Johannes Thomas Schneider^1,2, Martin Haas², Wolfgang Ruhm¹, Juergen Hennig², and Peter Ullmann^1
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.

William A Grissom¹, Chen Dong¹, Laura Sacolick¹, and Mika W Vogel^1
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.

Martijn Anton Cloos^1,2, Nicolas Boulant¹, Guillaume Ferrand², Michel Luong², Christopher J Wiggins¹, Denis Le Bihan¹, and Alexis Amadon^1
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 k_T-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.

Mohammad Mehdi Khalighi¹, Manojkumar Saranathan², William Grissom³, Adam B. Kerr⁴, Ron Watkins², and Brian K. Rutt^2
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.

Shaihan J Malik¹, Hanno Homann², Peter Börnert³, and Joseph V Hajnal^1
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.

Jon-Fredrik Nielsen¹, Daehyun Yoon², Neal Anthony Hollingsworth³, Katherine Lynn Moody⁴, Mary Preston McDougall^3,4, Steven M Wright^3,4, and Douglas C Noll^1
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.

Cem Murat Deniz^1,2, Leeor Alon^1,2, Ryan Brown¹, Hans-Peter Fautz³, Daniel K Sodickson¹, and Yudong Zhu^1
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.

Johannes Thomas Schneider^1,2, Martin Haas², Stéphanie Ohrel¹, Heinrich Lehr¹, Wolfgang Ruhm¹, Hans Post¹, Jürgen Hennig², and Peter Ullmann^1
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.

Cungeng Yang¹, Weiran Deng¹, Vijayanand Alagappan², Lawrence L. Wald³, and Victor Andrew Stenger^1
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.

Hai Zheng¹, Tiejun Zhao², Yongxian Qian³, Tamer Ibrahim^1,3, and Fernando Boada^1,3
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.