ELECTRONIC POSTER - ismrm

1 Imaging Division, UMC utrecht, Utrecht, Utrecht, Netherlands, 2 UMC Utrecht, 3 Mathematics, Utrecht
University, 4 Radiotherapy, UMC Utrecht
Multi dimensional spatially selective excitation (mDSSE) RF pulse design aims to homogenize the magnetization over a given region
of interest. After discretizing the solution of the Bloch Equation under the small flip angle approximation, the problem is to find an
optimal numerical solution to the least squares problem argmin{||Ax-b||2 } (1) where the matrix A is the discretization of the integral
operator as in [1], x and b the vectors corresponding to the requested RF pulses and desired magnetization respectively. In this paper
we present an algorithm to solve (1) in a fast and stable way. From recent works it appears that a smooth RF pulse profile improves
accuracy in the magnetization obtained from a transmit system. The speed achieved by the algorithm is exploited to add regularization
terms to (1) in order to optimize the smoothness of the solution.
15:00 4942. Detection of Human Mononuclear Cells Labelled with Micron-Sized Iron Oxide
Particles Using the Sub-Pixel Enhancement of Nonuniform Tissue (SPENT) Sequence
Bernard Siow 1 , David W. Carmichael 2,3 , Johannes Riegler 4 , Daniel Alexander 1 , Mark
Lythgoe 4 , Roger Ordidge 3
1 Centre for Medical Image Computing, University College London, London, United Kingdom; 2 Institute of
Neurology, University College London, United Kingdom; 3 Department of Medical Physics and Bioengineering,
University College London, United Kingdom; 4 Centre for Advanced Biomedical Imaging, University College
London, United Kingdom
The Sub-pixel Enhancement of Nonuniform Tissue (SPENT) sequence applies a 2pi phase dispersion across each voxel: the net phase
of spins in magnetically homogeneous voxels would be equal to zero and thus no signal would be generated. If there are sub-pixel
inhomogeneities, then the net phase of spins in a voxel is not zero and thus signal is seen. In this study, human mononuclear cells
labelled with micron-sized iron oxide particles, which creates sub-voxel perturbations in the field, are scanned with a spin-echo
SPENT sequence producing positive contrast images. SPENT provides directional information, as well as the potential for
quantification.
Topics in RF Design
Hall B Monday 14:00-16:00 Computer 116
14:00 4943. ΦFA CUP: PHase Based Flip Angle Calibration Using the P0 Pulse for Proton MRI
at 7T
Davide Santoro 1 , Tomasz Lindel, 1,2 , Matthias Dieringer, 1,3 , Wolfgang Renz, 1,4 , Thoralf
Niendorf, 1,5
1 Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine, Berlin, Germany; 2 (PTB),
Physikalisch Technische Bundesanstalt, Berlin, Germany; 3 Franz-Volhard Klinik, Clinic for Cardiology,
Charité Berlin, University Medicine, Berlin, Germany; 4 Siemens Healthcare, Erlangen, Germany;
5 Experimental and Clinical Research Center (ECRC), Charité Campus Buch, Humboldt-University, Berlin,
Germany
We demonstrate the applicability of a rapid 3D-B1+ mapping method for proton MRI at 7T. The method is based on the acquisition
of two phase images where the effective flip angle is encoded in the phase of the non-slice selective rectangular composite pulse used
as excitation in a gradient recall echo. On phantoms our method compares well with the double angle method, and is approximately 40
times faster. 3D FA map of human brain acquired in 102s are presented.
14:30 4944. Spatial RF Pulse Design in Local Rotating Frame
Seung-Kyun Lee 1
1 GE Global Research, Niskayuna, NY, United States
In magnetic resonance, a gradient- and voxel-dependent rotating frame, which we call a “local rotating frame” can eliminate all
longitudinal magnetic fields in magnetization dynamics and therefore significantly simplifies the theory and practice of spatial RF
pulse design. When the gradient waveform is pre-determined, as is the case in most existing numerical RF design methods, the frame
transformation is completely straightforward, and removes need for repeated calculation of the same gradient effects as RF pulse is
iteratively updated. After introducing basic theoretical elements of the new frame approach, we demonstrate its usefulness in two
examples. First, we demonstrate calculation of the residual dephasing in slice selective excitation caused by nonlinearity of the Bloch
equations by analytical integration of the equations in the local rotating frame. Second, we show that numerical integration of the
Bloch equations is made significantly faster in the new frame due to the lack of strong longitudinal field. We discuss the relevance of
the new approach in the context of iterative RF design in parallel transmit.