Echo-planar imaging of intravoxel incoherent motion

The recently established single-shot technique of echo-planar imaging of intravoxel incoherent motion (IVIM) for determining and imaging the variations of microscopic motions of water has been applied to studies of water perfusion in phantoms and to in vivo studies of diffusion and perfusion in cat and human brains. The phantom results demonstrate that perfusion levels comparable with those found in vivo have easily observable and reproducible effects on signal amplitude that are consistent with previous IVIM theory. Reliable measurements of the diffusion coefficient in various types of brain tissue have been obtained. The results for white matter are consistent with the existence of anisotropic diffusion in oriented bundles of myelinated nerve fibers. The results for gray matter can be fitted to the IVIM theory and suggest a value of up to 14% for the fraction of the signal contributed by randomly perfusing fluid in normal cerebral cortex.     

Diffusion-weighted MR imaging of extraaxial tumors

The clinical usefulness of the application of spin-echo diffusion-weighted imaging in the evaluation of extraaxial cysts and epidermoid tumors is demonstrated in a series of 15 patients. Apparent diffusion coefficient (ADC) images based on intravoxel incoherent motion (IVIM) were obtained with a maximum gradient b value = 100 s/mm2. Lesion ADC was qualitatively compared to external phantoms. In all cases, epidermoid tumors revealed reduced ADC values similar to that of normal brain tissue. On the other hand, all cysts had ADC similar to the stationary water phantom. Lesion delineation was improved due to the replacement of normal pulsatile (very high ADC) cisternal CSF. Direct quantitative measurements of ADC using this technique may not be possible due to unavoidable motion artifact.     

A versatile flow phantom for intravoxel incoherent motion MRI

Although there have been many advancements in cancer research, much is still unknown about the heterogeneous tumor microenvironment. Diffusion-weighted MRI has proven to be a viable and versatile microstructural probe. Diffusion-weighted sequences specifically sensitive to intravoxel incoherent motion (IVIM) have seen a recent resurgence of interest as they promise to provide a valuable window on the vascular microenvironment. To understand, test, and optimize IVIM-sensitive approaches, a complex flow phantom was constructed to mimic certain characteristics of the tumor microenvironment such as tortuous microvasculature, heterogeneous vascular permeability, and interstitial fluid pressure buildup. Results using this phantom on a clinical scanner platform confirmed IVIM sensitivity to microscopic flow effects. Biexponential fitting of signal decay curves enabled quantitative extraction of perfusion fraction, IVIM-related pseudodiffusivity, and tissue diffusivity. Parametric maps were also generated, illustrating the potential utility of IVIM-sensitive imaging in clinical settings. The flow phantom proved to be an effective test-bed for validating and optimizing the IVIM-MRI technique to provide surrogate markers for microvascular properties.     

IVIM-DWI技术在肝脏弥漫性病变中的研究进展

在生物组织中体素内不相干运动(IVIM)包括体素内水分子扩散和微循环灌注,IVIM双指数模型可以精确描述DWI信号衰减与b值的关系,分别获取反映组织水分子扩散和微循环灌注的参数。近年来,IVIM成像在肝脏弥漫性病变的检测与分级中应用越来越多。主要阐述肝脏IVIM成像的原理、影响因素及可重复性以及在弥漫性病变中的诊断价值。     

胎盘的功能MRI研究进展

胎盘功能不良是妊娠失败的主要原因,可导致多种产前和产后综合征,对成年女性健康会造成长期负面的影响。近年来通过对动物模型和人类胎盘的功能MRI研究,在很大程度上提高了对胎盘的结构、血流、血氧状态、代谢特征以及妊娠并发症的认识。采用的成像技术包括动态对比增强、动脉自旋标记、血氧水平依赖与氧增强、扩散加权成像与体素内不相干运动、磁共振波谱成像等,分别研究胎盘的氧合状态、组织微结构及代谢。就各种成像技术的基本原理、动物模型和人类胎盘的研究进展、技术限度与挑战以及未来潜在的应用领域等予以综述。     

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.     

Diffusion-weighted MR imaging of the brain: value of differentiating between extraaxial cysts and epidermoid tumors

This study demonstrates the use of diffusion-weighted MR imaging in improving the specificity of the diagnosis of extraaxial brain tumors. Three surgically proved lesions (one arachnoid cyst and two epidermoid tumors) and two nonsurgically proved lesions (arachnoid and ependymal cysts) were evaluated with T1- and T2-weighted spin-echo studies followed by intravoxel incoherent motion (IVIM) MR imaging. The IVIM images of the lesions were displayed as an apparent diffusion coefficient (ADC) image obtained at 0.65 G/cm (maximum gradient b value = 100 sec/mm2) and compared with external oil and water phantoms. The ADC of arachnoid cysts was similar to stationary water whereas the ADC of epidermoid tumors was similar to brain parenchyma, indicating the solid nature and the slower diffusion rate of the epidermoid tumors. Cisternal CSF demonstrated uniformly high ADC, primarily because of bulk flow, which enhanced image contrast. Improved delineation of postsurgical changes was also possible. Our preliminary results show that diffusion-weighted MR imaging can be useful in distinguishing between arachnoid cysts and epidermoid tumors.     

Diffusion-weighted MR imaging of the brain: value of differentiating between extraaxial cysts and epidermoid tumors

This study demonstrates the use of diffusion-weighted MR imaging in improving the specificity of the diagnosis of extraaxial brain tumors. Three surgically proved lesions (one arachnoid cyst and two epidermoid tumors) and two nonsurgically proved lesions (arachnoid and ependymal cysts) were evaluated with T1- and T2-weighted spin-echo studies followed by intravoxel incoherent motion (IVIM) MR imaging. The IVIM images of the lesions were displayed as an apparent diffusion coefficient (ADC) image obtained at 0.65 G/cm (maximum gradient b value = 100 sec/mm2) and compared with external oil and water phantoms. The ADC of arachnoid cysts was similar to stationary water whereas the ADC of epidermoid tumors was similar to brain parenchyma, indicating the solid nature and the slower diffusion rate of the epidermoid tumors. Cisternal CSF demonstrated uniformly high ADC, primarily because of bulk flow, which enhanced image contrast. Improved delineation of postsurgical changes was also possible. Our preliminary results show that diffusion-weighted MR imaging can be useful in distinguishing between arachnoid cysts and epidermoid tumors.     

Assessment of microvessel perfusion of pituitary adenomas: a feasibility study using turbo spin-echo-based intravoxel incoherent motion imaging

European Radiology - To evaluate the feasibility of assessment of microvessel perfusion of pituitary adenomas with intravoxel incoherent motion (IVIM) imaging using single-shot turbo...     

Intravoxel incoherent motion imaging using steady-state free precession

IVIM MR imaging is a method which generates images of diffusion and perfusion in vivo. Until now, intravoxel incoherent motion (IVIM) images have been obtained using spin-echo sequences with extragradient pulses, resulting in long acquisition times (typically 2 x 8 min 32 s). A new method is proposed here, using steady-state free precession (SSFP), which allows IVIM images to be obtained in a couple of minutes. Phantom studies showed that the sensitivity of SSFP to IVIMs is much greater than that of spin echoes. In vivo images are shown.     

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.     

Extension of the intravoxel incoherent motion model to non‐gaussian diffusion in head and neck cancer

Purpose:

To extend the intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) model to restricted diffusion and to simultaneously quantify the perfusion and restricted diffusion parameters in neck nodal metastases.

Materials and Methods:

The non‐Gaussian (NG)‐IVIM model was developed and tested on diffusion‐weighted MRI data collected on a 1.5‐Tesla MRI scanner from eight patients with head and neck cancer. Voxel‐wise parameter quantification was performed by using a noise‐rectified least‐square fitting method. The NG‐IVIM, IVIM, Kurtosis, and ADC (apparent diffusion coefficient) models were used for comparison. For each voxel, within the metastatic node, the optimal model was determined using the Bayesian Information Criterion. The voxel percentage preferred by each model was calculated and the optimal model map was generated. Monte Carlo simulations were performed to evaluate the accuracy and precision dependency of the new model.

Results:

For the eight neck nodes, the range of voxel percentage preferred by the NG‐IVIM model was 2.3–79.3%. The optimal modal maps showed heterogeneities within the tumors. The Monte Carlo simulations demonstrated that the accuracy and precision of the NG‐IVIM model improved by increasing signal‐to‐noise ratio and b value.

Conclusion:

The NG‐IVIM model characterizes perfusion and restricted diffusion simultaneously in neck nodal metastases. J. Magn. Reson. Imaging 2012;36:1088–1096. © 2012 Wiley Periodicals, Inc.     

Renal artery stenosis: in vivo perfusion MR imaging

The intravoxel incoherent motion (IVIM) model of perfusion and diffusion imaging was applied to an in vivo canine model of unilateral renal artery stenosis and was compared with relative renal blood flow determination with radioactive microspheres. The percentage relative renal blood flow as determined with radioactive microspheres correlated closely with the percentage apparent diffusion coefficient. If this method can be adapted to human imaging, it may provide a noninvasive means for detecting renal artery stenosis.     

IVIM双指数、拉伸指数模型对子宫内膜癌分型的价值

目的 探讨体素内不相干运动（IVIM）双指数、拉伸指数模型各参数在Ⅰ型与Ⅱ型子宫内膜癌（EC）中的鉴别诊断价值。 方法 回顾性分析51例经手术病理证实的Ⅰ型（31例）与Ⅱ型（20例）EC病人的IVIM成像资料。由2名医师在IVIM各参数伪彩图上分别测量慢速表观扩散系数（ADC-slow）、快速表观扩散系数（ADC-fast）、灌注分数（f）、扩散分布指数（DDC）和扩散异质性指数（α）。采用组内相关系数（ICC）评价2名医师测量参数的一致性。采用Fisher确切概率法比较Ⅰ型与Ⅱ型EC的病理类型及分级分期。绘制受试者操作特征（ROC）曲线,分析2组间有统计学差异的参数及其诊断效能。 结果 2名医师所测数据的一致性较好（均ICC≥0.75）。Ⅰ型组的ADC-slow、DDC值均高于Ⅱ型组,ADC-fast值低于Ⅱ型组（均P<0.05）。ADC-slow值为0.500×10^-3 mm²/s时,鉴别Ⅰ型和Ⅱ型EC的AUC为0.926,敏感度、特异度、准确度分别为80.0%、90.3%、86.3%。 结论 双指数模型的参数ADC-slow、ADC-fast值和拉伸指数模型的参数DDC值均有助于鉴别Ⅰ型与Ⅱ型EC,其中ADC-slow值鉴别诊断效能更大。     

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。     

MRI对上皮性卵巢癌组织学分型的诊断应用

常规MRI可从形态学角度评估上皮性卵巢癌（EOC）组织学分型,扩散加权成像（DWI）、体素内不相干运动成像（IVIM）及动态增强MRI（DCE-MRI）等功能成像技术可进行定量后处理分析。基于MRI的影像组学通过全面分析Ⅰ型和Ⅱ型EOC的影像特征,在术前可以预测EOC分型,有助于制定个体化治疗方案及评估预后。就DWI、IVIM、DCE-MRI及影像组学在鉴别EOC分型中的应用进行综述。     

体素内不相干运动成像对胰腺癌的诊断价值

目的:探究DWI单指数模型、体素内不相干运动成像双指数模型及拉伸指数模型对胰腺癌诊断的临床应用价值。方法:回顾性收集分析经证实胰腺癌30例;同期检查胰腺正常者25例,行3.0T MRI-DWIeDWI,HR DWI扫描。测量记录两组单指数模型、双指数模型、拉伸指数模型各参数值。结果:ADC.Standard ADC,D-mono,D*-mono,D*-Bi,DDC、α值差异均有统计学差异(P<0.05)。α值ROC曲线下面积>0.9,AUC为0.905,ADC,D-moono,D*-mono,D*-Bi,DDC值ROC曲线下面积在0.75~0.9之间,AUC分别为0.725,0.873,0.899,0.773,0.767,standard ADC曲线下面积>0.5,AUC值0.538结论:IVIM多个参数对PC的诊断效能较常规DWI序列ADC值更高;其中IVIM序列拉伸指数模型的α值是鉴别胰腺癌与正常组织的最佳参数.     

Influence of respiratory motion in body diffusion weighted imaging under free breathing (examination of a moving phantom)

Diffusion weighted imaging (DWI) theoretically aims to detect random motion over small distances, such as Brownian motion. Therefore, breath-hold scanning has been considered the only way to acquire DWI in the body without artifacts from bulk motion. Recent reports suggest that non-breath-hold scanning is feasible. The purpose of this study was to evaluate the influence of respiratory motion on DWI using a moving phantom model. Our results showed that the difference in apparent diffusion coefficient (ADC) was less than 10% between a static phantom and a moving phantom. There was no relation between the speed and stroke of the moving phantom and the calculated ADC. The results indicate that stable motion such as calm respiration does not cause signal loss on DWI, in contrast to intra-voxel incoherent motion (IVIM). The images obtained using this method showed high resolution and signal-to-noise ratio (SNR), suitable for three-dimensional display of the lesion.     

Magnetic resonance: perfusion and diffusion imaging

Summary The use of magnetic resonance imaging to detect normal and pathological problems of perfusion and diffusion is reviewed. Motion sensitised spin-echo images can be used to detect changes in slow flow velocity within a voxel (intravoxel coherent motion (IVCM) as well as intravoxel incoherent motion (IVIM) effects attributable to both diffusion and perfusion. Changes have been identified in a variety of brain diseases in the absence of changes in conventional images but the techniques are very vulnerable to motion artefact of all types. More rapid and more sensitive approaches using steady state free precision and echo-planer imaging are being investigated. Anisotropic diffusion imaging enables white matter tracts to be demonstrated within the brain and spinal cord as a function of their direction because diffusion of water across axons is much more restricted than it is along them. This technique provides a unique method for localisation of lesions and displays obvious changes in disease in which diffusion becomes less restricted.     

Dynamic phantom with heart,lung, and blood motion for initial validation of MRI techniques

Purpose:

To develop an anthropomorphic phantom to simulate heart, lung, and blood motion. Magnetic resonance imaging (MRI) is sensitive to image distortion and artifacts caused by motion. Imaging phantoms are used to test new sequences, but generally, these phantoms lack physiological motion. For the validation of new MR‐based endovascular interventional and other techniques, we developed a dynamic motion phantom that is suitable for initial in vitro and more realistic validation studies that should occur before animal experiments.

Materials and Methods:

An anthropomorphic phantom was constructed to model the thoracic cavity, including respiratory and cardiac motions, and moving blood. Several MRI methods were used to validate the phantom performance: anatomical scanning, rapid temporal imaging, digital subtraction angiography, and endovascular tracking. The quality and nature of the motion artifact in these images were compared with in vivo images.

Results:

The closed‐loop motion phantom correctly represented key features in the thorax, was MR‐compatible, and was able to reproduce similar motion artifacts and effects as seen in in vivo images. The phantom provided enough physiological realism that it was able to ensure a suitable challenge in an in vitro catheter tracking experiment.

Conclusion:

A phantom was created and used for testing interventional catheter guiding. The images produced had similar qualities to those found in vivo. This phantom had a high degree of appropriate anthropomorphic and physiological qualities. Ethically, use of this phantom is highly appropriate when first testing new MRI techniques prior to conducting animal studies. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.