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体素内不相干运动（IVIM）扩散系数（D）和灌注分数（f）的估算方法比较 ...

已有 8742 次阅读2019-6-17 16:11 |个人分类:医学影像|系统分类:医学科学| IVIM

体素内不相干运动（IVIM）扩散系数（D）和灌注分数（f）的估算方法比较
Comparison of methods for estimation of the intravoxel incoherent motion (IVIM) diffusion coefficient (D) and perfusion fraction (f).
OBJECTIVE: Intravoxel incoherent motion (IVIM) shows great potential in many applications, e.g., tumor tissue characterization. To reduce image-quality demands, various IVIM analysis approaches restricted to the diffusion coefficient (D) and the perfusion fraction (f) are increasingly being employed. In this work, the impact of estimation approach for D and f is studied. 
体素内不相干运动（IVIM）在许多应用中显示出巨大的潜力，例如肿瘤组织表征。为了降低图像质量要求，越来越多地采用限于扩散系数（D）和灌注分数（f）的各种IVIM分析方法。 在这项工作中，研究了IVIM估算方法对D和f值的影响。
MATERIALS AND METHODS: Four approaches for estimating D and f were studied: segmented IVIM fitting分段IVIM拟合, least-squares fitting of a simplified IVIM model (sIVIM)简化IVIM模型（sIVIM）的最小二乘拟合, and Bayesian fitting of the sIVIM model using marginal posterior modes or posterior means 以及使用边缘后验众数或后验均值的sIVIM模型的贝叶斯拟合. The estimation approaches were evaluated in terms of bias and variability as well as ability for differentiation between tumor and healthy liver tissue using simulated and in vivo data. 
RESULTS: All estimation approaches had similar variability and ability for differentiation and negligible bias, except for the Bayesian posterior mean of f, which was substantially biased. Combined use of D and f improved tumor-to-liver tissue differentiation compared with using D or f separately. 
DISCUSSION: The similar performance between estimation approaches renders the segmented one preferable due to lower numerical complexity and shorter computational time.
由于较低的数值复杂度和较短的计算时间，IVIM估算方法之间的类似性能使得分段的方法更可取。
Superior tissue differentiation when combining D and f suggests complementary biologically relevant information.联合D和f得到了优异的组织区分图像，表明互补的生物学相关信息。




Effects of diffusion and pseudo-diffusion on the MRI signal.
扩散和伪扩散对MRI信号的影响。

Random water displacements (either from individual molecular motion or from collective water incoherent flow at voxel level) results in a exponential decay of the signal amplitude with the degree of field gradient encoding (b value). 
随机水分子位移（来自单个分子运动或来自体素水平的集体水分子不相干流动）导致随着场梯度编码程度（b值）升高而出现信号幅度的指数衰减。

The tissue diffusion and blood flow component contribute separately to the signal,resulting to a biexponential shape. However, as D*≫D the IVIM (blood flow) effect appears as a deviation of the tissue diffusion signal decay at low b values.
组织扩散和血流分量分别对信号起作用，形成双指数形状。 然而，当D*»D时，IVIM（血流）效应表现为在低b值下组织扩散信号衰减的偏离。

Jalnefjord O, Andersson M, Montelius M, Starck G, Elf AK, Johanson V, Svensson J, Ljungberg M.Comparison of methods for estimation of the intravoxel incoherent motion (IVIM) diffusion coefficient (D) and perfusion fraction (f).MAGMA. 2018 Dec;31(6):715-723. doi: 10.1007/s10334-018-0697-5. Epub 2018 Aug 16. PMID: 30116979

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回复 hyc3140 2019-6-17 17:43
D* is usually expected to be at least one order of magnitude greater than D.
D *通常预计比D大至少一个数量级。

Two major approaches for estimating only D and f have been proposed: one is based on a specialized model-fitting procedure, and one uses a special case of the IVIM model (Eq. 1),both assuming b values in certain ranges and that D* ≫ D.
已经提出了仅估计D和f的两种主要方法：一种基于专门的模型拟合程序，一种使用IVIM模型的特殊情况（方程1），两种方法均假设b值在特定范围内并且D *»D.
回复 hyc3140 2019-6-18 09:30
Intravoxel incoherent motion (IVIM) imaging has gained increased interest during the last years because of its applicability in abdominal imaging.
过去几年中，由于其在腹部成像中的适用性，体素内不相干运动（IVIM）成像引起了越来越多的关注。

during the last years（在过去几年中，在最近几年）
in the past few years （在过去几年中）
over the past few years（过去几年中）
回复 hyc3140 2019-6-18 10:03
Numerous studies concerning IVIM model fitting have been performed, and the Bayesian approach has shown great promise. However, none of the previous studies has, to our knowledge, aimed to assess the impact of the methodology used in the Bayesian model fitting, most importantly including the choices of prior distribution and central tendency measure, which often differ among studies
回复 hyc3140 2019-6-19 17:22
While diffusion and blood microcirculation are two completely different physical phenomena taking place at very different spatial and temporal scales, apparent motion randomness can be seen, instead, in the geometry of the vessel network where blood circulates.
虽然扩散和血液微循环是在非常不同的空间和时间尺度上发生的两种完全不同的物理现象，但是在血液循环的血管网络的几何形状中可以看到明显的运动随机性。

In other word,randomness results from the collective motion of blood water molecules in the network, flowing from one capillary segment to the next.
换句话说，随机性来自网络中血液水分子的集体运动，从一个毛细血管段流向下一个毛细血管段。

this collective movement may be viewed as a pseudodiffusion process where average displacements, l, would now correspond to the mean capillary segment length and the mean velocity, v, would be that of blood in the vessels.
这种集体运动可以被视为一种假扩散过程，其中平均位移L现在对应于平均毛细血管段长度，平均速度V将是血管中的血液。
回复 hyc3140 2019-6-19 20:14
Extending Einstein's diffusion equation to pseudodiffusion provides a value for the pseudodiffusion coefficient, D* = lv/6,which is around 10-8m2/s taking for L 100 μm and for v 1 mm/s.
将爱因斯坦的扩散方程拓展至伪扩散，则可得到伪扩散系数值，D* = lv/6，约为10-8m2/s，取L 100μm和v 1mm/s。

This is where luck strikes: D* is close enough to D, which means that diffusion MRI is sensitive to both diffusion and blood microcirculation, both resulting, separately, in a monoexponential decay of the MRI signal with the b value.
此为妙处所在（这就是运气所在之处）：D*足够接近D，这意味着扩散MRI对扩散和血液微循环都敏感，两者分别导致MRI信号随b值呈单指数衰减。

However, the blood microcirculation component decays 10 times faster, which allows the two independent phenomena to be separated.
然而，血液微循环成分（较血管外组织内水分子）衰减快10倍，利用该特点可将这两个独立的现象进行区分。

Interestingly, the value for D* is found similar across other species which have been scanned, which is not completely surprising as small animals have shorter capillary segments, but higher flow velocities.
有趣的是，在被扫描的其他物种中发现D*的值相似，这并不足奇，因为小动物毛细血管段较短，但流速较高。
回复 hyc3140 2019-6-19 20:49
The overall MRI signal attenuation, S/So, is simply the sum of the tissue and blood components (assuming water exchange between blood and tissues is negligible during the encoding time, a hypothesis which
still needs to be investigated), taking the shape of a biexponential decay:
Sb/S0 = f exp [(-b (D*+Dblood) + (1-f) exp (-b D)

总的MRI信号衰减（S/So）仅仅是组织和血液成分的总和（假设在编码时间内血液和组织之间的水交换可忽略不计，这一假设仍需要进行研究），采取的形式为 双指数衰减：Sb/S0 = f exp [(-b (D*+Dblood) + (1-f) exp (-b D)

where f is the flowing blood fraction, D is the water diffusion coefficient in the tissue and Dblood the water diffusion coefficient in blood(diffusion of individual water molecules in blood obviously occurs in addition to the blood circulation driven IVIM effect).
其中f是流动的血液分数，D是组织中的水扩散系数，Dblood是血液中的水扩散系数（除血液循环驱动的IVIM效应外，显然存在血液中各个水分子的扩散）。

However, given the small fraction of the flowing blood (a few %) compared to the overall tissue water content, the perfusion driven IVIM signature appears more as a deviation as small b values of the tissue diffusion driven monoexponential signal decay.
然而，与总组织水含量相比，考虑到流动血液只占很小一部分（很少 ％），在小b值时，灌注驱动的IVIM特征更像是组织扩散驱动的单指数信号衰减的偏离。

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