Abstract
Low-cost and portable magnetic resonance imaging (MRI) may make this imaging modality more accessible. Permanent-magnet-array is an option to supply a static magnetic field (B-field) with portability, low cost, and no power consumption. However, it has low field strength. Moreover, it does not have linear gradients, thus the signals and the images are not linked by the Fourier transformation as they are in a conventional system. The B-field generated by an array and called spatial-encoding-magnetic-field (SEM), is spatially non-linear and always on. Such an SEM, in terms of the field strength, direction, homogeneity, pattern and its field pattern variation, is related to the image quality. This relation is crucial because it can be used to guide the magnet and system design for high image quality and portability. However, it has not been systematically studied. In this paper, the characteristics of the SEMs from different magnet array designs are identified. Due to the non-linearity of the SEMs, local structural similarity (SSIM) index is proposed to evaluate the region-dependent image quality, and local k-space is applied to analyze the region-dependent effects of these SEMs on image reconstruction. Moreover, point spread function is applied to analyze the overall effect of the SEMs on the quality of reconstructed images. Besides the intrinsic effects of the SEMs, those of the external factors, e.g. the receive coil sensitivity, are analyzed. This study identifies the unique characteristics of the SEMs in a permanent-magnet-array-based MRI system, and offers methods to analyze the unique relation between the image quality and the field. It can not only guide the magnet designs but also trigger more design ideas, e.g., the design of the mechanical movement of the magnet array, and that of the static magnetic field shimming coils, paving the way towards a low-field MRI system with practical portability.
Highlights
Magnetic resonance imaging (MRI) shows the advantages of good soft tissue contrast, non-ionizing radiation, a capability to image an arbitrary plane, etc
The relation of an encoding field pattern with a rotation, and a mechanical movement in general, and the image reconstruction will be further investigated and elaborated by generalizing the three field patterns from known magnet arrays in Fig. 3 to more general patterns that represent some of the main types of field patterns, and can be achieved based on known magnet arrays
This paper presents a detailed study on the characteristics of the encoding magnetic field generated by permanent magnet arrays, in terms of its effect on the quality of image reconstruction in an MRI system
Summary
Magnetic resonance imaging (MRI) shows the advantages of good soft tissue contrast, non-ionizing radiation, a capability to image an arbitrary plane, etc. In an MRI scanner with expected portability and reduced cost when a permanent magnet/magnet array is used to generate a spatially non-linear SEM, the SEM has unique characteristics compared to those reported in the PatLoc framework Such an SEM is always on and its variation is not obtained by electrically switching but by, for example, mechanical movements. The characteristics of SEMs generated by different permanent magnet arrays are identified The effects of these encoding fields and their pattern variations on image reconstruction are detailed. The field patterns generated by the Halbach magnet array, the IO ring-pair magnet array, and the irregular-shaped IO ring-pair magnet array in Fig. 2 are used as examples for the investigation
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