ISMRM 21st Annual Meeting & Exhibition ○ 20-26 April 2013 ○ Salt Lake City, Utah, USA
SCIENTIFIC SESSION
B1 Mapping & Corrections Tuesday 23 April 2013
Room 355 EF | 16:00 - 18:00 | Moderators: | Vasily L. Yarnykh, Xiaohong Joe Zhou
16:00 | 0365. | DREAM B1+ Mapping for Short-T2 and -T2* Components Kay Nehrke1, Maarten J. Versluis2, Andrew Webb2, and Peter Börnert1,2 1Philips Research Laboratory, Hamburg, Germany, 2Leiden University Medical Center, Leiden, Netherlands A new acquisition scheme based on virtual stimulated echoes has been developed for the DREAM B1+ mapping approach, which allows compensation of both, T2and T2*, effects. The new option has been studied on phantoms and in vivo at 7T. For the phantom experiments, rubber phantoms were employed, showing that the sequence allows accurate B1+ mapping for a short T2 in the order of a millisecond. For the in vivo experiments, B1+ maps of the brain were acquired, showing that the contrast between brain tissue and CSF is reduced for the new sequence option.
16:12 | 0366. | 3D B1 Mapping for Short T2* Spins Using Radial Gradient Echo with Gradient Offset Independent Adiabaticity Naoharu Kobayashi1 and Michael Garwood2 1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, United States, 2University of Minnesota, Minneapolis, Minnesota, United States Obtaining the B1 map is necessary for a number of MRI methods, including T1 measurement. We introduce a new approach to actual flip angle imaging (AFI) that achieves 3D B1 mapping even when T2* is short. The approach utilizes COncurrent Dephasing and Excitation (CODE) with Gradient Offset Independent Adiabaticity (GOIA). CODE is a short TE radial GRE sequence that is relatively robust compared to some other ultra-short TE sequences. The use of GOIA pulses in CODE significantly reduces the peak power and SAR.
16:24 | 0367. | Hybrid Excitation and Bloch-Siegert Encoding Pulses for Short-TE 3D |B1+| Mapping Marcin Jankiewicz1 and William A. Grissom2 1MRC/UCT Medical Imaging Research Unit, Department of Human Biology, University of Cape Town, Observatory, Western Cape, South Africa, 2Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, United States We introduce a new hybrid excitation and off-resonance B1+-encoding pulse that enables Bloch-Siegert B1+ mapping with a much shorter TE, enabling mapping of short-T2 species.
16:36 | 0368. | Unexpected Lateral Asymmetry in TSE Image Contrast Explained: Tissues with Short T2 Show Extreme Sensitivity to B1 Inhomogeneity Jaco J.M. Zwanenburg1, Fredy Visser1,2, Jeroen Hendrikse1, and Peter R. Luijten1 1Radiology, University Medical Center Utrecht, Utrecht, Netherlands, 2Philips Healthcare, Best, Netherlands In heavily T2-weighted FLAIR images at 7T, a lateral asymmetry was observed in the contrast of the pulvinar, a nucleus of the thalamus. Due to the small distance between the bilateral pulvinars, B1 inhomogeneity seemed unlikely as explanation for the asymmetry in contrast. In this work, however, we show with simulations and experiments that TSE trains exhibit a highly T2 dependent sensitivity to B1 inhomogeneity, which leads to asymmetry in image contrast (rather than in signal intensities alone). This finding urges for careful interpretation of TSE images (also at clinical field strengths), and highlights the need for precise B1 shimming.
16:48 | 0369. | Multiband Spokes Pulses and Design Algorithm for B1+ Inhomogeneity-Compensated Multislice Excitation at 7T Anuj Sharma1, Samantha J. Holdsworth2, Rafael O'Halloran2, Eric Aboussouan2, Anh Tu Van2, Murat Aksoy2, Julian R. Maclaren2, Roland Bammer2, Victor Andrew Stenger3, and William A. Grissom1 1Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, United States, 2Radiology, Stanford University, Stanford, California, United States, 3Medicine, University of Hawaii, Honolulu, Hawaii, United States An approach to designing patient-tailored 3D spokes excitation RF pulses that simultaneously excite multiple slices and compensate transmit RF field inhomogeneity at high static field is presented. An interleaved greedy and local optimization algorithm for spokes pulse design was extended to the multiband spokes pulse design problem. In-vivo results demonstrate the effectiveness of the designed multiband spokes pulses in reducing flip angle inhomogeneities caused by field inhomogeneity at 7T.
17:00 | 0370. | B1 Estimation Using Adiabatic Refocusing: BEAR Kalina V. Jordanova1, Dwight G. Nishimura1, and Adam B. Kerr1 1Electrical Engineering, Stanford University, Stanford, CA, United States A novel phase-based B1 estimation method using adiabatic refocusing (BEAR) is proposed and validated experimentally at 1.5T. The method exhibits a linear relationship between phase and B1 that is insensitive to off-resonance, T1, and T2. Using this method, B1 mapping can be localized to a slice or 3D volume, with a spin-echo acquisition that is appropriate for fast projection measurements. These properties make BEAR an ideal candidate for use in robust transmitter gain calibration of nominally homogeneous volumetric RF coils.
17:12 | 0371. | Spatial Phase Encoding Using a Bloch-Siegert Shift Gradient Ralf Kartäusch1, Florian Fidler1, Toni Drießle1, Thomas Kampf2, Thomas C. Basse-Lüsebrink1,2, Uvo C. Hoelscher1, Peter M. Jakob1,2, and Xavier Helluy2 1MRB Research Center for Magnetic-Resonance-Bavaria, Würzburg, Bavaria, Germany, 2EP5, Universität Würzburg, Würzburg, Bavaria, Germany In this work we introduce a robust RF only spatial encoding method, which is mimicking conventional phase encoding using B0-gradients by exploiting the properties of spatially dependent Bloch-Siegert phase shifts induced by a RF gradient coil. Being a RF only encoding technique it has immunity against eddy currents and very short switching time. The application of BS-gradients are not restricted in principle to spatial phase encoding and could be for example applied to flow or diffusion measurements.
17:24 | 0372. | Improved Bloch-Siegert Based B1 Mapping by Reducing Off-Resonance Shift Qi Duan1, Peter van Gelderen1, and Jeff H. Duyn1 1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, Maryland, United States This abstract investigates fast simultaneous B0/B1+ mapping by Bloch-Siegert shift via lowering the off-resonance frequency of this pulse, as theoretical analysis indicated that the sensitivity of Bloch-Siegert based B1+ mapping can be substantially improved when irradiating closer to resonance. Using optimized irradiation pulse shape and gradient crushers to minimize direct excitation effects, in vivo experiments on human brain at 7T confirmed the improved sensitivity available with this approach operating with peak B1+ much larger than the frequency offset. This improved sensitivity translated into an 80% reduction in B1+ estimation errors, without increasing tissue heating.
17:36 | 0373. | Bloch-Siegert Shift B1 Mapping with Multi-Band Excitation Tiejun Zhao1, Yujuan Zhao2, Anthony DeFranco2, and Tamer S. Ibrahim2 1Siemens Healthcare; Siemens Medical Solutions USA, Inc., Pittsburgh, PA, United States, 2University of Pittsburgh, Pittsburgh, PA, United States Multi-band (MB) excitation for data acquisition was successfully incorporated into the Bloch-Siegert Shift B1 mapping sequence. The B1 maps from the MB excitation were validated against the conventional Bloch-Siegert data acquisition and were found to be highly comparable with conventional Bloch-Siegert method. The SAR reduction with a factor of two and the slice coverage increased by three folds were demonstrated with the MB methods.
17:48 | 0374. | Compensating Temporal B1 Field Inhomogeneities Using Paired Self-Compensated Spin-Lock Pulses Bogdan G. Mitrea1, Axel J. Krafft1, Ruitian Song1, Ralf B. Loeffler1, and Claudia M. Hillenbrand1 1Radiological Sciences, St Jude Children's Research Hospital, Memphis, TN, United States Spin-lock prepared acquisitions generate a new image contrast that provides molecular-level information from biological systems. Because spin-lock pulses are usually long, amplitude changes in the B1 field can occur (e.g. due to thermal effects of the RF amplifier). Such changes can create imaging artifacts that will introduce errors in the T1 quantification. Here, we present an improved approach that uses paired self-compensating spin-lock pulses to reduce artifacts from temporal inhomogeneities of the B1 field while preserving the compensation for B0 inhomogeneities.