Improvements in RF Shimming in High Field MRI Using High Permittivity Materials With Low Order Pre-Fractal Geometries

Ultra-high field MRI is an area of great interest for clinical research and basic science due to the increased signal-to-noise, spatial resolution and magnetic-susceptibility-based contrast. However, the fact that the electromagnetic wavelength in tissue is comparable to the relevant body dimensions...

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Bibliographic Details
Published inIEEE transactions on medical imaging Vol. 35; no. 8; pp. 1837 - 1844
Main Authors Schmidt, Rita, Webb, Andrew
Format Journal Article
LanguageEnglish
Published United States IEEE 01.08.2016
The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subjects
Arrays
Dielectric constant
Electromagnetic Phenomena
Field strength
Fractals
Geometry
High field MRI
high permittivity dielectric pad
Homogeneity
Humans
Magnetic fields
Magnetic Resonance Imaging
Magnetics
Mathematical model
Permittivity
Radio frequency
RF inhomogeneity
RF shimming
Simulation
Wavelengths
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Summary:Ultra-high field MRI is an area of great interest for clinical research and basic science due to the increased signal-to-noise, spatial resolution and magnetic-susceptibility-based contrast. However, the fact that the electromagnetic wavelength in tissue is comparable to the relevant body dimensions means that the uniformity of the excitation field is much poorer than at lower field strengths. In addition to techniques such as transmit arrays, one simple but effective method to counteract this effect is to use high permittivity "pads". Very high permittivities enable thinner, flexible pads to be used, but the limiting factor is wavelength effects within the pads themselves, which can lead to image artifacts. So far, all studies have used simple continuous rectangular/circular pad geometries. In this work we investigate how the wavelength effects can be partially mitigated utilizing shaped pad with holes. Several arrangements have been simulated, including low order pre-fractal geometries, which maintain the overall coverage of the pad, but can provide better image homogeneity in the region of interest or higher sensitivity depending on the setup. Experimental data in the form of in vivo human images at 7T were acquired to validate the simulation results.
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ISSN:0278-0062
1558-254X
DOI:10.1109/TMI.2016.2531120