Design of biplanar shim coils for ultra-low field MRI

Abstract

Ultra-low field magnetic resonance imaging (MRI) is a popular research topic in the field of mobile medical imaging. We construct a 50 mT ultra-low field MRI system for real-time monitoring of intracranial bleeding. The region of interest is a spherical space with a diameter of 200 mm, which is sufficiently large for a brain tissue. The homogeneity of the magnetic field determines the signal-to-noise ratio of MRI directly. First, the distribution of the main magnetic field is measured and expanded by using orthogonal spherical harmonic functions. Second, a harmonic coefficient method is used to design active shim coils of the first two orders, with each coil generating a specific harmonic magnetic field. Lastly, the contour distribution of the stream function is utilized to obtain the winding of the shim coils. Results show that each shim coil can generate a pure harmonic magnetic field within an error of 5%. The magnetic field uniformity is reduced from 137 ppm to 28 ppm by these shim coils.