B0 & B1 Mapping
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Thursday 10 May 2012
Room 210-211  13:30 - 15:30 Moderators: Laura Sacolick, Kyunghyun Sung

13:30 0604.   
Relative B1+ mapping directly from k-space for rapid Multi-Transmit calibration
Francesco Padormo1, Shaihan J. Malik1, and Jo V. Hajnal1
1Imaging Sciences Department, MRC Clinical Sciences Centre, Hammersmith Hospital, Imperial College, London, United Kingdom

We present a method of obtaining relative multi-channel B1+ information directly from k-space measurements. This approach, which is based on ideas from the SPIRIT technique, is fast, low in SAR and produces maps free of anatomy and noise. The method can also extract full receive coil profiles form the same data. We demonstrate that this technique is capable of full 3D calibration of a whole body 8 channel transmit RF system in 14s plus the time to acquire a single channel B1+ map. Up to 8 receiver channels can be calibrated using the same processing with no further data acquisition.

13:42 0605.   DREAM - A Novel Approach for Robust, Ultra-Fast, Multi-Slice B1 Mapping
Kay Nehrke1, and Peter Börnert1
1Philips Research Laboratories, Hamburg, Germany

A novel multi-slice B1 mapping approach dubbed DREAM (Dual Refocusing Echo Acquisition Mode) is proposed, which derives a 2D B1 map from a single, ultra-short acquisition of about 130 ms duration, which is more than an order of magnitude faster than most existing B1 mapping techniques. Moreover, the B1 phase and B0 are delivered additionally for free. The performance of the approach is demonstrated in vivo by B1 mapping experiments of brain and abdomen at 3T.

13:54 0606.   Very fast volumetric B1+ mapping at 7 Tesla using DREAM
Peter Börnert1, Kay Nehrke1, Maarten Versluis2, and Andrew Webb2
1Philips Research Laboratories, Hamburg, Germany, 2C.J. Gorter Center for high field MRI, Leiden University Medical Center, Department of Radiology, Leiden, Netherlands

With the move towards higher fields, RF homogeneity problems caused by wave propagations effects have become obvious, which can compromise clinical diagnosis. The knowledge about the actual B1+ field is a prerequisite for compensation and can also be used to estimate electric tissue parameters and E-field components and SAR at ultra high fields. All known B1+ mapping techniques are inefficient hampering application especially in parallel transmit set-ups. To overcome these limitations, in this work, a new, very fast, simple and safe B1+ mapping approach for ultra-high field imaging is introduced allowing volumetric B1+ brain mapping in less than 10s.

14:06 0607.   Adiabatic pulse design for Bloch-Siegert B1+ Mapping
Mohammad Mehdi Khalighi1, Brian K Rutt2, and Adam B Kerr3
1Global Applied Science Lab, GE Healthcare, Menlo Park, California, United States, 2Radiology Deaprtment, Stanford University, Stanford, California, United States, 3Electrical Engineering Department, Stanford University, Stanford, California, United States

B1+ mapping by the Bloch-Siegert (B-S) method has been shown to be fast and accurate; however, it suffers from high SAR and long TE. We have developed a new adiabatic B-S RF pulse design method, which achieves more B-S B1+ measurement sensitivity for a given pulse width, SAR and T2* than previous B-S pulse designs. A 2ms adiabatic B-S pulse generates 2.5 times more angle to noise ratio maps in the brain compared to 6ms conventional Fermi pulse with the same SAR. The adiabatic B-S pulse performance was validated both in phantoms and in vivo.

14:18 0608.   
Analysis of B1 mapping by Bloch Siegert Shift
Esra Abaci Turk1,2, Yusuf Ziya Ider1, and Ergin Atalar1,2
1Electrical and Electronics Engineering Department, Bilkent University, Ankara, Turkey, 2UMRAM, Bilkent University, Ankara, Turkey

In this study, B1 mapping by the Bloch-Siegert shift is analyzed with simulations and experiments. The importance of the pulse duration and the crusher gradients are investigated. It is shown that the off-resonance pulse duration is necessary for an accurate B1 mapping and also crusher gradients have to be used in order to remove the echo originating from tilting off-slice spins by the off-resonance pulse.

14:30 0609.   Implementation and Validation of Fast Whole-Brain B1 Mapping Based on Bloch-Siegert Shift and EPI Readout
Qi Duan1, Souheil J. Inati2, Peter van Gelderen1, Sunil Patil3, and Jeff H. Duyn1
1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States, 2Functional MRI Facility, NIMH, National Institutes of Health, Bethesda, MD, United States, 3Center for Applied Medical Imaging, Siemens Corporation, Corporate Research, Baltimore, MD, United States

Mitigation of transmit field inhomogeneity at high field greatly benefits from subject specific B1 mapping. However, the generally long scan times of mapping techniques limit their practical use. Here, modifications to the Bloch Siegert (BS) B1 mapping method are implemented to allow whole brain B1mapping within 40s. The effectiveness of these modifications, including an improved gradient scheme, improved BS pulse, and EPI readout, is demonstrated in phantoms and human brain.

14:42 0610.   Simultaneous Mapping of B1 and Flip Angle by Combined Bloch-Siegert, Actual Flip-angle Imaging (BS-AFI)
Samuel A. Hurley1, Pouria Mossahebi2,3, Kevin M. Johnson1, and Alexey A. Samsonov3
1Medical Physics, University of Wisconsin, Madison, WI, United States, 2Biomedical Engineering, University of Wisconsin, Madison, WI, United States,3Radiology, University of Wisconsin, Madison, WI, United States

Methods to map B1 and flip angle are often used interchangeably without regard to the subtle but important differences between these two parameters. We present a novel combination of Actual Flip-angle Imaging (AFI) and Bloch-Siegert B1 mapping to measure them simultaneously, and demonstrate how this can improve the accuracy of quantitative magnetization transfer (MT) measurements.

14:54 0611.   
Bias in Breast B0 mapping; shimming lipid rich parts of the body at 7T
Vincent Oltman Boer1, Mariska P Luttje1, Peter R Luijten1, and Dennis W.J. Klomp1
1Radiology, UMC Utrecht, Utrecht, Utrecht, Netherlands

B0 field maps can be obtained rapidly using dual echo gradient echo sequences. However in lipid rich tissue this is complicated due to the presence of multiple spectral components, hence requiring many more echoes and thereby increasing scantime. Here we demonstrate that by incorporating the seven most intense lipid resonances in the breast, in-phase echo times can be obtained enabling B0 field mapping with dual echo gradient echo imaging in the human breast at 7T without a bias in lipid tissue.

15:06 0612.   Rapid Slice-by-Slice Calculation of Susceptibility-Induced B0 Map in the Fourier Domain permission withheld
Seung-Kyun Lee1, and Ileana Hancu1
1GE Global Research, Niskayuna, NY, United States

We demonstrate a 2D Fourier-based method to rapidly calculate susceptibility-induced B0 inhomogeneity in individual slices. Compared to the 3D Fourier method, this method has dramatically reduced computer memory requirement and attains comparable speed of calculation when B0 map is desired in a small number of slices, as is often the case in automatic shim decision. The method is applied to calculation of susceptibility-induced B0 field in an axial bilateral breast slice based on susceptibility-segmented 3D anatomical image of a female volunteer.

15:18 0613.   A method for efficient and robust estimation of low noise, high dynamic range B0 maps
Joseph Dagher1, Ali Bilgin2, Timothy Reese3, and Georges El Fakhri1
1Nuclear Medicine and Molecular Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States, 2Departments of Biomedical Engineering and, Electrical and Computer Engineering, University of Arizona, Tucson, AZ, United States, 3Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States

We propose a joint acquisition-processing solution to the problem of field map estimation. Our optimizer carefully chooses three echo times that guarantee robust and accurate field map estimation using the reconstruction algorithm over an arbitrary spectral range of inhomogeneity values. We show that our method is not subject to the traditional accuracy-robustness trade-off. The resulting implications include: improved robustness by removing the bound on the shortest echo time difference, enhanced spectral estimation over a large dynamic range of inhomogeneity values, and eliminating the need for phase unwrapping. Phantom experiments confirm these conclusions.