Spatial encoding with nonlinear magnetic fields has recently drawn attention for its potential to achieve faster gradient field switching within safety limits, tailored resolution in regions of interest, and improved parallel imaging using encoding fields that complement the sensitivity profiles of RF receive arrays. Proposed methods can broadly be divided into those that employ phase encoding (PatLoc and COGNAC) and those that acquire nonlinear projections (O-Space, Null Space imaging, and 4D-RIO). Projection methods typically reconstruct images using iterative algorithms to backproject the data by exploiting the full encoding matrix. For this reason, they are more sensitive than phase encoding methods to systematic errors introduced by imperfect knowledge of the encoding trajectory.
In the present work, voxel-wise phase evolution is mapped at each acquired point in an O-Space trajectory using a variant of chemical shift imaging, capturing all spin dynamics caused by encoding fields, eddy currents, and pulse timing. Phase map calibration is then applied to data acquired from a high-power, 12 cm, Z2 insert coil with a home-built 8-channel RF transmit-receive array on a 3T human scanner. We show the first experimental proof-of-concept O-Space images on in vivo and phantom samples, paving the way for more in-depth exploration of O-Space and other nonlinear projection imaging methods. We also use quadratic phase preparation with a Cartesian pulse sequence to image form a localized field-of-view within the sample. This extends a previously presented method ("GradLoc") by creating localization with a 3-D encoding field and also by combining it with parallel imaging for scan acceleration.
|Advisor:||Constable, R. Todd|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 73/12(E), Dissertation Abstracts International|
|Subjects:||Biomedical engineering, Medical imaging|
|Keywords:||Nonlinear magnetic fields, Parallel imaging, Quadratic fields, Spatial encoding|
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