【作者】 李晓东；

【导师】 辛学刚；

【作者基本信息】 华南理工大学 ， 生物医学工程， 2020， 硕士

【摘要】 磁共振成像(magnetic resonance imaging,MRI)技术是临床应用中不可替代的影像学工具,用于产生MR图像的信号主要来自于氢质子。在生物组织内,水分子是重要的富含氢质子的成像物质。大量水分子在组织微结构中扩散运动,即在细胞内或者细胞外微环境中扩散,也在细胞内外之间进行扩散,后者表现为跨细胞膜水交换。因此,基于水扩散和跨膜水交换的MRI方法将能提供关于活体状态下的组织微结构的重要功能信息。扩散加权成像(diffusion weighted imaging,DWI)技术对水扩散敏感,DWI技术的一个关键特征是通过表观扩散系数(apparent diffusion coefficient,ADC)量化水分子的可移动性。使用DWI技术进行临床疾病诊断时,病变过程通常伴随着ADC参数的变化。科学家们已经提出一些假设解释ADC参数的改变,这些假设包括细胞膜渗透性、细胞内空间的体积分数、细胞外空间的弯曲度以及细胞内空间。然而,尚缺乏实测实验研究定量地评估上述不同的组织微结构特性对ADC测量的影响。针对这一现状,本课题设计了一系列具有可调参数的物理体模执行实验评估。纤维渗透性、纤维内空间的体积分数和聚乙烯吡咯烷酮(polyvinylpyrrolidone,PVP)浓度分别描述细胞膜渗透性、细胞内空间的体积分数和扩散系数。在实验设定的体模参数下,ADC数值范围在0.986μm~2/ms到2.097μm~2/ms之间。实验中观察到ADC数值随着纤维渗透性的降低(平均孔径从5nm降低到3 nm)、纤维内空间体积分数的增加(从0.297到0.356)以及纤维内空间PVP浓度的增加(从0%到40%w/w)而降低,最高百分比降低分别为5.1%、11.3%和30.0%。结果表明了纤维内体积分数和PVP浓度对ADC作用的相互依赖。这些结果可能预示着细胞内体积分数和细胞内扩散系数对ADC测量的组合效应不可被忽视。在传统DWI序列的基础上,本课题开发并实现了用于对跨膜水交换进行成像的过滤交换成像(filter exchange imaging,FEXI)方法,该方法通过表观交换速率(apparent exchange rate,AXR)量化水交换效应。此外,我们提出将双重聚自旋回波(twice-refocused spin echo,TRSE)梯度设计方案加入FEXI序列,用于补偿涡流效应。本课题开发的FEXI方法实现了对水交换速率的准确测量,加入涡流补偿方案后,测量精确性进一步提高。更多还原

【Abstract】 Magnetic resonance imaging(MRI)technology is an irreplaceable imaging tool in clinicalapplications.The signals for producing the MR images are mainly from the hydrogen protons.In biological tissues,water molecules are the important imaging substances rich in hydrogen protons.A large number of water molecules diffuse in the tissue microstructure,that is,in the intracellular or extracellular microenvironment,and also between the intracellular and extracellular microenvironment.The latter is the water exchange across the cell membrane.Therefore,the MRI methods based on water diffusion and water exchange across the cell membrane will provide important functional information about tissue microstructures in vivo.Diffusion weighted imaging(DWI)technology is sensitive to water diffusion.A key characteristic of DWI is quantification by the apparent diffusion coefficient(ADC).When using DWI technology for clinical diagnosis,the pathological processes usually accompany with the changes in the ADC parameters.Some potential hypotheses have been proposed to explain the decrease in ADC,including the change in membrane permeability,intracellular volume fraction(IVF),tortuosity of extracellular spaces,and intracellular diffusivity.However,no experimental study has been conducted to quantitatively assess the effects of these microstructural properties on the ADC measurements.In view of this situation,we designed a series of physical phantoms with adjustable parameters to conduct an experimental study.Three phantom parameters(i.e.,fiber permeability,volume fraction,and polyvinylpyrrolidone(PVP)concentration of the intra-fiber spaces)were used to characterize three cellular tissue parameters(i.e.,membrane permeability,intracellular volume fraction,and diffusivity),respectively.At given phantom parameters,the ADC values range from 0.986μm~2/ms to 2.097μm~2/ms.Decreased ADC with the decreased fiber permeability(the mean pore diameter decreases from 5 nm to 2 nm),increased volume fraction of the intra-fiber spaces(from 0.297 to 0.356),and increased PVP concentration of the intra-fiber spaces(up to 40%(w/w))were observed,and the percentage decrease were up to 5.1%,11.3%,and 30.0%,respectively.Experimental results demonstrate that the effect of the volume fraction of the intra-fiber spaces on the ADC measurements depends on the PVP concentration of the intra-fiber spaces,and vice versa.These results suggest that the combined effect of intracellular volume fraction and diffusivity on the ADC measurements cannot be ignored.Based on traditional DWI technology,we designed and implemented the filter exchange imaging(FEXI)method for imaging water exchange across the cell membrane.The parameter to quantify the water exchange effect is the apparent exchange rate(AXR).Furthermore,we presented to add a double-refocused spin echo(TRSE)gradient scheme to the FEXI sequence to compensate the eddy current effect.The FEXI method designed in this subject realizes the accurate measurement of the water exchange rate.After adding the eddy current compensation scheme,the measurement accuracy is further improved.更多还原