Research On The Design Of Small-size RF Coils For Magnetic Resonance Imaging

Radio-frequency(RF) coils are the key component of the magnetic resonance imaging system. RF coils as transmit coils transmit radio frequency pulse signal required in the magnetic resonance imaging system, which produce a uniform stability of the radio frequency magnetic field of Larmor frequency in the imaging region to stimulate the spinning nucleus produces the magnetic resonance signal. RF coils as receiving coils receive magnetic resonance signal that the spinning nucleus produce, which should have a high receiving signal-to-noise ratio and ensure the homogeneity of RF receive magnetic field.This thesis takes the magnetic resonance imaging for fruit fly as the research background, which improves the traditional design method of RF coils and applies it to the design of small-size cylindrical RF coils and small-size shielded RF coils system.The first section of the thesis discusses the design method of small-size cylindrical RF coils. The target field approach is used for finding current densities distributed on cylindrical RF coils surface. Stream function method is applied to discrete current density as the expression of Fourier series. The ill-conditioned integral equation is solved by introducing the minimum curvature constraint function and minimum divergence of current density constraint function which related to the method of Tikhonov regularization. By using software programming and simulation,Fourier coefficient is derived, which is taken into the expressions of current density and magnetic field intensity to derive the winding pattern of RF coils and magnetic field intensity in region of interest(ROI).In the second section of the thesis, the design method of small-size cylindrical RF coils is extended. According to the design principle of the shielded RF coils system, the shielded coils which is coaxial with the primary coils and has current densities distributed on its surface is designed. A small-size shielded radio RF coils system is constituted. By applying the design method of small-size cylindrical RF coils, the RF coils system is solved, which derives the winding pattern of primarycoils and shielded coils with magnetic field intensity in the interior target region of primary coils and exterior target region of shielded coils. The influence of resizing the RF coils and selecting the constraint factors on the winding pattern of RF coils and the homogeneity of the induced magnetic field in the ROI is discussed.Research indicates that the RF coils designed by this method can generate a uniform magnetic field in the region of interest. The constraint factors can change the winding pattern of the coils and the homogeneity of the induced magnetic field in the ROI. Different sizes of RF coils own their appropriate constraint factors values. The selection of penalty factors needs to consider the simplicity of the winding pattern of RF coils and the homogeneity of the induced magnetic field in the ROI.The innovation points of this thesis are as follows:(1) the small-size cylindrical RF coils and small-size shielded RF coils system are designed by applying the target field approach. By combining minimum divergence of current density constraint function and minimum curvature constraint function, the ill-integral equation is restrained.(2) In the actual coils processing, the influence of selecting the constraint factors on the winding pattern of RF coils and the homogeneity of the induced magnetic field in the ROI is discussed in detail.