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B A S T I E N   G U E R I N ,   P H .   D .

MRI technology and neuroimaging at the A. A. Martinos Center for Biomedical Imaging

W h a t ' s   N e w
[6/19/2014] MRM paper is out on SAR reduction for patients with deep brain stimulation using parallel transmission MRI.
My collaborator Yigitcan Eryaman has a paper this month in MRM on the use of the transmit degree-of-freedom of pTx coils for reduction of SAR at the tip of deep brain stimulation implants. The idea of using pTx to reduce heating risks in DBS patients is not completely new, but the method of this paper has the unique advantage of not requiring an accurate simulation of the patient and the implant, which is extremely difficult to perform as the lead geometry is not generally known. Instead, Yigitcan computes appropriate modes of excitation of the pTx array that lead to a small B1+ artefact, which is used as a surrogate for SAR. Check it out...

[6/13/2014] New MRM paper on pTx pulse design for Simultaneous Multi-Slice (SMS).
I have a new paper in MRM this month about the
design of pTx RF pulses for SMS. In this work, I show that local SAR and peak power constraints can be included in the pTx SMS design problem much like in "standard" single slice imaging. The advantage of enforcing these constraints explicitely in the SMS design problem is much more pronouced however, because of the large SAR and power demands of these pulses. Simulations in this paper show that this strategy allows the pulse design algorithm to utilize the power amplifiers at their maximum performance while satisfying all system and safety limits, which in turns dramatically improves the quality of the multi-slice flip-angle excitations. 

[4/22/2014] Two new MRM papers on SAR reduction using "dark modes" and comparison of pTx coils using EM simulation.
My collaborator Yigitcan Eryaman's paper on reduction of global and local SAR using "dark modes" of a transmit arrayt is out in early view
of MRM. The idea of this work is to build a transmit array using both elements that create B1+ (loops) as well as elements that do not (dipoles). Excitation of the dipole elements is optimized for SAR reduction, not for flip-angle mitigation. My paper on simulation of pTx coils is also out today.This paper is the result of a 3 years collaboration with Siemens Healthcare that was aimed to determine the "best" parallel transmit geometry for body imaging at 3 T.  This is an enormous amount of work, for which I developed an electromagnetic simulation framework with my collaborator Peter Serano, which is based on the co-simulation technique of Mickael Kozlov and Robert Turner. I also had to develop a pTx pulse design strategy that constrains power and global and local SAR. Check this work out, there are many "learned lessons" in it that will be of interest to coil builders and pulse designers!

[3/24/2014] MRM paper on SAR constrained pTx pulse design is editor's pick!
My paper "
Local specific absorption rate (SAR), global SAR, transmitter power, and excitation accuracy trade‐offs in low flip‐angle parallel transmit pulse design", published online last year in Magnetic Resonance in Medicine was one of 2 papers picked by the editor for the April issue.

[2/18/2014] New MRM paper on realistic head phantom.
Nadine Graedel's new paper on the temperature mapping of pTx pulses's SAR distributions using a realistic head phantom is now available in early view of the journal Magnetic Resonance in Medicine.

[2/10/2014] 2014 ISMRM abstracts
So the results are up and it looks like our pTx collaboration between MGH and MIT is going to be well represented in Milan next May. Oral presentations will include (titles are not exact, you can search for the author last name in the ISMRM website):
  • Ultimate SNR and SNR calculations in non-uniform body models (Bastien Guerin);
  • A new framework for decoupling pTx coils with many channels (Zohaib Mahmood)
  • PTx pulse design for patients with deep brain stimulation implants (Yigitcan Eryaman);
  • High throughput evaluation of pTx coils using ultra-fast electromagnetic simulations (Jorge Villena);
  • Fast, robust design of time-optimal gradient trajectories (Mathias Davids).
We have also several posters and e-posters (again, titles not exact...):
  • Large flip-angle Kt point pTx pulse design with local SAR constraints (Filiz Yetisiz);
  • Validation of local SAR reduction in patients with deep brain stimulation implants using pTx (Bastien Guerin);
  • SAR-constrained pTx pulse design robust to inaccuracies in the RF transmit chain (Adrian Martin);
  • Joint design of RF and gradient waveforms for fast pTx inner volume excitation based on a general parameterization of gradient trajectories (Mathias Davids).
The Wald lab has many more exciting abstracts that will be presented there, too many to list. Check the complete list on the ISMRM website!