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.     

Error model for reduction of cardiac and respiratory motion effects in quantitative liver DW‐MRI

Diffusion‐weighted images of the liver exhibit signal dropout from cardiac and respiratory motion, particularly in the left lobe. These artifacts cause bias and variance in derived parameters that quantify intravoxel incoherent motion. Many models of diffusion have been proposed, but few separate attenuation from diffusion or perfusion from that of bulk motion. The error model proposed here (Beta*LogNormal) is intended to accomplish that separation by modeling stochastic attenuation from bulk motion as multiplication by a Beta‐distributed random variate. Maximum likelihood estimation with this error model can be used to derive intravoxel incoherent motion parameters separate from signal dropout, and does not require a priori specification of parameters to do so. Liver intravoxel incoherent motion parameters were derived for six healthy subjects under this error model and compared with least‐squares estimates. Least‐squares estimates exhibited bias due to cardiac and respiratory gating and due to location within the liver. Bias from these factors was significantly reduced under the Beta*LogNormal model, as was within‐organ parameter variance. Similar effects were appreciable in diffusivity maps in two patients with focal liver lesions. These results suggest that, relative to least‐squares estimation, the Beta*LogNormal model accomplishes the intended reduction of bias and variance from bulk motion in liver diffusion imaging. Magn Reson Med 70:1460–1469, 2013. © 2012 Wiley Periodicals, Inc.     

Shear stiffness estimation using intravoxel phase dispersion in magnetic resonance elastography.

Dynamic MR elastography (MRE) is a phase-contrast technique in which the periodic shear motion of an object is encoded as variations in the phase of the reconstructed images. An alternative MRE method is presented whereby waves are depicted as intensity variations in the magnitude images due to intravoxel phase dispersion (IVPD). A theoretical framework is developed to model how the IVPD magnitude data are related to the underlying shear wave motion, and how they can be used to estimate shear stiffness. The results are shown in a series of phantom experiments to demonstrate that IVPD MRE complements phase-contrast MRE.     

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.     

Intravoxel incoherent motion in body diffusion-weighted MRI: reality and challenges

OBJECTIVE: Diffusion-weighted MRI is increasingly applied in the body. It has been recognized for some time, on the basis of scientific experiments and studies in the brain, that the calculation of apparent diffusion coefficient by simple monoexponential relationship between MRI signal and b value does not fully account for tissue behavior. However, appreciation of this fact in body diffusion MRI is relatively new, because technologic advancements have only recently enabled high-quality body diffusion-weighted images to be acquired using multiple b values. There is now increasing interest in the radiologic community to apply more sophisticated analytic approaches, such as those based on the principles of intravoxel incoherent motion, which allows quantitative parameters that reflect tissue microcapillary perfusion and tissue diffusivity to be derived. CONCLUSION: In this review, we discuss the principles of intravoxel incoherent motion as applied to body diffusion-weighted MRI. The evidence for the technique in measuring tissue perfusion is presented and the emerging clinical utility surveyed. The requisites and challenges of quantitative evaluation beyond simple monoexponential relationships are highlighted.     

体素内不相干运动的扩散加权像在肾病中的研究进展

传统扩散加权成像（DWI）对脑部及腹腔脏器病变具有诊断功能,对肾脏等具有丰富微循环组织的病变诊断仍有局限性。体素内不相干运动的扩散加权像（IVIM-DWI）通过分离作用弥补了DWI的不足,对肾功能的评估和慢性肾病及肿瘤性肾病的诊断起影像支持作用。本文就IVIM-DWI在临床上的运用及其原理进行详细的阐述。     

肝脏多b值扩散心率因素的评价

目的:评价心电门控技术心率因素对肝脏体素内不相干运动成像(IVIM)图像质量及参数值的影响.方法:选取20名接受心电门控IVIM DWI序列扫描的志愿者,设定6个b值(范围0,50,100,150,300,600s/mm2),按受检者心率分为2组,每组10例,实验组受检者心率≤70次/分,对照组受检者心率≥80次/分.由两名从事腹部MR影像诊断5年以上的高年资医师对所得MR图像进行分析,测量并分析图像SNR、ADC、IVIM各参数值,并对IVIM-DWI图像质量进行评价.结果:实验组所得图像伪影最少,解剖结构清晰,图像质量较高,不同b值时实验组SNR值均大于对照组,两组间差异有统计学意义(P＜0.05).两组ADC、D、D*、f值中,仅肝左叶的ADC值[实验组(1.432±0.299)×10-3mm2/s;对照组(1.945±0.543)×10-3 mm2/s]和D值[实验组(1.372±0.378)×10-3mm2/s;对照组(1.617±0.361)×10-3mm2/s]在两组间的差异有统计学意义(P值分别为0.000、0.013).结论:肝脏IVIM心电门控成像技术具有重要的临床价值,低心率心电门控IVIM DWI成像可得到较好的图像质量和稳定可靠的参数值.     

Enhancing pancreatic adenocarcinoma delineation in diffusion derived intravoxel incoherent motion f‐maps through automatic vessel and duct segmentation

Diffusion‐based intravoxel incoherent motion imaging has recently gained interest as a method to detect and characterize pancreatic lesions, especially as it could provide a radiation‐ and contrast agent‐free alternative to existing diagnostic methods. However, tumor delineation on intravoxel incoherent motion‐derived parameter maps is impeded by poor lesion‐to‐pancreatic duct contrast in the f‐maps and poor lesion‐to‐vessel contrast in the D‐maps. The distribution of the diffusion and perfusion parameters within vessels, ducts, and tumors were extracted from a group of 42 patients with pancreatic adenocarcinoma. Clearly separable combinations of f and D were observed, and receiver operating characteristic analysis was used to determine the optimal cutoff values for an automated segmentation of vessels and ducts to improve lesion detection and delineation on the individual intravoxel incoherent motion‐derived maps. Receiver operating characteristic analysis identified f = 0.28 as the cutoff for vessels (Area under the curve (AUC) = 0.901) versus tumor/duct and D = 1.85 μm²/ms for separating duct from tumor tissue (AUC = 0.988). These values were incorporated in an automatic segmentation algorithm and then applied to 42 patients. This yielded clearly improved tumor delineation compared to individual intravoxel incoherent motion‐derived maps. Furthermore, previous findings that indicated that the f value in pancreatic cancer is strongly reduced compared to healthy pancreatic tissue were reconfirmed. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Magnetic resonance diffusion/perfusion phantom experiments

Recently, several models for determining microcirculatory parameters using magnetic resonance imaging have been proposed. These include the intravoxel incoherent motion (IVIM) model, the intravoxel coherent motion (IVCM) model, and various tracer models. In order to evaluate these models before extension into physiological systems, phantom studies were used to assess model assumptions, measurement uncertainties, and sensitivity to changes in perfusion. Emphasis is placed on the IVIM model, but the techniques discussed could be extended to evaluation of other models as well. An overview of considerations in pulse sequence development, phantom design, and data interpretation is presented for a variety of phantoms ranging in complexity from stationary volumes of fluid and mechanically pumped phantoms to isolated animal kidneys and finally to an in vivo animal model.     

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.     

DWI技术在肝外胆管细胞癌诊断、分期和 疗效评估中的研究进展

肝外胆管细胞癌（EHCC）是第二大原发性肝胆系统肿瘤,其恶性程度高,预后不良。扩散加权成像（DWI）是反映组织水分子扩散运动的常用无创性成像方法。近年来,DWI及体素内不相干运动（IVIM）、扩散张量成像（DTI）及扩散峰度成像（DKI）等衍生技术已广泛应用于EHCC的诊断、病理分期预测和监测以及疗效评估。就DWI及其衍生序列对EHCC应用的研究进展以及局限性和应用前景予以综述。     

Improving relative anisotropy measurement using directional correlation of diffusion tensors.

A method employing directional correlation of the diffusion tensor, directional-correlation weighted relative anisotropy (DRA), was developed to improve the accuracy of estimated relative anisotropy (RA). The intravoxel directional correlation was established on the same voxel between two identically acquired diffusion tensor images, and the correlation coefficient derived from tensor dot product was employed as the weighting factor applied in the calculation of RA. The effect of noise influence was reduced since the random noise between repeated scans is not directionally correlated. The RA and the inter- and intravoxel DRA estimations were examined on rat brains in vivo. The background noise alters the direction of eigenvectors and the magnitude of eigenvalues. The dispersion angle between repeatedly obtained eigenvectors, representing the extent of directional alteration of eigenvectors, depends on the tissue anisotropy as well as the signal-to-noise ratio (SNR) of the source images. Current results demonstrate that the intravoxel DRA improves the accuracy of RA estimation, increases the relative contrast of gray and white matter, and avoids the partial volume effect commonly seen in the intervoxel operations.     

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...     

Theoretical and experimental evaluation of phase-dispersion effects caused by brain motion in diffusion and perfusion MR imaging

We investigated intravoxel phase dispersion caused by pulsatile brain motion in diffusion spin-echo pulse sequences. Mathematical models were used to describe the spatial and temporal velocity distributions of human brain motion. The spatial distribution of brain-tissue velocity introduces a phase spread over one voxel, leading to signal loss. This signal loss was estimated theoretically, and effects on observed diffusion coefficient and perfused capillary fraction were assessed. When parameters from a diffusion pulse sequence without motion compensation were used, and ECG triggering with inappropriate delay times was assumed, the maximal signal loss caused by brain-motion-induced phase dispersion was predicted to be 21%. This corresponds to a 95% overestimation of the diffusion coefficient, and the perfusion-fraction error was small. Corresponding calculations for motion-compensated pulse sequences predicted a 1% to 1.5% signal loss due to undesired phase dispersion, whereas experimental results indicated a signal loss related to brain motion of 4%.     

Contrast-enhanced three-dimensional MR angiography of neck vessels: does dephasing effect alter diagnostic accuracy?

The aim of this study was to evaluate diagnostic accuracy of contrast-enhanced MRA (CEMRA) compared with digital subtraction angiography (DSA) in studying neck vessels of 48 patients. In three groups of patients, we used three MRA protocols differing for voxel size to assess if intravoxel dephasing effects could modify accuracy of CEMRA. Accuracy and correlation with DSA results were calculated in all patients and separately in the three groups. A qualitative analysis of the likeness between morphology of the stenosis in CEMRA and DSA images was also assessed. In all patients accuracy and agreement with DSA were 96% and k=0.85 in subclavian arteries, 96% and k=0.84 in vertebral artery, 97% and k=0.88 in common carotid arteries, and 94% and k=0.86 in internal carotid arteries. In the three groups accuracy and agreement with DSA did not show any significant difference. Qualitative analysis of CEMRA and DSA images revealed a better agreement in depicting the morphology of stenosis using a smaller voxel size. The CEMRA represents a powerful tool for the non-invasive evaluation of neck vessels. Overestimation trend of CEMRA is confirmed and the reduction of voxel size, decreasing the dephasing intravoxel effect, allows to have a better overlapping of stenosis morphology on CEMRA compared with DSA, but it does not yield diagnostic gain in the stenosis grading. Electronic Publication     

Use of magnetic particles for sensitizing MR images to blood flow.

Magnetic resonance (MR) images made with the IVIM (intravoxel incoherent motion) technique for demonstrating tissue microcirculation are limited in sensitivity because of the small volume of blood involved. This limitation may be overcome by incorporating magnetic particles into the flow. The magnetic perturbation caused by the particles extends beyond the walls of the capillary and affects a much larger volume than that of the flowing material. Imaging experiments conducted with an artificial capillary system for renal dialysis, containing large magnetic particles, showed that signal intensity decreased with increasing flow rate through the dialysis bundle and with increasing particle concentration. Predictions of the effect based on a theoretical model of spin dephasing in the field of a magnetic dipole agreed with the experimental data. The results hold promise for development of the technique in vivo.     

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。     

The capillary network: a link between IVIM and classical perfusion.

MR measurements based on motion encoding gradients, such as intravoxel incoherent motion imaging, could provide, in principle, information on flowing blood volume and blood velocity. This note shows that, in addition, the knowledge of the capillary network organization may provide a link between these measurements and those obtained by conventional and MR perfusion techniques based on tracer uptake by tissues.