RF Pulse Design
Monday 20 April 2009
Room 314 16:30-18:30

Moderators:

 Peter Börnert and Kawin Setsompop
 
16:30  170.

Sequential Optimal Spoke Selection for Spoke Trajectory Based RF Pulses Design in Parallel Excitation
   

Chao Ma1, Dan Xu2, Kevin F. King2, Zhi-Pei Liang1
1Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA; 2Applied Science Laboratory, GE Healthcare, Milwaukee, WI, USA
    A new method to select optimal spoke locations for spoke trajectory based RF pulses design is presented. The problem is formulated as a combinatorial optimization problem, which is then efficiently solved using a greedy algorithm with recursive evaluation of the cost function. Simulation results and experimental results on a 3T GE Excite scanner with two-channel parallel excitation system demonstrate that the proposed method outperformed existing methods with improved computational efficiency.
     
16:42 171. Sparse Parallel Transmit Pulse Design Using Orthogonal Matching Pursuit Method
    Dong Chen1,2, Folkmar Bornemann1, Mika W. Vogel2, Laura I. Sacolick2, Guido Kudielka2, Yudong Zhu3
1Center for Mathematical Science, Technical University of Munich, Munich, Germany; 2Imaging Technologies, GE Global Research Europe, Munich, Germany; 3New York University Langone Medical Center, New York, NY, USA
    Parallel RF transmit offers additional degrees of freedom in excitation pulse design. One application of that is accelerating multidimensional selective excitation by under-sampling the excitation k space. Where to ideally under-sample the k space with optimal sparsity is determined by the target profile, B1 maps and error tolerance, which is known a priori. We propose an adaptive k space sparsifying method, which exploits this prior-knowledge using a greedy-wise algorithm that simultaneously sparsify over all Tx coils and significantly reduces the pulse duration. The method was validated with Bloch simulations and parallel transmit phantom imaging experiments.
     
16:54 172. Coping with Off-Resonance Effects and Gradient Imperfections in Parallel Transmission Experiments
    Johannes Thomas Schneider1,2, Martin Haas2, Jürgen Hennig2, Sven Junge1, Wolfgang Ruhm1, Peter Ullmann1
1Bruker BioSpin MRI GmbH, Ettlingen, Germany; 2Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany
    For parallel spatially-selective excitation / TransmitSENSE the exact matching of the RF pulses and the gradient waveforms to be played out simultaneously is crucial in order to achieve high spatial accuracy and selectivity. In particular, off-resonance effects and gradient-system imperfections may significantly disturb this interplay resulting in poor spatial definition and artifacts. Therefore, in this study we investigated different methods to determine and compensate for such effects. It is shown that using measured global k-space trajectories, off-resonance maps and local phase evolutions for trajectory pre-calibration and accordingly adapted pulse calculations allow very precise excitation despite of experimental imperfections.
     
17:06 173. Parallel Transmission Method for Susceptibility Artifact and B1+ Inhomogeneity Reduction
    Cungeng Yang1, Weiran Deng1, Vijay A. Alagappan2, Lawrence L. Wald2, V. Andrew Stenger1
1Department of Medicine, University of Hawaii, Honolulu, HI, USA; 2A.A. Martinos Center for Biomedical Imaging, Harvard Medical School, Charlestown, MA, USA
    Susceptibility artifacts and B1+ inhomogeneity are major limitations in high field MRI. Although 3D RF pulses have been shown to be useful for reducing B1+ inhomogeneity with parallel transmission, susceptibility artifacts remain a problem. Parallel z-shim is a promising technique for reducing the through-plane signal loss susceptibility artifact without sacrificing temporal resolution and implementation ease. We present a parallel transmission, z-shim 3D tailored RF pulse for simultaneously reducing susceptibility artifacts and B1+ field inhomogeneity. The method is demonstrated in vivo in T2*-weighted human brain imaging at 3T. Reduced susceptibility artifacts and improved B1+ homogeneity is observed.
     
17:18  174. Joint Design of Excitation and Refocusing Pulses for Fast Spin Echo Sequences in Parallel Transmission
    Dan Xu1, Kevin F. King1
1Applied Science Laboratory, GE Healthcare, Waukesha, WI, USA
    Parallel transmission of RF pulses has been shown to be effective in correcting B1 inhomogeneity. However, there has been no fast spin echo application with parallel transmission yet due to the lack of ways of designing parallel transmit pulses that satisfy the CPMG condition. We pose the design of excitation and refocusing pulse pair as a joint design problem under the optimal control framework, where the optimal pulse pair is sought jointly to drive the magnetization vector to its desired state while maintaining the CPMG condition. Bloch simulation results are shown to demonstrate the superior performance of the proposed method.
     
17:30 175. Robust Parallel Excitation Pulse Design
    William Allyn Grissom1, Adam B. Kerr2, Pascal Stang2, Greig Scott2, John Pauly2
1Electrical Engineering and Radiology, Stanford University, Stanford, CA, USA; 2Electrical Engineering, Stanford University, Stanford, CA, USA
    In contrast to parallel receive, parallel excitation requires separate power amplifiers for each channel, which can be cost-prohibitive. Several groups are currently investigating the use of low-cost amplifiers as a solution to this problem. However, these amplifiers can have difficulty tracking steep RF envelope changes. In this work, we demonstrate a regularization technique for parallel excitation pulse design that results in pulses that are robust to amplifier non-idealities, with minimal impact on excitation accuracy. We demonstrate experimentally that excitation accuracy is improved using the new technique compared to unregularized pulses.
   

 
17:42 176. SAR Hotspot Reduction by Temporal Averaging in Parallel Transmission
    Ingmar Graesslin1, Julia Weller1, Ferdinand Schweser2, Bjoern Annighoefer3, Sven Biederer4, Ulrich Katscher1, Tim Nielsen1, Paul Harvey5, Peter Börnert1
1Philips Research Europe, Hamburg, Germany; 2IDIR / University Clinics, Jena, Germany; 3TU Hamburg-Harburg, Hamburg, Germany; 4University of Lübeck, Lübeck, Germany; 5Philips Healthcare, Best, Netherlands
   

This paper presents a novel approach for local SAR reduction by exploitation of the temporal degree of freedom of multi-shot imaging sequences. It is based on successive application of RF pulses with the same target excitation pattern, but different spatial distributions of SAR, levelling out by time averaging.

The concept was validated by simulations. Both computation of RF pulses and spatial SAR distributions were significantly accelerated implementing it on a graphics-processing unit. The local SAR calculation was carried out in real-time for a whole body bio-mesh.
     
17:54 177. Spectral-Spatial Pulse Design for Through-Plane Phase Precompensatory Slice Selection in T2*-Weighted Functional MRI
    Chun-yu Yip1, Daehyun Yoon2, Valur Olafsson2, Sangwoo Lee3, William A. Grissom4, Jeffrey A. Fessler2, Douglas C. Noll5
1A.A. Martinos Center, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, USA; 2Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA; 3GE Healthcare, Waukesha, WI, USA; 4Stanford University, Palo Alto, CA, USA; 5Biomedical Engineering, University of Michigan, Ann Arbor, MI, USA
   

T2*-weighted functional MR images suffer from signal loss artifacts caused by the magnetic susceptibility differences between air cavities and brain tissues.

We propose a novel spectral-spatial pulse design that is slice-selective and capable of mitigating the signal loss.

The two-dimensional spectral-spatial pulses create precompensatory phase variations that counteract through-plane dephasing, relying on the assumption that resonance frequency offset and through-plane field gradient are spatially correlated. The pulses can be precomputed prior to functional MRI experiments and used repeatedly for different slices in different subjects.
     
18:06 178. Design of High-Bandwidth Adiabatic RF Pulses Using the Shinnar Le-Roux Algorithm
   

Priti Balchandani1, John Pauly2, Daniel Spielman1
1Radiology, Stanford University, Stanford, CA, USA; 2Electrical Engineering, Stanford University, Stanford, CA, USA
    We present a new, systematic method for designing adiabatic RF pulses that utilizes the Shinnar Le-Roux (SLR) algorithm for pulse design. By overlaying a sufficient amount of quadratic phase across the spectral profile prior to the inverse SLR transform, we generate RF pulses that exhibit the required spectral characteristics as well as adiabatic behavior. The addition of quadratic phase distributes RF energy more uniformly, resulting in reduced SAR pulses. The method enables the pulse designer to specify spectral profile parameters and the degree of quadratic phase. Simulations and phantom experiments demonstrate that pulses generated using this new method behave adiabatically.
     
18:18 179. A VERSE Algorithm with Additional Acoustic Noise Constraints
    Sebastian Schmitter1, Marco Mueller1, Wolfhard Semmler1, Michael Bock1
1Medical Physics in Radiology, German Cancer Research Center, Heidelberg, Germany
   

In this work a general VERSE algorithm is presented, that reduces both the B1 amplitude (and thus SAR) and the acoustic noise of the gradient for arbitrary RF pulses. The resonance frequencies of the gradient coils are additional constraints in this algorithm. The Fourier transform of the VERSE gradient shape is adapted to these resonance frequencies. The method is demonstrated for SINC rf pulses. A noise reduction of more than 13 dB compared to non-optimized VERSE pulses is measured, whereas the effects of the VERSE algorithm on the slice profile is negligible.