Design Of Biplanar Shim Coils For Ultra-low Field Nuclear Magnetic Resonance

Cerebrovascular disease is a rapidly developing disease with high risk and high mortality.The current monitoring means include Glasgow Coma Scale(GCS),cerebral Computed tomography(CT)imaging,electrical impedance tomography and intracranial pressure monitoring.GCS depends on the vital signs to observe the score,so there exit inevitable subjectivity and delay.CT imaging machine is large in volume and has high demands for radiation protection,which make it impossible to monitor at the bedside.Electrical impedance tomography fails to meet the requirements for the disease diagnosis because of its low-resolution image,contour blur and large noise.Intracerebral pressure monitoring needs to implant a probe into ventricle to monitor the pressure,and it is unable to reflect intracranial hematoma and edema enlargement in a short time.For brain diseases with high risk,such as stroke,real-time monitoring at the bedside and early indication of changes in the lesion area play key roles in rescue and treatment,which is urgently needed in hospital.Ultra-low Magnetic Resonance Imaging(ULMRI)is made of permanent magnets and does not demand a cooling system.It is lightweight and easy to move.Therefore,an ULMRI is appropriate for rapid and acute recognition of sudden hematoma enlargement or rebleeding.ULMRI will provide material base and professional equipment for diagnosis of intracranial disease and a monitoring system for bedside use.An ultra-low field magnetic resonance device has high requirements for magnetic field homogeneity considering its low main magnetic field value,thereby affecting the signal-to-noise ratio of images directly.However,the error of the magnet manufacturing and installation is unavoidable.Therefore,shimming is essential to make fine corrections of magnetic field.This paper study a method for the design of biplanar shim coils for ultra-low field nuclear magnetic resonance.The content includes the following parts:(1)Magnetic field measurement and harmonic analysis.To measure the original magnetic field distribution in the target area accurately,a self-made magnetic field measurement tool is designd based on the principle of nuclear magnetic resonance.The uniformity of the measured magnetic field is 137 ppm.The main magnetic field is expanded by using orthogonal spherical harmonic functions and the influences of the harmonics on the magnetic field homogeneity are analyzed,which determine the order of shimming.(2)Design of biplanar shim coils in ULMRI.A harmonic coefficient method based on target field method is used to design active shim coils of the first two orders,with each coil generating a specific harmonic magnetic field.In addition,the contour distribution of the stream function is utilized to obtain the winding of the shim coils.Some fine-tunings of structure are needed for each shim coil has only one current source.Each loop of the coil is connected.To simulate the magnetic field produced by the tuned shim coil in the target area,models are established.The magnetic field deviation is within 5%.(3)Shimming experiment.Due to the large target area of this study,RF coil is redesigned with solenoid.The uniformity of RF coil is 5.17%,which meets the requirement.Besides,a shimming system consisting of spectrometer,RF coil,duplexer and shimming coil is built.The half width of the spectrum line of the sample signal is used to reflect the uniformity of the magnetic field.The uniformity of the magnetic field is reduced from 137 ppm to 28 ppm after shimming.Finally,the influence of coil error and coil position offset on the coil magnetic field is simulated and analyzed.The conclusion is that the center line alignment should be paid attention to when the coil is installed.In this paper,a series of works on the design of the biplanar active shim coils for ultra-low field magnetic resonance have been carried out,and the homogeneity of the main magnetic field has been successfully improved,which ensures the image quality and signal-to-noise ratio for ULMRI.