同时多层成像技术联合肝脏MR扩散成像的应用及展望

同时多层（SMS）成像技术与多种MRI序列联合应用可明显缩短成像时间。MR扩散成像,如常规扩散加权成像（DWI）、体素内不相干运动成像（IVIM）、扩散张量成像（DTI）和扩散峰度成像（DKI）能反映组织内水分子扩散、血流灌注、组织结构复杂性等微观特征,在肝脏病变检测和辅助定性中有重要价值。SMS与MR扩散成像联合后可明显缩短成像时间,利于各种MR扩散成像在肝脏中的广泛应用。综述SMS对肝脏扩散成像扫描速度的提升效率、对影像质量和定量参数的影响,以期推动SMS成像技术在临床中的广泛应用。     

腹部IVIM-DWI应用研究及进展

体素内不均一运动(IVIM)是指MR扩散加权成像(DWI)上体素内信号衰减同时包括真性水分子扩散和毛细血管网中随机血流微循环灌注,导致表观扩散系数(ADC)值反映的信息有限。采用多b值可获取系列DWI影像,根据双指数模型拟合,可同时获得组织的扩散和灌注信息,更全面地分析组织扩散成像数据。IVIM-DWI目前已广泛应用于肝脏、胰腺、肾脏、前列腺等脏器,就该技术在腹部MR成像中的应用研究现状及进展予以综述。     

体素内相位不相干运动MR成像在头颈部的应用进展

组织中水分子的运动不仅包括纯水分子的扩散,同时还包括毛细血管灌注。体素内相位不相干运动（IVIM）同时考虑扩散及灌注两种运动成分,能够更准确地描述体素内的运动。肿瘤内血管生成是恶性肿瘤的特征之一,IVIM灌注参数可以反映组织血流灌注情况,有助于鉴别诊断良恶性肿瘤。了解疾病治疗前后扩散及灌注的变化,对疗效的预测有一定指导。对正常组织灌注的研究,有助于观察生物学功能。介绍IVIM的基本原理,并对近几年IVIM在头颈部的应用进行综述。      

体素内不相干运动扩散加权成像在肾脏的应用进展

体素内不相干运动扩散加权成像(IVIM-DWI)假设组织内水分子扩散运动为非高斯分布,并基于双指数模型得到扩散相关参数。与常规DWI比较,IVIM-DWI不仅能够更加真实地反映组织内水分子扩散运动,并且能够反映组织的微循环灌注。肾脏的毛细血管网及小管系统丰富,因此肾脏的水分子扩散状态和微循环灌注容易受多种因素的影响而发生改变。综述IVIM-DWI在原发性肾脏疾病(肾肿瘤、肾血管疾病等)、继发性肾病(糖尿病肾病、输尿管梗阻性继发性肾病等)及肾移植中的应用进展。     

IVIM在胶质瘤中的研究进展及临床应用

胶质瘤微血管生成在肿瘤发生及发展过程中具有重要作用,与胶质瘤的分级及预后密切相关,因此准确了解肿瘤的微循环血流动力学特征是十分必要的.磁共振灌注成像技术可在无创、无辐射的情况下反映微循环灌注情况,已广泛应用于临床及科研中.体素内不相干运动(IVIM)成像技术是在扩散成像基础上发展的磁共振成像技术,其不仅能反应组织扩散的...     

IVIM-DWI在肿瘤性病变中的应用进展

扩散加权成像(diffusion weighted imaging,DWI)为单指数模型,其信号衰减同时受真性水分子扩散及微血管灌注的影响,并不能反映真实弥散,体素内不相干运动(in intravoxel incoherent motion,IVIM)基于双指数数学模型同时反映水分子真实弥散及微灌注,更全面、精确的反映肿瘤组织微观结构的复杂性,已成为近年来功能MRI研究的热点。笔者主要就IVIM在各系统肿瘤中的应用进展进行综述。     

IVIM扩散加权成像在前列腺癌中的应用研究

前列腺癌是老年男性常见疾病,前列腺癌早期诊断是治疗和预后关键。MRI能较好显示前列腺解剖结构和相邻软组织关系,D WI是目前无创性检测活体水分子运动方法。近年来,在常规D WI基础上开发多b值体素内不相干运动（IVIM）扩散加权成像序列不仅能反映组织水分子扩散信息,亦能反映组织灌注信息。本文旨在综述IVIM-DWI原理、特点及在前列腺癌诊断中的应用。     

体素内不相干运动成像技术在颅脑中的应用

1988年,Le Bihan提出了一种能同时反映生物组织中扩散和灌注信息的成像方法 ,即体素内不相干运动(intravoxel incoherent motion,IVIM)DWI。近年来,随着高场强MRI和Stejskal-Tanner梯度线圈的出现,IVIM成像技术逐渐发展成熟,被广泛应用于全身各类病变诊断中,尤其是在肿瘤诊断方面。其中,IVIM在颅脑缺血性病变及脑肿瘤的影像学诊断中尤其重要。本文旨在对IVIM成像原理、技术要点、参数生理学意义及其在颅脑病变中的临床应用简要综述。     

DWI在肝脏占位性病变应用的现状

扩散加权成像(DWI)是利用水分子的扩散运动特性进行成像的技术,使MRI对人体的研究深入到分子水平,反映人体组织的微观几何结构以及细胞内外水分子的运动等变化。现就DWI在肝脏占位性病变诊断和疗效评估中的应用现状及其与MR常规序列的比较予以综述。     

DWI在肝脏占位性病变应用的现状

扩散加权成像（DWI）是利用水分子的扩散运动特性进行成像的技术,使MRI对人体的研究深入到分子水平,反映人体组织的微观几何结构以及细胞内外水分子的运动等变化.现就DWI在肝脏占位性病变诊断和疗效评估中的应用现状及其与MR常规序列的比较予以综述.     

Separation of diffusion and perfusion in intravoxel incoherent motion MR imaging

Intravoxel incoherent motion (IVIM) imaging is a method the authors developed to visualize microscopic motions of water. In biologic tissues, these motions include molecular diffusion and microcirculation of blood in the capillary network. IVIM images are quantified by an apparent diffusion coefficient (ADC), which integrates the effects of both diffusion and perfusion. The aim of this work was to demonstrate how much perfusion contributes to the ADC and to present a method for obtaining separate images of diffusion and perfusion. Images were obtained at 0.5 T with high-resolution multisection sequences and without the use of contrast material. Results in a phantom made of resin microspheres demonstrated the ability of the method to separately evaluate diffusion and perfusion. The method was then applied in patients with brain and bone tumors and brain ischemia. Clinical results showed significant promise of the method for tissue characterization by perfusion patterns and for functional studies in the evaluation of the microcirculation in physiologic and pathologic conditions, as, for instance, in brain ischemia.     

Intravoxel incoherent motion imaging using spin echoes

The purpose of this paper is to review the basic principles of diffusion measurement with spin echoes. These principles can be combined with those of MR imaging to generate maps of diffusion coefficients. Diffusion imaging can be extended to imaging of other intravoxel incoherent motions (IVIM), such as blood microcirculation. Some of the technical problems encountered when implementing IVIM imaging are presented.     

Diffusion-weighted imaging of pancreatic cancer

Magnetic resonance imaging (MRI) is a reliable and accurate imaging method for the evaluation of patients with pancreatic ductal adenocarcinoma (PDAC). Diffusion-weighted imaging (DWI) is a relatively recent technological improvement that expanded MRI capabilities, having brought functional aspects into conventional morphologic MRI evaluation. DWI can depict the random diffusion of water molecules within tissues (the so-called Brownian motions). Modifications of water diffusion induced by different factors acting on the extracellular and intracellular spaces, as increased cell density, edema, fibrosis, or altered functionality of cell membranes, can be detected using this MR sequence. The intravoxel incoherent motion (IVIM) model is an advanced DWI technique that consent a separate quantitative evaluation of all the microscopic random motions that contribute to DWI, which are essentially represented by molecular diffusion and blood microcirculation (perfusion). Technological improvements have made possible the routine use of DWI during abdominal MRI study. Several authors have reported that the addition of DWI sequence can be of value for the evaluation of patients with PDAC, especially improving the staging; nevertheless, it is still unclear whether and how DWI could be helpful for identification, characterization, prognostic stratification and follow-up during treatment. The aim of this paper is to review up-to-date literature data regarding the applications of DWI and IVIM to PDACs.     

Liver fibrosis: an intravoxel incoherent motion (IVIM) study

Purpose:

To characterize longitudinal changes in molecular water diffusion, blood microcirculation, and their contributions to the apparent diffusion changes using intravoxel incoherent motion (IVIM) analysis in an experimental mouse model of liver fibrosis.

Materials and Methods:

Liver fibrosis was induced in male adult C57BL/6N mice (22–25 g; n = 12) by repetitive dosing of carbon tetrachloride (CCl₄). The respiratory‐gated diffusion‐weighted (DW) images were acquired using single‐shot spin‐echo EPI (SE‐EPI) with 8 b‐values and single diffusion gradient direction. True diffusion coefficient (D_true), blood pseudodiffusion coefficient (D_pseudo), and perfusion fraction (P_fraction) were measured. Diffusion tensor imaging (DTI) was also performed for comparison. Histology was performed with hematoxylin‐eosin and Masson's trichrome staining.

Results:

A significant decrease in D_true was found at 2 weeks and 4 weeks following CCl₄ insult, as compared with that before insult. Similarly, D_pseudo values before injury was significantly higher than those at 2 weeks and 4 weeks after CCl₄ insult. Meanwhile, P_fraction values showed no significant differences over different timepoints. For DTI, significant decrease in ADC was observed following CCl₄ administration. Fractional anisotropy at 2 weeks after CCl₄ insult was significantly lower than that before insult, and subsequently normalized at 4 weeks after the insult. Liver histology showed collagen deposition, the presence of intracellular fat vacuoles, and cell necrosis/apoptosis in livers with CCl₄ insult.

Conclusion:

Both molecular water diffusion and blood microcirculation contribute to the alteration in apparent diffusion changes in liver fibrosis. Reduction in D_true and D_pseudo values resulted from diffusion and perfusion changes, respectively, during the progression of liver fibrosis. IVIM analysis may serve as valuable and robust tool in detecting and characterizing liver fibrosis at early stages, monitoring its progression in a noninvasive manner. J. Magn. Reson. Imaging 2012;36:159–167. © 2012 Wiley Periodicals, Inc.     

Could intravoxel incoherent motion diffusion-weighted magnetic resonance imaging be feasible and beneficial to the evaluation of gastrointestinal tumors histopathology and the therapeutic response?

Gastrointestinal tumors (GTs) are among the most common tumors of the digestive system and are among the leading causes of cancer death worldwide. Functional magnetic resonance imaging (MRI) is crucial for assessment of histopathological changes and therapeutic responses of GTs before and after chemotherapy and radiotherapy. A new functional MRI technique, intravoxel incoherent motion (IVIM), could reveal more detailed useful information regarding many diseases. Currently, IVIM is widely used for various tumors because the derived parameters (diffusion coefficient, D; pseudo-perfusion diffusion coefficient, D*; and perfusion fraction, f) are thought to be important surrogate imaging biomarkers for gaining insights into tissue physiology. They can simultaneously reflect the microenvironment, microcirculation in the capillary network (perfusion) and diffusion in tumor tissues without contrast agent intravenous administration. The sensitivity and specificity of these parameters used in the evaluation of GTs vary, the results of IVIM in GTs are discrepant and the variability of IVIM measurements in response to chemotherapy and/or radiotherapy in these studies remains a source of controversy. Therefore, there are questions as to whether IVIM diffusion-weighted MRI is feasible and helpful in the evaluation of GTs, and whether it is worthy of expanded use.     

Perfusion-driven Intravoxel Incoherent Motion (IVIM) MRI in Oncology: Applications,Challenges, and Future Trends

Recent developments in MR hardware and software have allowed a surge of interest in intravoxel incoherent motion (IVIM) MRI in oncology. Beyond diffusion-weighted imaging (and the standard apparent diffusion coefficient mapping most commonly used clinically), IVIM provides information on tissue microcirculation without the need for contrast agents. In oncology, perfusion-driven IVIM MRI has already shown its potential for the differential diagnosis of malignant and benign tumors, as well as for detecting prognostic biomarkers and treatment monitoring. Current developments in IVIM data processing, and its use as a method of scanning patients who cannot receive contrast agents, are expected to increase further utilization. This paper reviews the current applications, challenges, and future trends of perfusion-driven IVIM in oncology.     

肾脏缺血再灌注损伤的功能MRI实验研究进展

肾脏缺血再灌注损伤是导致急性肾损伤和移植肾功能延迟恢复的重要因素,严重者甚至会发生急性肾衰竭。扩散加权成像（DWI）、体素内不相干运动（IVIM）、血氧水平依赖（BOLD）、动脉自旋标记（ASL）、纵向弛豫时间定量成像（T1 mapping）等功能MRI能够无创、敏感、多次地监测不同程度的肾损伤,提供水分子扩散、微循环、血流灌注及血氧水平等微观信息的动态变化,为了解肾脏缺血再灌注损伤的发生机制、早期诊断、预后评估等提供更多信息。就肾脏缺血再灌注损伤的功能MRI实验研究进展予以综述。     

Intravoxel incoherent motion diffusion imaging of the liver: Optimal b-value subsampling and impact on parameter precision and reproducibility

PurposeTo increase diffusion sampling efficiency in intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI) of the liver by reducing the number of diffusion weightings (b-values).Materials and methodsIn this IRB approved HIPAA compliant prospective study, 53 subjects (M/F 38/15, mean age 52 ± 13 y) underwent IVIM DWI at 1.5 T using 16 b-values (0–800 s/mm²), with 14 subjects having repeat exams to assess IVIM parameter reproducibility. A biexponential diffusion model was used to quantify IVIM hepatic parameters (PF: perfusion fraction, D: true diffusion and D*: pseudo diffusion). All possible subsets of the 16 b-values were probed, with number of b values ranging from 4 to 15, and corresponding parameters were quantified for each subset. For each b-value subset, global parameter estimation error was computed against the parameters obtained with all 16 b-values and the subsets providing the lowest error were selected. Interscan estimation error was also evaluated between repeat exams to assess reproducibility of the IVIM technique in the liver. The optimal b-values distribution was selected such that the number of b-values was minimal while keeping parameter estimation error below interscan reproducibility error.ResultsAs the number of b-values decreased, the estimation error increased for all parameters, reflecting decreased precision of IVIM metrics. Using an optimal set of 4 b-values (0, 15, 150 and 800 s/mm²), the errors were 6.5, 22.8 and 66.1% for D, PF and D* respectively. These values lie within the range of test–retest reproducibility for the corresponding parameters, with errors of 12.0, 32.3 and 193.8% for D, PF and D* respectively.ConclusionA set of 4 optimized b-values can be used to estimate IVIM parameters in the liver with significantly shorter acquisition time (up to 75%), without substantial degradation of IVIM parameter precision and reproducibility compared to the 16 b-value acquisition used as the reference.     

Can the IVIM model be used for renal perfusion imaging?

Objective: Renal perfusion imaging may provide information about the hemodynamic significance of a renal artery stenosis and could improve noninvasive characterization when combined with angiography. It was proposed previously that diffusion sequences could provide useful perfusion indices based on the intravoxel incoherent motion (IVIM) model. Owing to motion artifacts, diffusion imaging has been restricted to relatively immobile organs like the brain. With the availability of single-shot echo-planar imaging (EPI) our purpose was to evaluate the IVIM model in renal perfusion. Methods and material: Eight volunteers underwent diffusion-sensitive magnetic resonance (MR) imaging of the kidneys using a spin echo (SE) EPI sequence. The diffusion coefficients determined by a linear regression analysis and fits to the IVIM function were calculated. Results and conclusion: Our preliminary experience does not support the possibility of obtaining perfusion information using the IVIM model in the kidneys.