Time-Multiplexed RF Transmission to Improve B1 Homogeneity in High Field MRI
1
조회
0
다운로드
0
인용
0
참고문헌
키워드
초록
To improve B1 homogeneity in high field MRI, the RF power is applied to the transmit array coil elements sequentially in the time-multiplexed way. Since only a single coil element is activated in a time-multiplexing slot, the global standing wave formation in the human body is greatly suppressed. The time-multiplexing slot width is on the order of micro seconds, hence, high-order-harmonic slices can be placed far from the transmit coil and simultaneous multiple slice selection can be avoided. The B1 homogeneities of a birdcage coil and an eight-channel transmit array coil have been compared through finite difference time domain simulations. The simulation results indicate that the proposed technique can reduce the peak-to-peak B1 inhomogeneity down to one fourth of the transmission with a birdcage coil on the central plane of the human head model at 3 T. The mimicking experiments at 3 T, eight separate experiments with a single coil element activated and image reconstruction by combining the eight images, also show promising results. It is expected that the proposed technique has some advantages over other B1 improving methods in real practice since simple RF switching circuitries are only necessary and electromagnetic coupling between the coil elements is out of concern in its realization.
Abstract
To improve B1 homogeneity in high field MRI, the RF power is applied to the transmit array coil elements sequentially in the time-multiplexed way. Since only a single coil element is activated in a time-multiplexing slot, the global standing wave formation in the human body is greatly suppressed. The time-multiplexing slot width is on the order of micro seconds, hence, high-order-harmonic slices can be placed far from the transmit coil and simultaneous multiple slice selection can be avoided. The B1 homogeneities of a birdcage coil and an eight-channel transmit array coil have been compared through finite difference time domain simulations. The simulation results indicate that the proposed technique can reduce the peak-to-peak B1 inhomogeneity down to one fourth of the transmission with a birdcage coil on the central plane of the human head model at 3 T. The mimicking experiments at 3 T, eight separate experiments with a single coil element activated and image reconstruction by combining the eight images, also show promising results. It is expected that the proposed technique has some advantages over other B1 improving methods in real practice since simple RF switching circuitries are only necessary and electromagnetic coupling between the coil elements is out of concern in its realization.
참고문헌 (27)
- T. Vaughan, "9.4T human MRI: preliminary results", Magn. Reson. Med, 56, pp.1274~1282 (2006)
- P. Bottomley, "RF magnetic field penetration, phase shift and power dissipation in biological tissue: implications for NMR imaging", Phys. Med. Biol, 23, pp.630~643 (1978)
- J.T. Vaughan, "High frequency volume coils for clinical NMR imaging and spectroscopy", Magn. Reson. Med, 32, pp.206~218 (1994)
- D.C. Alsop, "A spiral volume coil for improved RF field homogeneity at highstatic magnetic field strength", Magn. Reson. Med, 40, pp.4954~ (1988)
- T.S. Ibrahim, "Effect of RF coil excitation on field inhom- ogeneity at ultra high fields: a field optimized TEM resonator", Magn. Reson. Imag, 19, pp.1339~1347 (2001)
- J.T. Vaughan, "Efficient high-frequency body coil for high-field MRI", Magn.Reson.Med.vol.52pp.851859, 52, pp.851859~ (2004)
- S. Conolly, "A selective adiabatic spin-echo pulse", J. Magn. Reson, 83, pp.324~334 (1989)
- R. Deichmann, "RF inhomogeneity compensation in structural brain imaging", Magn. Reson. Med, 47, pp.398~402 (2002)
- S. Saekho, "Small tip angle three-dimensional tailored radiofrequency slab-select pulse for reduced B1 inhomogeneity at 3T", Magn. Reson. Med, 53, pp.479~484 (2005)
- S. Saekho, "Fast-kz three-dimensional tailored radiofrequency pulse for reduced B1 inhomogeneity", Magn. Reson. Med, 55, pp.719~724 (2006)
- W. Mao, "Exploring the limits of RF shimming for high-field MRI of the human head", Magn. Reson. Med, 56, pp.918922~ (2006)
- C.M. Collins, "Combination of optimized transmit arrays and some receive array reconstr- uction methods can yield homogeneous imagesat very high frequencies", Magn. Reson. Med, 54, pp.1327~1332 (2005)
- Y.D.Zhu, "Parallel excitation with an array of transmit coils", Magn. Reson. Med, 51, pp.775~784 (2004)
- P. Ullmann, "Experimental analysis of parallel excitation using dedicated coil setups and simultaneous RF transmission on multiple channels", Magn. Reson. Med, 54, pp.994~1001 (2005)
- U. Katscher, "Transmit SENSE", Magn. Reson. Med, 49, pp.144~150 (2003)
- U. Katscher, "Theoretical and numerical aspects of transmit SENSE", IEEE Trans. Med. Imaging, 23, pp.520~525 (2004)
- K. Setsompop, "Parallel RF transmission with eight channels at 3 Tesla", Magn. Reson. Med, 56, pp.1163~1171 (2006)
- Z. Zhang, "Reduction of transmitter B1 inhomogeneity with transmit SENSE slice-select pulses", Magn. Reson. Med, 57, pp.842~847 (2007)
- C.M. Collins, "Array-optimized composite pulse for excellent whole-brain homogeneity in high-field MRI", Magn. Reson. Med, 57, pp.470~474 (2007)
- C.M. Collins, "Strengths and limitations of pulsing coils in an array sequentially to avoid RF interference in high field MRI", in Proc.13th Annual Meeting of ISMRM, Miami Beach, FL, USA, pp.816~ (2005)
- C.M. Collins, "Multi-coil composite pulses for whole-brain homogeneity improved over RF shimming alone", in Proc.14th Annual Meeting of ISMRM, Seattle, USA, pp.702~ (2006)
- D.I.Hoult, "The principle of reciprocity in signal strength calculations-A mathematical guide", Concepts Magn. Reson, 12, pp.173~187 (2000)
- J. Wang, "RF shimming considering both excitation homogeneity and SAR", in Proc. 15th Annual Meeting of ISMRM, Berlin, Germany, pp.1022~ (2007)
- A.C. Zelinski, "Designing RF pulses with optimal specific absorption rate (SAR) characteristics and exploring excitation fidelity, SAR and duration tradeoffs", in Proc. 15th Annual Meeting of ISMRM, Berlin, Germany, pp.1699~ (2007)
- X. Wu, "SAR analysis for transmit SENSE at 7T with a human head model", in Proc. 15th Annual Meeting of ISMRM, Berlin, Germany, pp.3350~ (2007)
- W. Mao, "Calculation of SAR for transmit coil arrays", Concepts Magn. Reson. Part B, 31B, pp.127~131 (2007)
- C.A.T. Van den Berg, "Simul- taneous B1+ homogenization and specific absorption rate hotspot suppression using a magnetic resonance phase array transmit coil", Magn. Reson. Med, 57, pp.577~586 (2007)