TY - JOUR A1 - Schlamann, M. A1 - Yoon, M.-S. A1 - Maderwald, S. A1 - Pietrzyk, T. A1 - Bitz, Andreas A1 - Gerwig, M. A1 - Forsting, M. A1 - Ladd, S. C. A1 - Ladd, M. E. A1 - Kastrup, O. T1 - Auswirkungen der Magnetresonanztomografie auf die Elektrophysiologie des motorischen Kortex: eine Studie mit transkranieller Magnetstimulation T1 - Effects of MRI on the electrophysiology of the motor cortex: a TMS study JF - RöFo - Fortschritte auf dem Giebiet der Röntgenstrahlen und der bildgebenden Verfahren Y1 - 2009 U6 - https://doi.org/10.1055/s-0028-1109038 SN - 1438-9029 VL - 181 IS - 3 SP - 215 EP - 219 PB - Thieme CY - Stuttgart ER - TY - JOUR A1 - Theysohn, Jens M. A1 - Kraff, Oliver A1 - Eilers, Kristina A1 - Andrade, Dorian A1 - Gerwig, Marcus A1 - Timmann, Dagmar A1 - Schmitt, Franz A1 - Ladd, Mark E. A1 - Ladd, Susanne C. A1 - Bitz, Andreas T1 - Vestibular effects of a 7 Tesla MRI examination compared to 1.5 T and 0 T in healthy volunteers JF - PLoS one N2 - Ultra-high-field MRI (7 Tesla (T) and above) elicits more temporary side-effects compared to 1.5 T and 3 T, e.g. dizziness or “postural instability” even after exiting the scanner. The current study aims to assess quantitatively vestibular performance before and after exposure to different MRI scenarios at 7 T, 1.5 T and 0 T. Sway path and body axis rotation (Unterberger's stepping test) were quantitatively recorded in a total of 46 volunteers before, 2 minutes after, and 15 minutes after different exposure scenarios: 7 T head MRI (n = 27), 7 T no RF (n = 22), 7 T only B₀ (n = 20), 7 T in & out B₀ (n = 20), 1.5 T no RF (n = 20), 0 T (n = 15). All exposure scenarios lasted 30 minutes except for brief one minute exposure in 7 T in & out B₀. Both measures were documented utilizing a 3D ultrasound system. During sway path evaluation, the experiment was repeated with eyes both open and closed. Sway paths for all long-lasting 7 T scenarios (normal, no RF, only B₀) with eyes closed were significantly prolonged 2 minutes after exiting the scanner, normalizing after 15 minutes. Brief exposure to 7 T B₀ or 30 minutes exposure to 1.5 T or 0 T did not show significant changes. End positions after Unterberger's stepping test were significantly changed counter-clockwise after all 7 T scenarios, including the brief in & out B₀ exposure. Shorter exposure resulted in a smaller alteration angle. In contrast to sway path, reversal of changes in body axis rotation was incomplete after 15 minutes. 1.5 T caused no rotational changes. The results show that exposure to the 7 Tesla static magnetic field causes only a temporary dysfunction or “over-compensation” of the vestibular system not measurable at 1.5 or 0 Tesla. Radiofrequency fields, gradient switching, and orthostatic dysregulation do not seem to play a role. Y1 - 2014 U6 - https://doi.org/10.1371/journal.pone.0092104 SN - 1932-6203 VL - 9 IS - 3 PB - PLOS CY - San Francisco ER - TY - JOUR A1 - Umutlu, L. A1 - Maderwald, S. A1 - Kinner, S. A1 - Kraff, O. A1 - Bitz, Andreas A1 - Orzada, S. A1 - Johst, S. A1 - Wrede, K. A1 - Forsting, M. A1 - Ladd, M. E. A1 - Lauenstein, T. C. A1 - Quick, H. H. T1 - First-pass contrast-enhanced renal MRA at 7 Tesla: initial results JF - European Radiology Y1 - 2013 U6 - https://doi.org/10.1007/s00330-012-2666-0 SN - 1432-1084 VL - 23 IS - 4 SP - 1059 EP - 1066 PB - Springer CY - Berlin ER - TY - JOUR A1 - Fiedler, Thomas M. A1 - Ladd, Mark E. A1 - Clemens, Markus A1 - Bitz, Andreas T1 - Safety of subjects during radiofrequency exposure in ultra-high-field magnetic resonance imaging JF - IEEE Letters on Electromagnetic Compatibility Practice and Applications N2 - Magnetic resonance imaging (MRI) is one of the most important medical imaging techniques. Since the introduction of MRI in the mid-1980s, there has been a continuous trend toward higher static magnetic fields to obtain i.a. a higher signal-to-noise ratio. The step toward ultra-high-field (UHF) MRI at 7 Tesla and higher, however, creates several challenges regarding the homogeneity of the spin excitation RF transmit field and the RF exposure of the subject. In UHF MRI systems, the wavelength of the RF field is in the range of the diameter of the human body, which can result in inhomogeneous spin excitation and local SAR hotspots. To optimize the homogeneity in a region of interest, UHF MRI systems use parallel transmit systems with multiple transmit antennas and time-dependent modulation of the RF signal in the individual transmit channels. Furthermore, SAR increases with increasing field strength, while the SAR limits remain unchanged. Two different approaches to generate the RF transmit field in UHF systems using antenna arrays close and remote to the body are investigated in this letter. Achievable imaging performance is evaluated compared to typical clinical RF transmit systems at lower field strength. The evaluation has been performed under consideration of RF exposure based on local SAR and tissue temperature. Furthermore, results for thermal dose as an alternative RF exposure metric are presented. Y1 - 2020 SN - 2637-6423 U6 - https://doi.org/10.1109/LEMCPA.2020.3029747 VL - 2 IS - 3 SP - 1 EP - 8 PB - IEEE CY - New York, NY ER - TY - CHAP A1 - Bitz, Andreas A1 - Kraff, O. A1 - Orzada, S. A1 - Maderwald, S. A1 - Brote, I. A1 - Ladd, M. T1 - Experimental and Numerical Assessment of RF Safety of Transmit Coils at 7 Tesla T2 - ISMRM workshop on MR safety 2010 : RF heating of the human in MRI : workshop series. The Washington County Historic Courthouse, Stillwater, Minnesota, USA, 15 - 17 October 2010 Y1 - 2010 SN - 978-1-62276-088-6 SP - 195 ER - TY - JOUR A1 - Orzada, Stephan A1 - Solbach, Klaus A1 - Gratz, Marcel A1 - Brunheim, Sascha A1 - Fiedler, Thomas M. A1 - Johst, Sören A1 - Bitz, Andreas A1 - Shooshtary, Samaneh A1 - Abuelhaija, Asjraf A1 - Voelker, Maximilian N. A1 - Rietsch, Stefan H. G. A1 - Kraff, Oliver A1 - Maderwald, Stefan A1 - Flöser, Martina A1 - Oehmingen, Mark A1 - Quick, Harald H. A1 - Ladd, Mark E. T1 - A 32-channel parallel transmit system add-on for 7T MRI JF - Plos one Y1 - 2019 U6 - https://doi.org/10.1371/journal.pone.0222452 ER - TY - JOUR A1 - Rietsch, Stefan H. G. A1 - Pfaffenrot, Viktor A1 - Bitz, Andreas A1 - Orzada, Stephan A1 - Brunheim, Sascha A1 - Lazik-Palm, Andrea A1 - Theysohn, Jens M. A1 - Ladd, Mark E. A1 - Quick, Harald H. A1 - Kraff, Oliver T1 - An 8-channel transceiver 7-channel receive RF coil setup for high SNR ultrahigh-field MRI of the shoulder at 7T JF - Medical Physics Y1 - 2017 U6 - https://doi.org/10.1002/mp.12612 SN - 0094-2405 IS - Article in press PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Orzada, Stephan A1 - Bitz, Andreas A1 - Johst, Sören A1 - Gratz, Marcel A1 - Völker, Maximilian N. A1 - Kraff, Oliver A1 - Abuelhaija, Ashraf A1 - Fiedler, Thomas M. A1 - Solbach, Klaus A1 - Quick, Harald H. A1 - Ladd, Mark E. T1 - Analysis of an integrated 8-Channel Tx/Rx body array for use as a body coil in 7-Tesla MRI JF - Frontiers in Physics Y1 - 2017 U6 - https://doi.org/10.3389/fphy.2017.00017 SN - 2296-424X N1 - Article number 17 VL - 5 IS - Jun ER - TY - JOUR A1 - Orzada, Stephan A1 - Fiedler, Thomas M. A1 - Bitz, Andreas A1 - Ladd, Mark E. A1 - Quick, Harald H. T1 - Local SAR compression with overestimation control to reduce maximum relative SAR overestimation and improve multi-channel RF array performance JF - Magnetic Resonance Materials in Physics, Biology and Medicine N2 - Objective In local SAR compression algorithms, the overestimation is generally not linearly dependent on actual local SAR. This can lead to large relative overestimation at low actual SAR values, unnecessarily constraining transmit array performance. Method Two strategies are proposed to reduce maximum relative overestimation for a given number of VOPs. The first strategy uses an overestimation matrix that roughly approximates actual local SAR; the second strategy uses a small set of pre-calculated VOPs as the overestimation term for the compression. Result Comparison with a previous method shows that for a given maximum relative overestimation the number of VOPs can be reduced by around 20% at the cost of a higher absolute overestimation at high actual local SAR values. Conclusion The proposed strategies outperform a previously published strategy and can improve the SAR compression where maximum relative overestimation constrains the performance of parallel transmission. Y1 - 2020 SN - 1352-8661 U6 - https://doi.org/10.1007/s10334-020-00890-0 IS - 34 (2021) SP - 153 EP - 164 PB - Springer CY - Heidelberg ER - TY - JOUR A1 - Orzada, S. A1 - Maderwald, S. A1 - Poser, B. A. A1 - Johst, S. A1 - Kannengiesser, S. A1 - Ladd, M. E. A1 - Bitz, Andreas T1 - Time-interleaved acquisition of modes: an analysis of SAR and image contrast implications JF - Magnetic Resonance in Medicine N2 - s the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time-interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different Burn:x-wiley:07403194:media:MRM23081:tex2gif-stack-1 modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in Burn:x-wiley:07403194:media:MRM23081:tex2gif-stack-2 homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T2, and T2*-weighted imaging and, if a specified homogeneity criterion is met, in T1-weighted imaging as well. Y1 - 2012 U6 - https://doi.org/10.1002/mrm.23081 SN - 1522-2594 VL - 67 IS - 4 SP - 1033 EP - 1041 PB - Wiley-Liss CY - New York ER - TY - JOUR A1 - Kraff, Oliver A1 - Bitz, Andreas A1 - Kruszona, Stefan A1 - Orzada, Stephan A1 - Schaefer, Lena C. A1 - Theysohn, Jens M. A1 - Maderwald, Stefan A1 - Ladd, Mark E. A1 - Quick, Harald H. T1 - An eight-channel phased array RF coil for spine MR imaging at 7 T JF - Investigative Radiology Y1 - 2009 U6 - https://doi.org/10.1097/RLI.0b013e3181b24ab7 SN - 1536-0210 VL - 44 IS - 11 SP - 734 EP - 740 PB - Lippincott Williams & Wilkins ER - TY - JOUR A1 - Fiedler, Thomas M. A1 - Ladd, Mark E. A1 - Bitz, Andreas T1 - SAR Simulations & Safety JF - NeuroImage Y1 - 2017 U6 - https://doi.org/10.1016/j.neuroimage.2017.03.035 SN - 1053-8119 IS - Epub ahead of print PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Fiedler, Thomas M. A1 - Ladd, Mark E. A1 - Bitz, Andreas T1 - RF safety assessment of a bilateral four-channel transmit/receive 7 Tesla breast coil: SAR versus temperature limits JF - Medical Physics Y1 - 2017 U6 - https://doi.org/10.1002/mp.12034 N1 - This article is corrected by: Errata: Erratum: “RF safety assessment of a bilateral four-channel transmit/receive 7 Tesla breast coil: SAR versus tissue temperature limits” [Med. Phys. 44(1), 143–157 (2017)] Volume 44, Issue 2, 772 VL - 44 IS - 1 SP - 143 EP - 157 ER - TY - JOUR A1 - Fiedler, Thomas M. A1 - Orzada, Stephan A1 - Flöser, Martina A1 - Rietsch, Stefan H. G. A1 - Schmidt, Simon A1 - Stelter, Jonathan K. A1 - Wittrich, Marco A1 - Quick, Harald H. A1 - Bitz, Andreas A1 - Ladd, Mark E. T1 - Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specificabsorption rate, tissue temperature, and thermal dose JF - NMR in Biomedicine N2 - In this study, the performance of an integrated body-imaging array for 7 T with 32 radiofrequency (RF) channels under consideration of local specific absorption rate (SAR), tissue temperature, and thermal dose limits was evaluated and the imaging performance was compared with a clinical 3 T body coil. Thirty-two transmit elements were placed in three rings between the bore liner and RF shield of the gradient coil. Slice-selective RF pulse optimizations for B1 shimming and spokes were performed for differently oriented slices in the body under consideration of realistic constraints for power and local SAR. To improve the B1+ homogeneity, safety assessments based on temperature and thermal dose were performed to possibly allow for higher input power for the pulse optimization than permissible with SAR limits. The results showed that using two spokes, the 7 T array outperformed the 3 T birdcage in all the considered regions of interest. However, a significantly higher SAR or lower duty cycle at 7 T is necessary in some cases to achieve similar B1+ homogeneity as at 3 T. The homogeneity in up to 50 cm-long coronal slices can particularly benefit from the high RF shim performance provided by the 32 RF channels. The thermal dose approach increases the allowable input power and the corresponding local SAR, in one example up to 100 W/kg, without limiting the exposure time necessary for an MR examination. In conclusion, the integrated antenna array at 7 T enables a clinical workflow for body imaging and comparable imaging performance to a conventional 3 T clinical body coil. KW - body imaging at 7 T MRI KW - thermal dose KW - tissue temperature KW - transmit antenna arrays Y1 - 2022 U6 - https://doi.org/10.1002/nbm.4656 SN - 0952-3480 (Print) SN - 1099-1492 (Online) VL - 35 IS - 5 SP - 1 EP - 17 PB - Wiley ER - TY - JOUR A1 - Fiedler, Thomas M. A1 - Orzada, Stephan A1 - Flöser, Martina A1 - Rietsch, Stefan H. G. A1 - Quick, Harald H. A1 - Ladd, Mark E. A1 - Bitz, Andreas T1 - Performance analysis of integrated RF microstrip transmit antenna arrays with high channel count for body imaging at 7 T JF - NMR in Biomedicine N2 - The aim of the current study was to investigate the performance of integrated RF transmit arrays with high channel count consisting of meander microstrip antennas for body imaging at 7 T and to optimize the position and number of transmit ele- ments. RF simulations using multiring antenna arrays placed behind the bore liner were performed for realistic exposure conditions for body imaging. Simulations were performed for arrays with as few as eight elements and for arrays with high channel counts of up to 48 elements. The B1+ field was evaluated regarding the degrees of freedom for RF shimming in the abdomen. Worst-case specific absorption rate (SARwc ), SAR overestimation in the matrix compression, the number of virtual obser- vation points (VOPs) and SAR efficiency were evaluated. Constrained RF shimming was performed in differently oriented regions of interest in the body, and the devia- tion from a target B1+ field was evaluated. Results show that integrated multiring arrays are able to generate homogeneous B1+ field distributions for large FOVs, espe- cially for coronal/sagittal slices, and thus enable body imaging at 7 T with a clinical workflow; however, a low duty cycle or a high SAR is required to achieve homoge- neous B1+ distributions and to exploit the full potential. In conclusion, integrated arrays allow for high element counts that have high degrees of freedom for the pulse optimization but also produce high SARwc , which reduces the SAR accuracy in the VOP compression for low-SAR protocols, leading to a potential reduction in array performance. Smaller SAR overestimations can increase SAR accuracy, but lead to a high number of VOPs, which increases the computational cost for VOP evaluation and makes online SAR monitoring or pulse optimization challenging. Arrays with interleaved rings showed the best results in the study. KW - body imaging at UHF MRI KW - integrated transmit coil arrays KW - VOP compression Y1 - 2021 U6 - https://doi.org/10.1002/nbm.4515 SN - 0952-3480 (ISSN) SN - 1099-1492 (eISSN) VL - 34 IS - 7 PB - Wiley CY - Weinheim ER - TY - CHAP A1 - Bitz, Andreas A1 - Klomp, D.W. A1 - Ladd, M.E. T1 - Experimental and numerical determination of SAR and temperature distribution of a human endorectal coil for MR imaging of the prostate at 7T (903.) T2 - 16th annual ISMRM scientific meeting and exhibition 2008 : Toronto, Ontario, Canada, 3 - 9 May 2008 Y1 - 2009 SN - 978-1-61567-196-0 PB - Curran CY - Red Hook, NY ER -