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Magnetic Resonance Materials in Physics, Biology and Medicine

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22-11-2022 | Magnetic Resonance Imaging | Research Article

Concept for gradient-free MRI on twin natural slices

Author: Gordon E. Sarty

Published in: Magnetic Resonance Materials in Physics, Biology and Medicine | Issue 4/2023

Abstract

Objective

The design of an MRI for use in space requires that the hardware be kept to an absolute minimum in terms of mass, complexity, and power. In addition, NASA requirements are that the external stray field needs to be less than 3.2 Gauss, 7 cm from the MRI enclosure.

Theory

RF encoding designs with Halbach magnets offer the best chance of meeting those requirements. Spatially non-uniform magnetic fields with foliations of isomagnetic surfaces, or natural slices, may be used to provide slice selection, and to reduce further the hardware complexity, for TRansmit Array Spatial Encoding (TRASE) Magnetic Resonance Imaging (MRI) or potentially for other radio frequency (RF) encoding methods. The design of such non-uniform magnetic fields in a Halbach configuration with built-in axial gradients leads to pairs of isomagnetic surfaces centered on either side of a central maximum field strength slice. If TRASE images from slices other than the central isomagnetic surface are desired, then the Nuclear Magnetic Resonance (NMR) signals originating from the twin natural slices must be separated during image reconstruction. Here, a design for simultaneously imaging on twin slices in such an inhomogeneous magnetic field using multiple receiver coils with spatially varying RF profiles is described mathematically and numerical simulation examples are given.

Design approach

To achieve RF encoding on the natural slices, at least three TRASE transmit coils are required. Here a solution with twisted solenoid coils is given. To achieve the twin slice separation at least two receive coils are required. Here a solution with two solenoids is given.

Discussion

The MRI design presented here uses a combination of RF encoding (TRASE), a spatial encoding magnetic field (SEM, pairs of natural slices) and receive coil spatial profiles to encode enough information into the NMR signal for image slice reconstruction. The design presented here enables using Halbach magnets with a built-in axial gradient to be used for MRI.

Conclusion

The result is a new gradient-free TRASE MRI design capable of imaging pairs of electronically selectable axial slices.

Available only for authorised users

Note that we do not distinguish between “Fourier transform” and “inverse Fourier transform” since that is a matter of arbitrary definition. Of course, it is important in practice to get the sign of the exponent correct, otherwise a mirror image of the intended image results.

Here, \(P\rho\) is a low resolution version of \(\rho\) with the resolution limited by the extent of k-space covered.

The units may be considered as meters for a head-size MRI; the 0.1779 number is one used for an actual MRI under development at the time of writing,

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Title: Concept for gradient-free MRI on twin natural slices
Author: Gordon E. Sarty
Publication date: 22-11-2022
Publisher: Springer International Publishing
Keywords: Magnetic Resonance Imaging
Magnetic Resonance Imaging
Published in: Magnetic Resonance Materials in Physics, Biology and Medicine / Issue 4/2023
Print ISSN: 0968-5243
Electronic ISSN: 1352-8661
DOI: https://doi.org/10.1007/s10334-022-01047-x

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Springer Medicine

Concept for gradient-free MRI on twin natural slices

Abstract

Objective

Theory

Design approach

Discussion

Conclusion

Keynote webinar | Spotlight on medication adherence

Springer Medicine

Abstract

Objective

Theory

Design approach

Discussion

Conclusion

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