Elliptical Halbach magnet and gradient modules for low-field portable MRI

TL;DR

This paper presents a novel elliptical Halbach magnet and gradient module design for portable MRI, achieving improved field homogeneity, larger field of view, and enhanced gradient performance suitable for point-of-care applications.

Contribution

It introduces an optimized elliptical Halbach magnet configuration and gradient coils, advancing portable MRI technology with better homogeneity, larger FoV, and efficient gradient performance.

Findings

Magnet generates 87 mT field with 5,700 ppm homogeneity in 20 cm FoV

Gradient efficiency reaches up to 0.8 mT/m/A with 12 mT/m maximum gradient

System weighs 216 kg and has heat dissipation around 25 W

Abstract

Objective. To develop methods to design the complete magnetic system for a truly portable MRI scanner for neurological and musculoskeletal (MSK) applications, optimized for field homogeneity, field of view (FoV) and gradient performance compared to existing low-weight configurations. Approach. We explore optimal elliptic-bore Halbach configurations based on discrete arrays of permanent magnets. In this way, we seek to improve the field homogeneity and remove constraints to the extent of the gradient coils typical of Halbach magnets. Specifically, we have optimized a tightly-packed distribution of magnetic Nd$_2$Fe$_14$B cubes with differential evolution algorithms, and a second array of shimming magnets with interior point and differential evolution methods. We have also designed and constructed an elliptical set of gradient coils that extend over the whole magnet length, maximizing the distance between the lobe centers. These are optimized with a target field method minimizing a cost function that considers also heat dissipation. Main result. We have employed the new toolbox to build the main magnet and gradient modules for a portable MRI scanner designed for point-of-care and residential use. The elliptical Halbach bore has semi-axes of 10 & 14 cm and the magnet generates a field of 87 mT homogeneous down to 5,700 ppm (parts per million) in a 20 cm diameter FoV, it weighs 216 kg and has a width of 65 cm and a height of 72 cm. Gradient efficiencies go up to around 0.8 mT/m/A, for a maximum of 12 mT/m with in 0.5 ms with 15 A & 15 V amplifier. The distance between lobes is 28 cm, significantly increased with respect to other Halbach-based scanners. Heat dissipation is around 25 W at maximum power, and gradient deviations from linearity are below 20% in a 20 cm sphere.