Slice-Selective Transmit Array Pulses for Improvement in Excitation Uniformity and Reduction of SAR
- 1 1Department of Bioengineering, The Pennsylvania State University College of Medicine, Hershey, USA
- 2 3Department of Radiology, Stanford University, Stanford, USA
- 3 2Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, USA
- 4 2Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, USA
- 5 Department of Bioengineering, The Pennsylvania State University College of Medicine, Hershey, USA
- 6 GE Healthcare, Aurora, USA
- 7 Department of Bioengineering, The Pennsylvania State University College of Medicine, Hershey, USA
Abstract
To overcome challenges of inhomogeneous transmit B 1 distribution and high specific energy absorption rate (SAR) in MRI, we compare slice-selective array-optimized composite pulse and RF shimming designed to both improve B 1 uniformity and reduce SAR using an 8-channel transmit head array loaded with a head model at various RF pulse excitation times, and compare results with standard quadrature voltage distribution at 3T (128 MHz) and 7T (300 MHz). The excitation uniformity was estimated throughout the 3D brain region and SAR was calculated for the whole head. The optimized composite pulse could produce significantly better homogeneity and significantly better homogeneity when SAR was not constrained, and both significantly better homogeneity and lower SAR when the pulse duration was allowed to be twice that of the quadrature or RF shimmed pulse. When the total pulse durations were constrained to the same length, the relative advantages of the optimized composite pulse for producing better homogeneity and lower SAR simultaneously were diminished. Using the optimization results, the slice-selective composite pulse sequence was implemented on a 3D MRI simulator currently under development, and showed both effective slice selection and improvement in excitation uniformity compared to a conventional quadrature driving method.
- [1] C. A. van den Berg, B. van den Bergen, H. Kroeze, L. W. Bartels and J. J. Lagendijk, “Simultaneous Homogenisation and SAR Hotspot Suppression by a Phased Array MR Transmit Coil,” Magnetic Resonance in Medicine, Vol. 57, No. 3, 2006, pp. 577-586.
- X. Wu, C. Akgun, J. T. Vaughan, K. Ugurbil and P.-F. Van de Moortele, “SAR Reduction in Transmit SENSE Using Adapted Excitation k-Space Trajectories,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 673.
- X. Wu, C. Akgun, J. T. Vaughan, K. Ugurbil and P.-F. Van de Moortele, “SAR Analysis for Transmit SENSE at 7T with a Human Head Model,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 3350.
- I. Graesslin, S. Biederer, F. Schweser, K.-H. Zimmermann, U. Katscher and P. Bornert, “SAR Reduction for Parallel Transmission Using VERSE and k-Space Filtering,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 674.
- T. S. Ibrahim and L. Tang, “A Whole-Body 7 Tesla RF Excitation Scheme with Much Improved Field Homogeneity and Local/Global SARs over Quadrature Excitation,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 1013.
- Z. Wang, J. C. Lin, W. Mao, W. Liu, M. B. Smith and C. M. Collins, “SAR and Temperature: Simulations and Comparison to Regulatory Limits for MRI,” Journal of Magnetic Resonance Imaging, Vol. 26, No. 2, 2007, pp. 437-441. doi:10.1002/jmri.20977
- Z. Wang, S.-H. Oh, M. B. Smith and C. M. Collins, “RF Shimming Considering both Excitation Homogeneity and SAR,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 1022.
- G. McKinnon, “RF Shimming and SAR Considerations with an Eight-Element 3T Body Coil,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 1085.
- A. C. Zelinski, K. Setsompop, V. Alagappan, B. A. Gagoski, L. M. Angelone, G. Bonmassar, U. Fontius, F. Schmitt, E. Adalsteinsson and L. L. Wald, “Pulse Design Methods for Reduction of Specific Absorption Rate in Parallel RF Excitation,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 1698.
- A. C. Zelinski, V. K. Goyal, L. Angelone, G. Bonmassar, L. L. Wald and E. Adalsteinsson, “Designing RF Pulses with Optimal Specific Absorption Rate (SAR) Characteristics and Exploring Excitation Fidelity, SAR and Pulse Duration Tradeoffs,” Proceedings of the 15th Annual Meeting of ISMRM, Berlin, 19-25 May 2007, p. 1699.