schmucker. Messung der Atemalkoholkonzentration zur Beurteilung des Einschwemmsyndroms bei urologischen Eingriffen unter Vollnarkose.. In Biomedizinische Technik / Biomedical Engineering, vol. 38(s1) , pp. 423-424, 1993
T. Schmidt, and O. Dössel. Testprozeduren für die Qualitätskontrolle von Vielkanal-Biomagnetometersystemen. In Biomedizinische Technik, vol. 38(s1) , pp. 189-190, 1993
Mit zunehmendem Einsatz von Vielkanal-Biomagnetometersystemen im klinischen Bereich gewinnt auch die Frage nach einer verläßlichen Qualitätskontrolle an Bedeutung. Immer häufiger werden die Ergebnisse einer biomagnetischen Messung zur Grundlage einer medizinischen Entscheidung gemacht. Besonders bei der Verwendung als prächirurgische Zusatzinformation stellt dies aber hohe Anforderungen an die Präzision und die Reproduzierbarkeit der Meßergebnisse. Mit dieser Zielrichtung wurde ein Meßprogramm erarbeitet, das die leistungsbestimmenden Parameter eines Biomagnetometers erfaßt und die realistischen Bedingungen in der klinischen Umgebung berücksichtigt. Im Vordergrund stehen dabei Messungen zum Rauschverhalten des Systems und zur Lokalisierungsgenauigkeit. Mit den vorgestellten Testprozeduren ist eine einfache, reproduzierbare und die relevanten Qualitätsaspekte abdeckende Überprüfung eines Vielkanal-Magnetometersystems möglich. Regelmäßige Anwendung deckt Fehlerquellen und damit verbundene Ungenauigkeiten auf und erlaubt die Zuverlässigkeit medizinischer Meßergebnisse einzuschätzen. Darüberhinaus werden die Ergebnisse verschiedener Systeme besser vergleichbar, was insbesondere bei multizentrischen Studien von Bedeutung ist.