| High-field magnetic resonance has become more widely used because of its advantages such as high resolution.Fetal magnetic resonance imaging(FMRI)develops rapidly in the high field with its special advantages,such as the ability to imaging the brain,lung or other complicated pathologies and reduction of amniotic fluid.However,due to the inhomogeneity phenomenon in the high field,it's easy to generate local hot spots s during imaging.The inhomogeneity of the transmission field B1+ affects the image quality in(FMRI)and cause the image fail to meet clinical needs.Due to lack of dedicated fetal imaging RF coils,the system body coil is often used to acquire fetal images.This setup is not optimized and offers limited sensitivity and image quality.Therefore,to obtain a high-quality image,it must be required the B1+ field generated by the transmitting coil is sufficiently homogeneity and the receiving coil has a sufficiently high signal-to-noise ratio(SNR).With the development of MRI RF coil technology,the research on improving the distribution of electromagnetic(EM)field for the propagation path of RF EM fields such as B1+ field has begun to emerge.High dielectric(HD)materials have been shown that high dielectric material has the ability to change the electromagnetic field distribution,and increase the imaging sensitivity and safety.The dielectric properties,geometry of the(HD)material and the position of the load relative to the RF coil are important factors in determining the distribution of the B1+ field.The main task of this paper is to design and use a HD material for fetus imaging.With accomplishing and designing the customized brackets for pregnant women and 22-channel radio frequency receiver coils,Based on the above work,we carried out research and conducted electromagnetic simulations of phantom and pregnant women and complete the 3T clinical testing study to improve the quality of fetus imaging under 3T fMRI.A method combining EM field simulation and circuit simulation was employed in this study,which can dramatically improve simulation efficiency for the optimization of the values of the lumped elements and corresponding EM field distribution.To validate the accuracy and efficiency of the method,a comparison study of the 3-channel dual-tuned RF coil simulation was conducted between the conventional simulation and proposed co-simulation approaches.In this work,firstly,the research of the subject evaluated the Influence of HD Material on the strength,homogeneity,transmission efficiency of B1+ field and the SAR under the high field strengths of the phantom load through electromagnetic simulation analysis.Then verify the simulation on a 3T MRI.Secondly,we modeled and numerically evaluated two typical positions of pregnant patient in MR scans and investigated the effect of high dielectric material to fetal imaging at 3T.The parameters evaluated include position of the high dielectric material,the distribution of the B field,and SAR.In order to maximize the benefits of using HD materials in fetal imaging,we investigate the fetal imaging performance with the use of the different size and thickness of the high dielectric pad through the electromagnetic simulation,and how B1+ field and SAR changes with the size of the dielectric material and ultimately determine an optimal design of HDM to improve fetal magnetic resonance imaging.The results could be used to guide the selection of appropriate size and thickness of dielectric pads in fetal magnetic resonance imaging in order to gain the highest performance.While using the HD materials to improve the distribution of B1+field,a 22-channel flexible coil array for fetal examinations was designed and manufactured to obtain the high quality fetal images.Compared with Siemens 6-channel body coil under the experiments loading the phantom,the proposed fetal coil array achieves significant improvements in imaging coverage,image SNR and parallel acceleration capability in MRI.showing its potential of the fetal coil array for the clinical diagnosis. |
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