The design of a light and compact magnet for MRI

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MRI is the best non-invasive diagnostic imaging modality providing high quality images of the entire body. However, its high costs and claustrophobic magnet geometry prevent a faster growth and broader applications. Therefore we propose to address these weaknesses by designing a new magnet suitable for MRI based on the so-called Halbach structure.****Halbach magnets have a cylindrical shape and comprise small magnetic pieces placed on a surface of a cylinder. Although the Halbach design has been known for over 30 years, a full analytical theory of the magnetic field produced by the magnet has been developed only for a very high length/diameter ratio, when the mathematical description of the field can be reduced to two dimensions (2D). A 3D mathematical model of the finite-length magnet ("long" magnet) with small length/diameter ratio (<1) has been proposed very recently (2014) but a magnet based on this theory has not yet been constructed. Computer programs solving semi-analytical field equations, using the finite elements or the boundary methods to calculate the field have been developed yet not without drawbacks. Furthermore magnet weight or its size have not been optimized so far. In addition, there is no perfect manufacturing process, yet none of the theoretical models include manufacturing errors, such as inconsistent segmentation of the magnetic pieces over the cylinder's surface or their imperfect position.****It is known that, for example, 2D segmentation causes creation of harmonics of the order of a multiple of the number of the segments. To deal with this common problem, the magnetic field over a sphere is analyzed using spherical harmonics. From this analysis, the placement of small compensating magnets is calculated to cancel the spherical harmonics and to improve magnet homogeneity. The field homogeneity may be also improved by correcting positions of the segments. However none of these methods is perfect. In our approach we propose to develop a method of shimming on a surface of a cylinder instead of a sphere, as traditionally used. We expect this approach will provide efficient shimming for "short" Halbach magnets.****In summary, we propose to develop a new design of a light, permanent magnet suitable for standard and "gradient free" MRI, co-developed by the applicant. As "gradient free" MRI does not require gradients of the magnetic field and associated amplifiers, it is feasible to produce inexpensive, light and silent MRI system for many new imaging applications.******