Reconstruction of MRI data encoded by multiple nonbijective curvilinear magnetic fields

Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan
Magnetic Resonance in Medicine (Impact Factor: 3.57). 10/2012; 68(4):1145-56. DOI: 10.1002/mrm.24115
Source: PubMed


Parallel imaging technique using localized gradients (PatLoc) uses the combination of surface gradient coils generating nonbijective curvilinear magnetic fields for spatial encoding. PatLoc imaging using one pair of multipolar spatial encoding magnetic fields (SEMs) has two major caveats: (1) The direct inversion of the encoding matrix requires exact determination of multiple locations which are ambiguously encoded by the SEMs. (2) Reconstructed images have a prominent loss of spatial resolution at the center of field-of-view using a symmetric coil array for signal detection. This study shows that a PatLoc system actually has a higher degree of freedom in spatial encoding to mitigate the two challenges mentioned above. Specifically, a PatLoc system can generate not only multipolar but also linear SEMs, which can be used to reduce the loss of spatial resolution at the field-of-view center. Here, we present an efficient and generalized image reconstruction method for PatLoc imaging using multiple SEMs without explicitly identifying the locations where SEM encoding is not unique. Reconstructions using simulations and empirical experimental data are compared with those using conventional linear gradients to demonstrate that the general combination of SEMs can improve image reconstructions. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.

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    • "Both SMASH/GRAPPA and SENSE, and indeed most of MRI, were designed to work with linear B 0 gradients. More recently, specialized non-linear gradient field coils [11] have been used with time varying gradient methods, like PatLoc [12] [13] [14], to reduce the ∆B max /∆t responsible for producing painful peripheral nerve stimulation. The efficiency of imaging with nonlinear B 0 encoding fields may be increased, for example in methods like O-space imaging [15] and null space imaging [16], by designing the B 1 fields to encode information complementary to that encoded by the SEM. "
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    ABSTRACT: Sequences that encode the spatial information of an object using nonlinear gradient fields are a new frontier in MRI, with potential to provide lower peripheral nerve stimulation, windowed fields of view, tailored spatially‐varying resolution, curved slices that mirror physiological geometry, and, most importantly, very fast parallel imaging with multichannel coils. The acceleration for multichannel images is generally explained by the fact that curvilinear gradient isocontours better complement the azimuthal spatial encoding provided by typical receiver arrays. However, the details of this complementarity have been more difficult to specify. We present a simple and intuitive framework for describing the mechanics of image formation with nonlinear gradients, and we use this framework to review some the main classes of nonlinear encoding schemes. © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part A 40A: 253–267, 2012.
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