Time

Prog #

Kay Nehrke¹, Peter Börnert¹

¹Philips Research Europe, Hamburg, Germany

MRI based B₁-mapping techniques potentially suffer from adverse error propagation in the range of small flip angles, resulting in noisy maps in regions of low B₁. However, on multi-transmit systems the coil basis functions used for optimizing the RF waveforms may be freely chosen as a result of the linear properties of the transmit system. Hence, the RF field may be adapted to a favourable operational range of the B₁-mapping technique by appropriate superposition of coils. In the present work, this approach was validated in phantom experiments, resulting in a strongly improved quality of the B₁-maps.

David Otto Brunner¹, Klaas Paul Pruessmann¹

¹University and ETH Zurich, Zurich, Switzerland

Recently developed methods for mitigating inhomogeneous RF excitation at ultra-high fields such as RF-shimming or Transmit-SENSE rely on accurate maps of the excitation field of all individual elements. These maps must be acquired within the preparation phase of an experiment and hence under severe time and power constraints. However, the sensitivity of most mapping methods is very low since the flip angle achieved using a single element for excitation is very low. We present a matrix approach of measuring the excitation fields with high accuracy for an entire 8-channel array within 40 s measurement time.

Adam Bruce Kerr¹, Hans-Peter Fautz², Mika W. Vogel², Patrick Gross², John Mark Pauly¹, Yudong Zhu³

¹Stanford University, Stanford, California , USA; ²GE, Munich, Germany; ³GE, Albany, New York, USA

Two slice-selective approaches for rapid estimation of B1 maps over a large dynamic range, including careful compensation of slice profile and off-resonance effects are presented.  The methods are evaluated on an eight-channel parallel transmit 3T body array, and validated by presenting a successful demonstration of a 3D slice-selective parallel transmit excitation.

Sha Zhao¹, ², Lloyd J. Gregory, ¹², Geoffery J. Parker¹

¹The University of Manchester, Manchester, UK; ²Salford Royal NHS Trust Fund, Manchester, UK

We report a B1 mapping method at 3 T, based on a magnetisation preparation Turbo FLASH sequence. A series of acquisitions with different nominal preparation angles are used to calculate B1 efficiency at every voxel. This method is accurate, fast, capable of 3D volume coverage and has a low RF irradiation to the subjects. It is also easy to implement.

Olof Dahlqvist Leinhard¹, ², Marcel Jan Bertus Warntjes¹, ², Peter Lundberg¹, ²

¹Linköping University, Linköping, Sweden; ²Center for Medical Image Science and Visualization (CMIV), Linköping, Sweden

A method for whole volume three dimensional B1 mapping is presented. The method is based on measurement of the effect caused by a saturation pre-pulse in turbo field echo (TFE) pulse sequence and produces B1 field maps covering the brain in 10 seconds.

Michael Schär¹, ², Evert-Jan P. A. Vonken¹, ³, Matthias Stuber¹

¹The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA; ²Philips Medical Systems, Cleveland, Ohio, USA; ³Utrecht University, Utrecht, Netherlands

Cardiac MRI and MRS at 3T are challenged by inhomogeneous B0- and B1-fields. A sequence is proposed to combine the acquisition of a B0- and a B1-map in a single breath-hold for fast determination of localized second order shim, resonance frequency F0, and RF power settings. It is shown in 5 volunteers that the determined shim and RF power settings allow to correct B0 field inhomogeneities and to accurately adjust the mean flip angle in a chosen region of interest in the heart.

Kai Zhong¹, Oliver Speck²

¹Otto-von-Guericke University , Magdeburg, Germany; ²Otto-von-Guericke University, Magdeburg, Germany

In this study, a new fast acquisition method based on the TESSA (Transition from Equilibrium into Steady State Acquisition) principle has been applied for simultaneous in vivo quantitation of B1 and T1 maps at 7 Tesla. This novel method utilizes the transient magnetization from equilibrium to steady state which otherwise is discarded and is self-contained, e.g. only a single acquisition is required to determine both B1 and T1. The new method has no specific requirement for the acquisition parameters TR and flip angle and has a 300% gain in speed. The TESSA principle could potentially stimulate further in vivo high field applications.

Jung-Jiin Hsu¹, Greg Zaharchuk¹, Gary H. Glover¹

¹Stanford University, Stanford, USA

In quantitative MRI, measuring both the flip angle of an RF pulse and the longitudinal relaxation time T1 are critically important for pixel intensity normalization and for calculating physical quantities especially in longitudinal studiesand in imaging at high field.  But conventional methods can only measure one ofthem and require long scan times.  In this work, two novel methods are developed which can acquire multi-slice scan data in about half the time needed for a conventional flip-angle measurement, and both the flip angle and T1 can be determined simultaneously.

Jang-Yeon Park¹, Michael Garwood¹

¹Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota, Minneapolis, Minnesota, USA

The most common B1 mapping approach is the double-angle method which uses the ratio of two images acquired with different flip angles, requiring TR >> T1.  Recently, several methods have been proposed to shorten TR.  Here, a new time-efficient method is introduced using π/2 and π hyperbolic secant pulses, where B1 map is determined by phase difference between two spin-echo images with opposite frequency sweeps.  It can shorten TR because it uses phase, not signal magnitudes, and 2D multi-slice B1 map can be obtained.  It can also avoid possible errors due to different slice profiles caused by different flip angles.

Dingxin Wang¹, ², Sven Zuehlsdorff², Andrew Larson¹

¹Northwestern University, Chicago, Illinois, USA; ²Siemens Medical Solutions, Chicago, Illinois, USA

In this study we present a 3D multi-slab double-angle method combined with multi-echo CPMG acquisition and compensated catalyzation pulses at the end of each TR which drive the longitudinal magnetization into steady-state enabling short TR for fast large volume RF field mapping.