不用b=0图像的活体组织体素内不相干运动分析:应用在肝脏纤维化评估的一个例子

Living tissue intravoxel incoherent motion(IVIM) diffusion MR analysis without b=0 image: an example for liver fibrosis evaluation

体素内不相干运动(intravoxel incoherent motion,IVIM)成像系列的组成一般由开始没有弥散成像梯度磁场的成像(b=0s/mm^2)及一系列不同强度(或不同持续时间)的弥散成像梯度磁场的成像。在人体的许多器官,比如肝脏,b值与成像信号的关系一般分为两部分:(1)低b值时信号快速下降与组织内的血液灌注导致的快速水分子移位有关;(2)高b值时信号相对较为缓慢下降与组织内的水分子弥散相关。这些关系大致表现为b值系列与信号的双指数衰减下降关系,并由三个参数来表示即Dslow(D)、Dfast(D*)、PF(f)。Dslow反映水分子弥散移位的快慢,Dfast反映血液灌注的水分子移位的快慢,PF反映灌注占的百分比。IVIM成像虽然理论上有许多优越性,但是以前常规描述的方式在临床医学中没有实际应用起来,其原因是公式拟合困难,测量结果不稳定。肝脏为富血供器官,适合IVIM观察,但是肝脏同时受到一些生理性运动的影响,比如呼吸运动以及心脏的跳动,左肝还受到胃内容物引起的磁敏感伪影的影响。最近我们发表了两个小型临床研究,第一个研究中有16位健康志愿者及33位乙型肝炎性肝纤维化患者(其中15例为1期肝纤维化);第二个研究中有26位健康志愿者及12位乙型肝炎性肝纤维化患者(其中4例为1期肝纤维化)。这两个研究中所有的健康人与肝纤维化患者均可以通过IVIM区别。而且,第二个研究中有4例活检病理无肝纤维化的患者,其IVIM结果与正常人类似。虽然迄今为止我们的IVIM扫描参数以及数据处理方式尚未最优化,我们推想,优良的结果是基于我们采取了下面的方法:(1)IVIM数据分析去除b=0的图像;(2)仔细选择b值分布及合理选择分段拟阀值b;(3)去除质量不好的图像;(4)将所有的IVIM参数考虑进临床判断。本文描述的IVIM评估肝脏纤维化的图像采集及图像后处理分析尚未最优化。可以预期,优化后的IVIM对于检出早期肝脏纤维化会有很高的敏感性及特异性。

With the reported intravoxel incoherent motion(IVIM)analyses,the diffusion image signal decay is computed starting from b=0 s/mm2 image and then increasingly higher b-values using a biexponential decay model.In many organs such as the liver,the relationship between b-value and diffusion signal is composed of two parts:①low b-values are associated with quick signal decay caused by fast moving water associated with blood perfusion;②high b-values are associated with slow signal decay caused by pure water diffusion.These associations are represented by three parameters associated with a bi-exponential model,i.e.Dslow(D),Dfast(D^*),PF(f).PF is the fraction of the pseudo-diffusion linked to microcirculation,Dslow is the true diffusion coefficient representing the pure molecular diffusion,and Dfast is the perfusion-related diffusion.Although the theory of IVIM is very appealing,the practical application in the liver is difficult due to the difficulties in curve fitting and instability of results.Due to its relatively high blood supply,the liver is a very suitable organ for IVIM study.However,the liver is in the meantime particularly affected by physiological motions such as respiration and heart beating;meanwhile,the left liver is also affected by susceptibility artefact due to contents in the stomach.Recently we published two small cohort studies.Study-1 had 16 healthy volunteers and 33 viral hepatitis-b liver fibrosis patients among them 15 patients had stage-1 liver fibrosis.Study-2 had 26 healthy volunteers and 12 viral hepatitis-b liver fibrosis patients among them 4 patients had stage-1 liver fibrosis.All patients and healthy volunteers can be separated by IVIM analysis.Interestingly,study-2 had four patients with biopsy showing no or minimal fibrosis,and these four subjects’IVIM measurements resembled healthy volunteers.While our current data acquisition and analysis still remain not optimal,we believe we were able to achieve these good results by the following measures:①IVIM analysis without b=0 data;②Carefully select b value distribution and the threshold b-value;③Discard poor image quality files;④Incorporating all IVIM parameters for consideration.We expect that with further improvement in data acquisition and post-processing,IVIM technique will have high sensitivity and specificity for early liver fibrosis evaluation.