Two‐dimensional imaging in a lightweight portable MRI scanner without gradient coils

Abstract

Purpose As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as Intensive Care Units, physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods We construct and validate a truly portable (<100kg) and silent proof-of-concept MRI scanner which replaces conventional gradient encoding with a rotating lightweight cryogen-free, low-field magnet. When rotated about the object, the inhomogeneous field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections which encode the iteratively reconstructed 2D image. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but is shown to be likely sufficient for many medical applications. Conclusion The presented MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating the gradient coil. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology.

Proposal for a permanent magnet system with a constant gradient mechanically adjustable in direction and strength

“…Secondly the homogeneity requirements for the gradient field are less demanding in image reconstruction as nicely demonstrated in Ref. 6 and from Figs. 4(c-f) it can be estimated that the area of a certain gradient homogeneity scales logarithmically with the number of used magnets.…”

“…If the first quadrupole is indicated by "Q1" and the second by "Q2" their combined field can be expressed by Eq. [6] as…”

“…However, shimming permanent magnets is still a demanding iterative process. In 2014 Zimmerman Cooley et al presented a method to produce MR-images utilizing the inhomogeneous magnetic field of a non-shimmed Halbach (6). This was done by collecting spatial information from exact knowledge of the field, mechanical rotation of the entire magnet, using an eight channel receiver array and image reconstruction via special software.…”

“…However, there is a slight problem with the proposed method in Ref. 6 because the image reconstruction is limited in regions of rather homogeneous fields (e.g. in the center).…”

“…Like in Ref. 6 only permanent magnets are involved, however, deliberately generating better defined inhomogeneities that are scalable in amplitude and direction. It is hoped that simple Fourier-transformation and Radon-transformation (filtered backprojection) will be sufficient to generate images by rotating either gradient or object between scans.…”