TY - CHAP A1 - Bitz, Andreas A1 - Alaydrus, M. A1 - Streckert, J. A1 - Hansen, V. T1 - Combination of the hybrid/sup [2]/-method and the FDTD-method for solution of boundary value problems with electrically large and high-resolution bodies T2 - IEEE Antennas and Propagation Society International Symposium (IEEE Cat. No.02CH37313) 2002 : San Antonio, TX, USA, 16-21 June Y1 - 2002 SN - 0-7803-7330-8 U6 - http://dx.doi.org/10.1109/APS.2002.1018209 SP - 278 EP - 281 PB - IEEE CY - Piscataway, NJ ER - TY - CHAP A1 - Bitz, Andreas A1 - Alaydrus, M. A1 - Streckert, J. A1 - Hansen, V.W. T1 - Absorption rates inside human body due to radiated electro-magnetic fields of multi-band base station antennas. Boundary value problems with electrically large and high-resolution bodies T2 - 24th BEMS Annual Meeting, Quebec City, Quebec, Canada, 23 - 27 June 2002 Y1 - 2002 SP - 40 EP - 41 PB - Bioelectromagnetics Society CY - Frederick, MD ER - TY - CHAP A1 - Bitz, Andreas A1 - Alaydrus, M. A1 - Streckert, J. A1 - Hansen, V.W. T1 - Base station antennas embedded in complex environments: rf exposure in controlled areas T2 - 23rd BEMS Annual Meeting, St. Paul, Minnesota, USA, 49-50, June 2001 Y1 - 2001 SP - 49 EP - 50 PB - Bioelectromagnetics Society CY - Frederick, MD ER - TY - CHAP A1 - Bitz, Andreas A1 - Streckert, J. A1 - Hansen, V.W. A1 - Stogbauer, F. ED - Hietanen, Maila ED - Jokela, Kari ED - Juutilainen, Jukka ED - Työterveyslaitos, T1 - Shielded RF-exposure system with an integrated LF-measuring system for in vitro experiments with cell-layers T2 - Proceedings of the Ebea 2001 : 5th International Congress of the European Bioelectromagnetics Association (Ebea) ; 6-8 September 2001, Marina Congress Center, Helsinki, Finland Y1 - 2001 SN - 9789518024401 SP - 67 EP - 68 PB - Finnish Institute of Occupational Health CY - Helsinki ER - TY - CHAP A1 - Bitz, Andreas A1 - Streckert, J.R. A1 - Hansen, V.W. A1 - Lerchl, A. ED - Danesy, Dorothea T1 - Freely moving or restrained animals in bioelec-tromagnetic experiments - pros and cons T2 - AP 2000 : Millennium Conference on Antennas & Propagation, Davos, Switzerland, 9 - 14 April 2000 Y1 - 2000 IS - Band 1 SP - 489 PB - ESA Publications Division, ESTEC CY - Noordwijk ER - TY - CHAP A1 - Bitz, Andreas A1 - Streckert, J. A1 - Hansen, V.W. A1 - Buschmann, J. T1 - RF exposure of non restrained animals in an overmoded radial waveguide T2 - 22nd BEMS Annual Meeting, Munich, Germany, 2000 Y1 - 2000 SP - 63 PB - Bioelectromagnetics Society CY - Frederick, MD ER - TY - JOUR A1 - Kobus, Thiele A1 - Bitz, Andreas A1 - Uden, Mark J. van A1 - Lagemaat, Miram W. A1 - Rothgang, Eva A1 - Orzada, Stephan A1 - Heerschap, Arend A1 - Scheenen, Tom W. J. T1 - In vivo 31P MR spectroscopic imaging of the human prostate at 7 T: safety and feasibility JF - Magnetic Resonance in Medicine N2 - 31P MR spectroscopic imaging of the human prostate provides information about phosphorylated metabolites that could be used for prostate cancer characterization. The sensitivity of a magnetic field strength of 7 T might enable 3D 31P MR spectroscopic imaging with relevant spatial resolution in a clinically acceptable measurement time. To this end, a 31P endorectal coil was developed and combined with an eight-channel 1H body-array coil to relate metabolic information to anatomical location. An extensive safety validation was performed to evaluate the specific absorption rate, the radiofrequency field distribution, and the temperature distribution of both coils. This validation consisted of detailed Finite Integration Technique simulations, confirmed by MR thermometry and Burn:x-wiley:07403194:media:MRM24175:tex2gif-stack-1 measurements in a phantom and in vivo temperature measurements. The safety studies demonstrated that the presence of the 31P endorectal coil had no influence on the specific absorption rate levels and temperature distribution of the external eight-channel 1H array coil. To stay within a 10 g averaged local specific absorption rate of 10 W/kg, a maximum time-averaged input power of 33 W for the 1H array coil was allowed. For transmitting with the 31P endorectal coil, our safety limit of less than 1°C temperature increase in vivo during a 15-min MR spectroscopic imaging experiment was reached at a time-averaged input power of 1.9 W. With this power setting, a second in vivo measurement was performed on a healthy volunteer. Using adiabatic excitation, 3D 31P MR spectroscopic imaging produced spectra from the entire prostate in 18 min with a spatial resolution of 4 cm3. The spectral resolution enabled the separate detection of phosphocholine, phosphoethanolamine, inorganic phosphate, and other metabolites that could play an important role in the characterization of prostate cancer. Y1 - 2012 U6 - http://dx.doi.org/10.1002/mrm.24175 SN - 1522-2594 VL - 68 IS - 6 SP - 1683 EP - 1695 PB - Wiley-Liss CY - New York ER - TY - JOUR A1 - Umutlu, Lale A1 - Bitz, Andreas A1 - Maderwald, Stefan A1 - Orzada, Stephan A1 - Kinner, Sonja A1 - Kraff, Oliver A1 - Brote, Irina A1 - Ladd, Susanne C. A1 - Schroeder, Tobias A1 - Forsting, Michael T1 - Contrast-enhanced ultra-high-field liver MRI: a feasibility trial JF - European Journal of Radiology Y1 - 2013 U6 - http://dx.doi.org/10.1016/j.ejrad.2011.07.004 SN - 0720-048X VL - 82 IS - 5 SP - 760 EP - 767 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Yazdanbakhsh, Pedram A1 - Solbach, Klaus A1 - Bitz, Andreas T1 - Variable power combiner for RF mode shimming in 7-T MR imaging JF - IEEE Transaction on Biomedical Engineering N2 - This contribution discusses the utilization of RF power in an MRI system with RF mode shimming which enables the superposition of circularly polarized modes of a transmit RF coil array driven by a Butler matrix. Since the required power for the individual modes can vary widely, mode-shimming can result in a significant underutilization of the total available RF power. A variable power combiner (VPC) is proposed to improve the power utilization: it can be realized as a reconfiguration of the MRI transmit system by the inclusion of one additional matrix network which receives the power from all transmit amplifiers at its input ports and provides any desired (combined) power distribution at its output ports by controlling the phase and amplitude of the amplifiers’ input signals. The power distribution at the output ports of the VPC is then fed into the “mode” ports of the coil array Butler matrix in order to superimpose the spatial modes at the highest achievable power utilization. The VPC configuration is compared to the standard configuration of the transmit chain of our MRI system with 8 transmit channels and 16 coils. In realistic scenarios, improved power utilization was achieved from 17% to 60% and from 14% to 55% for an elliptical phantom and a region of interest in the abdomen, respectively, and an increase of the power utilization of 1 dB for a region of interest in the upper leg. In general, it is found that the VPC allows significant improvement in power utilization when the shimming solution demands only a few modes to be energized, while the technique can yield loss in power utilization in cases with many modes required at high power level. Y1 - 2012 U6 - http://dx.doi.org/10.1109/TBME.2012.2205926 SN - 1558-2531 VL - 59 IS - 9 SP - 2549 EP - 2557 PB - IEEE CY - New York ER - TY - JOUR A1 - Orzada, Stephan A1 - Bitz, Andreas A1 - Schäfer, Lena C. A1 - Ladd, Susanne C. A1 - Ladd, Mark E. A1 - Maderwald, Stefan T1 - Open design eight-channel transmit/receive coil for high-resolution and real-time ankle imaging at 7 T JF - Medical Physics N2 - Purpose: At 1.5 T, real-time MRI of joint movement has been shown to be feasible. However, 7 T, provides higher SNR and thus an improved potential for parallel imaging acceleration. The purpose of this work was to build an open, U-shaped eight-channel transmit/receive microstrip coil for 7 T MRI to enable high-resolution and real-time imaging of the moving ankle joint. Methods: A U-shaped eight-channel transmit/receive array for the human ankle was built.urn:x-wiley:00942405:mp3399:equation:mp3399-math-0001-parameters and urn:x-wiley:00942405:mp3399:equation:mp3399-math-0002-factor were measured. SAR calculations of different ankle postures were performed to ensure patient safety. Inhomogeneities in the transmit field consequent to the open design were compensated for by the use of static RF shimming. High-resolution and real-time imaging was performed in human volunteers. Results: The presented array showed good performance with regard to patient comfort and image quality. High acceleration factors of up to 4 are feasible without visible acceleration artifacts. Reasonable image homogeneity was achieved with RF shimming. Conclusions: Open, noncylindrical designs for transmit/receive coils are practical at 7 T and real-time imaging of the moving joint is feasible with the presented coil design. Y1 - 2011 U6 - http://dx.doi.org/10.1118/1.3553399 SN - 2473-4209 VL - 38 IS - 3 SP - 1162 EP - 1167 PB - Wiley CY - Hoboken ER -