正常成人肾脏磁共振扩散加权成像研究

目的 通过测量正常成人肾脏磁共振扩散加权成像(DWI)时的表观弥散系数(ADC)值,为肾脏病变患者在DWI成像时提供正常对照标准.资料与方法 20例健康志愿者均行MRI常规平扫及DWI成像,DWI成像时b值分别采用0s/mm2、200s/mm2、500s/mm2、800s/mm2、1000s/mm2.结果 不同b值下双侧肾脏皮髓质ADC值及双侧肾脏平均ADC值差异有统计学意义(F=57.863～240.324,P=0.000),双侧肾脏皮髓质ADC值及平均ADC值均高于肝、脾及胰腺ADC值.双侧肾脏皮质在不同b值下的ADC值均高于髓质,差异有统计学意义(t=8.436～20.281,P=0.000).左侧肾脏皮、髓质ADC值及平均ADC值在b=500s/mm2、b=800s/mm2及1000s/mm2时均高于右侧,差异有统计学意义(b=800s/mm2时t=2.023,P=0.048;其余P=0.000).b=200s/mm2时双侧肾脏皮、髓质ADC值及平均ADC值之间差异无统计学意义(t=1.739,P=0.098;t=0.704,P=0.490;t=-0.314,P=0.757).结论 正常成人肾脏ADC值在不同b值下有差异,双侧肾脏平均ADC值亦存在差异.     

脑组织ADC值定量研究的影响因素分析

目的:探讨不同的MR成像技术对脑组织近似弥散系数(ADC)定量的影响.材料和方法:使用5种不同的弥散敏感系数(b值)(b=500、1 000、1 500、2 000、3 000s/mm2)在3T MR上对17名志愿者进行颅脑弥散加权成像(DWI)以及b=1 000s/mm2下的液体翻转衰减恢复弥散加权成像(FLAIR-DWI);其中10名志愿者完成3TMR检查后,再用1.5T MR进行b=1 000s/mm2的颅脑弥散加权成像.获得ADC图后,分别测量两侧半卵圆中心、扣带回、丘脑、胼胝体膝部、胼胝体压部及侧脑室体部的ADC值,并进行统计学分析.结果:b值的增加使脑组织的ADC值显著下降,FLAIR可以明显降低脑脊液对ADC值测量的污染;不同MR扫描仪对ADC值的定量无显著影响.结论:DWI参数的选择对ADC值的测量有着显著的影响,尤其是b值,最佳的b值应该为1 000s/mm2,因而在研究中必须始终保持扫描参数的一致;DWI前施加FLAIR抑止脑脊液信号明显减轻了脑脊液部分容积效应对ADC值测量的影响,这对于脑萎缩患者的ADC值测量有着十分重要的价值;同一厂家生产的不同场强的MR成像系统对ADC值定量研究无显著影响.     

不同弥散强度对脑组织ADC值及FA值测定的影响

探讨不同弥散强度对弥散张量成像中表观弥散系数值及部分各向异性指数值测定的影响.材料和方法:使用七个不同的弥散梯度(b值分别为b0=0、b1=500s/mm2、b2=1000s/mm2、b3=1500s/mm2、b4=2000s/mm2、b5=2500s/mm2和b6=3000s/mm2)对20名正常成人脑组织进行弥散张量成像,对7个不同的b值分别进行组合(组合方式为b0b1、b0b2、b0b3、b0b5、b0b6和b0b6)及后处理,分别获得其ADC图和FA图,然后在ADC图和FA图上分别测定尾状核头、内囊后肢、丘脑、半卵圆中心、侧脑室体部及胼胝体膝部和压部的数值并进行统计学分析.结果:脑内各个解剖部位的ADC值均随着弥散强度的增加而显著降低(P＜0.05);FA值不随弥散强度的改变而改变(P＞0.05).结论:FA值不随弥散强度的改变而发生改变;随着弥散强度的改变,ADC值也发生改变,在进行临床研究时要充分考虑弥散强度对ADC值测定的影响.     

健康成人肾脏不同部位的磁共振扩散加权成像研究

目的通过测量健康成人肾脏磁共振扩散加权成像（DWI）时的表观弥散系数（ADC）值,为肾脏病变患者在DWI检查时提供正常对照值标准。方法 20名无任何肾疾患的健康志愿者均经MRI常规平扫及DWI扫描,DWI扫描时扩散敏感系数b值分别采用0 s/mm2、200 s/mm2、500 s/mm2、800 s/mm2及1000 s/mm2。全部操作是采用EPI序列和呼吸门控软件进行的。结果不同b值下双侧肾脏不同部位皮、髓质ADC值差异有统计学意义,双侧肾脏不同部位皮质在不同b值下的ADC值均高于相应部位髓质,两者之间差异有统计学意义（P=0.000）。结论健康成人肾脏各不同部位的ADC值与b值相关,肾脏皮质ADC值高于相应部位的髓质。     

软组织肿瘤的MR弥散加权成像初步研究

目的:探讨MR DWI在软组织肿瘤定性诊断中的价值.方法:研究了23例软组织肿瘤DWI.弥散敏感系数b分别取300s/mm2、600s/mm2和900s/mm2,在三个方向施加弥散敏感梯度场得到DWI,获得肿瘤实质的ADC值.结果:①23例软组织肿瘤的DWI只有1例肌间脂肪瘤为混杂信号,其它均为高信号;②在b＝300s/mm2时,良、恶性肿瘤组的ADC值为(1.64±0.12)×10-3mm2/s、(1.86±0.19)×10-3mm2/s,良、恶性肿瘤组间的ADC值差异有显著性;在b＝600s/mm2时,良、恶性肿瘤组的ADC值分别为(1.42±0.13)×10-3mm2/s、(1.46±0.15)×10-3mm2/s,二者之间差异无显著性;在b＝900s/mm2时,良、恶性肿瘤组的ADC值分别为(1.10±0.12)×10-3mm2/s、(1.00±0.13)×10-3mm2/s,二者之间差异无显著性.结论:DWI信号对软组织肿瘤诊断价值有限.在b＝300s/mm2时,恶性软组织肿瘤的ADC值大于良性软组织肿瘤的ADC值.     

乳腺肿瘤弥散系数与分子生物学相关性研究

目的:探讨乳腺良恶性病变磁共振弥散成像与分子生物学之间的相关性。材料和方法:对36例原发性乳腺癌和12例良性乳腺病变患者术前行乳腺弥散成像(DWI),测定良恶性病变弥散系数(ADC)值,术后标本行免疫组织化学染色测定癌细胞CerbB-2、PCNA、P53的表达情况,并分析与磁共振弥散系数值的相关性。结果:34例乳腺癌及12例良性乳腺病变完成弥散成像。恶性肿瘤组ADC值(0.778±0.169)×10-3mm2/s明显小于良性病变ADC值(1.801±0.429)×10-3mm2/s(P<0.05,b=1000s/mm2)。ADC值与PCNA、P53表达呈负相关(r=-0.634,-0.493,P<0.05)。结论:乳腺良恶性病变的ADC值有显著性差异,乳腺癌的ADC值与分子生物学之间存在一定相关性。     

磁共振弥散加权成像在胃恶性肿瘤诊断中的应用价值

目的探讨磁共振弥散加权成像在胃恶性肿瘤诊断中的价值。方法收集临床病理确诊的胃恶性肿瘤患者48例作为病例组,其中胃腺癌38例,胃淋巴瘤10例。另随机选取健康志愿者20例作为对照组。所有患者均在治疗前行常规MRI(T1WI、T2WI)及弥散加权成像扫描(弥散敏感因子b值取0,800)。利用计算机软件直接测量肿瘤组织的信号强度值及ADC值,取其平均值。同样的方法获得对照组胃壁的ADC值,对获得的数据进行组间对照研究。结果常规MRI胃恶性肿瘤检出敏感性为91.7%(44/48);DWI成像检出的敏感性为100%(48/48);DWI病灶部位信号强度值明显高于对照组,胃恶性肿瘤平均信号强度值为345.33±170.10,正常胃壁平均信号强度值为149.80±36.24;在b值为800s/mm2时,胃恶性肿瘤平均ADC值是(0.87±0.11)×10-3 mm2/s,正常胃壁平均ADC值是(1.79±0.11)×10-3 mm2/s,两者之间差异有统计学意义(P0.05)。胃腺癌患者平均ADC值是(0.83±0.07)×10-3 mm2/s,胃淋巴瘤患者平均ADC值是(1.03±0.07)×10-3 mm2/s,两者之间差异具有统计学意义(P0.05)。结论磁共振弥散加权成像在胃恶性肿瘤诊断及鉴别诊断中具有明显的应用价值。     

MR DWI监测兔肝VX2肿瘤的实验研究

目的:探讨兔肝VX2肿瘤的MR扩散加权成像(DWI)的成像参数与成像特征。方法:10只新西兰大白兔(雄性,2.5±0.3 kg),9只经手术制作肝VX2肿瘤模型,分别于种植前、种植后第7天、第14天及第21天行MR DWI检查,b值分别设为100、200、300、400、500和600 s/mm2,使用EPI-SE序列,扫描时间为30 s。由扩散加权序列产生表观扩散系数图(ADC map)。兴趣区(ROI)以DWI为参考手工绘制,计算平均ADC值。以ADC值作为统计指标,使用随机区组的双因素方差分析及SNK法进行统计处理。结果:兔肝VX2瘤在DWI上呈明显高信号,边缘清楚。不同b值时正常肝脏与VX2瘤组织的ADC值差异有显著性意义(P<0.001)。随着b值的增大,病灶与周围肝组织对比度降低,同时病灶内部的ADC变异度降低。b值为400 s/mm2以上时,同一ROI所得的ADC值之间差异无统计学意义,b值100、200和300 s/mm2时,同一ROI所得的ADC值差异有统计学意义。结论:DWI在发现、追踪肝脏VX2瘤生长过程有重要价值,在肝脏DWI成像参数中b值选择400 s/mm2以下比较合适。     

MR扩散加权成像使用不同b值测量腹部脏器ADC值的比较

目的:探讨不同b值对MR扩散加权成像(DWI)时腹部实质性器官和肝脏病灶表观扩散系数(ADC)值测量结果的影响。方法:29例研究对象行DWI检查,包括正常对照组17例,恶性病变组12例(肝细胞癌6例,胆管细胞癌2例,转移癌4例)。对每例受试者分别采用2种扫描方案行DWI扫描,方案A采用b值为0、500和1000s/mm2,方案B采用b值为0、200和400s/mm2。测量并比较两种方案DWI扫描时腹部各器官及肝脏病灶的ADC值。结果:采用A方案时正常肝脏的ADC值为(1.50±0.11)×10-3mm2/s,采用B方案时为(1.13±0.19)×10-3mm2/s,两者差异有极显著性意义(t=6.058,P<0.001)。采用这2种扫描方案其它器官的ADC值测量结果:脾脏分别为(0.94±0.08)×10-3mm2/s和(0.86±0.08)×10-3mm2/s,胰腺分别为(1.43±0.13)×10-3mm2/s和(1.36±0.15)×10-3mm2/s,肾皮质分别为(2.19±0.15)×10-3mm2/s和(2.21±0.19)×10-3mm2/s,肝内恶性病灶分别为(1.11±0.14)×10-3mm2/s和(1.02±0.13)×10-3mm2/s;除肾皮质外,脾脏、胰腺和肝脏病灶的两次ADC值测量结果间的差异均有显著性意义(P<0.05),b值差大时的测量结果均明显高于b值差小者。结论:DWI成像时b值的不同会影响ADC值的测量结果,大b值差时的测量结果较小b值差时更准确且稳定。     

MR扩散加权成像评价兔VX2肝肿瘤射频消融疗效的实验研究

目的 通过对兔VX2肝肿瘤模型作动物实验,探讨MR扩散加权成像(diffusion-weighted imaging, DWI)在评价射频消融疗效的可行性与可靠性.方法 9例兔VX2肝肿瘤模型做射频消融术(radiofrequency ablation, RFA),治疗后1周做常规MRI检查确定肿瘤平面后,选择b值(扩散敏感度)为100 s/mm2,300 s/mm2和1000 s/mm2分别做水扩散加权成像扫描,与病理作比较分析正常肝组织、残留肿瘤灶和消融坏死灶的DWI图、表观扩散系数(apparent diffusion coefficient,ADC)差异.结果 由于受患兔呼吸与心跳影响,DWI图像均存在不同程度伪影.以DWI图像上正常肝组织信号作为对比,对应于病理观察到的存活肿瘤组织区域在DWI图像上表现为更低信号,而消融坏死灶区域表现为高信号.而在选定平面所重建的ADC图像上则刚好相反,存活区域色彩鲜明,而消融坏死区域色彩暗淡.正常肝组织、存活肿瘤组织与消 融坏死组织的ADC值(单位:×10-3mm2/s)分别为1.658±0.866、2.011±0.925、0.709±0.511(b＝100 s/mm2);1.242±0.651、1.436±0.425、0.529±0.387(b＝300 s/mm2);0.819±0.357、1.136±0.217、0.558±0.332(b＝1000 s/mm2).在同一b值下,ADC值由高到低依次为存活肿瘤组织、正常肝组织与消融坏死组织,而存活肿瘤组织与消融坏死组织在同一b值下均存在显著性差异;随着b值的升高,不同组织的ADC值普遍降低,在b值分别为100 s/mm2和1000 s/mm2时,同一组织的ADC测量值也存在统计学差异(消融坏死组织除外),消融坏死组织在不同b值下测量ADC值无显著性差异.结论 MR扩散加权成像能从微观、功能成像的角度区别消融坏死灶与肿瘤残留灶,是有发展潜力的新型RFA评价手段,但图像质量有待改进.     

Increased diffusion sensitivity with hyperechos.

It is shown that the introduction of a 180 degrees refocusing pulse into a standard diffusion weighted stimulated echo sequence is equivalent to the simplest hyperecho sequence with identical diffusion weighting but equal or greater signal-to-noise (SNR) and thus equal or greater diffusion contrast. For high b-value imaging, the hyperecho sequence thus possesses the high diffusion contrast in the presence of small T(1)/T(2) ratios characteristic of stimulated echo sequences but with less than the 50% loss in SNR that is associated with the stimulated echo. For low b-value imaging, the hyperecho signal converges to that of the standard spin echo. The advantages of the two-pulse diffusion weighted hyperecho sequence are demonstrated theoretically. Experimental results are shown in the application to high angular resolution diffusion encoding (HARD) in normal human brain.     

Is the “biexponential diffusion” biexponential?

Diffusion-weighted signal from the brain, S, deviates from monoexponential dependence on the b-factor. This property is often referred to as biexponential diffusion, since the corresponding model fits data well. The aim of this study is to examine the necessity of using the biexponential model in homogeneous voxels under isotropic diffusion weighting up to b = 2.5 ms/microm(2). The model is compared to the cumulant expansion of ln S in a power series in b, which takes its origin in fundamental properties of the diffusion-weighted signal, but diverges at large b. The absence of statistically significant evidences for the biexponential diffusion is demonstrated in gray matter. The cumulant expansion terminated after the term b(2) describes data equally well with fewer adjustable parameters. The biexponential model is preferable in voxels with a partial volume of CSF.     

扩散张量成像导出量对扩散时间依赖关系的实验研究

目的研究扩散张量导出量与扩散时间的关系。方法保持扩散敏感梯度磁场强度不变，使用8个不同的扩散时间对11名被试者进行扩散张量成像扫描得到脑部的各向异性与各向同性信息，计算出各个感兴趣区的平均扩散率与各向异性分数后进行比较。结果不同扩散时间对应的平均扩散率有显著性差异（P〈0．05），而各向异性分数无显著性差异（P〉0．05）。结论扩散时间对扩散张量成像导出量中的各向异性分数无影响，对平均扩散率有影响。原因是细胞内外水分子扩散性质不同。     

The tensor distribution function

Diffusion weighted magnetic resonance imaging is a powerful tool that can be employed to study white matter microstructure by examining the 3D displacement profile of water molecules in brain tissue. By applying diffusion‐sensitized gradients along a minimum of six directions, second‐order tensors (represented by three‐by‐three positive definite matrices) can be computed to model dominant diffusion processes. However, conventional DTI is not sufficient to resolve more complicated white matter configurations, e.g., crossing fiber tracts. Recently, a number of high‐angular resolution schemes with more than six gradient directions have been employed to address this issue. In this article, we introduce the tensor distribution function (TDF), a probability function defined on the space of symmetric positive definite matrices. Using the calculus of variations, we solve the TDF that optimally describes the observed data. Here, fiber crossing is modeled as an ensemble of Gaussian diffusion processes with weights specified by the TDF. Once this optimal TDF is determined, the orientation distribution function (ODF) can easily be computed by analytic integration of the resulting displacement probability function. Moreover, a tensor orientation distribution function (TOD) may also be derived from the TDF, allowing for the estimation of principal fiber directions and their corresponding eigenvalues. Magn Reson Med 61:205–214, 2009. © 2008 Wiley‐Liss, Inc.     

一种新的用于磁共振扩散加权波谱术的扩散敏感梯度磁场施加方案

目的 提出一种基于PRESS的用于磁共振扩散加权波谱的扩散敏感梯度磁场施加方案.方法 在PRESS序列中的2个π射频脉冲两侧各施加一对交错的扩散敏感梯度磁场.结果 本方案有效减小涡流,利用PRESS序列中2个π射频脉冲之间的时间,延长了扩散梯度磁场脉冲的持续时间.即使在扩散权重较大的情况下,Naa,Cr及Cho的峰仍十...     

Anisotropic anomalous diffusion assessed in the human brain by scalar invariant indices

A new method to investigate anomalous diffusion in human brain, inspired by the stretched‐exponential model proposed by Hall and Barrick, is proposed here, together with a discussion about its potential application to cerebral white matter characterization. Aim of the work was to show the ability of anomalous diffusion indices to characterize white matter structures, whose complexity is only partially accounted by diffusion tensor imaging indices. MR signal was expressed as a stretched‐exponential only along the principal axes of diffusion; whereas, in a generic direction, it was modeled as a combination of three stretched‐exponentials. Indices to quantify the tissue anomalous diffusion and its anisotropy, independently of the experiment reference frame, were derived. Experimental results, obtained on 10 healthy subjects at 3T, show that the new parameters are highly correlated to intrinsic local geometry when compared with Hall and Barrick indices. Moreover, they offer a different contrast in white matter regions when compared with diffusion tensor imaging. Specifically, the new indices show a higher capability to discriminate among areas of the corpus callosum associated to different distribution in axonal densities, thus offering a new potential tool to detect more specific patterns of brain abnormalities than diffusion tensor imaging in the presence of neurological and psychiatric disorders. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo diffusion tensor imaging of human calf muscle.

PURPOSE: To investigate a tetrahedral diffusion gradient encoding scheme to measure the diffusion tensor in vivo for human calf muscle. MATERIALS AND METHODS: The theoretical TE which maximizes the signal-to-noise ratio (SNR) of the diffusion images was derived for both the orthogonal and tetrahedral sampling strategies and the SNR advantage verified experimentally. A diffusion echo-planar imaging (EPI) sequence was used to image five volunteers. Gradient cycling and geometric averaging was performed to eliminate cross-terms between the imaging and diffusion gradients. RESULTS: Trace diffusion coefficients in human muscle are spatially invariant and have low intersubject variability (<4%). Images of the off-diagonal terms confirm the anisotropy of muscle, and fiber orientation maps were derived from these off-diagonal images. A noninvariant index of anisotropy, A(ratio) (average value: 1.28), was found to be less susceptible to noise than the invariant index. CONCLUSION: This technique is robust and can be readily implemented on clinical scanners with EPI capabilities.