首发抑郁症患者治疗前后的静息态fMRI研究

目的:应用磁共振弥散张量成像(DTI)探讨首发抑郁症患者药物治疗前后脑白质异常的变化特点.方法:采用3.0T磁共振成像仪,对13例首发抑郁症患者(分别于治疗前及抗抑郁药物治疗8周后)和14例性别、年龄相匹配的正常志愿者进行DTI检查.运用SPM2分析软件,采用基于体素的分析方法,比较首发抑郁症患者药物治疗前后全脑弥散各向异性分数(FA)值的差异及变化.结果:首发抑郁症患者的右侧胼胝体膝部、内囊膝部、右侧下纵束所属部分区域FA值明显低于正常对照组(P＜0.001),经过药物治疗症状缓解后,FA值明显恢复(P＜0.001).结论:治疗有效的首发抑郁症患者脑白质的微观病理改变是可逆的;DTI可以动态观察抑郁症患者脑白质的微观病理改变.     

基于体素分析的首发青少年抑郁症全脑DTI研究

【摘要】目的：运用磁共振弥散张量成像（DTI）技术探讨首发未治疗青少年抑郁症患者的脑白质微结构变化特点。方法：前瞻性纳入35例首发未治疗青少年抑郁症患者（抑郁症组）和年龄、性别及受教育程度相匹配的26例健康志愿者（对照组）进行DTI扫描,运用FSL软件计算全脑白质各向异性分数（FA）,应用基于体素分析（VBA）方法比较两组间全脑白质FA差异,并分析抑郁症组脑白质微结构改变与临床数据的相关性。结果：抑郁症组17项汉密尔顿抑郁量表（HAMD-17）评分高于对照组（P＜0.01）。与对照组相比,抑郁症组脑桥、左侧小脑前叶白质FA值升高（簇水平P＜0.05,FWE校正）,未发现白质FA值减低的脑区;脑桥FA值与HAMD-17评分呈负相关（r=-0.352,P=0.038）。结论：青少年抑郁症患者脑桥小脑环路受损,在疾病早期出现脑白质FA值升高,可能是神经纤维代偿性再生所致。     

慢性高原病的脑磁共振弥散张量成像研究

目的：应用3.0T磁共振弥散张量成像（diffusion tensor imaging,DTI）技术,评价慢性高原病患者脑白质的各向异性分数（fractional anisotrophy,FA）。方法：本研究共包括9例慢性高原病患者和10例正常志愿者,行常规MRI扫描及DTI扫描,测定DTI图像上不同脑白质区的FA值,并比较两组FA值的差异。结果：两组常规MR检查未见明确差异,DTI示慢性高原病组双侧额叶及海马旁白质、胼胝体膝部FA值较正常志愿者组降低,差异具有统计学意义（P〈0.05）。结论：慢性高原病患者部分脑白质区FA值下降,提示脑白质纤维束完整性受损。     

皮质下动脉硬化性脑病的扩散张量成像分析

目的 探讨扩散张量成像(DTI)技术在皮质下动脉硬化性脑病(SAE/BD)中的应用价值.方法 对经临床诊断的40例BD患者和40例健康老年人行常规MRI及DTI检查,测量BD组常规MRI表现阳性病灶区、BD组表现正常脑白质区及对照组正常脑白质3组平均弥散系数(DCavg)值、各向异性分数(FA)值,进行统计学分析,并研究二者之间的相关性.结果 BD病灶区FA值降低、DCavg值增高,与对照组脑白质差异均有统计学意义(P＜0.05);BD正常脑白质区FA值与正常对照组相比有所降低、DCavg值有所增高,且差异有统计学意义(P＜0.05),3组的DCavg值与FA值均具有相关性.结论 DTI检查BD,其DCavg、FA值显示出特征性的改变,即DCavg值增高,FA值降低,并能够发现常规MRI检查正常的脑白质微结构改变.     

Application of TBSS-based multiple DTI index analysis in amnestic MCI

目的:采用基于纤维束空间统计分析(TBSS)的方法分析遗忘型轻度认知障碍(aMCI)患者DTI多参数图,以研究其全脑白质纤维束损害的特点以及可能的病理机制.方法:选取29例aMCI患者和30名正常老年人(正常对照组),对全脑进行1.5T磁共振DTI序列扫描.采用FSL软件对被试的DTI扫描数据进行预处理后对aMCI组、正常对照组的部分各向异性、平均弥散、轴向弥散、径向弥散图进行基于体素的全脑非参数统计学比较.结果:与正常对照组比较,aMCI组的双侧皮质脊髓束、双侧扣带束中部、双侧胼胝体、左侧内囊前肢、左侧外囊、双侧下额枕束、左侧上纵束、左侧膝状体内视路、右侧钩束、右侧视放射出现FA值减低;胼胝体膝部及体部、左侧皮质脊髓束、左侧内囊前后肢、左侧外囊、左侧上纵束出现MD值增加;DR值增高的区域与FA值类似;DA图未发现异常;左侧白质改变多于右侧.aMCI组全脑白质各参数图与简易智能精神状态检查量表(MMSE)评分没有相关性.结论:相对于正常老年人,aMCI患者脑内多发纤维束出现损害,其DTI多参数图改变特点提示白质纤维束的损害可能主要是由退行性改变(retrogenesis)导致的.DTI各参数图的改变可能无法反映患者总体认知功能障碍的严重程度,但这还需要进一步的大样本研究.     

多发性硬化的正常表现脑白质的DTI定量

目的：应用磁共振弥散张量成像（DTI）技术，定量研究多发性硬化（MS）患者在常规磁共振上表现正常的脑白质（NAWM），以及探讨其相关的微观病理改变。材料和方法：采用3．0T磁共振仪，对34例Ms患者和25例性别年龄相匹配的正常志愿者均进行DTI检查。分别测量MS组和对照组的9个不同部位脑白质的平均弥散率（MD）和部分各向异性指数（FA值），这9个部位包括胼胝体膝部、体部、压部、内囊后肢、侧脑室旁白质、额叶白质、顶叶白质、枕叶白质以及小脑中脚。比较两组NAWM之间的MD和FA值是否存在差异。结果：MS组的NAWM的MD值均高于对照组（P〈0．05），以胼胝体体部、内囊、侧脑室旁及额叶白质、顶叶白质、枕叶白质更为显著（P〈0．01）；MS组的FA值与对照组比较，胼胝体体部、顶叶白质、额叶白质、枕叶白质及内囊、侧脑室旁发现明显降低（P〈0．05），以后4个部位更为显著（P〈0．01），而胼胝体膝部、压部和小脑结合臂有降低趋势，但无统计学意义。结论：应用DTI定量研究可以探测到多发性硬化的NAWM所出现的微观病理改变，表现为水分子的平均弥散幅度明显升高，以幕上明显，并且胼胝体体部、内囊后肢等白质纤维明显失去正常的方向性。DTI在对白质损伤程度的量化评估中具有重要的价值。     

磁共振扩散张量成像在评估帕金森病脑内白质改变中的应用

目的 利用磁共振扩散张量成像技术(diffusion tensor imaging,DTI)评价帕金森病患者脑白质纤维束与正常老年组间的差异.方法 对23例临床诊断为原发性帕金森病(Parkinson's disease, PD)患者和相匹配的22例正常老年人行DTI成像,采用人工勾画感兴趣区(region of interest,ROI)的方法在DTI参数图,包括部分各向异性图(fractional anisotropy,FA)和平均扩散系数图(mean diffusivity,MD)上测量脑内主要白质纤维束的FA值及MD值,采用t检验的方法进行组间比较.结果 PD组双侧钩束、后扣带束及上纵束的FA值低于对照组(P＜0.05),双侧后扣带束及右侧上纵束的MD值高于正常组(P＜0.05).结论 PD的病理改变不仅存在于黑质纹状体系统,也累及白质纤维.     

梗阻性脑积水的弥散加权成像及弥散张量成像研究

目的探讨磁共振弥散加权成像(DWI)与弥散张量成像(DTI)对梗阻性脑积水的诊断价值。方法 25例梗阻性脑积水患者和30例志愿者均行3.0T磁共振常规序列以及DWI和DTI扫描,重建出ADC图、MD图、FA图、RA图、VR图及AI图,并对各参数图进行测量和记录,所得数据进行统计学分析。结果 30例志愿者及25例梗阻性脑积水患者分别测量双侧侧脑室周围脑白质、胼胝体膝部及压部区域的ADC、MD、FA、RA、VR及AI进行测量,ADC图中胼胝体膝部两组ADC值差异具有统计学意义(P<0.05),双侧侧脑室外侧脑白质两组所测得MD、FA、VR、AI值均有统计学意义(P<0.05),两组胼胝体压部RA值有统计学差异(P<0.05),两组间胼胝体膝部所有测得DTI各参数值均无统计学差异(P>0.05)。结论 DWI和DTI在脑积水的诊断中具有重要的诊断价值。     

磁共振弥散张量成像联合弥散张量纤维束成像在早期弥漫性轴索损伤中的应用

目的探讨磁共振弥散张量成像(DTI)与弥散张量纤维束示踪技术(DTT)对早期弥漫性轴索损伤(DAI)的诊断应用价值。方法对22例DAI早期(伤后10天内)患者(DAI组)及12例正常志愿者(对照组)分别行常规MR扫描及弥散张量成像扫描。在FA图上分别测量DAI组及对照组双侧半球白质感兴趣区(胼胝体压部、胼胝体膝部、内囊前肢、内囊后肢)部分各向异性(FA)值,将两组感兴趣区(ROI)平均FA值的差异进行比较,并对DAI组FA值与临床GCS评分进行相关性分析。运用弥散张量纤维束成像显示通过病灶的纤维束特征。结果病变组与对照组比较,DAI早期FA值(胼胝体压部0.647±0.069、胼胝体膝部0.615±0.043、内囊前肢0.541±0.065、内囊后肢0.639±0.035)较对照组(胼胝体压部0.748±0.045、胼胝体膝部0.729±0.058、内囊前肢0.622±0.038、内囊后肢0.667±0.027)FA值显著降低(P0.001)。胼胝体压部FA值的变化与GCS评分呈正相关(r=0.736,P=0.001)。DTT较好显示感兴趣区白质纤维束形态,更直观显示脑白质损伤程度。结论 DTI联合DTT是DAI患者检查的敏感方法,FA值评估是DAI程度的重要依据,DDT可观察神经纤维束受损范围。     

首发精神分裂症幻听患者的全脑白质变化研究

目的 探讨首发精神分裂症(SCH)幻听患者的全脑白质微细结构的改变.方法 分别对25例首发SCH幻听患者和25例正常对照者行扩散张量成像(DTI)扫描,采用基于纤维束示踪的空间统计分析(TBSS)方法分析2组全脑各向异性(FA)图,使用非参数的随机排列检验(random permutation test)对2组全脑DTI的FA值进行统计分析比较,得出有差异的脑区.结果 与正常组相比,首发SCH幻听组左侧额眶回与顶下小叶的白质FA值明显降低,差异具有统计学意义(P＜0.05).结论 TBSS方法能较全面、准确地分析首发SCH幻听患者早期脑白质细微结构的改变.     

首发抑郁症患者治疗前后悲伤体验的fMRI研究

目的:探讨首发抑郁症患者基于体素的全脑形态改变特点.方法:采用3.0T磁共振机,研究32例抑郁症患者及32例性别、年龄相匹配的正常志愿者.运用SPM2分析软件,采用基于体素的形态学方法,进行全脑比较分析.结果:抑郁症患者右侧海马、右侧杏仁核及右侧枕中回体积较正常对照组缩小(P<0.05,FDR-correct-ed).结论:首发抑郁症患者右侧海马、右侧杏仁核及右侧枕中回体积有明显缩小倾向.     

Minimum detectable difference of MR diffusion maps in acute ischemic stroke

PURPOSE: To determine the minimum detectable difference (MDD) and investigate variability of region-of-interest (ROI) analysis of apparent diffusion coefficient (ADC) and fractional anisotropy (FA) in acute ischemic stroke. MATERIALS AND METHODS: Ten patients with acute stroke (<24 hours) and moderate-to-large infarcts were imaged using a fast diffusion tensor technique. Four observers repeated three trials, during which each of two ROI types (free-hand polygon and ellipse) were drawn in white and gray matter (WM and GM) on FA and ADC maps. Analysis-of-variance techniques examined tissue and ROI type effects as well as inter- and intraobserver variability. F-tests examined the variability differences between ROI types. RESULTS: The MDD for ADC was 0.160 x 10(-3) mm(2) s(-1) in WM and 0.212 x 10(-3) mm(2) s(-1) in GM. The FA MDD was 0.19 in WM and 0.10 in GM. Tissue but not ROI type affected the mean values for both ADC and FA maps. Intraobserver reliability was substantial, while interobserver reliability was poor-to-moderate. No variability differences were found by ROI types. CONCLUSION: The MDD for WM and GM in normal and ischemic tissue were calculated. Inter- and intraobserver variability and tissue type affect ROI analysis of ADC and FA maps.     

123I-ADAM binding to serotonin transporters in patients with major depression and healthy controls: a preliminary study.

The serotonergic system may play an important role in the pathophysiology of major depressive disorder (MDD). Few imaging studies have examined serotonin transporter (SERT) binding in patients with MDD. We hypothesized that SERT binding activity may be altered in patients with MDD. This study compared SERT binding in patients with MDD with that in healthy controls. METHODS: We studied SERT activity in 7 patients (22-50 y old) with moderate to severe MDD and 6 healthy controls (24-56 y old) using (123)I-labeled 2-((2-((dimethylamino)methyl) phenyl)thio)-5-iodophenylamine (ADAM) and SPECT brain imaging. Subjects underwent SPECT 4 h after intravenous administration of 185 MBq (5 mCi) of (123)I-ADAM. Images were reconstructed in the axial plane, and region-of-interest demarcations were placed on the midbrain, medial temporal region, and basal ganglia region. RESULTS: (123)I-ADAM binding to SERT in the midbrain was significantly lower (P = 0.01) in MDD patients (1.81 +/- 0.07) than in controls (1.95 +/- 0.13). Age-adjusted (123)I-ADAM binding in the midbrain correlated significantly with scores on the Hamilton Depression Rating Scale (r = 0.82; P = 0.02). A significant negative correlation was observed between (123)I-ADAM SERT binding in the midbrain and age in the healthy control group (r = 0.98; P = 0.0002). SERT binding in the basal ganglia or medial temporal regions of interest did not significantly differ between groups. CONCLUSION: The findings from this preliminary study suggest the possibility of decreased SERT binding in the midbrain region of patients with MDD, with the degree of decrease correlating with the severity of depressive symptoms. There also appears to be an age-related decline in midbrain (123)I-ADAM SERT binding in healthy subjects.     

Brain microstructural abnormalities revealed by diffusion tensor images in patients with treatment-resistant depression compared with major depressive disorder before treatment

Treatment-resistant depression (TRD) is a therapeutic challenge for clinicians. Despite a growing interest in this area, an understanding of the pathophysiology of depression, particularly TRD, remains lacking. This study aims to detect the white matter abnormalities of whole brain fractional anisotropy (FA) in patients with TRD compared with major depressive disorder (MDD) before treatment by voxel-based analysis using diffusion tensor imaging. A total of 100 patients first diagnosed with untreated MDD underwent diffusion tensor imaging scans. 8 weeks after the first treatment, 54 patients showed response to the medication, whereas 46 did not. Finally, 20 patients were diagnosed with TRD after undergoing another treatment. A total of 20 patients with TRD and another 20 with MDD before treatment matched in gender, age, and education was enrolled in the research. For every subject, an FA map was generated and analyzed using SPM5. Subsequently, t-test was conducted to compare the FA values voxel to voxel between the two groups (p < 0.001 [FDR corrected], t > 7.57, voxel size > 30). Voxel-based morphometric (VBM) analysis was performed using T1W images. Significant reductions in FA were found in the white matter located in the bilateral of the hippocampus (left hippocampus: t = 7.63, voxel size = 50; right hippocampus: t = 7.82, voxel size = 48). VBM analysis revealed no morphological abnormalities between the two groups. Investigation of brain anisotropy revealed significantly decreased FA in both sides of the hippocampus. Although preliminary, our findings suggest that microstructural abnormalities in the hippocampus indicate vulnerability to treatment resistance.     

Diffusion-tensor MR imaging in children with developmental delay: preliminary findings

PURPOSE: To determine whether diffusion-tensor magnetic resonance (MR) imaging can depict abnormalities in patients with a diagnosis of developmental delay but structurally normal brain MR imaging results. MATERIALS AND METHODS: Twenty pediatric patients who received a diagnosis of developmental delay underwent brain MR examinations, including diffusion-tensor MR imaging. The MR findings in these patients were compared with those in 10 age-matched neurodevelopmentally healthy children. Diffusion constant (Dav) and anisotropy were measured bilaterally in regions of interest in the centrum semiovale, corona radiata, internal capsule, corpus callosum, and subcortical white matter of the frontal and parieto-occipital lobes. By using a one-tailed Student t test in the positive direction for Dav and in the negative direction for anisotropy and P <.05 to indicate a significant difference, the Dav and anisotropy values for children with developmental delay were compared with those for children who were neurodevelopmentally healthy. RESULTS: The children with developmental delay had significant increases in Dav in all measured structures (P, <.001 to <.03). Significant decreases in anisotropy were detected in all white matter fiber tracts studied (P, <.001 to <.03) except the posterior limb of the internal capsule. CONCLUSION: In the children with developmental delay, diffusion-tensor MR imaging depicted decreases in anisotropy and increases in Dav in the white matter fiber tracts, which appeared to be normal at conventional MR imaging.     

脑星形胶质细胞瘤周围区域的磁共振DTI研究

目的：采用扩散张量成像（DTI）技术研究脑星形胶质细胞瘤周围区域的细微结构改变,探讨其确定肿瘤边界的价值。方法：将28例经手术病理证实的星形胶质细胞瘤患者分为低度恶性星形细胞瘤和恶性胶质瘤两组。利用DTI检测肿瘤区、肿瘤周围区和健侧脑组织区,计算各向异性（FA）和平均扩散系数（ADC）及脑白质纤维束的情况。结果：28例星形胶质细胞瘤的3个不同区域DTI数值有显著性差异。周围区域DTI各值的均数在低度恶性和恶性胶质瘤之间进行比较,ADC、FA值的差异均有统计学意义。FA图显示低度恶性星形胶质细胞瘤多以推移周围邻近白质为主,与邻近白质边界清楚,而恶性胶质瘤组则以破坏与浸润白质束为主,边界相对模糊。结论：应用DTI成像可能对确定星形胶质细胞瘤的边界具有一定的临床意义。     

复发-缓解型多发性硬化投射纤维束定量DTI

目的：探讨复发-缓解型多发性硬化（RRMS）患者投射纤维定量扩散张量改变.方法：20例RRMS患者及年龄和性别相匹配的20例健康志愿者（对照组）行MRI扫描,获取常规MRI及DTI图像,分别进行测量分析,比较2组投射纤维的部分各向异性指数（FA）和平均扩散系数（MD）值的变化.主要纤维束包括丘脑前辐射（atr）、丘脑上辐射（str）、丘脑后辐射（ptr）、皮质延髓束（cpt）、皮质脊髓束（cst）等.结果：RRMS组患者与健康对照组的FA值比较：ptr（左侧：0.541±0.141 vs 0.628±0.153;右侧：0.512±0.079 vs 0.560±0.102）、cpt/atr（左侧：0.421±0.118 vs 0.503±0.104;右侧：0.437±0.064 vs 0.512±0.102）、cpt/ptr（左侧：0.405±0.056 vs 0.500±0.078;右侧：0.427±0.057 vs 0.496±0.083）、cpt/cst/str（左侧：0.476±0.115 vs 0.554±0.056;右侧：0.462±0.095 vs 0.520±0.074）,RRMS组的FA值均较健康对照组低,2组间差异有统计学意义（P〈0.05）;RRMS组患者atr的FA值（左侧：0.538±0.103,右侧：0.542±0.092）与对照组（左侧：0.564±0.087;右侧0.568±0.116）比较,差异无统计学意义（P〉0.05）.RRMS组患者与健康对照组的MD值比较：cpt/ptr（左侧：0.928±0.102 vs 0.853±0.105;右侧：0.949±0.165 vs 0.859±0.141）、cpt/cst/str（左侧：0.811±0.137 vs 0.772±0.093;右侧：0.868±0.167 vs 0.784±0.128）的MD值差异有统计学意义（P〈0.05）,RRMS组的MD值较健康对照组增高;RRMS组患者atr（左侧：0.898±0.143 vs 0.868±0.118;右侧：0.850±0.164 vs0.835±0.074）、ptr（左侧：0.874±0.203 vs 0.829±0.103;右侧：0.847±0.172 vs 0.802±0.075）、cpt/atr（左侧：0.856±0.187 vs 0.805±0.161;右侧：0.829±0.246 vs 0.820±0.093）的MD值和健康对照比较差异均无统计学意义（P〉0.05）.结论：多发性硬化患者双侧ptr、cpt/atr、cpt/ptr、cpt/cst/str存在FA值减小,双侧cpt/ptr、cpt/cst/str存在MD值的增高,提示在投射纤维中上述白质纤维束存在结构改变,DTI可以作为反映RRMS投射纤维微观病理改变的敏感检查指标.     

Intraoperative diffusion-tensor MR imaging: shifting of white matter tracts during neurosurgical procedures--initial experience

PURPOSE: To prospectively evaluate the location of white matter tracts with diffusion-tensor imaging (DTI) during neurosurgical procedures. MATERIALS AND METHODS: Ethical committee approval and signed informed consent were obtained. A 1.5-T magnetic resonance imager with an adapted rotating surgical table that is placed in a radiofrequency-shielded operating theater was used for pre- and intraoperative imaging. DTI was performed by applying an echo-planar imaging sequence with six diffusion directions in 38 patients (20 female patients, 18 male patients; age range, 7-77 years; mean age, 45.6 years) who were undergoing surgery (35 craniotomy and three burr hole procedures). Color-encoded maps of fractional anisotropy were generated by depicting white matter tracts. A rigid registration algorithm was used to compare pre- and intraoperative images. RESULTS: Intraoperative DTI was technically feasible in all patients, and no major image distortions occurred in the areas of interest. Pre- and intraoperative color-encoded maps of fractional anisotropy could be registered; these maps depicted marked and highly variable shifting of white matter tracts during neurosurgical procedures. In the 27 patients who underwent brain tumor resection, white matter tract shifting ranged from an inward shift of 8 mm to an outward shift of 15 mm (mean shift +/- standard deviation, outward shift of 2.5 mm +/- 5.8). In 16 (59%) of 27 patients, outward shifting was detected; in eight (30%), inward shifting was detected. In eight patients who underwent temporal lobe resections for drug-resistant epilepsy, shifting was only inward and ranged from 2 to 14 mm (9 mm +/- 3.3). In two of the three patients who underwent burr hole procedures, outward shifting occurred. CONCLUSION: Intraoperative DTI can depict shifting of major white matter tracts that is caused by surgical intervention.     

Diffusion tensor imaging of patients with HIV and normal-appearing white matter on MR images of the brain

BACKGROUND AND PURPOSE: HIV enters the CNS early in the course of infection and produces neuropsychiatric impairment throughout the course of illness, which preferentially affects the subcortical white matter. The development of a neuroimaging marker of HIV may allow for the earliest detection of cognitive impairment. The purpose of this study was to determine whether MR diffusion tensor imaging can detect white matter abnormalities in patients who have tested positive for HIV. METHODS: Ten patients with HIV (eight men and two women; mean age, 42 years) underwent MR imaging of the brain with MR diffusion tensor imaging, which included routine fluid-attenuated inversion recovery and fast spin-echo T2-weighted imaging. Diffusion constants and anisotropy indices were calculated from diffusion tensor maps. Peripheral viral load, Centers for Disease Control staging, and cluster of differentiation 4 levels were determined. RESULTS: All patients had normal results of MR imaging of the brain, except for mild atrophy. Four of 10 patients had undetectable viral loads. These patients were receiving highly active antiretroviral therapy. The diffusion constant and anisotropy were normal. Four of 10 patients had viral loads between 10,000 and 200,000. Diffusion anisotropy in the splenium and genu was significantly decreased (P < .02). The diffusion constant of the subcortical white matter was elevated in the frontal and parietooccipital lobes (11%). Two of 10 patients had viral loads >400,000. Anisotropy of the splenium was half normal (P < .0004) and of the genu was decreased 25% (P < .002). The average diffusion constant was diffusely elevated in the subcortical white matter. CONCLUSION: Calculating the diffusion constant and anisotropy in the subcortical white matter and corpus callosum in patients with HIV detected abnormalities despite normal-appearing white matter on MR images and nonfocal neurologic examinations. Patients with the highest diffusion constant elevations and largest anisotropy decreases had the most advanced HIV disease. Patients with the lowest viral load levels, who had normal anisotropy and diffusion constants, were receiving highly active antiretroviral therapy.