| Magnetic resonance imaging (MRI) is a kind of tomographical imaging technology without ionizing radiation, using radiofrequency excitation pulse with a certain frequency emitted by radio frequency (RF) coil to excite the nuclei in the body’s tissues (typically hydrogen protons) to produce nuclear magnetic resonance (NMR) signal. The NMR signals undergoing gradient encoding have space position information of body tissue, then the NMR signals are collected by receiving coil. NMR signals received by receiver coil are then treated with the reconstructions of the computer system and be used for clinical diagnosis of MR images. Clinicians may use MR images of tissues and organs inside human body structure and physiological characteristics for diagnosis and analysis. At present, MRI has been widely applied in clinic and diagnosis of diseases, also applied to the research of the field of life sciences.Compared to ultrasound, CT (computed tomography) and imaging methods such as X-ray, MRI has a lot of the unique advantages of imaging. Firstly, MRI has lots of imaging parameters, proton resonance frequency, transverse relaxation time, and the longitudinal relaxation time, etc. Therefore using MRI to check diseases provides more diagnostic information to clinicians so as to obtain more accurate diagnosis for patients; Secondly, MRI can do the tissue imaging on any direction. MR can be on the premise of not changing the body position for patients with fault imaging in different directions, thereby to clearly observe the tissue anatomy and also obtain pathological changes of the three-dimensional tracking in time; Thirdly, as the mechanism of MRI is use of specific frequency of external RF excitation pulse to stimulate the body’s internal nucleus to produce NMR signal, so there is no ionizing radiation and the body’s tissues can be scanned by MRI safely; Fourthly, MRI has good diagnostic values for fetus. Because MRI has excellent resolution on soft tissue and a wide range of imaging vision, so in fetus MRI the physiological structure of fetus can be clearly observed and valuable diagnostic information can also be discovered; In addition, the gas and bone artifacts will not happen in the MRI, so MRI is advantageous in the diagnosis of diseases.Compared with MRI at low field, MRI at high field offers the higher time, spatial resolution, and signal-to-noise ratio (SNR). So nowadays the development of the MR equipments is progressing towards high field and even ultra-high field. With the rapid increase of field strength, MRI in the nerves, blood vessels, such as joint application shows better diagnosis, therefore the rise of field strength can further enhance the quality of MRI. The current field strengths of clinical MRI equipment are mainly 1.5 and 3T. MR equipment at 3T gives better diagnosis effect. High field MRI, however, is still facing a lot of problems waiting to be solved. As the body’s tissues have different electromagnetic parameters (electrical conductivity and dielectric constant), so as the body’s tissues receiving the RF excitation pulse from outside of the human body with variety of different tissues will have different electromagnetic parameters and produce resonance medium effect, also known as antistatic effect. This effect can lead to different degrees of attenuation imposed by RF excitation pulse in human tissues, so as to make the B1+ field distribution in the tissue of human body become uneven. So, even if the B1+ field produced in free space can be evenly distributed, but after being loaded with the body’s tissues, the distribution of the B1+ field will become extremely uneven. B1+ field non-uniformity will directly reduce the signal-to-noise ratio, contrast, uniformity and specific tissue inhibiting effect of MRI which reduces the MRI clarity to a large extent. It will reduce the diagnostic value of image and is not convenient to clinical doctors to make accurate diagnosis for the diseases. In addition, the human tissues imposed by external high strength RF pulse which interacts with human tissues with the antistatic effect and standing wave effect can cause the RF energy gathered in human tissue. RF energy deposition will inevitably lead to local temperature rise in the local tissue, and may produce potential thermal damage harm to human body. On the international academic field, SAR (special absorption rate, SAR) is usually adopted to measure the RF pulse energy level absorbed by tissues. The organizations also set up corresponding SAR values absorption safety standard. Therefore the uneven B1+ field and high SAR which may cause unsafe factors have to be faced in the use of high field MRI.In this study, we focus on fetus MRI. MRI for the fetus can provide additional supplementary for ultrasound diagnosis. MRI shows high values in diagnosis of the central nervous system (CNS), neck mass, face, chest, abdomen and sterile infection in the fetus. Especially for the diagnosis of fetal CNS, MRI has high image resolution and can provide clinicians with a high diagnostic value. At present, MRI for the fetus is still mainly in the field strength of 1.5T, mainly considering a very strict security restriction of SAR for fetus. High SAR will inevitably cause fetal tissue temperature to rise and will affect the development of the baby’s growth. Some researchs have used the high field MRI and want to take full use of high spatial resolution and SNR to improve the effect of MRI in the diagnosis of fetus. These studies all need to focus on SAR of the fetus and pregnant woman firstly to consider the security issue. With the development of MR technology in the future, early diagnosis of fetal diseases in the security of high field imaging is one of the important research topics.B1+ field and SAR analysis in the research of MRI requires numerical analysis method in the field of practical engineering. This problem needs to consider irregular boundary, the structure of the complex material, so specific electromagnetic field numerical analysis method is needed to solve this electromagnetic field problem. Commonly used numerical analysis methods include finite-difference time-domain (FDTD), the method of moments (MoM), the finite element method (FEM), and finite integral technology (FIT). FDTD method is adopted in this paper as the numerical analysis method. It is a finite differential instead of time domain differential expression of Maxwell curl equation and then get the finite difference form of the electromagnetic field components. The electromagnetic field numerical analysis method is not restricted by the geometry of the medium, and easy for modeling complex medium. FDTD numerical method has high response, wide band which can be calculated in the time domain after using Fourier transform to get the whole frequency band response. Thus this numerical analysis method is very suitable for the simulation of RF coil to tune and make the coil to arrive resonance state. The principle of MoM is firstly under way to the discretization operation of the area with the existence of current distribution. Bigger memory is needed, therefore this method does not apply to such complex medium load on human body tissues for analysis. And in the analysis of MoM, the main time spent on the calculation is to solve the algebraic equations. So the size of the matrix in the method directly affects the final computer memory size and will directly affect the speed of calculation. FEM numerical method can be divided into three solving processes:pre-treatment, solving calculation and rear. Each link can be standardized. However, complexity and lengthy of FEM calculation will make this method just hard to be solved in the area. Divisions of prime number and node numbers are more, so you need to obtain initial data in a wide range and finally several big equations will occupy larger computer memory which will make the whole numerical solving process long. Therefore, if FEM method is applied to human tissue electromagnetic field calculation, it will increase memory required and thus reduce efficiency of numerical calculation. In this study, the need for accuracy, convenience in the analysis of the interaction of radio frequency electromagnetic field with human tissue, the FDTD numerical analysis method is adopted in this study to analyze electromagnetic field interaction between human body and HDM.Before the use of FDTD numerical analysis for analyzing the RF pulse which interacts with human body load, it is needed to establish a human 3D model. In practice, there are two ways to build a 3D model of the human body. One is based on the actual clinical scan data used for reconstruction, the other is using numerical model. This article about normal female pelvic model illustrates the first method of building 3D model of human body. This method is based on the computed tomography images of human body. First of all, using the precise artificial segmentation and volume rendering and then the 3D model is set up. Then using the three-dimensional model for finite element subdivision and triangles for optimization, and corresponding electromagnetic properties are given to different organizations to establish the 3D electromagnetic model. Numerical model is used to expound the second building method of electromagnetic model in this paper. This paper uses the real adult female pelvic model combined with a numerical model of fetus to establish 13-week pregnant female pelvic electromagnetic model, which contains the uterus, placenta, amniotic fluid, fetal body, and fetal brain.At present a lot of researches show that the use of HDM can change the B1+ field distribution in MRI. So the quality of MRI at the high field (3T) or ultra high field (7T) can be improved with using HDM. Research shows that in high field the application of dielectric material such as water and ultrasonic gel can reduce the artifacts created during the process of MRI and enhance the signal strength of the MRI. New HDM such as calcium titanate, barium titanate can significantly change the distribution of the B1+ field around the area of HDM. This means that the efficiency of radio frequency field emission can be significantly improved if only the areas located around the surrounding area of HDM in terms of MRI. Therefore, RF energy needed can get a certain degree of reduction in the same radio frequency effect. Fetus MRI at 3T faces the main challenge of SAR which may be beyond safety standard and the use of HDM can be used to solve this problem. In this paper, the research is also aiming to improve the safety of the fetus imaging, and do the research of reducing SAR by using HDM.In this study, FDTD numerical analysis method is used to study the SAR reductions using HDM of different shapes, thicknesses, dielectric constants at 1.5 and 3T in fetus MRI. Actual human body 3D electromagnetic model of adult woman pelvic is mixed with a 13-week fetal numerical model to build a 13-week pregnant female pelvic electromagnetic model in our simulations. The reductions of SAR values can be obtained by the placement of appropriate HDM under certain conditions of MRI, thereby significantly improving the safety of fetus MRI. From the SAR distributions in the human tissue, it can be seen SAR has been significantly reduced in a very optimal situation of use of HDM and no new hotpots has been generated. We also consider the corresponding situation of BI+, field uniformity in the case of optimal dielectric constant for each shape of HDM. The results show that the B1+ field uniformity is slightly lower after adding HDM in the interest area (region of interest, ROI), but the deterioration of the B1+ field will not be huge influence on the actual image quality. Through this study, we can see that the HDM in the use of MRI are beneficial to the safety of fetal MRI. But there is still a lot of work waiting to be done. Firstly, using HDM to reduce the SAR in the MRI to improve the safety of the imaging is not absolute. From the FDTD simulation results, it can be seen that the use of HDM can improve the SAR values in some cases.This article firstly introduces the background of this research, research status, and the structure of this article which focuses on the development of MRI system, performance, advantages and disadvantages and the present situation in the clinical application. Basic physical principle of MRI, imaging quality parameters, commonly used imaging sequences, and then hardware composition of MRI system are introduced. Then basic principle of electromagnetic field numerical analysis methods which tells how to build the 3D electromagnetic model used in the FDTD simulation are talked. Then several commonly used electromagnetic field numerical analysis methods are talked with the emphasis on the FDTD method which is used in this research. Firstly, the important application value of fetus MRI in clinic are talked and then analysis of many challenges in the current fetus MRI are given. And from these challenges, it can be seen that security is important for the implementation of fetus MRI. Finally, the work which has been done during the progress of the research is summarized, the work for the next step is also discussed at the same time. |
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