Research On Design Methods Of The Deployable Gradient Coils In MRI

Magnetic resonance imaging(MRI)is a non-invasive imaging technology which can reflect the internal structure of an object.In the past few decades,MRI is widely used in medical diagnosis,biological research,and material science.In recent years,MRI has been used in imaging at the microscale with spatial resolutions less than 100 ?m.Therefore,Magnetic Resonance Microscopy(MRM)is one of the important trends in the development of magnetic resonance imaging.There are some challenges for micro scale magnetic resonance imaging.On the one hand,there is a challenge to image mass-limited or volume-limited samples due to the huge reduction in signal-to-noise ratio(SNR);On the other hand,a higher imaging spatial resolution is required to provide smaller features for the micro scale magnetic resonance imaging.Therefore,improving the imaging spatial resolution and signal sensitivity is critical for MRM.According to the principle of magnetic resonance imaging,the SNR of imaging is mainly limited by the signal of RF coils and the strength of main magnetic field.Imaging spatial resolution can be improved by increasing the gradient strength of the magnetic field generated by the gradient coils.Imaging with micro coils which closely conform to small samples is a straightforward way to improve the spatial resolution and SNR.Therefore,miniaturization of the coils of the magnetic resonance system has become a hot topic.This thesis will focus on the design of deployable micro gradient coils in MRI.The design of gradient coils for magnetic resonance imaging(MRI)system is a multiple objectives optimization problem,which usually needs to deal with a couple of conflicting design objectives,such as the stored magnetic energy,power consumption,and target linear gradient distribution.These design requirements are conflicting and usually there is no unique optimal solution which is capable of minimizing all objectives simultaneously.Therefore,the gradient coils design needs to be optimized reasonably with the tradeoff among different design objectives.Based on the developable property of the cylindrical surface of gradient coils and the stream function method,the multiple objectives optimization problem is analyzed by using the Pareto optimization method in this paper.The corresponding values of the stored magnetic energy,power consumption,and target linear gradient distribution regarding to the performance of gradient coils are discussed respectively.The quantitative relationships of each design requirements are analyzed in Pareto solution space,where Pareto optimal solutions can be intuitively found by dealing efficiently with tradeoff among different coil properties.The optimization results show that there are multiple available solutions in the convex Pareto solution space under the constraints that the linear gradient deviation is less than 5%,and that the magnetic stored energy and power dissipated are both no more than a user-specified value.With the proposed approach,coil designers can have a reasonable overview of gradient coil design about the achievable performance of some specific properties and the competing or compatible relationships among coils properties.Thereby,a suitable design of the gradient coils for a given requirement of MRI application can be chosen reasonably.Even though the current density technique is successful for the design of MR gradient coils,it also has some limitations.Firstly,the current distribution derived by these methods must be approximated by discrete wires or current paths.As for small numbers of wires,the approximation to the current distribution is poor,resulting in deviation of the calculated magnetic fields.Secondly,return paths must be introduced to assure that equal currents flow in all paths.The return currents in these wires produce a smaller negative field which reduces somewhat the gradient achievable,and lead to a longer geometry for the current-carrying surface,and a higher inductance.Thirdly,the complexity of the configuration increases the difficulty of manufacturing gradient coils for microscale MRI.Therefore,a topology optimization method based on the density method is proposed for designing gradient coils of magnetic resonance microscopy.Different from the popular stream function method,the design variables of the proposed method are the distribution of conductive material,which can avoid post-processing errors that occur when approximating the continuous current density by discrete wires in stream function approaches.And a voltage-driven transverse gradient coil is proposed to avoid introducing a coil-winding pattern and simplifies the coil configuration.The feasibility and accuracy of the method are verified through the design of z-gradient and y-gradient coils on a cylindrical surface.Numerical examples illustrate the influence of the optimization model and design parameters to final coil configurations.Even though the proposed method still is in its infancy,the preliminary design results have shown that it has advantages that the stream method cannot match,and provides more possibilities for improving the performance of gradient coils.It is desirable that a topologically optimized design can be fabricated reliably by a certain manufacturing process.To fulfill the requirement for manufacturing,one recent trend is to directly consider the manufacturing characteristics in the optimization process to achieve length scale on the optimized design and thus ensure prototype manufacturability.In this paper,a novel structure length scale control method is proposed based on the morphological operators,such as dilation and erosion.The effectiveness of the proposed method is verified by the optimized designs of mechanical tensile structures and cantilever structures.As to the gradient coil design,the gradient coils with larger volume fraction of conductive material may lead to eddy currents between the coils,which will affect magnetic field linearity.In order to avoid large eddy currents,the length scale control method is utilized in the design of gradient coils.And a topological configuration composed of multiple thin wires is obtained.The design results show that this method can control the coil size effectively.