Dielectric Shimming

Dielectric Shimming: Optimization Techniques for Dielectric Pads in MRI

Gerlach, M.M.J.

Remis, R.F. (mentor)

Electrical Engineering, Mathematics and Computer Science

Microelectronics

Circuits and Systems

2016-07-29

Dielectric shimming is proven to be very useful in increasing the homogeneity of the 𝐵1+ field in high field MRI. Current optimization and design techniques for dielectric pad parameters are slow. The goal of this thesis is to find a fast and accurate pad design and optimization technique. Two new techniques are proposed. The first, a method that simply uses inspection by solving the forward problem in a relatively fast way. The other proposed technique follows a more analytical approach to find the optimal permittivity and conductivity of a pad in a couple of iterative steps with a Gauss-Newton method. This last technique uses a new proposed approach to predict the phase of the 𝐵1+ field in a direct fashion. These techniques provide fast and accurate simulation results for a two-dimensional abdominal body slice placed in a 3T MRI scanner for different pad scenarios. From these results it can be concluded that both proposed techniques generate comparable pads, which are able to increase the homogeneity of the 𝐵1+ field. A comparison between the two techniques is made. The Gauss-Newton method provides a fast, robust and accurate optimization technique for large scale problems, but offers less flexibility and insight to the data compared to the method via inspection. The flexibility of the method via inspection and the insight it provides is shown for different scenarios (pad location, multiple pads, pad shape, pad thickness), where the effect of the optimal permittivity and conductivity on the homogeneity of the resulting 𝐵1+ field is simulated. Even the maximum allowed SAR can be incorporated in this pad optimization technique.

http://resolver.tudelft.nl/uuid:d6e968d5-be6a-440c-97e2-bc4842358b6f

Student theses

master thesis

(c) 2016 Gerlach, M.M.J.