Localization by nonlinear phase preparation and k‐space trajectory design

Abstract

A technique is described to localize MR signals from a target volume using nonlinear pulsed magnetic fields and spatial encoding trajectories designed using local k-space theory. The concept of local k-space is outlined theoretically, and this principle is applied to simulated phantom and cardiac MRI data in the presence of surface and quadrupolar gradient coil phase modulation. Phantom and in vivo human brain images are obtained using a custom, high-performance quadrupolar gradient coil integrated with a whole-body 3-T MRI system to demonstrate target localization using three-dimensional T 2 *-weighted spoiled gradient echo, two-dimensional segmented, multiple gradient encoded spin echo, and threedimensional balanced steady-state free precession acquisitions. This method may provide a practical alternative to selective radiofrequency excitation at ultra-high-field, particularly for steady-state applications where repetition time (TR) must be minimized and when the amount of energy deposited in human tissues is prohibitive. There are several limitations to the approach including the spatial variation in resolution, high frequency aliasing artifacts, and spatial variation in echo times and contrast. Magn Reson Med 67:1620-1632,

Monoplanar gradient system for imaging with nonlinear gradients

“…Combining nonlinear SEMs with linear gradients enables novel fast imaging strategies. One example of such a technique is localization by nonlinear phase preparation, which has been recently demonstrated by Witschey et al [30]. First in-vivo results from cardiac imaging by use of localization by nonlinear phase preparation have been reported elsewhere [31].…”

A z‐gradient array for simultaneous multi‐slice excitation with a single‐band RF pulse

“…Although the proposed methods create locally linear gradient fields, these fields can also be regarded as N‐SEMs in general. N‐SEMs have proven to be useful in many aspects of both the reception and excitation phases of an imaging sequence. Therefore, increased hardware complexity is useful for many other purposes in addition to multi‐slice excitation with a single‐band RF pulse.…”

Reconstruction of MRI data encoded by multiple nonbijective curvilinear magnetic fields

“…Additionally, it has been shown that PatLoc system can be used to achieve small FOV imaging (20) and parallel transmission (21,22). However, the price to pay includes the complexity in hardware availability, control, and image reconstruction.…”