The relationship between diffusion anisotropy and time of onset after stroke

Hemodialysis-Related Acute Brain Injury Demonstrated by Application of Intradialytic Magnetic Resonance Imaging and Spectroscopy

SIGNIFICANCE STATEMENT

Hemodialysis (HD) results in reduced brain blood flow, and HD-related circulatory stress and regional ischemia are associated with brain injury over time. However, studies to date have not provided definitive direct evidence of acute brain injury during a HD treatment session. Using intradialytic magnetic resonance imaging (MRI) and spectroscopy to examine HD-associated changes in brain structure and neurochemistry, the authors found that multiple white (WM) tracts had diffusion imaging changes characteristic of cytotoxic edema, a consequence of ischemic insult and a precursor to fixed structural WM injury. Spectroscopy showed decreases in prefrontal N -acetyl aspartate (NAA) and choline concentrations consistent with energy deficit and perfusion anomaly. This suggests that one HD session can cause brain injury and that studies of interventions that mitigate this treatment's effects on the brain are warranted.

BACKGROUND

Hemodialysis (HD) treatment-related hemodynamic stress results in recurrent ischemic injury to organs such as the heart and brain. Short-term reduction in brain blood flow and long-term white matter changes have been reported, but the basis of HD-induced brain injury is neither well-recognized nor understood, although progressive cognitive impairment is common.

METHODS

We used neurocognitive assessments, intradialytic anatomical magnetic resonance imaging, diffusion tensor imaging, and proton magnetic resonance spectroscopy to examine the nature of acute HD-associated brain injury and associated changes in brain structure and neurochemistry relevant to ischemia. Data acquired before HD and during the last 60 minutes of HD (during maximal circulatory stress) were analyzed to assess the acute effects of HD on the brain.

RESULTS

We studied 17 patients (mean age 63±13 years; 58.8% were male, 76.5% were White, 17.6% were Black, and 5.9% were of Indigenous ethnicity). We found intradialytic changes, including the development of multiple regions of white matter exhibiting increased fractional anisotropy with associated decreases in mean diffusivity and radial diffusivity-characteristic features of cytotoxic edema (with increase in global brain volumes). We also observed decreases in proton magnetic resonance spectroscopy-measured N -acetyl aspartate and choline concentrations during HD, indicative of regional ischemia.

CONCLUSIONS

This study demonstrates for the first time that significant intradialytic changes in brain tissue volume, diffusion metrics, and brain metabolite concentrations consistent with ischemic injury occur in a single dialysis session. These findings raise the possibility that HD might have long-term neurological consequences. Further study is needed to establish an association between intradialytic magnetic resonance imaging findings of brain injury and cognitive impairment and to understand the chronic effects of HD-induced brain injury.

CLINICAL TRIALS INFORMATION

NCT03342183 .

Investigation of white matter and grey matter alteration in the monkey brain following ischemic stroke by using diffusion tensor imaging

Background

Investigation of stroke lesion has mostly focused on grey matter (GM) in previous studies and white matter (WM) degeneration during acute stroke is understudied. In the present study, monkeys were utilized to investigate the alterations of GM and WM in the brain following ischemic occlusion using diffusion tensor imaging (DTI).

Methods

Permanent middle cerebral artery occlusion (pMCAO) was induced in rhesus monkeys (n=6) with an interventional approach. Serial DTI was conducted on a clinical 3T in the hyperacute phase (2-6 hours), 48, and 96 hours post occlusion. Regions of interest in GM and WM of lesion areas were selected for data analysis.

Results

Mean diffusivity (MD), radial diffusivity (RD), and axial Diffusivity (AD) in WM decreased substantially during hyperacute stroke, as similar as those seen in GM. No obvious fractional anasotropy (FA) changes were seen in GM and WM during hyper acute phase. until 48 hours post stroke when significant fiber losses were oberved also. Pseudo-normalization of MD, AD, and RD was seen at 96 hours. Pathological changes of WM and GM were observed in ischemic areas at 8, 48, and 96 hours post stroke. Relative changes of MD, AD and RD of WM were correlated negatively with infarction volumes at 6 hours post stroke.

Conclusion

The present study revealed the microstructural changes in gray matter and white matter of monkey brains during acute stroke by using DTI. The preliminary results suggest axial and radial diffusivity (AD and RD) may be sensitive surrogate markers to assess specific microstructural changes in white matter during hyper-acute stroke.

Glymphatic Dysfunction in Patients With Ischemic Stroke

Objectives: Rodent experiments have provided some insight into the changes of glymphatic function in ischemic stroke. The diffusion tensor image analysis along the perivascular space (DTI-ALPS) method offers an opportunity for the noninvasive investigation of the glymphatic system in patients with ischemic stroke. We aimed to investigate the changes of glymphatic function in ischemic stroke and the factors associated with the changes. Materials and Methods: A total of 50 patients (mean age 56.7 years; 30 men) and 44 normal subjects (mean age 53.3 years; 23 men) who had preoperative diffusion-tensor imaging for calculation of the analysis along the perivascular space (ALPS) index were retrospectively included. Information collected from each patient included sex, age, time since stroke onset, infarct location, hemorrhagic change, infarct volume, infarct apparent diffusion coefficient (ADC), infarct fractional anisotropy (FA), and ALPS index of both hemispheres. Interhemispheric differences in ALPS index (infarct side vs. contralateral normal side) were assessed with a paired t-test in all patients. ALPS index was normalized by calculating ALPS ratios (right-to-left and left-to-right) for comparisons between patients and normal subjects. Comparisons of ALPS ratios between patients and normal subjects were performed using analysis of covariance with adjustments for age and sex. Linear regression analyses were performed to identify factors associated with the ALPS index. Results: In patients, the mean ALPS index ipsilateral to infarct was 1.162 ± 0.126, significantly lower (P < 0.001) than that of the contralateral side (1.335 ± 0.160). The right-to-left ALPS index ratio of patients with right cerebral infarct was 0.84 ± 0.08, significantly lower (P < 0.001) than that of normal subjects (0.95 ± 0.07). The left-to-right ALPS ratio of patients with left cerebral infarct was 0.92 ± 0.09, significantly (P < 0.001) lower than that of normal subjects (1.05 ± 0.08). On multiple linear regression analysis, time since stroke onset (β = 0.794, P < 0.001) was the only factor associated with the ALPS index. Conclusion: The ALPS index showed lower values in ischemic stroke suggesting impaired glymphatic function. Following initial impairment, the ALPS index increased with the time since stroke onset, which is suggestive of glymphatic function recovery.

Diffusion weighted imaging in acute ischemic stroke: A review of its interpretation pitfalls and advanced diffusion imaging application

Diffusion weighted imaging (DWI) is a widely used imaging technique to evaluate patients with stroke. It can detect brain ischemia within minutes of stroke onset. However, DWI has few potential pitfalls that should be recognized during interpretation. DWI lesion could be reversible in the early hours of stroke and the entire lesion may not represent ischemic core. False negative DWI could lead to diagnosis of DWI negative stroke or to a missed stroke diagnosis. Ischemic stroke mimics can occur on DWI with non-cerebrovascular neurological conditions. In this article, the history of DWI, its clinical applications, and potential pitfalls for use in acute ischemic stroke are reviewed. Advanced diffusion imaging techniques with reference to Diffusion Kurtosis Imaging and Diffusion Tensor Imaging that has been studied to evaluate ischemic core are discussed.

Dynamics of Water Diffusion Changes in Different Tissue Compartments From Acute to Chronic Stroke-A Serial Diffusion Tensor Imaging Study

Background and Purpose: The immediate decrease of the apparent diffusion coefficient (ADC) is the main characteristic change of water diffusion in acute ischemic stroke. There is only limited information on the time course of diffusion parameters in different tissue compartments of cerebral ischemia.

Materials and Methods: In a longitudinal study, we examined 21 patients with acute ischemic stroke by diffusion tensor imaging within 5 h after symptom onset, 3 h later, 2 days, and 1 month after symptom onset. Acute diffusion lesion and the fluid-attenuated inversion recovery (FLAIR) after 2 days were used as volumes of interest to define persistent core, lesion growth, and reversible acute diffusion lesion. For all diffusion parameters ratios between the stroke lesion VOIs and the mirror VOIs were calculated for each time point. ADC ratio, fractional anisotropy ratios, and eigenvalues ratios were measured in these volumes of interest and in contralateral mirror regions at each time points.

Results: In the persistent core, ADC ratio (0.772) and all eigenvalues ratios were reduced on admission up to 1 day after stroke and increased after 1 month (ADC ratio 1.067). Within the region of infarct growth time course of diffusion parameter changes was similar, but delayed. In the brain area with reversible diffusion lesion, a partial normalization of diffusion parameters over the time was observed, while after 1 month diffusion parameters did not show the signature of healthy brain tissue. There were significantly different trends for all parameters over time between the three tissue compartments.

Conclusion: Diffusion tensor imaging displays characteristic changes of water diffusion in different tissue compartments over time in acute ischemic stroke. Even regions with reversible diffusion lesion show diffusion signatures of persisting tissue alterations.

Comparison of spatial normalization strategies of diffusion MRI data for studying motor outcome in subacute-chronic and acute stroke

A common means of studying motor recovery in stroke patients is to extract Diffusion Tensor Imaging (DTI) parameters from the corticospinal tract (CST) and correlate them with clinical outcome scores. To that purpose, conducting group-level analyses through spatial normalization has become a popular approach. However, the reliability of such analyses depends on the accuracy of the particular registration strategy employed. To date, most studies have employed scalar-based registration using either high-resolution T1 images or Fractional Anisotropy (FA) maps to warp diffusion data to a common space. However, more powerful registration algorithms exist for aligning major white matter structures, such as Fiber Orientation Distribution (FOD)-based registration. Regardless of the strategy chosen, automatic normalization algorithms are prone to distortions caused by stroke lesions. While lesion masking is a common means to lessen such distortions, the extent of its effect on tract-related DTI parameters and their correlation with motor outcome has yet to be determined. Here, we aimed to address these concerns by first investigating the effect of common T1 and FA-based registration as well as novel FOD-based registration algorithms with and without lesion masking on lesion load and DTI parameter extraction of the CST in datasets typically acquired for subacute-chronic and acute stroke patients. Second, we studied how differences in these procedures influenced correlation strength between CST damage (through DTI parameters) and motor outcome. Our results showed that, for high-quality subacute-chronic stroke data, FOD-based registration captured significantly higher lesion loads and significantly larger FA asymmetries in the CST. This was also associated with significantly stronger correlations in motor outcome with respect to T1 or FA-based registration methods. For acute data acquired in a clinical setting, there were few observed differences, suggesting that commonly employed FA-based registration is appropriate for group-level analyses.

Cytotoxic edema at onset in West syndrome of unknown etiology: A longitudinal diffusion tensor imaging study

OBJECTIVE

To clarify longitudinal changes in white matter microstructures from the onset of disease in patients with West syndrome (WS) of unknown etiology.

METHODS

Diffusion tensor imaging (DTI) was prospectively performed at onset and at 12 and 24 months old in 17 children with WS of unknown etiology. DTI was analyzed using tract-based spatial statistics (TBSS) and tract-specific analysis (TSA) of 13 fiber tracts, and fractional anisotropy (FA) and mean diffusivity (MD) were compared with those of 42 age-matched controls. Correlations of FA and MD with developmental quotient (DQ) at age 24 months were analyzed. Multiple comparisons were adjusted for using the false discovery rate (q-value).

RESULTS

TBSS analysis at onset showed higher FA and lower MD in the corpus callosum and brainstem in patients. TSA showed lower MD in bilateral uncinate fasciculi (UF) (right: q < 0.001; left: q = 0.03) at onset in patients. TBSS showed a negative correlation between FA at onset and DQ in the right frontal lobe, whereas FA at 24 months old exhibited a positive correlation with DQ in the diffuse white matter. MD for bilateral UF at 24 months old on TSA correlated positively with DQ (q = 0.04, both).

SIGNIFICANCE

These findings may indicate the existence of cytotoxic edema in the immature white matter and dorsal brainstem at onset, and subsequent alterations in the diffuse white matter in WS of unknown etiology. Microstructural development in the UF might play important roles in cognitive development in WS.

Comprehensive analysis of early fractional anisotropy changes in acute ischemic stroke

BACKGROUND AND PURPOSE

Cerebral ischemia leads to a rapid decrease of the apparent diffusion coefficient. For fractional anisotropy both increase and decrease have been reported in acute ischemic stroke. Aim of this study was to characterize early water diffusion changes in a homogenous group of acute stroke patients and to clarify the issue of early fractional anisotropy changes and their relation to time from symptom onset.

METHODS

MRI data of patients with acute ischemic stroke examined by diffusion tensor imaging within 8h after symptom were analyzed. We calculated fractional anisotropy, eigenvalues and the isotropic and anisotropic components of the diffusion tensor. The values were calculated as ratios between the ischemic lesion and a mirror region in the unaffected side and correlated with clinical parameters.

RESULTS

We included 63 patients: 49% female, mean age 69 ± 14 years, median NIHSS on admission 9 (IQR 4-14). For the whole sample, mean fractional anisotropy was increased (ratio: 1.083 ± 0.168), while all other diffusion parameters were decreased. Both the isotropic and anisotropic component of the diffusion tensor were decreased with a more pronounced decrease of the isotropic component (ratios: isotropic = 0.730 ± 0.106, anisotropic = 0.788 ± 0.127; p<0.001). There was no correlation of fractional anisotropy with time from symptom onset. Looking at individual patients, fractional anisotropy was increased in 70%. There were no differences in clinical characteristics between patients with increased and decreased fractional anisotropy.

CONCLUSION

Fractional anisotropy increase in acute stroke results from a more pronounced decrease of the isotropic diffusion component and is not related to time from symptom onset. Thus, fractional anisotropy is not helpful as a surrogate marker of lesion age in the very first hours of stroke.

Differentiation of the Infarct Core from Ischemic Penumbra within the First 4.5 Hours, Using Diffusion Tensor Imaging-Derived Metrics: A Rat Model

Objective

To investigate whether the diffusion tensor imaging-derived metrics are capable of differentiating the ischemic penumbra (IP) from the infarct core (IC), and determining stroke onset within the first 4.5 hours.

Materials and Methods

All procedures were approved by the local animal care committee. Eight of the eleven rats having permanent middle cerebral artery occlusion were included for analyses. Using a 7 tesla magnetic resonance system, the relative cerebral blood flow and apparent diffusion coefficient maps were generated to define IP and IC, half hour after surgery and then every hour, up to 6.5 hours. Relative fractional anisotropy, pure anisotropy (rq) and diffusion magnitude (rL) maps were obtained. One-way analysis of variance, receiver operating characteristic curve and nonlinear regression analyses were performed.

Results

The evolutions of tensor metrics were different in ischemic regions (IC and IP) and topographic subtypes (cortical, subcortical gray matter, and white matter). The rL had a significant drop of 40% at 0.5 hour, and remained stagnant up to 6.5 hours. Significant differences (p < 0.05) in rL values were found between IP, IC, and normal tissue for all topographic subtypes. Optimal rL threshold in discriminating IP from IC was about -29%. The evolution of rq showed an exponential decrease in cortical IC, from -26.9% to -47.6%; an rq reduction smaller than 44.6% can be used to predict an acute stroke onset in less than 4.5 hours.

Conclusion

Diffusion tensor metrics may potentially help discriminate IP from IC and determine the acute stroke age within the therapeutic time window.

Axial Diffusivity of the Corona Radiata at 24 Hours Post-Stroke: A New Biomarker for Motor and Global Outcome

Fractional anisotropy (FA) is an effective marker of motor outcome at the chronic stage of stroke yet proves to be less efficient at early time points. This study aims to determine which diffusion metric in which location is the best marker of long-term stroke outcome after thrombolysis with diffusion tensor imaging (DTI) at 24 hours post-stroke. Twenty-eight thrombolyzed patients underwent DTI at 24 hours post-stroke onset. Ipsilesional and contralesional FA, mean (MD), axial (AD), and radial (RD) diffusivities values were calculated in different Regions-of-Interest (ROIs): (1) the white matter underlying the precentral gyrus (M1), (2) the corona radiata (CoRad), (3) the posterior limb of the internal capsule (PLIC) and (4) the cerebral peduncles (CP). NIHSS scores were acquired at admission, day 1, and day 7; modified Rankin Scores (mRS) at 3 months. Significant decreases were found in FA, MD, and AD of the ipsilesional CoRad and M1. MD and AD were also significantly lower in the PLIC. The ratio of ipsi and contralesional AD of the CoRad (CoRad-rAD) was the strongest diffusion parameter correlated with motor NIHSS scores on day 7 and with the mRS at 3 months. A Receiver-Operator Curve analysis yielded a model for the CoRad-rAD to predict good outcome based on upper limb NIHSS motor scores and mRS with high specificity and sensitivity. FA values were not correlated with clinical outcome. In conclusion, axial diffusivity of the CoRad from clinical DTI at 24 hours post-stroke is the most appropriate diffusion metric for quantifying stroke damage to predict outcome, suggesting the importance of early axonal damage.

Randomized clinical trial of dialysate cooling and effects on brain white matter

Hemodialysis is associated with significant circulatory stress that could produce recurrent and cumulative ischemic insults to multiple organs, such as the brain. We aimed to characterize hemodialysis-induced brain injury by longitudinally studying the effects of hemodialysis on brain white matter microstructure and further examine if the use of cooled dialysate could provide protection against hemodialysis-associated brain injury. In total, 73 patients on incident hemodialysis starting within 6 months were randomized to dialyze with a dialysate temperature of either 37°C or 0.5°C below the core body temperature and followed up for 1 year. Brain white matter microstructure was studied by diffusion tensor magnetic resonance imaging at baseline and follow-up (38 patients available for paired analysis). Intradialytic hemodynamic stress was quantified using the extrema points analysis model. Patients on hemodialysis exhibited a pattern of ischemic brain injury (increased fractional anisotropy and reduced radial diffusivity). Cooled dialysate improved hemodynamic tolerability, and changes in brain white matter were associated with hemodynamic instability (higher mean arterial pressure extrema points frequencies were associated with higher fractional anisotropy [peak r=0.443, P<0.03] and lower radial diffusivity [peak r=-0.439, P<0.02]). Patients who dialyzed at 0.5°C below core body temperature exhibited complete protection against white matter changes at 1 year. Our data suggest that hemodialysis results in significant brain injury and that improvement in hemodynamic tolerability achieved by using cooled dialysate is effective at abrogating these effects. This intervention can be delivered without additional cost and is universally applicable.

Structural integrity of the contralesional hemisphere predicts cognitive impairment in ischemic stroke at three months

After stroke, white matter integrity can be affected both locally and distally to the primary lesion location. It has been shown that tract disruption in mirror's regions of the contralateral hemisphere is associated with degree of functional impairment. Fourteen patients suffering right hemispheric focal stroke (S) and eighteen healthy controls (HC) underwent Diffusion Weighted Imaging (DWI) and neuropsychological assessment. The stroke patient group was divided into poor (SP; n = 8) and good (SG; n = 6) cognitive recovery groups according to their cognitive improvement from the acute phase (72 hours after stroke) to the subacute phase (3 months post-stroke). Whole-brain DWI data analysis was performed by computing Diffusion Tensor Imaging (DTI) followed by Tract Based Spatial Statistics (TBSS). Assessment of effects was obtained computing the correlation of the projections on TBSS skeleton of Fractional Anisotropy (FA) and Radial Diffusivity (RD) with cognitive test results. Significant decrease of FA was found only in right brain anatomical areas for the S group when compared to the HC group. Analyzed separately, stroke patients with poor cognitive recovery showed additional significant FA decrease in several left hemisphere regions; whereas SG patients showed significant decrease only in the left genu of corpus callosum when compared to the HC. For the SG group, whole brain analysis revealed significant correlation between the performance in the Semantic Fluency test and the FA in the right hemisphere as well as between the performance in the Grooved Pegboard Test (GPT) and the Trail Making Test-part A and the FA in the left hemisphere. For the SP group, correlation analysis revealed significant correlation between the performance in the GPT and the FA in the right hemisphere.

Feasibility of in vivo quantitative magnetic resonance imaging with diffusion weighted imaging, T2-weighted relaxometry, and diffusion tensor imaging in a clinical 3 tesla magnetic resonance scanner for the acute traumatic spinal cord injury of rats: technical note

STUDY DESIGN

Prospective longitudinal study.

OBJECTIVE

To verify the feasibility of performing in vivo quantitative magnetic resonance imaging evaluation of moderate traumatic spinal cord injury (SCI) in rats using a clinical 3T scanner.

SUMMARY OF BACKGROUND DATA

Animal models of human diseases are essential for translational medicine. Potential treatments of SCI are evaluated in 2 ways: anatomical and functional. Advanced magnetic resonance sequences allow a noninvasive assessment of the spinal cord depicting both. This study describes and validates a very reproducible, feasible, affordable, and reliable method, designed to be applied in commercial 3T equipment, using a novel stereotactic device for spinal cord, leading to a readily available assessment of the progression of damage generated after traumatic SCI in rats.

METHODS

Four Long-Evans female rats were injured with a New York University weight-drop device to produce the SCI by contusion at thoracic level 10. All animals were placed in a fixation system, using a commercial wrist antenna to obtain magnetic resonance imaging data of the relaxometry time, apparent diffusion coefficient, and fractional anisotropy. Three sets of data obtained before SCI and 1 and 4 weeks after injury were compared.

RESULTS

The data showed a progressive decline in fractional anisotropy measurements after SCI comparing baseline versus the 1-week period (P < 0.001) and baseline versus the 4-week period (P < 0.019), with a significant progressive increase in apparent diffusion coefficient values and T2 after SCI only in the baseline versus the 4-week period (P < 0.045 and P < 0.024, respectively).

CONCLUSION

Our results helped us to validate a novel method to acquire highly reproducible and reliable quantitative biomarkers of traumatic SCI in vivo by using a 3T clinical MR scanner coupled with a novel stereotactic device for rats.

LEVEL OF EVIDENCE

N/A.

Brain connectivity plasticity in the motor network after ischemic stroke

The motor function is controlled by the motor system that comprises a series of cortical and subcortical areas interacting via anatomical connections. The motor function will be disturbed when the stroke lesion impairs either any of these areas or their connections. More and more evidence indicates that the reorganization of the motor network including both areas and their anatomical and functional connectivity might contribute to the motor recovery after stroke. Here, we review recent studies employing models of anatomical, functional, and effective connectivity on neuroimaging data to investigate how ischemic stroke influences the connectivity of motor areas and how changes in connectivity relate to impaired function and functional recovery. We suggest that connectivity changes constitute an important pathophysiological aspect of motor impairment after stroke and important mechanisms of motor recovery. We also demonstrate that therapeutic interventions may facilitate motor recovery after stroke by modulating the connectivity among the motor areas. In conclusion, connectivity analyses improved our understanding of the mechanisms of motor recovery after stroke and may help to design hypothesis-driven treatment strategies and sensitive measures for outcome prediction in stroke patients.

Diffusion tensor imaging of sports-related concussion in adolescents

Concussion is among the least understood neurologic injuries. The impact of concussion on the adolescent brain remains largely unknown. This study sought to establish short-term changes in white-matter integrity after sports-related concussion in adolescents, and examine the association between changes in white-matter integrity and a clinical measure of concussion. Twelve adolescents, aged 14-17 years with a sports-related concussion within 2 months, and 10 age-matched adolescents with no history of concussion were evaluated with the Sports Concussion Assessment Tool 2 and diffusion tensor imaging. Two measures compared the two groups: fractional anisotropy and mean diffusivity. Whole-brain fractional anisotropy values significantly increased (F(1,40) = 6.29, P = 0.010), and mean diffusivity values decreased (F(1,40) = 4.75, P = 0.036), in concussed athletes compared with control participants. Total scores on the Sports Concussion Assessment Tool 2 were associated with whole-brain fractional anisotropy. Mean diffusivity values with lower scores were associated with higher fractional anisotropy (R(2) = 0.25, P = 0.017) and lower mean diffusivity (R(2) = 0.20, P = 0.038). We provide evidence of structural changes in the integrity of white matter in adolescent athletes after sports-related concussion.

Diffusion tensor imaging, permanent pyramidal tract damage, and outcome in subcortical stroke

BACKGROUND

Studies in chronic stroke patients suggest that diffusion tensor imaging (DTI) parameters of the pyramidal tract (PT) relate to residual motor function. We performed a prospective controlled study to evaluate if the DTI parameters tract volume (TV) and fractional anisotropy (FA) in patients with acute subcortical infarcts are correlated with permanent PT damage and clinical outcome after 6 months.

METHODS

We acquired DTI in 18 stroke patients with subcortical ischemic infarcts either affecting the PT (PT group, n = 12) or not (non-PT group, n = 6) and in 7 age- and risk factor-matched controls at median times of 12 and 180 days. The PT was isolated using tractography and tract volume ratios (R(TV)) and FA ratios (R(FA)) were calculated (affected tract/unaffected tract). Ratios were compared within and between groups at initial and follow-up time points, as well as in tract portions above and below the infarcts, and were correlated to Rivermead Motor Function Test (RMFT) scores.

RESULTS

Mean R(FA) and R(TV) of the PT group were smaller than those of both non-PT and control groups initially and at follow-up (p < 0.01). Tract portions above the infarct had lower R(TV) than below (p < 0.05). There was no significant change in R(FA) and R(TV) over time for the whole tract or tract portions. R(FA) and R(TV) both were highly correlated with initial and follow-up RMFT scores.

CONCLUSIONS

DTI parameters of PT integrity acquired within the first weeks after acute subcortical stroke measure permanent ischemic PT damage and are highly correlated with residual motor function in the acute and chronic stage.

How does water diffusion in human white matter change following ischemic stroke?

PURPOSE

Temporal evolution of the water apparent diffusion coefficients (ADC) parallel (ADC parallel) and perpendicular (ADC perpendicular) to the human white matter tract following ischemia has not been investigated systematically. We attempted to quantify the evolution of ADC parallel and ADC perpendicular and examine whether it can be interpreted by a model of ischemic edema.

METHODS

We retrospectively selected 53 patients with ischemic lesions involving the posterior limb of the internal capsule (PLIC) and placed regions of interest in the right and left PLIC on ADC maps. We performed regression analysis of lesion-to-contralateral ratios of ADC parallel and ADC perpendicular against the time (t = 1-1600 h) from onset. We then fitted the estimated time courses of ADC parallel and ADC perpendicular obtained from the analysis to a model of nerve tissue composed of cylinders (axons) and spheres corresponding to isotropic structures, particularly focal cytoplasmic swellings of glial cells and axons seen in ischemic white matter.

RESULTS

The evolution of ADC perpendicular and ADC parallel differed. The estimated time course of ADC parallel in microm(2)*ms(-1) was 0.64 + 0.88 exp (-0.24t) for 1 < t < 54 h and 0.00059t + 0.61 for t >or= 54 h (contralateral normal value, 1.52). That of ADC perpendicular was 0.19-0.063 exp (-0.24t) for 1 < t < 54 h and 0.00040t + 0.17 for t >or=54 h (normal value 0.22). The model fitted to these values showed that the volume of the cylinders decreased, that of the spheres increased, and extracellular volume changed little from one hour to approximately one day after stroke onset.

CONCLUSION

In the human PLIC, ADC parallel continued to decrease from one hour to a few days after stroke onset, and ADC perpendicular tended to increase. The temporal evolution could be interpreted by progression of the focal cytoplasmic swelling of glial cells and axons previously observed in animal studies.

Assessment of arcuate fasciculus with diffusion-tensor tractography may predict the prognosis of aphasia in patients with left middle cerebral artery infarcts

INTRODUCTION

It is often clinically difficult to assess the severity of aphasia in the earliest stage of cerebral infarction. A method enabling objective assessment of verbal function is needed for this purpose. We examined whether diffusion tensor (DT) tractography is of clinical value in assessing aphasia.

METHODS

Thirteen right-handed patients with left middle cerebral artery infarcts who were scanned within 2 days after stroke onset were enrolled in this study. Magnetic resonance data of ten control subjects were also examined by DT tractography. Based on the severity of aphasia at discharge, patients were divided into two groups: six patients in the aphasic group and seven in the nonaphasic group. Fractional anisotropy (FA) and number of arcuate fasciculus fibers were evaluated. Asymmetry index was calculated for both FA and number of fibers.

RESULTS

FA values for the arcuate fasciculus fibers did not differ between hemispheres in either the patient groups or the controls. Number of arcuate fasciculus fibers exhibited a significant leftward asymmetry in the controls and the nonaphasic group but not in the aphasic group. Asymmetry index of number of fibers was significantly lower (rightward) in the aphasic group than in the nonaphasic (P = 0.015) and control (P = 0.005) groups. Loss of leftward asymmetry in number of AF fibers predicted aphasia at discharge with a sensitivity of 0.83 and specificity of 0.86.

CONCLUSIONS

Asymmetry of arcuate fasciculus fibers by DT tractography may deserve to be assessed in acute infarction for predicting the fate of vascular aphasia.

Progression of pathological changes in the middle cerebellar peduncle by diffusion tensor imaging correlates with lesser motor gains after pontine infarction

BACKGROUND

Wallerian degeneration in pyramidal tract following supratentorial stroke has been detected by some studies using diffusion tensor imaging (DTI), but the Wallerian degeneration in middle cerebellar peduncle after pontine infarction and its potential clinical significance remain to be confirmed.

METHODS

Seventeen patients with a recent focal pontine infarct underwent 3 DTIs at week 1 (W1), week 4 (W4), and week 12 (W12) after onset. Seventeen age-matched and gender-matched controls underwent DTI one time. Mean diffusivity and fractional anisotropy (FA) were measured in the basis pontis and bilateral middle cerebellar peduncles. Neurological deficit, motor deficit, functional independence, and limbs ataxia were assessed with the National Institutes of Health (NIH) Stroke Scale, Fugl-Meyer scale, Barthel Index, and the second part of International Cooperative Ataxia Rating Scale.

RESULTS

FA values at the bilateral middle cerebellar peduncles decreased significantly from W1 to W12 progressively (P<.01). The patients improved on the NIH Stroke Scale, Fugl-Meyer scale, and Barthel Index over time (P<.01). Greater absolute value of percentage reduction of FA at the bilateral middle peduncles, however, was associated with the less absolute value of percentage reduction of the NIH Stroke Scale and less increase in the Fugl-Meyer scale, as well as greater ataxia over time.

CONCLUSIONS

Wallerian degeneration in the middle cerebellar peduncle revealed by DTI may hinder the process of neurological recovery following a focal pontine infarct.

Elevations of diffusion anisotropy are associated with hyper-acute stroke: a serial imaging study

Diffusion tensor imaging (DTI) studies of human ischemic stroke within 24 h of symptom onset have reported variable findings of changes in diffusion anisotropy. Serial DTI within 24 h may clarify these heterogeneous results. We characterized longitudinal changes of diffusion anisotropy by analyzing discrete ischemic white matter (WM) and gray matter (GM) regions during the hyperacute (2.5-7 h) and acute (21.5-29 h) scanning phases of ischemic stroke onset in 13 patients. Mean diffusivity (MD), fractional anisotropy (FA) and T2-weighted signal intensity were measured for deep and subcortical WM and deep and cortical GM areas in lesions outlined by a > or =30% decrease in MD. Average reductions of approximately 40% in relative (r) MD were observed in all four brain regions during both the hyperacute and acute phases post stroke. Overall, 9 of 13 patients within 7 h post symptom onset showed elevated FA in at least one of the four tissues, and within the same cohort, 11 of 13 patients showed reduced FA in at least one of the ischemic WM and GM regions at 21.5-29 h after stroke. The fractional anisotropy in the lesion relative to the contralateral side (rFA, mean+/-S.D.) was significantly elevated in some patients in the deep WM (1.10+/-0.11, n=4), subcortical WM (1.13+/-0.14, n=4), deep GM (1.07+/-0.06, n=1) and cortical GM (1.22+/-0.13, n=5) hyperacutely (< or =7 h); however, reductions of rFA at approximately 24 h post stroke were more consistent (rFA= 0.85+/-0.12).

Time course of diffusion imaging in acute brainstem infarcts

PURPOSE

To study the time course of diffusion imaging at the lesion site in brainstem infarcts.

MATERIALS AND METHODS

Sequential MR scans were acquired from 24 patients with brainstem infarcts. Diffusion-weighted images (DWI), T(2)-weighted images (T(2)w), maps of apparent diffusion coefficient, and maps of fractional anisotropy were generated from each MR scan. A trend function was fitted to these measurements to model an objective, general time course of the studied parameters.

RESULTS

Apparent diffusion coefficient (ADC) continuously decreased over time until a transition time around 45 hours; afterwards a continuous increase took place. After the 14th day ADC reached values similar to the ADC of the intact contralateral side (pseudonormalization) and then further increased. Fractional anisotropy (FA) decreased continuously over 3 to 6 months.

CONCLUSION

Times of transition and pseudonormalization of ADC were longer than described for territorial hemispheric infarcts and describe the acute to subacute phase of brainstem ischemia. In contrast, the continuous decline of FA over 3 to 6 months indicates a chronic process of change of histological structures in brainstem ischemia, and may be regarded as an indicator of the chronic phase.