| Magnetic Resonance Imaging (MRI) represents a widespread type of brand-new imaging technology dating back to 1978. Compared with other imaging equipment, it has many distinct advantages, such as multiple imaging parameters, non-ionizing radiation and so on. Nowadays, MRI has been widely used at clinic and holds a key role in the diagnosis of various illnesses. MRI is viewed as a kind of tomography imaging technology physically based on nuclear magnetic resonance. To be specific, it comes from the interaction between nucleus system in a static magnetic field and electromagnetic waves with specific frequency. After stimulated by radio-frequency pulse, nucleus system will tend to be relaxed companied with an emergency of MRI signal. Relative space position of different nucleus is identified by exploring, in vivo, spatial information-concluded MRI signal produced by the gradient field, then, subjects are imaged. Considering the such plenty of advantages of MRI including high resolution, multiple imaging parameters, exposure on the human body without injury and applicable to the study on the structure and metabolism of living organisms, it has been widely applied to life scientific research and clinical diagnosis.Currently, the common medical imaging methods mainly embrace X-ray, CT, ultrasound and MR. MR is thought of as a superior one among these, which holds fives merits. (1) It has a high contrast resolution on soft tissue, fine structures of human body, such as muscle, tendons, and fascia, can be imaged well. (2) Multiple tomography imaging is available, tissues and organs can be manifested in three-dimensional space. (3) Multiple imaging parameters can be ensured, such many parameters as transverse relaxation time, longitudinal relaxation time, proton density and liquid flow can be adjusted each other with a combination of other techniques to gain more diagnostic message. (4) Radio frequency (RF) pulse has no radiation damage to human body, proton motivated by RF electromagnetic field is invariably kept in a transition between high and low energy state. (5) Pains and risks aroused by intubation and intravenous contrast agent can be alleviated, heart, and vessels can be imaged without the contrast medium. Meanwhile, MRI has also widely employed in pregnant women and fetal imaging with the merits of high resolution, non-ionizing radiation and multiple parameters. Compared with other common methods, MR provides more valuable diagnostic information.In contrast, MR also has its limits, which mainly comprise five aspects:(1) duo to a high cost, it takes more examination charge than other common imaging tools. (2) Limited by relaxation time, it takes much more time to collect data, bringing about a low-speed imaging. (3) MR is susceptible in the course of acquiring image, more requirements can be found in order to obtain a good image--patients are required of putting all metal objects off, other equipment for monitoring and rescue are not involved. Therefore MR is non-available for patients with cardiac pacemaker and critical illness. (4) MR is readily influenced by artifacts produced by patients’ breathing, swallowing and body movements. (5) Thermal damage is a potential risk, during the MR scanning, RF electromagnetic wave and body tissue will interact to produce thermal damage. More specifically, the security of SAR needs more attention. Internationally, specific absorption rate (SAR) with specific RF is used to measure the absorption of RF energy, offering relative safety criteria. In accordance to the given benchmark, SAR value is divided into the SAR value of whole body, SAR value of parts of the body and SAR value of local body. SAR value of whole body or SAR value of parts of the body refers to the mean value of RF energy that absorbed by human body. However, local SAR serves as a RF energy deposition in 10 or 1 grams of tissues within a defined time. However, challenges are also existed, such as the mother’s body feels less comfortable, especially for the SAR security in the fetal MRI. Recently published reports have demonstrated that the appropriate placement of the high dielectric material (HDM) between the human tissues and RF coil is able to reduce the SAR to some extent.Due to the local SAR value cannot be directly measured in vivo, thus, utilizing the numerical electromagnetic models for the study of local SAR is usually used in the related research. The study of numerical electromagnetic model of human anatomy in bio-electromagnetic field has been performed for more than three decades years, especially in electromagnetic behavior inside the human body in RF field. Thus, numerical electromagnetic model is utilized to the study on SAR value. Various electromagnetic models have been introduced in the literature characterized by different model resolution and number of segmented tissue types. For example, the Virtual Family, which comprises four anatomical whole body models (34 year old male,26 year old female,11 year old female,6 year old male), was widely used’ in simulation study, and the NORMAN, which consists of about 9 million voxels, with a resolution of 2 mm in the adult phantom, segmented into 37 tissue types, was introduced by Peter J Dimbylow, etc. However the above-mentioned human numerical electromagnetic model is not ideal for the calculation of local SAR with respect to subject-specific patient, because it has been proved that the obtained local SAR varied with different individual due to the various body geometric shape.Therefore, to obtain the accurate local SAR distribution in a high field MRI, the establishment of more accurate numerical electromagnetic models is needed. For example, to establish a subject-specific electromagnetic model, in which a library of MRI images and tissue volumes were used as a foundation, viable metrics were established to evaluate the models’ similarity for the guidance for the selection of the matching model. The image registration techniques were used to adjust the corresponding property of the library of MRI images and tissue volumes to obtain the final subject-specific voxel model. This method greatly improved the accuracy of the estimation of the local SAR, but it still had some limitations, including insufficient number of meshes and shapes of human tissue in the established library of MRI images. Xin introduced a method to establish a subject-specific electromagnetic model using CT image for the analysis of the radiofrequency field in MRI. Usually, the obtained model was not precise enough if only one kind of modality image (CT or MR) as source is utilized, due to the deficient resolution of soft tissues in CT images and the unsatisfied resolution of tissues containing little water in MR images.The thesis puts forward a fine personalized magnetic model which contains nine tissues. The SEMCAD software was used to simulate electromagnetism in the 3 T field for a purpose of gaining B1 field and SAR value. The application of HDM on fetal MR based on that model in terms of SAR value is also investigated. The SEMCAD software was also employed to improve the security of imaging in the 3 T field. The results were analyzed before calculating reduction extent. In conclusion, HDM with proper thickness and dielectric coefficient is placed on proper position of pregnant women model to make sure the security of SAR value in fetal MR imaging.We get the conclusion that electromagnetic model identified from hybrid imaging modalities, compared with that from single image modality, can provide us with a more precise prediction of B1 field and SAR value. It is also conductive to analyze local SAR in high field MRI, further evaluate the absorption of RF energy in the wild electromagnetic field. Hopefully, manual segmentation can be transferred into automatic segmentation. And, the model is established in a more effective way. In the study on decreasing SAR value by HDM in fetal MR imaging which based on that hybrid imaging modalities, the conclusion is that HDM with proper thickness and dielectric coefficient offers an evident lower of SAR value. In follow-up studies, the HDM is promised to be a simple and efficient method with lower cost to reduce SAR value of fetal MR imaging in 3 T. |
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