| Human tissues will have definite dielectric properties, namely the electrical property and the magnetic property under the electromagnetic field, represented by therelative permittivity ε and the conductivity o. Magnetic property, namely the permeability of biological tissue, is nearly same vacuum magnetic permeability, so it can be regarded as a constant. However, in the time varying electromagnetic field, the dielectric properties of the human body are closely related to the frequency of the electric field and the complex permittivity of biological tissues can be expressed asετ(ω)= ε’ (ω)-jε"(ω), where the real part of ε’(ω)is the relative permittivity and the imaginary parts "(ω)is that loss factor. Usually, it is difficult to solve the biological tissue loss factor, which often can be represented by the biological capacity of energy consumption with the equivalent conductivity σeff((ω)= ε"(ω)ε0+σ, where ε0 is the dielectric constant of vacuum. A large number of scientific research shows that the dielectric properties of human tissues are also related to the concentration of ions, the protein content and the ratio of water to free water, the tissue temperature and other factors. When the physiological or pathological status of the human body changed, the dielectric properties of the tissue will also change. Therefore, the dielectric properties of biological tissues are not only related to the frequency, but also depended on the water content, tissue type and tissue structure.RF electromagnetic field uniformed distribution in free space will become inhomogeneities in human tissues with different electromagnetic properties (permeability, conductivity and permittivity) in MR systems. Furthermore, with the increase of static magnetic field, especially in the high field magnetic resonance, RF electromagnetic wave length closed to tissue size, will resulting in standing wave effect, which lead to RF field distortion. The inhomogeneous B1 field will reduce the image contrast, signal to noise ratio, and the reliability of the image diagnosis. B1 mapping technique is based on the conventional magnetic resonance imaging technology, by detecting magnetic signal including RF field spatial distribution information to calculate the B1 field distribution in human tissue and it is also a magnetic resonance research hot spot in recent years. B1 mapping methods can be categorized into two group:phase-based and magnitude-based. Phase-based B1 mapping method causes variations in the phase of an image that are dependent on the strength of the B1 field, and use those variations to map flip angle. The BS method is included in this analysis as a phase-based method. Magnitude-based method typically uses a ratio of signal intensities between the acquired images including DAM, satTFL, and AFI. All investigated sequences use two images S1 and S2, which are combined in most cases by subtraction or division to form a third image S3 related to the magnitude. Knowledge of the spatial distribution of the transmitted radiofrequency (RF) field is meaningful for MR research and engineering applications. First, for the evaluation of the quality of RF coil, the B1 field generated by the RF coil directly determines the quality of the magnetic resonance image. High quality RF coils will produce a higher uniformity of the B1 field, and then produce a higher quality of the magnetic resonance image. The B1 distribution measured by B1 mapping technique can be used as an indicator to judge the quality of RF coils. Second, in parallel imaging, we often need to adjust the B1 field generated by each channel in order to obtain a higher uniformity of the B1 field. The B1 field distribution measured by B1 mapping technique can provide the reference direction for the adjustment of B1 field in each channel.In addition, the uniformity of B1 field can be improved by the B1 uniform field technique, and the local SAR value can be reduced. Third, the inhomogeneous B1 field will likely cover the difference of T1, T2 and proton weighted image between lesion tissue and normal tissue, which will reduce the reliability of T1, T2 and proton weighted image diagnosis. Therefore, the accurate measurement of B1 field is of great significance for T1, T2 and proton weighted images. Finally, as one of the basic dielectric properties of magnetic resonance tomographic imaging technology, imaging quality of B1 RF field directly affects the dielectric properties of magnetic resonance tomography algorithm. The robust of B1 mapping technology is very important for the following MR EPT algorithm.Magnetic resonance electrical properties tomography can reconstruct the dielectric properties of human tissue by measuring the distribution of RF field which carried the dielectric characteristics of biological tissue. In 1991, Haacke et al found that the homogenous distribution of radio frequency field will not be uniform in human tissue, which is the result of the interaction between the electromagnetic field and the dielectric properties of human tissues. Therefore, Haacke et al believed that the dielectric properties of human tissue may be obtained from the distribution of radio frequency field. In 2003, Wen et al. found a direct relationship between the RF field and the electrical properties in the high field magnetic resonance, and proposed a new electrical properties algorithm based on Helmholtz equation. In 2009, Katscher et al. proposed a new method called MR EPT for measuring the dielectric properties of human tissues based on the Ampere’s law, and realized the measurement of the dielectric properties of the human head in 3T magnetic resonance. The quality of dielectric properties image is determined by B1 technique and MR EPT algorithm. Although MR EPT technology currently still has many shortcomings, MR EPT can non-invasively obtain the dielectric properties images of human tissue without injecting current into the tissues. In recent years, MR EPT is one of the hot spots in magnetic resonance research.In this study, some of the most prominent B1 mapping techniques and MR EPT are examined both theoretically and in experiments. The B1 field scale factor (R) for DAM, satTFL and FDTD simulation in phantom and human head were calculated, and then the performance of DAM and satTFL was evaluated using the mean relative difference (MRD). The results suggested that the difference of R between above three methods is less than 10% in the tissues with low dielectric properties; on the contrary, the value of R of DAM is higher than that of satTFL and FDTD, in the tissues with high dielectric properties such as Cerebrospinal fluid, and the value of MRE for DAM is 20%. In the MR EPT experiments, we found that, in low dielectric properties of phantom, the reconstruction error for permittivity and conductivity were 0.01 and 0.003 S/m; in the high dielectric properties of phantom, the reconstruction error for permittivity and conductivity were 0.1 and 0.01 S/m; in human head, the reconstruction error for permittivity and conductivity were 2.5 and 0.03 S/m; The results of this paper can be provide some reference to select the appropriate B1 mapping techniques, while providing basic technical support to promote the practical process MR EPT technology. |
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