Quadrature-driving Birdcage Coil Design For 7.0T Magnetic Resonance Imaging

With its comparative advantages of non-invasive,high resolution,non-radiative and so on,magnetic resonance imaging(MRI)technology has been more and more widely used in modern life.The upper limit of a MRI system mainly depends on the strength of its static magnetic field.At present 3 Tesla or lower field MRI human-body system is most popular in commercial use,meanwhile,system at higher magnetic fields,like 7 Tesla can provide better SNR which will increase partial resolution or reduce the time for imaging.There are some limits nowadays,which include unproven safeness and stagnant of some key technologies,restrict the development of the ultra-high field MRI system for human use.On the other hand,ultra-high field animal MRI has been developed rapidly for some frontier scientific research.RF coils are the front end of MRI system,which is essential to the signal sensitivity.Compared with the surface coil,the birdcage coil has excellent structural symmetry,which can make the RF excitation field to be well distributed,and is widely used in commercial MRI instruments.The birdcage coil excited by the orthogonal excitation can induce a circular polarized magnetic field,which further improves the uniformity of the magnetic field and the sensitivity of the signal.However,the complexity of the birdcage coil structure increases the difficulty of the tuning of the coil,and the uniformity of the RF coil has a high demand on the design of the coil.In the orthogonal coil isolation in two channels will have a greater impact in experimental results,the unreasonable design of the tuning and decoupling structure will make the coil unbalanced,causing a serious distortion of the imaging signal,so the design of a tuning and isolation circuit in a quadrature birdcage coil needs to focus on the issue.On the other hand,the complexity of the coil structure in quadrature birdcage coil has been enhanced in ultra high field,such as antenna effect and parasitic capacitance in the 7.0T MRI system.Based on the theory of magnetic resonance and RF circuit principle,using CST software modeling and simulation aided design,we provide a systematic way to design the quadrature-driving birdcage coil for ultra-high field.A 8 leg high-pass quadrature-driving transceiving birdcage coil is developed for 7T animal MRI system.The tuning and decoupling structure,as well as the tuning method have been carefully studied on ultra high field quadrature birdcage coil.The experiment and test results show that the coil can work at 285MHz to 320MHz,in the working frequency band good matching(S11<-20dB,S22<-20dB)and high isolation(without Cable Trap S21<-15dB)can be achieved.The Q value without load reached 162.A water phantom and a fresh lemon have been finally used to test the coil on the Aglient 7.0T animal MRI system,the results show that good B1 magnetic field uniformity,SNR and SAR of the coil have been achieved.The images generated with the coil have good uniformity(>90%)and clear details.