Design of a multilayer Halbach permanent magnet for human finger NMR detection

Abstract

Permanent magnet is the key component of nuclear magnetic resonance (NMR) instrument. Based on the principle of Halbach array, this study designs and optimizes a multilayer Halbach permanent magnet for human finger NMR detection. The quality factor (QF) is introduced to represent the performance of magnetic field, firstly. The impact on magnetic performance, caused by various magnet heights, is analyzed using Finite Element Method(FEM). In order to compensate the end effect, the radial adjustment method is proposed, combining the adjustments of the radial size and the radial position of the magnet blocks. Comparing the effect on magnetic field of three different plans, the results show that combining the adjustments of inner radius and outer radius shims the field dramatically. Finally the designed construction is stacked by 11 layers of magic rings and each layer is composed of 8 trapezoid-shaped magnets. The magnetic field of the center cylindrical region (ϕ20 × 20 mm) of the structure is calculated, whose final strength achieves up to 447 mT and homogeneity is within 500 ppm. The designed magnet structure is manufactured and the measurement is finished in the end. The proposed construction has the advantages of small size and high uniformity, which has an extensive application prospect in portable NMR system.

New Halbach System Optimization Design

“…As a result, the magnetic block with trapezoidal cross section can be used in this study to build the Halbach array. Figure 2 depicts the partial construction of the magnetic ringl; the height of the trapezoidal section is d = r out -r in , and the widths of the baseline is a =rintan(θ/2) and the topline is b =routtan(θ/2), respectively 14 .…”

A Novel Optimization Method for Halbach Magnet

“…The partial structure of the magnetic ring is shown in Fig. 2, and the height of the trapezoidal section d = r out -r in , the width of the baseline a and the topline b are a = r in tan(θ /2) and b = r out tan(θ /2), respectively 30 .…”

Ferrofluids to Improve Field Homogeneity in Permanent Magnet Assemblies

“…This is beneficial for compact systems that enable magnetic resonance measurements in flow, where a solenoid is wound around a liquid sample tube that runs through the bore [13,14]. Reported Halbach magnets have lengths ranging from 5 to 70 cm, with reported spatial field homogeneity values in the range of 20-2000 ppm [15][16][17][18][19]. This is not homogeneous enough for NMR spectroscopy based on chemical shifts, for which a homogeneity of the order of 0.1 ppm or better is desired [13,20] but it can be sufficient for other applications that employ magnetic resonance principles, such as flow measurements [21] or relaxometry [22].…”