开放式磁感应成像技术基础研究
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摘要
目前运用于临床的医学影像技术可分为两种:结构性成像和功能性成像。结构性成像包括:X射线成像,CT,磁共振成像,超声成像。功能性成像主要包括:放射性核素显像,功能性核磁共振成像,脑磁图,电阻抗成像技术。当前,临床急需一种能够进行功能性成像的便携式床边实时检测设备,上述各种方法只有电阻抗成像技术能够提供连续时间功能性监护成像。然而,由于电阻抗成像技术需要向被测物体内注入电流,颅骨层的低电导率限制了电阻抗成像技术在颅脑疾病功能性成像方面的应用。所以,本论文根据磁场能够穿透低电导率颅骨的特点,针对颅脑疾病的实时监护,研究一种新型的无接触式功能性成像技术:开放式磁感应成像。论文内容包括以下几个方面:
①开放式磁感应成像的正问题理论研究。正问题研究主要包括两方面:一是采用基于有限元的A -φ法分析二维场域内的涡流分布情况。二是参照地质勘测分层模型的思路,提出生物体组织分层介质模型的假设,采用基于区域划分的Gauss-Legendre数值积分算法计算分层介质的开放式磁感应成像正问题。
②开放式磁感应成像逆问题研究。采用改进遗传算法计算分层开放式磁感应成像的逆问题。对遗传算法作了如下改进:根据个体之间的广义汉明距离限制,产生初始种群;采取精英保留策略和基于压差的排序方法进行选择操作;根据相关性配对方法,选择进行交叉的两个体,根据有效交叉区域的限制,确定单点交叉的基因座位置,根据个体适应度的大小,确定自适应交叉概率;根据进化代数,自适应调节变异概率。对开放式磁感应成像进行了仿真试验,结果表明:对于半径为10mm的探头,能对表皮下20mm的范围内,出血范围大约为两倍探头尺寸的电导率异常情况能够比较准确的进行重构。
③测量系统研制。介绍了开放式磁感应成像的硬件测量系统的各组成模块和设计原理。针对头颅,设计了传感器呈球面分布的多通道开放式磁感应成像设备。
④实验研究。实验研究包括两个主要内容:一是系统性能指标测试实验,确定测量系统的灵敏度,稳定性,以及噪声水平。测试结果表明:系统的相位灵敏度优于幅值灵敏度。所以我们确定以相位作为成像数据的来源。系统的相位灵敏度为0.185~o / S /m。通过稳定性测试实验,引入参考通道,大大减小了系统的相位噪声。系统的相位噪声在±0.05°之间。二是水槽实验,通过一系列水槽实验研究了开放式磁感应成像系统的成像效果。通过变频实验发现,激励频率越高,越有利于提高图像的分辨率。检测深度的实验表明,现有水槽系统的探测深度大约为20mm左右。最后,通过覆盖骨头的琼脂实验,证明了磁感应成像系统对低电导率骨骼的穿越能力,为将来实现临床颅脑疾病监护奠定了实验基础。
There are two main kind of clinical medicinal imaging technology. One is the structural imaging, and the other is the functional imaging. The former includes X-ray, CT, MRI and ultra-sound imaging. And the latter includes the radionuclidic imaging(PET), fMRI, Magnetoencephalography(MEG) and EIT. And now there is a urgent demand in the clinical application for a portable real-time monitoring functional imaging system on the bedside. All the mentioned imaging method can not satisfy this demand but EIT, but EIT employs a pair of electrodes to inject current into the measured objects, the low-conductivity characteristic of the skull layer limits its application in the brain disease functional imaging. Because the magnetic field can pass through the low-conductivity skull layer almost without attenuation, in this paper a contactless functional imaging method: Open Magnetic Induction Tomography is introduced. The research content includes following four parts:
①Forward problem of the open magnetic induction tomography. There are two parts of the forward problem research. A -φmethod based on FEM is employed to study the 2-D eddy current distribution. Referring the separated-layer idea in the geologic research, an assumption of multi-layer biological tissue model is presented. And a Gauss-Legendre numerical integral based on the region-separated idea is employed to calculate the forward problem of the open magnetic induction tomography.
②Inverse problem of the open magnetic induction tomography. A modified GA is employed to calculate the inverse problem. The modification of GA includes the following aspects. The initial population is generated according to the generalized Humming distance limitation. The elitist remaining method and the ranking algorithm based on pressure difference are used in selection. The two crossover individuals are selected according to the relative partner method. And gene position of the single-point crossover is located referring the limitation of the active crossover region. The crossover operation probability is adjusted according to the fitness value, and the variation operation probability is adjusted according to the iterative number. The simulation result indicates: a system equips a sensor with 10mm radius can reconstruct a conductivity abnormal region of two times larger as the sensor’s size within 20mm under the top surface.
③Measurement system research. All circuits and their designing principles are introduced. To simulate the human head, a multi-channel measurement system is constructed.
④Experiment research. There are two steps of the experiment research. One is the performance measurement of the MIT system including sensitivity, stability and the noise level. The result indicates that the sensitivity of phase is higher than that of the magnitude, so the phase data of the signal is recorded as the source of the imaging. The phase sensitivity of the system is 0.185~o / S /m . In order to improve the stability of the system and minimize the phase noise, a reference channel is employed. The phase noise range is±0.05°. The other is the water tank experiment. The results of the frequency-changing experiment indicate that the imaging quality can be improved by increasing the exciting frequency. The depth experiment indicates that our system can detect conductivity abnormal region within 20mm under the surface. And the imaging result of the agar model covered by a pig bladebone indicates that the magnetic induction tomography can detect conductivity distribution under the skull. And these experiments construct a base for the future clinical brain disease monitoring.
①开放式磁感应成像的正问题理论研究。正问题研究主要包括两方面:一是采用基于有限元的A -φ法分析二维场域内的涡流分布情况。二是参照地质勘测分层模型的思路,提出生物体组织分层介质模型的假设,采用基于区域划分的Gauss-Legendre数值积分算法计算分层介质的开放式磁感应成像正问题。
②开放式磁感应成像逆问题研究。采用改进遗传算法计算分层开放式磁感应成像的逆问题。对遗传算法作了如下改进:根据个体之间的广义汉明距离限制,产生初始种群;采取精英保留策略和基于压差的排序方法进行选择操作;根据相关性配对方法,选择进行交叉的两个体,根据有效交叉区域的限制,确定单点交叉的基因座位置,根据个体适应度的大小,确定自适应交叉概率;根据进化代数,自适应调节变异概率。对开放式磁感应成像进行了仿真试验,结果表明:对于半径为10mm的探头,能对表皮下20mm的范围内,出血范围大约为两倍探头尺寸的电导率异常情况能够比较准确的进行重构。
③测量系统研制。介绍了开放式磁感应成像的硬件测量系统的各组成模块和设计原理。针对头颅,设计了传感器呈球面分布的多通道开放式磁感应成像设备。
④实验研究。实验研究包括两个主要内容:一是系统性能指标测试实验,确定测量系统的灵敏度,稳定性,以及噪声水平。测试结果表明:系统的相位灵敏度优于幅值灵敏度。所以我们确定以相位作为成像数据的来源。系统的相位灵敏度为0.185~o / S /m。通过稳定性测试实验,引入参考通道,大大减小了系统的相位噪声。系统的相位噪声在±0.05°之间。二是水槽实验,通过一系列水槽实验研究了开放式磁感应成像系统的成像效果。通过变频实验发现,激励频率越高,越有利于提高图像的分辨率。检测深度的实验表明,现有水槽系统的探测深度大约为20mm左右。最后,通过覆盖骨头的琼脂实验,证明了磁感应成像系统对低电导率骨骼的穿越能力,为将来实现临床颅脑疾病监护奠定了实验基础。
There are two main kind of clinical medicinal imaging technology. One is the structural imaging, and the other is the functional imaging. The former includes X-ray, CT, MRI and ultra-sound imaging. And the latter includes the radionuclidic imaging(PET), fMRI, Magnetoencephalography(MEG) and EIT. And now there is a urgent demand in the clinical application for a portable real-time monitoring functional imaging system on the bedside. All the mentioned imaging method can not satisfy this demand but EIT, but EIT employs a pair of electrodes to inject current into the measured objects, the low-conductivity characteristic of the skull layer limits its application in the brain disease functional imaging. Because the magnetic field can pass through the low-conductivity skull layer almost without attenuation, in this paper a contactless functional imaging method: Open Magnetic Induction Tomography is introduced. The research content includes following four parts:
①Forward problem of the open magnetic induction tomography. There are two parts of the forward problem research. A -φmethod based on FEM is employed to study the 2-D eddy current distribution. Referring the separated-layer idea in the geologic research, an assumption of multi-layer biological tissue model is presented. And a Gauss-Legendre numerical integral based on the region-separated idea is employed to calculate the forward problem of the open magnetic induction tomography.
②Inverse problem of the open magnetic induction tomography. A modified GA is employed to calculate the inverse problem. The modification of GA includes the following aspects. The initial population is generated according to the generalized Humming distance limitation. The elitist remaining method and the ranking algorithm based on pressure difference are used in selection. The two crossover individuals are selected according to the relative partner method. And gene position of the single-point crossover is located referring the limitation of the active crossover region. The crossover operation probability is adjusted according to the fitness value, and the variation operation probability is adjusted according to the iterative number. The simulation result indicates: a system equips a sensor with 10mm radius can reconstruct a conductivity abnormal region of two times larger as the sensor’s size within 20mm under the top surface.
③Measurement system research. All circuits and their designing principles are introduced. To simulate the human head, a multi-channel measurement system is constructed.
④Experiment research. There are two steps of the experiment research. One is the performance measurement of the MIT system including sensitivity, stability and the noise level. The result indicates that the sensitivity of phase is higher than that of the magnitude, so the phase data of the signal is recorded as the source of the imaging. The phase sensitivity of the system is 0.185~o / S /m . In order to improve the stability of the system and minimize the phase noise, a reference channel is employed. The phase noise range is±0.05°. The other is the water tank experiment. The results of the frequency-changing experiment indicate that the imaging quality can be improved by increasing the exciting frequency. The depth experiment indicates that our system can detect conductivity abnormal region within 20mm under the surface. And the imaging result of the agar model covered by a pig bladebone indicates that the magnetic induction tomography can detect conductivity distribution under the skull. And these experiments construct a base for the future clinical brain disease monitoring.
引文
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