RF Pulse Design

Monday 22 April 2013

Sebastian Schmitter¹, Lance DelaBarre¹, Xiaoping Wu¹, Andreas Greiser², Dingxin Wang³, Kamil Ugurbil¹, and Pierre-Francois Van de Moortele^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, ²Siemens AG, Erlangen, Bavaria, Germany, ³Siemens Medical Solutions USA, Inc., Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Cardiac MRI may greatly benefit from ultra-high field (UHF) providing higher SNR and intrinsic contrast. However, shorter RF wavelength at UHF results in transmit B1 (B1+) and contrast variations through the heart. This problem has been addressed using multi-channel transmit coils and B1-shimming, but further improvements are expected by using parallel transmission (pTX) with multi-spoke RF-pulses as shown in brain and liver at 7T. However, this involves additional challenges, including rapid and robust multi-channel ECG-triggered B1+ calibration, and sensitivity to motion. Here, we investigate the impact of 2-spoke RF-pulse design on 7T cardiac imaging using a 16-channel pTX system.

Daniel A. Herzka¹, Haiyan Ding^2,3, and Michael Schar^4,5
1Biomedical Engineering, Johns Hopkins University, Baltimore, MD, United States, ²Biomedical Engineering, Tsinghua University, Beijing, China, ³Biomedical Engineering, Johns Hopkins School of Medicine, Baltimore, MD, United States, ⁴Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins School of Medicine, Baltimore, MD, United States, ⁵Clinical Science, Philips Healthcare, Cleveland, OH, United States

MRI scanners use an integrated birdcage coil to generate radio frequency (RF) excitation fields (B1+). It has been reported that at 3T variations in flip angle can range from 31 to 66% (B1+ inhomogeneities) over the left ventricle and average flip angles can be reduced by ~20% (RF power). Multi-channel transmit systems allow RF-shimming to locally improve the B1+ field. We quantify improvements in B1+ field homogeneity and average RF power resulting from dual-source parallel RF excitation in cardiac swine imaging. Correlation with animal size demonstrates that larger subjects suffer more B1+ field inhomogeneity and benefit more from RF shimming.

Anuj Sharma¹, Samantha J. Holdsworth², Rafael O'Halloran², Eric Aboussouan², Anh Tu Van², Julian R. Maclaren², Murat Aksoy², Victor Andrew Stenger³, Roland Bammer², and William A. Grissom^1
1Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, United States, ²Radiology, Stanford University, Stanford, California, United States, ³Medicine, University of Hawaii, Honolulu, Hawaii, United States

A new class of patient-tailored multiband RF pulses "kT-PINS" is presented that combines PINS multiband excitation pulses with kT-points 3D patient-tailored excitation pulses, enabling simultaneous excitation of multiple slices and compensation of transmit field inhomogeneities. An interleaved greedy and local optimization algorithm was developed to design the pulses. Phantom experiments demonstrate the effectiveness of the pulses in reducing flip angle inhomogeneity caused by transmit field inhomogeneity at 7T.

Xiaoping Wu¹, Sebastian Schmitter¹, Edward J. Auerbach¹, Steen Moeller¹, Kamil Ugurbil¹, and Pierre-Francois Van de Moortele^1
1CMRR, Radiology, University of Minnesota, Minneapolis, MN, United States

Simultaneous multi-band (MB) RF excitation, along with subsequent unaliasing via parallel imaging principles, provides an effective means to accelerate volume coverage along the slice direction. Recently, the approach has been exploited with significant success in functional and diffusion-weighted imaging studies of the brain. So far this technique has only been demonstrated in the context of single channel transmit. In this study, we extend this technique to multi-channel transmit and introduce parallel transmit (pTX) MB pulse design in order to tackle the issues of B1+ inhomogeneity at high and ultrahigh field strengths, and RF power deposition. The new extension is validated in the human brain at 7T and is demonstrated capable of providing good B1+ homogenization in addition to simultaneous MB excitation without necessitating the use of higher RF energy relative to a single channel application.

Benedikt A. Poser¹, Robert James Anderson¹, Peter Serano², Azma Mareyam², Bastien Guérin², Weiran Deng¹, Lawrence L. Wald², and Victor Andrew Stenger^1
1John A Burns School of Medicine, University of Hawaii, Honolulu, Hawaii, United States, ²Radiology, Massachusetts General Hospital, Charlestown, MA, United States

Simultaneous multi-slice (SMS) acquisitions have become popular for single-shot sequences such as BOLD and DW-EPI. SMS excitations are commonly achieved with multiband pulses. We explore parallel transmission (pTX) for SMS excitation, using differently frequency-shifted pulses on subsets of transmitters that define the excited slices. Using a dual-ring pTX coil and blippedCAIPIRINHA EPI, we show factor-2 SMS with conventional single-band pulses and factor-4 SMS using dual-band pulses on each ring. For pTX coils with inherent transmit-sensitivity encoding along the slice direction, the approach can reduce required RF power (global SAR) compared to MB pulses with as many frequency bands as slices applied on all coil elements.

Cem Murat Deniz^1,2, Leeor Alon^2,3, Ryan Brown³, Daniel K. Sodickson^2,3, and Yudong Zhu^2,3
1Department of Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York University, New York, NY, United States, ²Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States, ³Department of Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY, United States

Given the constraints imposed by the hardware resource in terms of RF power delivery and reflection handling capabilities, any practical RF pulse used on an MR scanner must be designed with those capabilities on mind. In parallel transmission, coupling and interaction taking place in the multi-port structure as well as in the subject can significantly affect individual channel RF power transmission and the demands placed upon the power amplifiers. By using pre-scan based multi-channel calibration, we designed parallel RF excitation pulses obeying the forward / reflected peak and average power limits of the RF power amplifier. Additionally, global SAR limits were incorporated in the RF pulse design. Results showed that the prediction capability of this new calibration method enables the design of parallel RF excitation pulses respecting strict and multifaceted power limits.

Florent Eggenschwiler¹, Kieran O'Brien², Rolf Gruetter^1,3, and José P. Marques^4
1EPFL, Laboratory for Functional and Metabolic Imaging, Lausanne, Vaud, Switzerland, ²University of Geneva, Department of Radiology, Geneva, Geneva, Switzerland, ³Universities of Geneva and Lausanne, École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland, ⁴University of Lausanne, Department of Radiology, Lausanne, Vaud, Switzerland

At high magnetic field strengths (B₀ > 3T), the inhomogeneous distribution of the B₁⁺ field causes undesirable signal and contrast variations of GM/WM across the brain. These artifacts impair the quality of T₂-weigthed images at high field and may mislead any medical diagnosis that depends on these images. In this study, short 3D tailored RF pulses (k_T-points) were combined with a variable flip angle TSE sequence to obtain T₂-weighted anatomical images with uniform contrast throughout the whole brain at 7T. A symmetric k-space trajectory ensured that the excitation profile associated with the k_T-points remained close to the predicted STA approximation over the large range of flip angles used in the TSE sequence and gave optimal contrast homogeneity. The proposed methodology was tested at 7T, on both single-channel and PTx systems and demonstrates very promising achievements in T₂-weighted brain imaging.

David Otto Brunner¹, Benjamin E. Dietrich¹, Matteo Pavan¹, and Klaas P. Pruessmann^1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Zurich, Switzerland

Pulsed NMR spectroscopy and imaging using stochastic1 or swept2 excitation during signal acquisition requires sets high requirements on the transmit chain in order to accurately deconvolve the received signal. Monitoring the RF pulse concurrently with the acquisition alleviates many of those requirements, however the computational effort is increased in particular if other confounding, such as propagation delay differences or temporally variable off-resonances need to be corrected. We present a fast FFT based reconstruction approach which is applied to SWIFT imaging using sideband excitation showing the benefits of the applied corrections.

Benoît Bourassa-Moreau¹, Guillaume Gilbert^1,2, and Gilles Beaudoin^1
1Department of Radiology, Centre Hospitalier de l'Université de Montréal, Montreal, Quebec, Canada, ²Philips Healthcare, Montreal, Quebec, Canada

In this work, we present a slice-selective pseudo-adiabatic excitation (pBIR4-S1s2) which offers a B1-insensitive excitation at an arbitrary flip angle in a comparatively short duration (<10 ms). Slice-selection is achieved by replacing the hard RF sub-pulses of the pseudo-adiabatic pulse by slice-selective sub-pulses (SLR) of the same time-integrated areas and driven with an oscillating gradient. Simulated magnetization response and experimental results are shown.

Rüdiger Stirnberg¹, Daniel Brenner¹, Tony Stöcker¹, and Nadim Jon Shah^1,2
1Institute of Neuroscience and Medicine - 4, Research Centre Juelich GmbH, Jülich, Germany, ²Department of Neurology, Faculty of Medicine, JARA, RWTH Aachen University, Aachen, Germany

A simple modification of a 3D-EPI sequence is proposed which renders additional time- and SAR-demanding fat suppression obsolete. Instead, using a rectangular excitation pulse yields robust fat suppression with minimal SAR if the pulse duration is selected carefully. This method, particularly useful for high field application, facilitates high-resolution functional imaging (fMRI) at temporal resolutions of two seconds or less. The basic concept and an expression for the optimal pulse duration (also valid for large flip angles) are introduced, validated in vivo at 3T and applied to finger tapping fMRI at 1.5mm isotropic resolution.