It's All RF: Pulses, Trains, Sweeps & Feedback

Room 718 A


Chairs: Charles H. Cunningham and Jürgen K. Hennig


Prog #

10:30 225. Young Investigator Award Finalist: Slice-Selective Tunable-Flip AdiaBatic Low Peak-Power Excitation (STABLE) Pulse

Priti Balchandani1, John Pauly1, Daniel Mark Spielman1

1Stanford University, Stanford, California , USA

Adiabatic pulses are useful in achieving uniform excitation profiles in the presence of B1 inhomogeneity.  Gradient modulation techniques for slice-selective adiabatic excitation have been introduced; however, the resultant pulses exceed the RF amplifier limits for most human scanners. We present an alternative gradient modulated approach for pulse design that achieves adiabatic slice-selection with significantly lower RF peak power requirements.  Our Slice-selective Tunable-flip AdiaBatic Low peak-power Excitation (STABLE) pulse consists of an oscillating gradient in conjunction with a BIR-4-like RF envelope that is sampled by many spatial subpulses.  Phantom and in vivo results demonstrate the adiabaticity and selectivity of the STABLE pulse.

10:50 226. Multiband Spectral-Spatial Design for High-Field and Hyperpolarized C-13 Applications

Adam Bruce Kerr1, Peder E.Z. Larson2, Michael Lustig1, Charles H. Cunningham3, Albert P. Chen2, Daniel B. Vigneron2, John Mark Pauly1

1Stanford University, Stanford, California , USA; 2UCSF, San Francisco, California , USA; 3U Toronto, Toronto, Canada

A novel approach for multiband spectral-spatial design  that iterates over feasible spectral sampling frequencies to determine the best design according to minimum-time, B1 or power criteria is presented.  SLR RF design is combined with FIR filter design based on convex optimization and spectral factorization to minimize the energy in transition and don’t-care regions.  Chemical-shift misregistration correction is also extended to support large-tip designs.  Examples appropriate for spectroscopic imaging of C-13 pyruvate metabolites or C-13 lactate imaging at 3T are presented and validated.

11:02 227. A Spiral Spectral-Spatial RF Pulse with Rotated Variable Density K-Space Trajectory

Weiran Deng1, Victor Andrew Stenger1

1University of Hawaii, Honolulu, Hawaii, USA

A spectral-spatial RF pulse with variable density spirals improve 2D spatial localization at the expense of increased aliasing outside the excitation field of view. By periodically rotating gradients, we found that slice aliasing can be significantly with in-vivo images.

11:14 228. SAR-Constrained Saturation Pulse Designs Based on B0 and B1 Maps

Kyunghyun Sung1, Krishna S. Nayak1

1University of Southern California, Los Angeles, California , USA

Complete and uniform saturation are important for quantitative cardiac imaging. The saturation effectiveness is sensitive to the homogeneities of both the static (B0) and radiofrequency (B1) magnetic fields. The B0 and B1 variation across the left ventricle has been previously measured and a train of weighted hard pulses was chosen to optimally saturate over the expected region in B0-B1 space (minimizing the average residual Mz). In in-vivo studies, compared to BIR-4 and conventional pulse train saturation, the proposed saturation pulse showed the lowest residual Mz over the left ventricle with p < 0.001.

11:26 229. Correction of RF Pulse Distortions, with Application in Radial Imaging Using SWIFT

Steen Moeller1, Curt Corum1, Djaudat Idiyatullin1, Ryan Chamberlein1, Michael Garwood1

1University of Minnesota, Minneapolis, Minnesota, USA

SWIFT (SWeep Imaging with Fourier Transform) is a new 3D radial sequence, utilizing frequency sweept pulses for simultaneous excitation and reception. Consistent Pertubations in the RF waveform different from the digital waveform creates bullseye artifact. A datadriven correction is proposed that effectively removes all of these pertubations. 3D Whole head SWIFT images is used to demonstrate the effectiveness of the proposed method.

11:38  230. The Virtual 180: Application to High Field Fast Spin Echo Imaging

Robert Marc Lebel1, Alan H. Wilman1

1University of Alberta, Edmonton, Canada

We present a method for FSE imaging with drastically reduced RF power; applications include high field and high resolution studies. We demonstrate that the magnetization coherence typically associated with 180  refocusing pulses may be achieved, temporarily, without high flip angles pulses. By encoding this “virtual 180  echo” into the central region of k-space, we obtain a short echo train TRAPS sequence. This permits long echo spacings as required for high resolution imaging. In this work we describe how to form a virtual 180 and present example protocols and resulting images obtained at 4.7T.

11:50 231. Exponential Excitation Pulses for Improved Water Content Mapping in the Presence of Background Gradients

Christine Preibisch1, Ralf Deichmann1

1University Frankfurt, Frankfurt, Germany

Introduction: Magnetic field gradients distort the exponential signal decay in multi-echo GE sequences, yielding erroneous values in water content maps based on extrapolation of the signal S(TE=0). Correct values can be achieved with exponentially shaped excitation pulses.Methods: Multi-echo GE images (phantom and volunteer) were acquired at 3T and simulations of saturation effects were performed for sinc and exponential excitation pulses.Results: Measurements and simulations show enhancement of S(TE=0) with increasing field gradient for sinc-shaped excitation pulses, but not for exponential pulses. Conclusion: Using exponential pulses increases the accuracy of water content mapping techniques in areas affected by field gradients.

12:02 232. Strongly Modulating Pulses: A New Method for Counteracting RF Inhomogeneity at High Fields

Nicolas Boulant1, Denis Le Bihan2, Alexis Amadon2

1Siemens, St Denis, France; 2CEA-Saclay, Gif sur Yvette, France

A new RF pulse method for counteracting B1 inhomogeneity for 3D imaging is presented. The technique makes use of a 2D {B0,B1} histogram to generate rapidly pulses where the flip angle is made uniform over the volume of interest. Measurements performed at 3 T show a reduction by a factor up to 15 of the standard deviation of the sine of the flip angle, compared to when using a standard square pulse calibrated by the scanner. Calculations tend to show less energy demands than for adiabatic BIR4 pulses.

12:14 233. Accelerated Radiation-Damping for Increased Spin Equilibrium (ARISE): A New Method for Controlling the Recovery of Longitudinal Magnetization

Susie Yi Huang1, 2, Thomas Witzel1, 2, Lawrence L. Wald1, 2

1Massachusetts General Hospital, Harvard Medical School, Charlestown, USA; 2Harvard-MIT Division of Health Sciences and Technology, Cambridge, USA

We introduce a method for accelerating the return of Mz to equilibrium that uses external feedback circuitry to strengthen the Radiation Damping (RD) field. This rotates the magnetization back to equilibrium faster than T1 relaxation. The method is characterized in phantom imaging and compared to numerical simulations. A 10ms feedback period in a crushed gradient echo sequence allowed >99% recovery of Mz in a long T1 phantom. Unlike SSFP, which is sensitive to off-resonance effects, the ARISE “flip-back” is self-generated and shows little off-resonance degradation, thereby offering a potentially useful building block for enhancing gradient echo sequences.