Imaging and Computing Seminar

Athanasios Polimeridis, EECS/RLE, MIT

Fast Integral Equation Solver for EM Analysis of High-Field MRI

We describe a fast method for estimating the fields in realistic human body models. Such simulations are needed for coil and RF pulse excitation design, specific absorption rate (SAR) prediction, and the design of high-field and parallel-transmission MRI. From an electromagnetic modeling perspective, the human body is a strongly inhomogeneous scatterer and the numerical analysis in this case is by no means trivial. In addition there is a strong drive in MRI technology for higher resolution, which translates to higher frequency. The combination of high dielectric contrast and high frequency suggests a Ć¢hostileĆ¢numerical solvers, most commonly based on partial differential equations methods, fail to meet the ever increasing requirements for ultra-fast computations. In this talk, we present a new class of volume integral equation methods that offer an ideal platform for customized fast algorithms where maximal use of a specific setting is possible. In the proposed approach the unknown physical quantities can be fairly represented on uniform tessellations, i.e., volumes decomposed into voxels, which is actually the natural setting for MRI data. In this case, the governing integral kernels are translational invariant and the associated matrix-vector products can be accelerated with the help of FFT. Also, the most computational intensive parts of our solver are embarrassingly parallelizable and involve simple operations that can be easily handled by GPU-accelerated libraries. The accuracy and efficiency of the proposed framework is demonstrated in a realistic scenario that considers electromagnetic scattering from a human body model. The remarkable performance of our solver can be attributed mainly to two key-features: the correct Galerkin discretization that leads to fast convergence of the iterative solver without the need of preconditioning, and the reduction of the original volume-volume integrals to purely surface-surface integrals with smoother kernels, which allows fast and accurate computation of the associated matrix element by means of DEMCEM, an open-source software. Our novel volume integral equation solver is currently being incorporated together with other in-house algorithms in a general computational framework for addressing a wide range of challenging applications in MRI device optimization; a project in collaboration with MRI experts from MIT and Harvard/MGH.

About the speaker:
Athanasios G. Polimeridis received the Diploma and the Ph.D. from the Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, in 2003 and 2008, respectively. From 2008 to 2012, he was a Post-Doctoral Research Associate at EPFL, Switzerland. Currently he is a Post-Doctoral Research Associate at MIT, where he is a member of the Computational Prototyping Group at the Research Laboratory of Electronics. His research interests revolve around computational methods for problems in physics and engineering (Electromagnetics, Casimir forces, MRI), with emphasis on the development and implementation of integral-equation based algorithms. In 2012, he was awarded a Swiss National Science Foundation Fellowship for Advanced Researchers.