Radial Imaging With Multipolar Magnetic Encoding Fields

Abstract

We present reconstruction methods for radial magnetic resonance imaging (MRI) data which were spatially encoded using a pair of orthogonal multipolar magnetic fields for in-plane encoding and parallel imaging. It is shown that a direct method exists in addition to iterative reconstruction. Standard direct projection reconstruction algorithms can be combined with a previously developed direct reconstruction for multipolar encoding fields acquired with Cartesian trajectories. The algorithm is simplified by recasting the reconstruction problem into polar coordinates. In this formulation distortion and aliasing become separate effects. Distortion occurs only along the radial direction and aliasing along the azimuthal direction. Moreover, aliased points are equidistantly distributed in this representation, and, consequently, Cartesian SENSE is directly applicable with highly effective unfolding properties for radio-frequency coils arranged with a radial symmetry. The direct and iterative methods are applied to simulated data to analyze basic properties of the algorithms and for the first time also measured in vivo data are presented. The results are compared to linear spatial encoding using a radial trajectory and quadrupolar encoding using a Cartesian trajectory. The direct reconstruction gives good results for fully sampled datasets. Undersampled datasets, however, show star-shaped artifacts, which are significantly reduced with the iterative reconstruction.

MR image reconstruction from generalized projections

“…Also, in radial PatLoc imaging, the Fourier analogues of the individual readouts can still be interpreted as image projections, where, however, the integration may be performed along multiple, bent isocontour lines of the effective encoding field. This has been shown before [cf, Figure in Schultz et al ].…”

Local field of view imaging for alias‐free undersampling with nonlinear spatial encoding magnetic fields

“…Reconstruction of the total FOV in one step from the total sampling pattern, as shown in Ref. ( for a radial trajectory with undersampling along the angular dimension, results in aliasing artifacts outside a circular region. For aliasing free reconstruction within this sub‐FOV, additional extraction of a radial sub‐pattern with shortened spokes to fulfill Nyquist sampling is required.…”

Development and Characterization of An Unshielded PatLoc Gradient Coil for Human Head Imaging

“…This was demonstrated with PatLoc only acquisitions using the linear z ‐gradient for slice selection and two non‐linear PatLoc spatial encoding magnetic fields (SEMs) for in plane encoding and was also applied to a radial sequence . Acceleration using parallel imaging techniques with reduced number of acquired k ‐space lines with higher acceleration factors was also successfully demonstrated .…”

“…Acceleration using parallel imaging techniques with reduced number of acquired k ‐space lines with higher acceleration factors was also successfully demonstrated . All these experiments, on phantoms and volunteers, had improved image resolution at the periphery but reduced resolution in the middle of the acquired images due to the characteristics of the SEMs . To overcome this limitation, PatLoc and linear SEMs are combined with the 4D‐RIO or North West EPI acquisition techniques.…”