Asymmetric Tapered Solenoid Designs for Halbach-Based Portable Magnetic Resonance Imaging

Abstract

Halbach array is a popular permanent magnet array used in head dedicated portable MRI. It supplies transversal main magnetic field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$B_{0}$</tex-math></inline-formula> ), and it works well with solenoid RF coils when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$B_{0}$</tex-math></inline-formula> is homogeneous. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$B_{0}$</tex-math></inline-formula> a Halbach array supplies is low. To compensate this, it is desired to have a high coil sensitivity of the solenoid coil that works in such a system. Hence in this paper, an asymmetrical tapered solenoid was proposed and an optimization was carried out for both the coil profile and pitches between turns for high coil sensitivity and homogeneity. Multi-objective genetic algorithm was used. B <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{1}$</tex-math></inline-formula> -field was calculated using Biot-Savart's law in the optimization process. The targeted field of view (FoV) is a 190 mm diameter of spherical volume (DSV) for head imaging. The optimized solenoid coils were simulated using frequency domain solver in CST Microwave studio, physically constructed, and compared with a reference coil of comparable dimensions. The optimal design shows B <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{1}$</tex-math></inline-formula> field increase of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 35% (calculation), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 33% (simulation) and minimal trade-off in homogeneity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 10% (calculation and simulation) within 190 mm DSV. For validation, the B <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{1}$</tex-math></inline-formula> sensitivity of the constructed coils were measured in the FoV on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$xy-$</tex-math></inline-formula> plane at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$z=0$</tex-math></inline-formula> . The measured results are in good agreement with the simulated and calculated results.