Parallel Transmission in Three Dimensions
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Tuesday May 10th
Room 511D-F  10:30 - 12:30 Moderators: Kawin Setsompop and V. Andrew Stenger

10:30 204.   Exploiting Phase Encoding Capabilities of Parallel Excitation for Improved Spatial Selectivity in Inner-Volume Imaging  
Johannes Thomas Schneider1,2, Martin Haas2, Wolfgang Ruhm1, Juergen Hennig2, and Peter Ullmann1
1Bruker BioSpin MRI GmbH, Ettlingen, Germany, 2Dept. of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany

In spatially-selective excitation (SSE) experiments with short repetition times, undesired transverse magnetization outside of specified target volumes, despite being excited with small tip-angles, may be strongly pronounced compared to magnetization generated with larger tip-angles inside the target volumes because of saturation effects and different steady states. This work proposes to apply phase modulation instead of amplitude modulation in SSE, i.e. global excitation with a homogenous tip-angle distribution while achieving spatial selectivity by generating excitation phases, which differ inside and outside the target volumes according to a certain encoding scheme. This approach yields improved selectivity and significant artifact reduction.

10:42 205.   3D Parallel Excitation Pulse Design Using Interleaved Sparse Approximation and Local Optimization 
William A Grissom1, Chen Dong1, Laura Sacolick1, and Mika W Vogel1
1GE Global Research, Munich, Germany

Determining optimal phase encoding locations for three dimensional parallel excitation spokes pulses is a non-trivial problem. Current algorithms can be classified as either sparse approximation-based methods, or methods that locally optimize the encoding locations. The two approaches have complementary strengths and weaknesses: sparse approximation-based methods approach global optimality when the target excitation phase is fixed and time-dependent effects (such as off-resonance) are ignored, while local methods can be performed jointly with target phase optimization and can account for time-dependent effects, but are only locally optimal. We introduce a new algorithm similar which interleaves greedy sparse approximation-based phase encoding selection with local gradient and target phase optimization. The new method is demonstrated using multiband pulse designs.

10:54 206.   Application of kT-points to human brain imaging at 7 Tesla 
Martijn Anton Cloos1,2, Nicolas Boulant1, Guillaume Ferrand2, Michel Luong2, Christopher J Wiggins1, Denis Le Bihan1, and Alexis Amadon1
1LRMN, CEA, DSV, I2BM, NeuroSpin, Gif-Sur-Yvette, ile-de-France, France, 2CEA, DSM, IRFU, Gif-Sur-Yvette, ile-de-France, France

Transmit-SENSE gives the opportunity to implement short excitation pulses with good flip-angle homogeneity at high field. Recently, a novel non-selective pulse design, referred to as kT-points, was presented which enables sub-millisecond pulses with excellent spatially uniform excitation properties over an extended volume. In this abstract, for the first time, application of this novel pulse design to human brain imaging at 7 Tesla is presented.

11:06 207.   Parallel Transmit using 3D Spokes RF Pulses for Improved B1+ Homogeneity over 3D Volumes 
Mohammad Mehdi Khalighi1, Manojkumar Saranathan2, William Grissom3, Adam B. Kerr4, Ron Watkins2, and Brian K. Rutt2
1Global Applied Science Laboratory, GE Healthcare, Menlo Park, California, United States, 2Department of Radiology, Stanford University, Stanford, California, United States, 3Imaging Technologies Lab, General Electric Global Research, Garching b. Munchen, Germany, 4Department of Electrical Engineering, Stanford University, Stanford, California, United States

B1+ inhomogeneity is a major issue at high field. We introduce a 3D spokes RF pulse designed to improve volumetric B1+ homogeneity; this is achieved by adding z-axis phase encoding to the more conventional 2D spokes pulse design. We show that 2ch pTx using our new 3D spokes pulses improves B1+ uniformity over a complete slab even compared to 2D spokes pulses, and more substantially (factor of 2) compared to conventional quadrature excitation. We have tested experimentally this new 3D spokes pulse concept on a 2-ch pTx-enabled 7T scanner and show the B1+ homogeneity benefits in phantoms and volunteer brain.

11:18 208.   Parallel Transmission design of multi-pulse sequences using Spatially Resolved Extended Phase Graphs (SREPG) 
Shaihan J Malik1, Hanno Homann2, Peter Börnert3, and Joseph V Hajnal1
1Robert Steiner MRI Unit, Imaging Sciences Department, MRC Clinical Sciences Centre, Hammersmith Hospital, Imperial College London, London, London, United Kingdom,2Institute of Biomedical Engineering, Karlsruhe Institute of Technology, Karlsruhe, Germany, 3Philips Research, Hamburg, Germany

The Extended Phase Graph (EPG) formalism is a powerful means for predicting MR signals in sequences involving multiple RF pulses. At high field (3T+) the RF fields (B1) vary strongly in space, so predictions by the EPG algorithm are not valid everywhere. Parallel transmission enhances control of B1 enabling flip angle distributions to be modulated in both space and time. The effect of changing fields in this way can be predicted with spatially resolved EPG (SREPG). We demonstrate that pulses may be independently optimised for fast spin echo using SREPG leading to solutions not achievable by RF shimming alone.

11:30 209.   Joint optimization of tip-down and tip-up RF pulses in small-tip (non-spin-echo) fast recovery imaging 
Jon-Fredrik Nielsen1, Daehyun Yoon2, Neal Anthony Hollingsworth3, Katherine Lynn Moody4, Mary Preston McDougall3,4, Steven M Wright3,4, and Douglas C Noll1
1Biomedical Engineering, University of Michigan, Ann Arbor, MI, United States, 2Electrical Engineering and Computer Science, University of Michigan, 3Electrical and Computer Engineering, Texas A&M University, 4Biomedical Engineering, Texas A&M University

In “fast-recovery” (FR) or driven-equilibrium steady-state imaging, the magnetization is tipped back toward the longitudinal axis at the end of each repetition interval (TR), with the aim of maximizing the acquired signal. Conventional FR imaging requires one or more spin-echo refocusing pulses, and hence heavy RF deposition. With the use of parallel RF transmission and 3D RF pulse design, it may be possible to replace the conventional spin-echo pulse train with a small-tip excitation pulse followed by a small-tip recovery (tip-up) pulse. We present a simple and effective approach for jointly optimizing the excitation and recovery pulses such that the residual (unwanted) transverse magnetization after the tip-up pulse is minimized.

11:42 210.   Parallel RF Pulse Design with Subject-Specific Global SAR Supervision 
Cem Murat Deniz1,2, Leeor Alon1,2, Ryan Brown1, Hans-Peter Fautz3, Daniel K Sodickson1, and Yudong Zhu1
1Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, New York, NY, United States, 2Sackler Institute of Graduate Biomedical Sciences, NYU School of Medicine, New York, NY, United States, 3Siemens Medical Solutions, Erlangen, Germany

Specific absorption rate (SAR) management and excitation homogeneity are critical aspects of parallel radio frequency (RF) transmission pulse design at ultra high magnetic field strength. The design of RF pulses for multiple channels is generally based on the solution of regularized least squares optimization problems, for which the regularization term is selected to control the integrated or peak pulse waveform amplitude. Unlike for single channel transmission systems, the SAR of parallel transmission systems is significantly influenced by interferences between the electric fields of the various transmit elements, which are not taken into account using conventional regularization terms. This work explores the effects upon SAR behavior of incorporating measurable electric field interactions into parallel transmission RF pulse design. The results of phantom experiments show that the global SAR during parallel transmission decreases when electric field interactions are incorporated in pulse design optimization.

11:54 211.   Parallel Spatially Selective Excitation Using Nonlinear Non-Bijective PatLoc Encoding Fields: Experimental Realization and First Results 
Johannes Thomas Schneider1,2, Martin Haas2, Stéphanie Ohrel1, Heinrich Lehr1, Wolfgang Ruhm1, Hans Post1, Jürgen Hennig2, and Peter Ullmann1
1Bruker BioSpin MRI GmbH, Ettlingen, Germany, 2Dept. of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany

The PatLoc technique uses non-linear, non-bijective magnetic fields in combination with RF receive sensitivity encoding for spatial encoding in MR image acquisition. Recently, a theoretical study based on simulations has demonstrated that the PatLoc benefits, such as locally increased spatial resolution, faster gradient switching and reduced peripheral nerve stimulation, can also be exploited for Parallel Spatially Selective Excitation (PEX). This work presents the first successful experimental realization of PEX based on PatLoc encoding fields and demonstrates that the ambiguities introduced into the excitation process by the non-bijectivity of these encoding fields can effectively be resolved by using parallel transmission techniques.

12:06 212.   Parallel Transmission with Spectral-Spatial Pulses for Susceptibility Artifact Correction 
Cungeng Yang1, Weiran Deng1, Vijayanand Alagappan2, Lawrence L. Wald3, and Victor Andrew Stenger1
1University of Hawaii, Honolulu, Hawaii, United States, 2General Electric Medical Systems, Waukesha, Wisconsin, United States, 3Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, Massachusetts, United States

Susceptibility induced signal loss is a major limitation in high field T2*-weighted MRI applications including BOLD fMRI. Spectral-spatial (SPSP) pulses have been shown to be very effective at reducing through-plane signal loss in axial slices using a single excitation. SPSP pulse design assumes a linear relationship between off-resonance frequency and through-plane susceptibility gradient Gs(f)=αf. This approximation holds well in more superior slice locations, however, inferior slices can have several regions that require different α’s. We propose the use of parallel excitation to apply unique SPSP pulses with each transmitter. The localization introduced by the transmission sensitivities compensates for the spatial distribution of the susceptibility gradients. The method is demonstrated in T2*-weighted human brain imaging at 3T with an eight-channel parallel transmission system.

12:18 213.   Through-plane Signal loss recovery and B1 inhomogeneity reduction in vivo at 7T using parallel transmission 
Hai Zheng1, Tiejun Zhao2, Yongxian Qian3, Tamer Ibrahim1,3, and Fernando Boada1,3
1Bioengineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2Siemens Medical Solutions, Pittsburgh, Pennsylvania, United States, 3Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States

T2*-weighted functional MRI images at ultra high field (UHF, >3T) is severely affected by through-plane signal loss and B1 inhomogeneity in areas near air/tissue interfaces in the brain. In this study, we demonstrate a parallel transmission technique to simultaneously mitigate signal loss and B1 inhomogenetiy based on the concatenation of multi-slices main magnetic field and B1+ maps during RF pulse design. Our results demonstrate that the approach is a practical means for the reduction of signal loss during in vivo T2*-weighted functional MRI at 7T.