Study On The Design Of Gradient Coils For Fully Open Permanent MRI System

This work is a part of the National Natural Science Foundation of China "key issues on magnet system of Anti-traditional medical magnetic resonance imaging device" under Grant 50377027.In present medicine applications, most magnetic resonace imaging (MRI) devices are traditional systems in which imaging regions are surrounded by main magnets. The main magnet of this kind of MRI system has huge volume, so that the whole equipment is very expensive, which makes small hospitals not able to afford it. Besides, the imaging region is located inside the magnet, which is not suitable for the use in intervention therapy and operation. Fully open permanent MRI system is a newly proposed device of which the imaging region has a slice shape and is located outside the magnet structure. The weight of permanent main magnet for the new type of MRI system is much less than that of magnet for the traditional MRI system. Thereore, the new MRI devide is not so expensive, and is fit to use more extensively. Gradient coils are important part of MRI system, the design of which is categorized as an inverse problem of electromagnetic field and has attracted many attentions recently. Furthermore, the gradient coils' design for the newly proposed permanent MRI system is a special inverse problem of electromagnetic field.The x(or z) gradient coil is designed to generate gradient magnetic fields which are parallel to the magnet's main field and vary linearly in region of interest along the x(ox z) axe of a Cartesian coordinates system. In this paper, the design methods of gradient coils for fully open permanent MRI device with slice imaging region are studied, and a novel target field method is proposed to design x and z uniplanar gradient coils. The theoretical optimal design of uniplanar gradient coil has been formulated based on R.Turner's target field method, the expression of coil energy and power dissipation can be acquired. The surface current density of the coil is represented by a two-dimensional Fourier series expansions, and the z component of magnetic field in region of interest can be obtained. A target function is constructed in terms of gradient coil's energy and power dissipation. The problem of solving current density is converted into that of Fourier coefficients. Appling stream function technique, the coil patterns are acquired. Employing the Biot-Savart law to the discrete current loops, the gradient magnetic field has been re-evaluated in order to validate the method proposed. The nonlinearity, inductance and resistance of the gradient coils designed with the methods roposed satisfy the design requirement.Eddy currents will be caused in ambient conducting parts because of the rapid switch speed of current pulse in gradient coils. The desired fast rise of the gradient magnetic filed is suppressed by the eddy currents so that the imaging process is affected. To diminish the influence of the eddy currents, the design method of self-shielded gradient coils is studied and the self-shielded uniplanar gradient coils are designed in this paper.The interaction between the current of the gradient coil and spatially static magnetic field components of main magnet generates a net Lorentz force, which causes problems of vibration and excess acoustic noise. The noise will make patients uneasy and does harm to patients sometimes. To avoid the generation of the noise, the design method of balanced Lorentz force gradient coils is studied and the gradient coils with cacelled net thrust force are designed.To revalidate the theory and method presented in this paper, an x planar finger gradient coil model is made and the z component of magnetic field in imaging region yielded by gradient coil is measured. The calculated results and measured results validate the method proposed.