Toward accurate diagnosis of white matter pathology using diffusion tensor imaging.

Diffusion tensor imaging (DTI) has been widely applied to investigate injuries in the central nervous system (CNS) white matter (WM). However, the underlying pathological correlates of diffusion changes have not been adequately determined. In this study the coregistration of histological sections to MR images and a pixel-based receiver operating characteristic (ROC) analysis were used to compare the axial (lambda( parallel)) and radial (lambda( perpendicular)) diffusivities derived from DTI and histological markers of axon (phosphorylated neurofilament, SMI-31) and myelin (Luxol fast blue (LFB)) integrity, respectively, in two different patterns of injury to mouse spinal cord (SC) WM. In contusion SC injury (SCI), a decrease in lambda( parallel) matched the pattern of axonal damage with high accuracy, but lambda( perpendicular) did not match the pattern of demyelination detected by LFB. In a mouse model of multiple sclerosis (MS), lambda( perpendicular) and lambda( parallel) did not match the patterns of demyelination or axonal damage, respectively. However, a region of interest (ROI) analysis suggested that lambda( perpendicular)-detected demyelination paralleled that observed with LFB, and lambda( parallel) decreased in both regions of axonal damage and normal-appearing WM (NAWM) as visualized by SMI-31. The results suggest that directional diffusivities may reveal abnormalities that are not obvious with SMI-31 and LFB staining, depending on the type of injury.     

双腔起搏器不同房室延迟间期及VVI模式起搏时心功能的变化

目的:观察双腔起搏器(DDD)不同房室延迟间期(AVD)及VVI起搏时的血流动力学及左心功能变化.方法:完全性房室传导阻滞植入DDD永久人工起搏器患者19例,男10例,女9例,年龄(66.79±15.52)岁,采用彩色多普勒超声心动图测量DDD起搏时不同AVD及VVI起搏时二尖瓣前向血流图(E、A峰),主动脉前向血流速度(AFV)及速度时间积分(AFVTI)、左心室收缩/舒张末期内径(LVESd/LVEDd)、左室射血分数(LVEF)、每搏输出量(SV)、心排血量(CO)等.预设心房起搏频率为75次/min, AVD自90 ms以30 ms步长依次递增至240 ms,最后程控为VVI模式.结果:(1)DDD起搏时血流动力学各参数随AVD变化而变化,当AVD为(163.30±30.41) ms时,E、A峰,AFV,AFVTI达最大;当AVD为240 ms时,部分患者E、A峰融合呈单峰;当以VVI起搏时,E峰高低不等,A峰间断、不规则出现,AFV、AFVTI较DDD起搏时明显减小. (2)DDD起搏时心功能各参数随AVD的递增而改善,达峰值后又逐渐下降;AVD为(163.30±30.41) ms时,CO达最大[(6.50±1.54) L/min],较AVD为90 ms[(5.83±1.61) L/min]、240 ms[(5.85±1.28) L/min]时分别增加11.5％、11.1％,较VVI起搏[(4.83±0.80) L/min]时增加25.9％.结论:(1)DDD起搏时左心血流动力学及心功能明显优于VVI模式;(2)DDD起搏时,心功能随不同AVD发生明显变化,对有心功能代偿不良者,优化AVD与选择起搏方式具有同样重要的临床意义.     

Anisotropy of NMR properties of tissues

Orientational anisotropy of T₂ and T₁ relaxation times, diffusion, and magnetization transfer has been investigated for six different tissues: tendon, cartilage, kidney, muscle, white matter, and optic nerve. Relaxation anisotropy was observed for tendon and cartilage, and diffusional anisotropy was measured in kidney, muscle, white matter, and optic nerve. All other NMR measurements of these tissues showed no orientational dependence. This pattern of NMR anisotropies can be interpreted from the underlying geometrical structures of the tissues.     

Mode switching failure during atrial flutter: the '2:1 lock-in' phenomenon.

AIMS: To evaluate incidence and mechanism of a special form of automatic mode switching (MS) failure in patients with atrial flutter. METHODS AND RESULTS: Retrospectively the charts of 134 patients implanted with dual chamber pacemakers with MS algorithms were reviewed. Seven patients (5.2%) were identified that presented with sustained rapid ventricular pacing resulting from atrial flutter with failure of automatic MS. Since this form of MS failure implies 2:1 tracking of atrial flutter, it was coined '2:1 lock-in'. A theoretical timing model was developed to clarify the mechanism of this special form of MS failure. Prerequisites for the '2:1 lock-in' phenomenon are: (1). the sum of the AV delay and the post ventricular blanking (PVAB) must be longer than the cycle length of the atrial flutter, (2). the tachycardia detection rate must be higher than half the atrial flutter rate and (3). the maximum tracking rate (MTR) must be higher than half the atrial flutter rate. Recommendations for programming in order to avoid this specific form of MS failure are made accordingly and parallel algorithms for flutter detection are discussed. CONCLUSION: '2:1 lock-in' is a typical form of MS failure in patients with atrial flutter and the mechanism is closely linked to the typical atrial sensing windows.     

Hyperpolarized (3)He MRI of mouse lung.

Hyperpolarized (3)He images of mouse lung are presented. Ventilation images and measurements of (3)He apparent diffusion coefficient (ADC) are reported in healthy mice, and preliminary studies of emphysema and lung cancer in mice are described using these techniques. The design and operation of an electronically controlled small-animal ventilator to deliver the hyperpolarized gas and control animal respiration are described. Images are acquired using an asymmetric gradient echo imaging method to enhance the signal-to-noise ratio of the rapidly diffusing (3)He. In mice with elastase-induced emphysema, the whole-lung average ADC is greater by approximately 25%, a statistically significant difference, compared to healthy animals. By contrast, mice exposed to cigarette smoke for up to 12 months reveal no statistically relevant increases in ADC, although emphysema was not confirmed in these mice. A study of lung cancer (melanoma) in mice is also presented. While tumors are shown to cause substantial ventilation defects in the lung, these defects appear confined to the cancerous regions and do not extend to large-scale regions of the lung distal to the tumors.     

Age-estimations of rats based on the average lateral diffusion constant of hepatocyte membrane proteins as revealed by fluorescence recovery after photobleaching

A method has been developed recently for measuring the average lateral diffusion constant of the proteins (D) in the cell membrane of hepatocytes in liver smears by fluorescence recovery after photobleaching (FRAP). A peroxide-induced autofluorescence (PIAF) of the membrane proteins was used as a fluorescent label. It has been established that D displays a significant negative linear correlation with age. The present paper describes age-estimations carried out on 12 male Fischer 344 rats (7–29 months of age) in so-called “blind experiments”: the operator knew only the sex of the rat, determined D from a small piece of the freshly removed liver, and estimated the age of the rat from the age-dependent regression line for D established previously on 16 other Fischer 344 male rats of various ages. There was a strong correlation of the estimated age with the actual one (r = 0.92), the slope of the regression line was 0.98 and its intercept differed from 0 by only 0.5 months. These results indicate that D may play a decisive role in the determination of membrane functions as predicted by the membrane hypothesis of aging.     

Imaging cortical association tracts in the human brain using diffusion-tensor-based axonal tracking.

Diffusion-tensor fiber tracking was used to identify the cores of several long-association fibers, including the anterior (ATR) and posterior (PTR) thalamic radiations, and the uncinate (UNC), superior longitudinal (SLF), inferior longitudinal (ILF), and inferior fronto-occipital (IFO) fasciculi. Tracking results were compared to existing anatomical knowledge, and showed good qualitative agreement. Guidelines were developed to reproducibly track these fibers in vivo. The interindividual variability of these reconstructions was assessed in a common spatial reference frame (Talairach space) using probabilistic mapping. As a first illustration of this technical capability, a reduction in brain connectivity in a patient with a childhood neurodegenerative disease (X-linked adrenoleukodystrophy) was demonstrated.     

Spatial normalization of diffusion tensor fields.

A method for the spatial normalization and reorientation of diffusion tensor (DT) fields is presented. Spatial normalization of tensor fields requires an appropriate reorientation of the tensor on each voxel, in addition to its relocation into the standardized space. This appropriate tensor reorientation is determined from the spatial normalization transformation and from an estimate of the underlying fiber direction. The latter is obtained by treating the principal eigenvectors of the tensor field around each voxel as random samples drawn from the probability distribution that represents the direction of the underlying fiber. This approach was applied to DT images from nine normal volunteers, and the results show a significant improvement in signal-to-noise ratio (SNR) after spatial normalization and averaging of tensor fields across individuals. The statistics of the spatially normalized tensor field, which represents the tensor characteristics of normal individuals, may be useful for quantitatively characterizing individual variations of white matter structures revealed by DT imaging (DTI) and deviations caused by pathology. Simulated experiments using this methodology are also described.     

晶状体和玻璃体切除术后二期后房型人工晶状体植入术疗效观察

目的 评价晶状体和玻璃体切除术后二期后房型人工晶状体植入术的疗效。方法 对42例晶状体和玻璃体切除术后3个月至6年的眼外伤患者行人工晶状体植入术。采用颞下方平坦部巩膜灌注，最大限度地利用残存的晶状体前囊为依托的二期后房型人工晶状体植入术。术后随访3个月以上。结果 无严重远期并发症，所有患者均达到或超过术前最佳矫正视力。术后裸眼视力0．2～0．4者10眼，0．5～0．9者28眼，≥1．0者4眼。结论 晶状体和玻璃体切除术后二期后房型人工晶状体植入术安全可靠，是目前矫正晶状体和玻璃体切除术后屈光不正的较好方法。     

Effects of Sampling Chamber Volume and Geometry on Aerodynamic Size Distributions of Metered-Dose Inhalation Aerosols Measured with the Andersen Cascade Impactor

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