Abstract
Dynamically adjustable permanent magnet arrays have been proposed to generate switchable magnetic fields for pre-polarisation in Ultra-Low Field magnetic resonance imaging. However, the optimal switching dynamics of the pre-polarisation magnetic field as well as the energy requirements, mechanical forces and stresses during switching of the pre-polarisation field have not been evaluated. We analysed these requirements numerically and estimated the magnetic resonance signal strength and image quality for two practical switching modes in an instrument suitable for scanning the human head. Von Mises stress analysis showed that although magnetic forces were significantly higher for two specific rungs, the structural integrity of magnet rungs would not be compromised. Our simulations suggest that a significantly higher signal yield is obtained by switching off the pre-polarisation field with the angular velocity in each rung dependent on its location.
Highlights
Adjustable permanent magnet arrays have been proposed to generate switchable magnetic fields for pre-polarisation in Ultra-Low Field magnetic resonance imaging
The studies described here deal with an ultra-low field (ULF) magnetic resonance imaging (MRI) instrument suitable for imaging the human head based on small permanent magnet arrays
We focus on two switching modes in a circular magnetic array, (a) one in which each magnet rung rotates with equal angular velocity, so that rotation stops at different times for different magnet rungs, and (b) one in which angular velocity varies with the location of the magnet in the array such that rotation of all magnets stops simultaneously
Summary
Adjustable permanent magnet arrays have been proposed to generate switchable magnetic fields for pre-polarisation in Ultra-Low Field magnetic resonance imaging. The optimal switching dynamics of the pre-polarisation magnetic field as well as the energy requirements, mechanical forces and stresses during switching of the pre-polarisation field have not been evaluated We analysed these requirements numerically and estimated the magnetic resonance signal strength and image quality for two practical switching modes in an instrument suitable for scanning the human head. Described pre-polarisation techniques entail the temporary placement of the sample within a strong homogeneous magnetic field to increase its net magnetisation, followed by rapid translation of the sample into a weak magnetic field for signal acquisition Such an approach is not feasible for human imaging due to the accelerations involved in moving the sample between high and low field locations[5,6]. We analysed the mechanical strains induced by the static magnetic forces and torques during acceleration/deceleration
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