Andrew Dewdney

Simultaneously driven linear and nonlinear spatial encoding fields in MRI

Spatial encoding in MRI is conventionally achieved by the application of switchable linear encoding fields. The general concept of the recently introduced PatLoc (Parallel Imaging Technique using Localized Gradients) encoding is to use nonlinear fields to achieve spatial encoding. Relaxing the requirement that the encoding fields must be linear may lead to improved gradient performance or reduced peripheral nerve stimulation. In this work, a custom‐built insert coil capable of generating two independent quadratic encoding fields was driven with high‐performance amplifiers within a clinical MR system. In combination with the three linear encoding fields, the combined hardware is capable of independently manipulating five spatial encoding fields. With the linear z‐gradient used for slice‐selection, there remain four separate channels to encode a 2D‐image. To compare trajectories of such multidimensional encoding, the concept of a local k‐space is developed. Through simulations, reconstructions using six gradient‐encoding strategies were compared, including Cartesian encoding separately or simultaneously on both PatLoc and linear gradients as well as two versions of a radial‐based in/out trajectory. Corresponding experiments confirmed that such multidimensional encoding is practically achievable and demonstrated that the new radial‐based trajectory offers the PatLoc property of variable spatial resolution while maintaining finite resolution across the entire field‐of‐view. Magn Reson Med, 2011.

Single-shot imaging with higher-dimensional encoding using magnetic field monitoring and concomitant field correction

PatLoc (Parallel Imaging Technique using Localized Gradients) accelerates imaging and introduces a resolution variation across the field‐of‐view. Higher‐dimensional encoding employs more spatial encoding magnetic fields (SEMs) than the corresponding image dimensionality requires, e.g. by applying two quadratic and two linear spatial encoding magnetic fields to reconstruct a 2D image. Images acquired with higher‐dimensional single‐shot trajectories can exhibit strong artifacts and geometric distortions. In this work, the source of these artifacts is analyzed and a reliable correction strategy is derived.

Monoplanar gradient system for imaging with nonlinear gradients

ObjectIn this paper we present a monoplanar gradient system capable of imaging a volume comparable with that covered by linear gradient systems. Such a system has been designed and implemented.Materials and methodsBuilding such a system was made possible by relaxing the constraint of global linearity and replacing it with a requirement for local orthogonality. A framework was derived for optimization of local orthogonality within the physical boundaries and geometric constraints. Spatial encoding of magnetic fields was optimized for their local orthogonality over a large field of view.ResultsA coil design consisting of straight wire segments was optimized, implemented, and integrated into a 3T human scanner to show the feasibility of this approach. Initial MR images are shown and further applications of the derived optimization method and the nonlinear planar gradient system are discussed.ConclusionEncoding fields generated by the prototype encoding system were shown to be locally orthogonal and able to encode a cylindrical volume sufficient for some abdomen imaging applications for humans.

Gradient waveform pre-emphasis based on the gradient system transfer function

The gradient system transfer function (GSTF) has been used to describe the distorted k‐space trajectory for image reconstruction. The purpose of this work was to use the GSTF to determine the pre‐emphasis for an undistorted gradient output and intended k‐space trajectory.