Research On Enhancing Magnetic Resonance Image Quality With High Permittivity Materials

Radio frequency(RF)coils are one of key parts in a magnetic resonance imaging(MRI)system as the components in the forefront for transmitting RF pulses and receiving signals in MRI.The homogeneity of B1+ affects the image homogeneity,and the sensitivity of receiving coils is one of main factors to determine the image signal to noise ratio(SNR),so the performance of RF coils plays a crucial role in MR image quality.Compared with other components in MRI systems,RF coils are easier to develop and optimize,so how to improve the performance of RF coils effectively has been one of the focuses of MRI system research.Traditional techniques to improve the performance of RF coils mainly include improving coil structures and reducing the conductor loss at present,however,these techniques are complicated or expensive.In recent years it has been found that properly placing high permittivity materials(HPMs)between the imaging object and RF coil can effectively improve the transmit efficiency and receive sensitivity of the coil which lead to higher SNR and lower special absorption rate(SAR).HPMs can be used as auxiliary devices of RF coils without altering RF coils or other components,they have become a new research hotspot in the field of MRI because they can be used flexibly and effectively enhance MR image quality.It has been shown that the precession frequency,the relative permittivity,thickness and position of HPMs have great influence on the enhancement effect of MR image quality.Therefore the research on enhancing MR image quality based on HPMs was carried out.Firstly a fast simulation method of the birdcage coil loaded with HPMs was proposed,which greatly shortened the simulation time compared with the traditional electromagnetic simulation method.Then a quantitative study was carried out on the influence of material geometry structure on MR images,and HPMs was also applied to animal MRI.The details are as follows:(1)A fast simulation method of the birdcage coil loaded with HPMs was proposed.This method included the lumped capacitance estimation and electromagnetic field and radio frequency circuit co-simulation,which could quickly complete the simulation of birdcage coils loaded with HPMs without manual iteration.Compared with traditional 3-D electromagnetic simulation optimization,this method could significantly shorten the total simulation time and accelerate the simulation design of RF coils.(2)A quantitative study on the effect of HPMs geometry structures on MR image quality was carried out.A birdcage coil was manufactured and BaTiO3 pads was prepared and tested,the effects of HPMs geometry structures on MR image quality were quantitatively studied through electromagnetic simulation and MRI experiments.The experimental results showed that the cylindrical and symmetrical annular sector cylindrical BaTiO3 pads effectively enhanced the image SNR and slightly reduced the inhomogeneity.Meanwhile,there was a significant linear correlation between the normalized image SNRr and the normalized mean of simulated transmit efficiency squared.Therefore,electromagnetic simulation results were able to quantitatively predict and evaluate the performance of RF coils loaded with HPMs,which accelerated the optimization design of HPMs and RF coils.(3)A research on the enhancement of animal MR image quality with HPMs was conducted.We applied lead zirconate titanate(PZT)ceramic pads to animal MRI and the effects of symmetrical annular sector cylindrical PZT pads on MR images were studied.The experimental results showed that the transmit power required by the coil with the same pulse sequence was reduced by about 29%and overall image SNR was improved by about 30%.Our research verified that the PZT pads had a great enhancement effect on animal MR images,which had a certain reference value for further research on enhancing MR image quality with HPMs.