WO2017219765A1 - 从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法 - Google Patents

从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法 Download PDF

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WO2017219765A1
WO2017219765A1 PCT/CN2017/083447 CN2017083447W WO2017219765A1 WO 2017219765 A1 WO2017219765 A1 WO 2017219765A1 CN 2017083447 W CN2017083447 W CN 2017083447W WO 2017219765 A1 WO2017219765 A1 WO 2017219765A1
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energy
field
tissue
electric field
magnetic
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辛学刚
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辛学刚
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
    • A61B5/7235—Details of waveform analysis
    • A61B5/7253—Details of waveform analysis characterised by using transforms
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/24—Arrangements or instruments for measuring magnetic variables involving magnetic resonance for measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/246—Spatial mapping of the RF magnetic field B1
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/288—Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T2207/00—Indexing scheme for image analysis or image enhancement
    • G06T2207/10—Image acquisition modality
    • G06T2207/10072—Tomographic images
    • G06T2207/10088—Magnetic resonance imaging [MRI]

Definitions

  • the present invention relates to the field of magnetic resonance imaging technology, and in particular, to a method for solving a tissue electrical characteristic distribution and a local specific absorption rate from an electromagnetic field energy propagation angle.
  • EPs Electrical properties
  • dielectric properties mainly refers to the electrical conductivity ⁇ and permittivity ⁇ of the tissue, which can reflect the physiological and pathological state of the tissue. Studies have shown that there are significant differences in electrical properties between normal tissues and tumor tissues. Therefore, by comparing the difference in the distribution of EPs between living tissue and normal tissue, we can understand the physiological and pathological state of the tissue, which is of great significance for the diagnosis of the disease and the early detection of the tumor.
  • MR EPT magnetic resonance human body electrical characteristic tomography
  • GE Global R&D Center also implemented tissue permittivity based on RF field amplitude and tissue conductivity tomography based on RF field phase in 3T MR, and further proposed MR EPT algorithm. Fast optimization calculation method.
  • Bin He and other scholars at the University of Minnesota Magnetic Resonance Center used the assumption that the human brain tissue was roughly symmetrical, and proposed the assumption that the RF emission magnetic field and the receiving magnetic field amplitude are roughly equivalent, and received from the RF multi-coil channel relying on the proton density distribution.
  • the proton density distribution of the whole brain is extracted from the obtained signal, and the amplitude and phase of the receiving field of each channel are calculated, and the transmitting magnetic field of each channel obtained by using the 7T ultra-high field multi-channel RF coil B 1 Mapping technology is adopted according to MR EPT.
  • the second-order differential algorithm realized the non-invasive EPs tomography of 7T MR human brain tissue.
  • Sodickson and other researchers at the Bernard & Irene Schwartz Biomedical Imaging Center of New York University in the United States proposed a local Maxwell Tomography (LMT) method to solve the problem that the phase of the 7T MR multi-channel coil transmit and receive fields cannot be directly measured.
  • LMT local Maxwell Tomography
  • the first type is: a simulation calculation method based on the human body electromagnetic model: the method applies a certain human body electromagnetic model, and is placed under a specific MR RF coil, and is set. A certain radio frequency scanning parameter is calculated, and the distribution of the B 1 field in the human tissue is calculated. Referring to the complex dielectric parameters of the isolated tissue, the radio frequency hotspot distribution of the tissue is obtained.
  • the second category is: MR scanning measurement calculation method based on linear MR human body complex dielectric parameter tomography (MR EPT) technology: in 2009, Katscher et al. proposed a method for determining the complex electrical conductivity and local RF hotspots; Voigt et al. In 2011, a linear measurement method for complex dielectric parameters of brain tissue, namely linear MR EPT, was proposed. Based on this research, an MR EPT-based RF hotspot distribution calculation method for living tissue was proposed in 2012, but only in the brain. The part of the living body MR scan has been verified, can not be applied to the whole body, and the calculation result error is about 20%, the accuracy is not ideal, and needs to be improved.
  • the existing MR EPT technology has the following problems: 1.
  • the operation process and reconstruction result are greatly affected by noise; 2.
  • the problem of large imaging error at the junction of different tissues cannot be solved; 3.
  • Imaging The resolution is poor and needs to be further improved.
  • the prior art adopts the approximation processing method, based on the assumption that the gradient of the tissue re-characteristics is zero; 2.
  • the reconstruction core algorithm is based on the second-order differential operation of Maxwell's electromagnetic equation, which The noise is amplified during the calculation process, which results in the calculation result being very sensitive to noise. 3.
  • the amplitude and phase information of the B 1 + field needs to be obtained, but the phase information of the B 1 + field cannot be obtained by measurement, and can only be estimated. Obtained, so the error based on the B 1 + field EPT reconstruction is large.
  • the biggest drawback of the first type of electromagnetic hotspot distribution simulation calculation method based on human body electromagnetic model is that the modeling work for establishing a personalized whole body electromagnetic model for a single patient individual is huge and cannot be promoted in practice, so it can only be used.
  • the universal human electromagnetic model which leads to individual differences cannot be measured.
  • the complex dielectric parameters of the isolated tissue are referenced, and there is a certain difference between the complex dielectric parameters of the isolated tissue and the complex dielectric parameters of the living tissue, and this difference also leads to calculation results. There is a deviation.
  • the second type of linear MR EPT-based MR scanning method for calculating the RF hotspot distribution requires that the measured complex dielectric parameters in the target region are much smaller than the electric field. This requirement is basically satisfied for the brain tissue mainly composed of soft tissue; but for the whole body tissue, it cannot be satisfied because the complex dielectric parameters of the whole body are different from each other.
  • an energy propagation angle from an electromagnetic field is provided. It is necessary to solve the problem of the distribution of the electrical properties of the tissue and the local specific absorption rate to overcome the shortcomings of the prior art.
  • the object of the present invention is to provide a method for solving the distribution of electrical properties and local specific absorption rate from the angle of electromagnetic field energy propagation, which avoids the deficiencies of the prior art, and solves the distribution of electrical properties and local specific absorption from the angle of electromagnetic field energy propagation.
  • the rate method has the characteristics of calculating orders and accurate results.
  • a method for solving the distribution of electrical properties of a tissue from the perspective of electromagnetic field energy propagation is provided.
  • the method is based on the fact that electromagnetic energy propagates from the periphery to the body under the action of the radio frequency coil, and the electric field and the magnetic field energy are alternately converted, and the energy is unlikely to be mutated.
  • the energy of a part of the electric field is converted into heat energy consumption;
  • the electromagnetic field emitted by the NMR radio frequency transmitting coil, after reaching the human body, the expression of the total energy of the electromagnetic field at each point is:
  • ⁇ is the angular frequency
  • ⁇ is the magnetic permeability
  • ⁇ is the permittivity
  • ⁇ is the conductivity
  • B is the magnetic induction
  • E is the electric field strength
  • “*” means taking the conjugate complex number. Represents magnetic field energy storage, Represents electric field energy storage, Indicates Joule heat loss;
  • the amplitude distribution of the magnetic field generated by the radio frequency emission B 1 + field at each point in the human body is obtained according to Obtaining the magnetic field energy of the B 1 + field at various points in the body; according to the electromagnetic interaction theory of Maxwell's equations, the energy of the electric field at the same time is obtained;
  • B out represents the magnetic induction intensity of the outer ring
  • B in represents the magnetic induction intensity of the inner ring
  • ⁇ out represents the electric field strength of the outer ring
  • B in and B out are obtained by the B 1 Mapping technique
  • the electric field energy stored at each point of the outer ring is converted into the magnetic field energy of each point corresponding to the inner ring, namely:
  • the loss tangent is obtained, and the loss tangent is the ratio of the electrical conductivity of the structure of the outer ring to the relative dielectric constant, and the relative dielectric constant is the permittivity and the angular frequency.
  • the product, the loss tangent is expressed as
  • the invention also provides a method for solving the local specific absorption rate of a tissue from the perspective of electromagnetic field energy propagation.
  • the method is based on the physical fact that the electromagnetic energy propagates from the periphery to the body under the action of the radio frequency coil, the electric field and the magnetic field energy are alternately converted, and the energy cannot be abruptly changed. In the process of mutual conversion between the electric field and the magnetic field energy, the energy of a part of the electric field will Converted into heat energy consumed;
  • the electromagnetic field emitted by the NMR radio frequency transmitting coil, after reaching the human body, the expression of the total energy of the electromagnetic field at each point is:
  • ⁇ is the angular frequency
  • ⁇ is the magnetic permeability
  • ⁇ is the permittivity
  • ⁇ is the conductivity
  • B is the magnetic induction
  • E is the electric field strength
  • “*” means taking the conjugate complex number. Represents magnetic field energy storage, Represents electric field energy storage, Indicates Joule heat loss;
  • the amplitude distribution of the magnetic field generated by the radio frequency emission B 1 + field at each point in the human body is obtained according to Obtaining the magnetic field energy of the B 1 + field at various points in the body; according to the electromagnetic interaction theory of Maxwell's equations, the energy of the electric field at the same time is obtained;
  • B out represents the magnetic induction intensity of the outer ring
  • B in represents the magnetic induction intensity of the inner ring
  • ⁇ out represents the electric field strength of the outer ring
  • B in and B out are obtained by the B 1 Mapping technique
  • the electric field energy stored at each point of the outer ring is converted into the magnetic field energy of each point corresponding to the inner ring, namely:
  • the method for solving the electrical property distribution and the local specific absorption rate from the electromagnetic field energy propagation angle of the invention solves the EPs from the perspective of the transmission and distribution of the magnetic resonance radio frequency electromagnetic field energy in the body, and only the radio frequency that can be directly measured is used in the calculation process.
  • the amplitude information of the field does not require the phase information of the RF field, which avoids the problem that the phase of the RF field cannot be directly measured.
  • the new method avoids the second-order differential operation method of the Maxwell equation adopted by the existing method, and can effectively improve the operation. The accuracy of the results.
  • the invention solves the distribution and locality of the electrical properties of the tissue from the angle of electromagnetic field energy propagation
  • the method of specific absorption rate is characterized by simple calculation and accurate result from the electromagnetic field energy, a new angle to solve the electrical properties of the tissue and the specific absorption rate of the tissue.
  • Embodiment 1 is a distribution diagram of amplitude and phase of a radio frequency field in a simulation experiment in Embodiment 3 of the present invention.
  • Example 3 is a distribution diagram of electrical conductivity and relative permittivity calculated in a simulation experiment of Example 3 of the present invention.
  • Embodiment 4 is a distribution diagram of electric field energy, magnetic field energy, heat loss, and total energy in a simulation experiment of Embodiment 3 of the present invention.
  • a method for solving the distribution of electrical properties of a tissue from the perspective of electromagnetic field energy propagation is based on the fact that electromagnetic energy propagates from the periphery to the body under the action of the radio frequency coil, and the electric field and the magnetic field energy are alternately converted, and the energy is unlikely to be abruptly changed. During the mutual conversion of the electric field and the magnetic field energy, the energy of a part of the electric field is converted into heat energy consumption;
  • ⁇ is the angular frequency
  • ⁇ is the magnetic permeability
  • ⁇ is the permittivity
  • ⁇ is the conductivity
  • B is the magnetic induction
  • E is the electric field strength
  • “*” means taking the conjugate complex number. Represents magnetic field energy storage, Represents electric field energy storage, Indicates Joule heat loss.
  • the amplitude distribution of the magnetic field generated by the radio frequency emission B 1 + field at each point in the human body is obtained according to The magnetic field energy of the B 1 + field at various points in the body is obtained; according to the electromagnetic interaction theory of Maxwell's equations, the energy of the electric field is obtained at the same time.
  • the electric field energy of the outer ring is converted into the magnetic field energy of the inner ring, namely:
  • the loss tangent is obtained, and the loss tangent is the ratio of the electrical conductivity of the structure of the outer ring to the relative dielectric constant, and the relative dielectric constant is the permittivity and the angular frequency.
  • the product, the loss tangent is expressed as
  • the ratio is That is, the value of the loss tangent is also known. Calculated above The results of distributed imaging are compared with the values of normal tissues, and you can find The location where the value is abnormal is an early discovery of the disease, such as early suspicious cancerous lesions, providing valuable spatial localization information, and supplemented by biopsy and other technical means to achieve early detection of the tumor, which has important clinical application value.
  • the method for solving the distribution of the electrical properties of the tissue from the electromagnetic field energy propagation angle of the invention solves the EPs from the perspective of the transmission and distribution of the electromagnetic field electromagnetic field energy in the body, and only uses the amplitude information of the directly measured RF field in the calculation process.
  • the phase information of the RF field is not needed, and the problem that the phase of the RF field cannot be directly measured is avoided.
  • the new method avoids the second-order differential operation of the Maxwell equation, and can effectively improve the accuracy of the calculation result.
  • the commonly used processing method is to propose that the local tissue electrical characteristics are uniform (ie, assuming that the gradient of the electrical properties of the local tissue is zero), and assuming that the radio frequency is ignored.
  • the cross-sectional component of the electric field and the longitudinal axis component of the RF magnetic field, that is, E x, y (r) ⁇ 0 and B z (r) ⁇ 0 are assumed.
  • the purpose of the existing MR EPT algorithm is to simplify the approximate calculation formula of the tissue EPs, and to form the existing MR EPT algorithm theory, such as the previously mentioned Voigt et al. This simplified processing results in an increase in measurement error in the presence of complex electromagnetic boundary conditions.
  • the method of the invention solves EPs from the perspective of electromagnetic field energy, and only applies the amplitude information of the magnetic field in the calculation process, avoiding the phase problem of the radio frequency field.
  • the phase problem is an unsolved problem in the existing MR EPT algorithm. Because the phase of the RF emission field is directly measured by engineering using the existing MR technology, many assumptions are also made in the process of solving the problem. In the case of the case, the approximate solution of the phase distribution is obtained, and the existing solution methods have large errors. Therefore, the method of the invention simultaneously avoids the second-order differential operation of the Maxwell equation, and can effectively improve the precision of the operation result.
  • a method for solving the local specific absorption rate of a tissue from the perspective of electromagnetic field energy propagation is based on the physical fact that electromagnetic energy propagates from the periphery to the body under the action of the radio frequency coil, and the electric field and the magnetic field energy are alternately converted, and the energy is unlikely to be mutated. During the mutual conversion of the electric field and the magnetic field energy, the energy of a part of the electric field is converted into heat energy.
  • ⁇ is the angular frequency
  • ⁇ is the magnetic permeability
  • ⁇ is the permittivity
  • ⁇ is the conductivity
  • B is the magnetic induction
  • E is the electric field strength
  • “*” means taking the conjugate complex number. Indicates magnetic field energy storage, Represents electric field energy storage, Indicates Joule heat loss.
  • the amplitude distribution of the magnetic field generated by the radio frequency emission B 1 + field at each point in the human body is obtained according to The magnetic field energy of the B 1 + field at various points in the body is obtained; according to the electromagnetic interaction theory of Maxwell's equations, the energy of the electric field is obtained at the same time.
  • the electric field energy of the outer ring is converted into the magnetic field energy of the inner ring, namely:
  • B out represents the magnetic induction of the outer ring
  • B in represents the magnetic induction of the inner ring
  • ⁇ out represents the electric field strength of the outer ring.
  • B in and B out are obtained by the B 1 Mapping technique.
  • the local specific absorption rate (Local SAR) of each point of each circle can be obtained, namely:
  • the method for solving the local specific absorption rate of the tissue from the electromagnetic field energy propagation angle of the invention solves the EPs from the perspective of the transmission and distribution of the electromagnetic field electromagnetic field energy in the body, and only the amplitude information of the directly measured RF field is used in the calculation process.
  • the phase information of the RF field is not needed, and the problem that the phase of the RF field cannot be directly measured is avoided; at the same time, the new method avoids the second-order differential operation of the Maxwell equation, and can effectively improve the accuracy of the calculation result.
  • the Local SAR distribution can be obtained according to the Local SAR calculation formula.
  • the method for solving the electrical property distribution and the local specific absorption rate from the electromagnetic field energy propagation angle of the invention solves the electrical properties of the tissue and the local specific absorption rate from the electromagnetic field energy, and has the characteristics of simple calculation and accurate results.
  • the correctness of the method of the present invention was verified by different angles such as simulation, phantom experiment and actual measurement of the human body.
  • the permittivity of the tissue can be calculated from equations (I) and (IV), and then the conductivity of the corresponding point can be found according to the tangent value of the loss angle, as shown in Fig. 3.
  • 4 is a distribution diagram of electric field energy, magnetic field energy, heat loss, and total energy in a simulation experiment of Embodiment 3 of the present invention.
  • a variety of dielectric materials capable of simulating the electrical properties of different tissues are used to fill a specific structural cavity to form electromagnetic boundary conditions similar to the interface of different human tissues, and a special phantom is produced.
  • Different scanning sequences were used in 3T magnetic resonance to obtain corresponding images.
  • was obtained by using B 1 Mapping technique. Then, the energy field of the nuclear magnetic resonance can be combined to solve the electrical characteristics and the local specific absorption rate.
  • the results prove that the method of the invention can obtain the electrical property distribution and the local specific absorption rate of the tissue simply and accurately.
  • the method of the present invention solves EPs from the perspective of electromagnetic field energy, and only applies the amplitude information of the magnetic field in the calculation process, avoiding the phase problem of the radio frequency field, because the phase is compared in the process of solving Complex and existing solution There are large errors; at the same time, the second-order differential operation of Maxwell's equation is avoided, which can effectively improve the accuracy of the calculation results.
  • the Local SAR distribution can be obtained according to the Local SAR calculation formula.
  • the method for solving the electrical property distribution and the local specific absorption rate from the electromagnetic field energy propagation angle of the invention solves the electrical properties of the tissue and the local specific absorption rate from the electromagnetic field energy, and has the characteristics of simple calculation and accurate results.

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Abstract

一种从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法，包括步骤：(1)计算核磁共振射频发射点发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量；(2)根据B 1 Mapping技术，得到射频发射产生的磁场B 1 +场的幅度分布，进而得到B 1 +场的能量；根据麦克斯韦方程组电磁互生理论，同时得到电场的能量；(3)通过比较内外圈各点之间的电磁场总能量差并进行计算得到各圈各点的局部比吸收率和得到各处损耗角正切值。从电磁场能量角度求解组织电特性和组织局部比吸收率的方式具有计算简单，结果精确的特点。

Description

从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法 技术领域
本发明涉及磁共振成像技术领域，特别是涉及一种从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法。
背景技术
电特性(简称EPs)又称介电特性，主要是指组织的电导率σ和电容率ε，该参数能够反映组织的生理、病理状态。已有研究证明正常组织和肿瘤组织存在明显的电特性差异。因此，通过比较活体组织与正常组织EPs分布图的差异，可以了解组织的生理、病理状态，对于疾病的诊断及肿瘤的超早期发现有十分重要的意义。
在成像的过程中，当磁场作用于人体组织时，活体组织会吸收射频能量。随着主磁场场强的增加，比吸收率也会增加。如果组织对射频能量吸收过多，超出人体自我调节的承受范围，就会出现局部灼伤，甚至体温调节紊乱，导致电解质代谢紊乱、神经系统紊乱等一系列问题。因此，量化测定超高场MRI全身组织Local SAR有重要的临床价值。
近年来，磁共振人体组织电特性断层成像(MR EPT)技术引起了科学家们极大的兴趣。该研究结合B1mapping技术，通过麦克斯韦方程导出。到目前为止，科学家们已在3T和7T磁共振下取得了一系列研究成果。主要如下所示:2009年，Philips欧洲研发 中心的Katscher等系统地研究了3T下的MR EPT技术，Katscher等提出了忽略RF磁场强度矢量中的Z方向分量，只利用另外两个分量来解析求解组织各处电导率和电容率的方法，并在鸟笼线圈中实现了电导率和电容率的活体测量，大大推动了3T MR EPT技术的发展。2011年，德国Karlsruhe生物医学工程学院的Voigt等在Katscher等的研究成果基础上，对3T RF场的幅度和相位做出了一定的假设，Voigt等基于一定的假设，提出了基于相位的电导率求解方法和基于磁场幅度的电容率求解方法。Voigt等提出的基于以上两种假设的MR EPT算法，经与Katscher等提出的算法比较，二者测量结果相差10％以内(但这并不意味着与真实值之间的误差也在10％以内)，但计算方法简化了很多，将MR EPT技术方法向临床实际应用推进了一步。GE全球研发中心的Bulumulla等在Voigt等人的工作基础上，亦在3T MR中实现了基于RF场幅度的组织电容率和基于RF场相位的组织电导率断层成像，并进一步提出了MR EPT算法的快速优化计算方法。美国明尼苏达大学磁共振中心的Bin He等学者在2013年利用人脑组织左右大致对称的特点，提出了RF发射磁场和接收磁场幅度大致相当的假设，从依托于质子密度分布的RF多线圈通道接收到的信号中抽取整个脑部的质子密度分布，并计算各通道接收场的幅度和相位，再结合采用7T超高场多通道RF线圈B1Mapping技术得到的各通道的发射磁场，依照MR EPT二阶微分算法，实现了7T MR人体脑部组织无创EPs断层成像。另有美国纽约大学Bernard&Irene  Schwartz生物医学成像中心的Sodickson等学者针对7T MR多通道线圈发射和接收场的相位无法直接测量的问题，提出了利用局部麦克斯韦断层成像(Local Maxwell Tomography,LMT)方法来求解各线圈通道的相位。他们将描述每一个通道的Maxwell电磁方程两边的实部和虚部分离，得到两组不同的方程。理论上，有5组以上发射和接收线圈通道的测量数据，就可以对质子密度分布、各发射通道和各接收通道的相位等未知量进行求解，并进而依照MR EPT二阶微分算法得到成像区域各像素点的组织EPs值分布。
与此同时，活体组织射频热点分布测定问题随着B0场的增高变得不可回避且日益重要，目前并未提出完善的解决方案，所以当前国际上相关研究非常活跃。现有的射频热点分布量化测定方法主要有两大类，第一类是：基于人体电磁模型的仿真计算方法：该方法应用一定的人体电磁模型，将其置于特定的MR射频线圈下，设置一定的射频扫描参数，计算B1场在人体组织内的分布，参考离体组织复介电参数，得到组织的射频热点分布。第二类是：基于线性MR人体组织复介电参数断层测量(MR EPT)技术的MR扫描测量计算方法：Katscher等在2009年提出了确定组织复电导率和局部射频热点的方法；Voigt等在2011年提出了脑部组织复介电参数的线性测量方法,即线性MR EPT，并在此研究基础上于2012年提出了针对活体组织的基于MR EPT的射频热点分布计算方，但是仅在脑部的活体MR扫描中得到验证，还不能应用到全身，并且计算结果误差在20％左右，准确度不够理想，还有待提高。知名教授Bin He领导的研究团队在2013年报道了在7T下测量脑部组织复介电参数的研究，为进一步采用MR EPT技术 计算射频热点分布提供支持。
关于MR EPT，现有的MR EPT技术中存在以下几个问题：1.运算过程和重建结果受噪声影响较大；2.无法解决不同组织交界处存在的成像误差较大等问题；3.成像分辨率较差，有待进一步提高。导致以上问题的原因有以下三个方面：1.现有技术均采用了近似处理的方法，基于组织复电特性梯度为零的假设；2.重建核心算法基于麦克斯韦电磁方程二阶微分运算，这就使得噪声在计算过程中被放大，导致计算结果对噪声非常敏感；3.需要得到B1 +场的幅度和相位信息，但B1 +场的相位信息是无法通过测量得到的，只能估计得到，因此基于B1 +场EPT重建误差较大。
关于Local SAR，第一类基于人体电磁模型的射频热点分布仿真计算方法的最大缺陷是：针对单个病人个体建立个性化全身电磁模型的建模工作量巨大，无法在实践中推广，因此只能采用通用人体电磁模型，这就导致个体差异无法测量。除此之外，计算电磁模型时参考离体组织的复介电参数，而离体组织的复介电参数与活体组织的复介电参数之间存在一定差异，这种差异也会导致计算结果出现偏差。第二类基于线性MR EPT的MR扫描计算射频热点分布的方法，均要求测量的目标区域内组织复介电参数的变化远远小于电场的变化。这种要求对以软组织为主的脑部组织而言，基本满足；但是对全身组织而言，就不能满足，因为全身范围内组织的复介电参数相互差异较大。
因此，针对现有技术不足，提供一种从电磁场能量传播角度 求解组织电特性分布及局部比吸收率的方法以克服现有技术不足甚为必要。
发明内容
本发明的目的在于避免现有技术的不足之处而提供一种从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法，该从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法具有计算接单、结果准确特点。
本发明的上述目的通过如下技术手段实现。
提供一种从电磁场能量传播角度求解组织电特性分布的方法，该方法基于以下物理事实：电磁能量在射频线圈的作用下，从四周向体内传播，电场和磁场能量交替转换，且能量不可能突变，在电场和磁场能量相互转换过程中，一部分电场的能量会转换成热能消耗掉；
核磁共振射频发射线圈发射出的电磁场，在到达人体后，各点电磁场的总能量的表达式为：
Figure PCTCN2017083447-appb-000001
其中，ω为角频率，μ是磁导率，ε是电容率，σ是电导率，B为磁感应强度，E为电场强度，“*”表示取共轭复数，
Figure PCTCN2017083447-appb-000002
表示磁场储能，
Figure PCTCN2017083447-appb-000003
表示电场储能，
Figure PCTCN2017083447-appb-000004
表示焦耳热损耗；
具体通过如下步骤进行：
(1)根据射频功放的功率大小和射频发射的时间，以及系统 的工作效率，计算核磁共振射频线圈发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量；
(2)根据B1Mapping技术，得到射频发射产生的磁场B1 +场在人体内各点的幅度分布，根据
Figure PCTCN2017083447-appb-000005
得到B1 +场在体内各点处的磁场能量；根据麦克斯韦方程组电磁互生理论，同时得到该处电场的能量；
(3)能量由外圈向内圈传递的过程中，由于热能的损耗，总能量不断减少，每圈对应各点之间减少的能量即是两圈之间各对应点之间B1 +场的能量的差，即：
Figure PCTCN2017083447-appb-000006
其中,Bout表示外圈的磁感应强度,Bin表示内圈的磁感应强度,Εout表示外圈电场强度；Bin和Bout通过B1Mapping技术得到；
外圈各点的电场储能转化为内圈对应各点的磁场能量，即：
Figure PCTCN2017083447-appb-000007
用式(Ⅱ)除以式(Ⅲ)，得到损耗角正切，损耗角正切为外圈各点所在部位组织的电导率与相对介电常数的比值，相对介电常数为电容率与角频率之积，损耗角正切的表达式为
Figure PCTCN2017083447-appb-000008
通过上述计算，得到成像区域内各处组织的电导率与相对介电常数的比值
Figure PCTCN2017083447-appb-000009
本发明同时提供一种从电磁场能量传播角度求解组织局部比吸收率的方法，
该方法基于以下物理事实：电磁能量在射频线圈的作用下，从四周向体内传播，电场和磁场能量交替转换，且能量不可能突变，在电场和磁场能量相互转换过程中，一部分电场的能量会转换成热能消耗掉；
核磁共振射频发射线圈发射出的电磁场，在到达人体后，各点电磁场的总能量的表达式为：
Figure PCTCN2017083447-appb-000010
其中，ω为角频率，μ是磁导率，ε是电容率，σ是电导率，B为磁感应强度，E为电场强度，“*”表示取共轭复数，
Figure PCTCN2017083447-appb-000011
表示磁场储能，
Figure PCTCN2017083447-appb-000012
表示电场储能，
Figure PCTCN2017083447-appb-000013
表示焦耳热损耗；
具体通过如下步骤进行：
(1)根据射频功放的功率大小和射频发射的时间，以及系统的工作效率，计算核磁共振射频线圈发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量；
(2)根据B1Mapping技术，得到射频发射产生的磁场B1 +场在人体内各点的幅度分布，根据
Figure PCTCN2017083447-appb-000014
得到B1 +场在体内各点处的磁场能量；根据麦克斯韦方程组电磁互生理论，同时得到该处电场的能量；
(3)能量由外圈向内圈传递的过程中，由于热能的损耗，总 能量不断减少，每圈对应各点之间减少的能量即是两圈之间各对应点之间B1 +场的能量的差，即：
Figure PCTCN2017083447-appb-000015
其中，Bout表示外圈的磁感应强度，Bin表示内圈的磁感应强度，Εout表示外圈电场强度；Bin和Bout通过B1Mapping技术得到；
外圈各点的电场储能转化为内圈对应各点的磁场能量，即：
Figure PCTCN2017083447-appb-000016
对各圈而言，电场和磁场能量相互转化，即：
Figure PCTCN2017083447-appb-000017
通过比较内外圈各点之间的电磁场总能量差，即
Figure PCTCN2017083447-appb-000018
得到各圈各点的热能损耗的值，即各圈各点的
Figure PCTCN2017083447-appb-000019
再利用人体组织各点处的组织密度ρ，得到各圈各点的局部比吸收率Local SAR，即：
Figure PCTCN2017083447-appb-000020
本发明的从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法，从磁共振射频发射电磁场能量在体内传输和分布的角度求解EPs，计算过程中只用到了可以直接测量得到的射频场的幅度信息，不需要射频场的相位信息，规避了射频场的相位无法直接测量的问题；同时新方法避免了现有方法采用的麦克斯韦方程的二阶微分运算的方法，可以有效的提高运算结果的精度。
本发明的从电磁场能量传播角度求解组织电特性分布及局部 比吸收率的方法，从电磁场能量——一个全新的角度求解组织电特性和组织局部比吸收率，具有计算简单，结果精确的特点。
附图说明
利用附图对本发明作进一步的说明，但附图中的内容不构成对本发明的任何限制。
图1是本发明实施例3的仿真实验中射频场幅度和相位的分布图。
图2是本发明实施例3的仿真实验中得到的SAR值分布图。
图3是本发明实施例3的仿真实验中计算得到的电导率和相对电容率的分布图。
图4是本发明实施例3的仿真实验中电场能量、磁场能量、热损耗和总能量的分布图。
具体实施方式
结合以下实施例对本发明作进一步描述。
实施例1。
一种从电磁场能量传播角度求解组织电特性分布的方法，该方法基于以下物理事实：电磁能量在射频线圈的作用下，从四周向体内传播，电场和磁场能量交替转换，且能量不可能突变，在电场和磁场能量相互转换过程中，一部分电场的能量会转换成热能消耗掉；
核磁共振射频发射点磁场的总能量的表达式为：
Figure PCTCN2017083447-appb-000021
其中，ω为角频率，μ是磁导率，ε是电容率，σ是电导率，B为磁感应强度，E为电场强度，“*”表示取共轭复数，
Figure PCTCN2017083447-appb-000022
表示磁场储能，
Figure PCTCN2017083447-appb-000023
表示电场储能，
Figure PCTCN2017083447-appb-000024
表示焦耳热损耗。
具体通过如下步骤进行：
(1)根据射频功放的功率大小和射频发射的时间，以及系统的工作效率，计算核磁共振射频线圈发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量；其中，反射回来的能量可通过测量得到。
(2)根据B1Mapping技术，得到射频发射产生的磁场B1 +场在人体内各点的幅度分布，根据
Figure PCTCN2017083447-appb-000025
得到B1 +场在体内各点处的磁场能量；根据麦克斯韦方程组电磁互生理论，同时得到该处电场的能量。
(3)能量由外圈向内圈传递的过程中，由于热能的损耗，总能量不断减少，每圈对应各点之间减少的能量即是两圈之间各对应点之间B1 +场的能量的差，即：
Figure PCTCN2017083447-appb-000026
外圈的电场能量转化为内圈的磁场能量，即：
Figure PCTCN2017083447-appb-000027
用式(Ⅱ)除以式(Ⅲ)，得到损耗角正切，损耗角正切为外圈各点所在部位组织的电导率与相对介电常数的比值，相对介 电常数为电容率与角频率之积，损耗角正切的表达式为
Figure PCTCN2017083447-appb-000028
通过上述计算，得到成像区域内各处组织的电导率与相对介电常数的比值
Figure PCTCN2017083447-appb-000029
由于正常人体组织的介电特性值是已知的，故其比值
Figure PCTCN2017083447-appb-000030
即损耗角正切的值，也是已知的。将上述计算得到的
Figure PCTCN2017083447-appb-000031
分布成像的结果与正常组织的值进行对比，就可以发现
Figure PCTCN2017083447-appb-000032
值异常所在部位，为疾病的早期发现，如早期可疑癌变病灶等，提供有价值的空间定位信息，再辅以活检等技术手段，就可以实现肿瘤的早期发现，具有重要的临床应用价值。
本发明的从电磁场能量传播角度求解组织电特性分布的方法，从磁共振射频发射电磁场能量在体内传输和分布的角度求解EPs，计算过程中只用到了可以直接测量得到的射频场的幅度信息，不需要射频场的相位信息，规避了射频场的相位无法直接测量的问题；同时新方法避免了麦克斯韦方程的二阶微分运算，可以有效的提高运算结果的精度。
相比现有的MR EPT的算法中，在计算MR射频电磁场时，普遍采用的处理方法是提出局部组织电特性是均匀的(即假设局部组织的电特性的梯度为零)，并且假设忽略射频电场的横截面分量和射频磁场的纵轴分量，即假设Ex,y(r)≈0和Bz(r)≈0。现有的 MR EPT算法这样处理的目的是为了简化得到组织EPs的近似计算公式，形成现有的MR EPT算法理论，比如之前提到的Voigt等给出的基于射频磁场幅度的电容率求解方法。这种简化处理在复杂电磁边界条件存在区域会导致测量结果误差增加。
本发明的方法从电磁场能量的角度求解EPs，计算过程中只应用了磁场的幅度信息，避开了射频场的相位问题。相位问题是现有MR EPT算法中未解决的难题，因为射频发射场的相位利用现有的MR技术，是无法在工程上直接测量得到的，在求解的过程中，同样是在做了许多假设的情况下求取相位分布近似解，且现有的求解方法都存在较大的误差。故，本发明的方法同时避免了麦克斯韦方程的二阶微分运算，可以有效的提高运算结果的精度。
实施例2。
一种从电磁场能量传播角度求解组织局部比吸收率的方法，该方法基于以下物理事实：电磁能量在射频线圈的作用下，从四周向体内传播，电场和磁场能量交替转换，且能量不可能突变，在电场和磁场能量相互转换过程中，一部分电场的能量会转换成热能消耗掉。
核磁共振射频发射点磁场的总能量的表达式为：
Figure PCTCN2017083447-appb-000033
其中，ω为角频率，μ是磁导率，ε是电容率，σ是电导率，B为磁感应强度，E为电场强度，“*”表示取共轭复数，
Figure PCTCN2017083447-appb-000034
表示磁 场储能，
Figure PCTCN2017083447-appb-000035
表示电场储能，
Figure PCTCN2017083447-appb-000036
表示焦耳热损耗。
具体通过如下步骤进行：
(1)根据射频功放的功率大小和射频发射的时间，以及系统的工作效率，计算核磁共振射频线圈发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量。
(2)根据B1Mapping技术，得到射频发射产生的磁场B1 +场在人体内各点的幅度分布，根据
Figure PCTCN2017083447-appb-000037
得到B1 +场在体内各点处的磁场能量；根据麦克斯韦方程组电磁互生理论，同时得到该处电场的能量。
(3)能量由外圈向内圈传递的过程中，由于热能的损耗，总能量不断减少，每圈对应各点之间减少的能量即是两圈之间各对应点之间B1 +场的能量的差，即：
Figure PCTCN2017083447-appb-000038
外圈的电场能量转化为内圈的磁场能量，即：
Figure PCTCN2017083447-appb-000039
对外圈而言：
Figure PCTCN2017083447-appb-000040
其中,Bout表示外圈的磁感应强度,Bin表示内圈的磁感应强度,Εout表示外圈电场强度。Bin和Bout通过B1Mapping技术得到。
通过比较内外圈各点之间的电磁场总能量差，即
Figure PCTCN2017083447-appb-000041
可以得到各圈各点的热能损耗的值，即各圈各点的
Figure PCTCN2017083447-appb-000042
再利用人体组织各点处的组织密度ρ(已知值)，即可得到各圈各点的局部比吸收率(Local SAR)值，即：
Figure PCTCN2017083447-appb-000043
本发明的从电磁场能量传播角度求解组织局部比吸收率的方法，从磁共振射频发射电磁场能量在体内传输和分布的角度求解EPs，计算过程中只用到了可以直接测量得到的射频场的幅度信息，不需要射频场的相位信息，规避了射频场的相位无法直接测量的问题；同时新方法避免了麦克斯韦方程的二阶微分运算，可以有效的提高运算结果的精度。在组织电特性分布的基础上，结合组织密度、组织处电场幅值、组织处电导率，按照Local SAR计算公式即可以得到Local SAR分布。
本发明的从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法，从电磁场能量——一个全新的角度求解组织电特性和组织局部比吸收率，具有计算简单，结果精确的特点。
实施例3。
为了验证本发明的准确性，通过仿真、体模实验和人体实际测量等不同角度，来验证本发明方法的正确性。
1)仿真
利用SEMCAD软件，设置核磁共振的频率为128MHZ，建立16通道的高通鸟笼线圈，将DUKE头部模型置于鸟笼线圈中。仿 真获取头部射频场B1 +场的分布，如图1所示。建立磁场由外向内的传播模型,将所得的B代入公式(Ⅱ)中,得到组织的热损耗值,又由于组织的密度是已知的,带入Local SAR的求解公式即可求得Local SAR的分布图，如图2所示。由公式(Ⅰ)和(Ⅳ)可计算得组织的电容率，再根据损耗角正切值，则可求出对应点的电导率，如图3所示。图4是本发明实施例3的仿真实验中电场能量、磁场能量、热损耗和总能量的分布图。
2)体模实验
采用各种能模拟不同组织电特性的介电材料，填充特定结构空腔，形成类似人体不同组织交界面的电磁边界条件，制成专用的体模。在3T的磁共振中采用不同的扫描序列，得到相应的图像，应用B1Mapping技术，求出射频场的幅度|B1 +|。然后，结合核磁共振的能量场求解电特性及局部比吸收率即可。
3)人体实测
通过仿真及体模实验验证基于核磁共振能量场求解电特性的准备性后，进行临床志愿者的人体测量实验。志愿者包含正常人群及肿瘤患者两类。
通过仿真、体模实验和人体实测，结果证明，本发明的方法能够简单、精确地得到组织的电特性分布和局部比吸收率。相比现有的MR EPT方法，本发明的方法从电磁场能量的角度求解EPs，计算过程中只应用了磁场的幅度信息，避开了射频场的相位问题，因为相位在求解的过程中，比较复杂，且现有的求解方法 都存在较大的误差；同时避免了麦克斯韦方程的二阶微分运算，可以有效的提高运算结果的精度。在组织电特性分布的基础上，结合组织密度、组织处电场幅值、组织处电导率，按照Local SAR计算公式即可以得到Local SAR分布。
本发明的从电磁场能量传播角度求解组织电特性分布及局部比吸收率的方法，从电磁场能量——一个全新的角度求解组织电特性和组织局部比吸收率，具有计算简单，结果精确的特点。
最后应当说明的是，以上实施例仅用以说明本发明的技术方案而非对本发明保护范围的限制，尽管参照实施例对本发明作了详细说明，本领域的普通技术人员应当理解，可以对本发明的技术方案进行修改或者等同替换，而不脱离本发明技术方案的实质和范围。

Claims (2)

  1. 一种从电磁场能量传播角度求解组织电特性分布的方法，其特征在于，该方法基于以下物理事实：电磁能量在射频线圈的作用下，从四周向体内传播，电场和磁场能量交替转换，且能量不可能突变，在电场和磁场能量相互转换过程中，一部分电场的能量会转换成热能消耗掉；核磁共振射频发射线圈发射出的电磁场，在到达人体后，各点电磁场的总能量的表达式为：
    Figure PCTCN2017083447-appb-100001
    其中，ω为角频率，μ是磁导率，ε是电容率，σ是电导率，B为磁感应强度，E为电场强度，“*”表示取共轭复数，
    Figure PCTCN2017083447-appb-100002
    表示磁场储能，
    Figure PCTCN2017083447-appb-100003
    表示电场储能，
    Figure PCTCN2017083447-appb-100004
    表示焦耳热损耗；
    具体通过如下步骤进行：
    (1)根据射频功放的功率大小和射频发射的时间，以及系统的工作效率，计算核磁共振射频线圈发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量；
    (2)根据B1Mapping技术，得到射频发射产生的磁场B1 +场在人体内各点的幅度分布，根据
    Figure PCTCN2017083447-appb-100005
    得到B1 +场在体内各点处的磁场能量；根据麦克斯韦方程组电磁互生理论，同时得到该处电场的能量；
    (3)能量由外圈向内圈传递的过程中，由于热能的损耗，总能量不断减少，每圈对应各点之间减少的能量即是两圈之间各对应点之 间B1 +场的能量的差，即：
    Figure PCTCN2017083447-appb-100006
    其中,Bout表示外圈的磁感应强度,Bin表示内圈的磁感应强度,Εout表示外圈电场强度；Bin和Bout通过B1Mapping技术得到；
    外圈各点的电场储能转化为内圈对应各点的磁场能量，即：
    Figure PCTCN2017083447-appb-100007
    用式(Ⅱ)除以式(Ⅲ)，得到损耗角正切，损耗角正切为外圈各点所在部位组织的电导率与相对介电常数的比值，相对介电常数为电容率与角频率之积，损耗角正切的表达式为
    Figure PCTCN2017083447-appb-100008
    通过上述计算，得到成像区域内各处组织的电导率与相对介电常数的比值
    Figure PCTCN2017083447-appb-100009
  2. 一种从电磁场能量传播角度求解组织局部比吸收率的方法，其特征在于，该方法基于以下物理事实：电磁能量在射频线圈的作用下，从四周向体内传播，电场和磁场能量交替转换，且能量不可能突变，在电场和磁场能量相互转换过程中，一部分电场的能量会转换成热能消耗掉；核磁共振射频发射线圈发射出的电磁场，在到达人体后，各点电磁场的总能量的表达式为：
    Figure PCTCN2017083447-appb-100010
    其中，ω为角频率，μ是磁导率，ε是电容率，σ是电导率，B为磁感 应强度，E为电场强度，“*”表示取共轭复数，
    Figure PCTCN2017083447-appb-100011
    表示磁场储能，
    Figure PCTCN2017083447-appb-100012
    表示电场储能，
    Figure PCTCN2017083447-appb-100013
    表示焦耳热损耗；
    具体通过如下步骤进行：
    (1)根据射频功放的功率大小和射频发射的时间，以及系统的工作效率，计算核磁共振射频线圈发射的总能量，减去系统反射回来的能量，得到人体组织内存在的电磁场的总能量；
    (2)根据B1 Mapping技术，得到射频发射产生的磁场B1 +场在人体内各点的幅度分布，根据
    Figure PCTCN2017083447-appb-100014
    得到B1 +场在体内各点处的磁场能量；根据麦克斯韦方程组电磁互生理论，同时得到该处电场的能量；
    (3)能量由外圈向内圈传递的过程中，由于热能的损耗，总能量不断减少，每圈对应各点之间减少的能量即是两圈之间各对应点之间B1 +场的能量的差，即：
    Figure PCTCN2017083447-appb-100015
    其中，Bout表示外圈的磁感应强度，Bin表示内圈的磁感应强度，Εout表示外圈电场强度；Bin和Bout通过B1Mapping技术得到；
    外圈各点的电场储能转化为内圈对应各点的磁场能量，即：
    Figure PCTCN2017083447-appb-100016
    对各圈而言，电场和磁场能量相互转化，即：
    Figure PCTCN2017083447-appb-100017
    通过比较内外圈各点之间的电磁场总能量差，即
    Figure PCTCN2017083447-appb-100018
    得到各圈各点的热能损耗的值，即各圈各点的
    Figure PCTCN2017083447-appb-100019
    再利用人体组织各点处的组织密度ρ，得到各圈各点的局部比吸收率Local SAR，即：
    Figure PCTCN2017083447-appb-100020
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