Apparent diffusion coefficient mapping of experimental focal cerebral ischemia using diffusion-weighted echo-planar imaging

Accelerated multi-b-value multi-shot diffusion-weighted imaging based on EPI with keyhole and a low-rank tensor constraint

PURPOSE

Multi-Shot (MS) Echo-Planar Imaging (EPI) may improve the in-plane resolution of multi-b-value DWI, yet it also considerably increases the scan time. Here we explored the combination of EPI with Keyhole (EPIK) and a calibrationless reconstruction algorithm for acceleration of multi-b-value MS-DWI.

METHODS

We firstly analyzed the impact of nonuniform phase accrual in EPIK on the reconstructed image. Based on insights gained from the analysis, we developed a calibrationless reconstruction algorithm based on a Space-Contrast-Coil Locally Low-Rank Tensor (SCC-LLRT) constraint for reconstruction of EPIK-acquired data. We compared the algorithm with a modified SPatial-Angular Locally Low-Rank (SPA-LLR) algorithm through simulations, phantoms, and in vivo study. We then compared EPIK with uniformly undersampled EPI for accelerating multi-b-value DWI in 6 healthy subjects.

RESULTS

Through theoretical derivations, we found that the reconstruction of EPIK with a SENSE-encoding-based algorithm, such as SPA-LLR, may cause additional aliasing artifacts due to the frequency-dependent distortion of the coil sensitivity. Results from simulations, phantoms, and in vivo study verified the theoretical finding by showing that the calibrationless SCC-LLRT algorithm reduced aliasing artifacts compared with SPA-LLR. Finally, EPIK with SCC-LLRT substantially reduced the ghosting artifacts compared with uniform undersampled multi-b-value DWI, decreasing the fitting errors in ADC (0.05 ± 0.01 vs 0.10 ± 0.01, P < 0.001) and IVIM mapping (0.026 ± 0.004 vs 0.06 ± 0.006, P < 0.001).

CONCLUSION

The SCC-LLRT algorithm reduced the aliasing artifacts of EPIK by using a calibrationless modeling of the multi-coil data. The dense sampling of k-space center offers EPIK a potential to improve image quality for acceleration of multi-b-value MS-DWI.

Infarct Evolution on MR-DWI After Thrombectomy in Acute Stroke Patients Randomized to Nerinetide or Placebo: The REPERFUSE-NA1 Study

BACKGROUND AND OBJECTIVES

The neuroprotectant nerinetide has shown promise in reducing infarct volumes in primate models of ischemia reperfusion. We hypothesized that early secondary infarct growth after endovascular therapy (EVT) (1) may be a suitable surrogate biomarker for testing neuroprotective compounds, (2) is feasible to assess in the acute setting using sequential MRI, and (3) can be modified by treatment with nerinetide.

METHODS

REPERFUSE-NA1 was a prospective, multisite MRI substudy of the randomized controlled trial ESCAPE-NA1 (ClinicalTrials.gov NCT02930018) that involved patients with acute disabling large vessel occlusive stroke undergoing EVT within 12 hours of onset who were randomized to receive intravenous nerinetide or placebo. Patients enrolled in REPERFUSE-NA1 underwent sequential MRI <5 hours post-EVT (day 1) and at 24 hours (day 2). The primary outcome was total diffusion-weighted MRI infarct growth early after EVT, defined as the lesion volume difference between day 2 and day 1. The secondary outcome was region-specific infarct growth in different brain tissue compartments. Statistical analyses were performed using the Mann-Whitney U test and multiple linear regression.

RESULTS

Sixty-seven of 71 patients included had MRI of sufficient quality. The median infarct volume post-EVT was 12.98 mL (IQR, 5.93-28.08) in the nerinetide group and 10.80 mL (IQR, 3.11-24.45) in the control group (p = 0.59). Patients receiving nerinetide showed a median early secondary infarct growth of 5.92 mL (IQR, 1.09-21.30) compared with 10.80 mL (interquartile range [IQR], 2.54-21.81) in patients with placebo (p = 0.30). Intravenous alteplase modified the effect of nerinetide on region-specific infarct growth in white matter and basal ganglia compartments. In patients with no alteplase, the infarct growth rate was reduced by 120% (standard error [SE], 60%) in the white matter (p = 0.03) and by 340% (SE, 140%) in the basal ganglia (p = 0.02) in the nerinetide group compared with placebo after adjusting for confounders.

DISCUSSION

This study highlights the potential of using MR imaging as a biomarker to estimate the effect of a neuroprotective agent in acute stroke treatment. Patients with acute large vessel occlusive stroke exhibited appreciable early infarct growth both in the gray matter and the white matter after undergoing EVT. Acknowledging relatively small overall infarct volumes in this study, treatment with nerinetide was associated with slightly reduced percentage infarct growth in the white matter and basal ganglia compared with placebo in patients not receiving intravenous alteplase and had no effect on the total early secondary infarct growth.

TRIAL REGISTRATION INFORMATION

ClinicalTrials.gov NCT02930018.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that for patients with acute large vessel ischemic stroke undergoing EVT, nerinetide did not significantly decrease early post-EVT infarct growth compared with placebo.

Deep learning-based automated lesion segmentation on mouse stroke magnetic resonance images

Magnetic resonance imaging (MRI) is widely used for ischemic stroke lesion detection in mice. A challenge is that lesion segmentation often relies on manual tracing by trained experts, which is labor-intensive, time-consuming, and prone to inter- and intra-rater variability. Here, we present a fully automated ischemic stroke lesion segmentation method for mouse T2-weighted MRI data. As an end-to-end deep learning approach, the automated lesion segmentation requires very little preprocessing and works directly on the raw MRI scans. We randomly split a large dataset of 382 MRI scans into a subset (n = 293) to train the automated lesion segmentation and a subset (n = 89) to evaluate its performance. We compared Dice coefficients and accuracy of lesion volume against manual segmentation, as well as its performance on an independent dataset from an open repository with different imaging characteristics. The automated lesion segmentation produced segmentation masks with a smooth, compact, and realistic appearance that are in high agreement with manual segmentation. We report dice scores higher than the agreement between two human raters reported in previous studies, highlighting the ability to remove individual human bias and standardize the process across research studies and centers.

Impact of mandated prospectively reported apparent diffusion coefficient values on the rates of positivity for clinically significant prostate cancer by PI-RADS score

BACKGROUND

Prior research has shown that retrospectively measured apparent diffusion coefficient (ADC) of prostate magnetic resonance imaging (MRI) lesions is associated with clinically significant prostate cancer (csPCa) on targeted biopsy suggesting that ADC should be measured and reported prospectively.

PURPOSE

To assess the impact of mandatory prospective measurement of ADC on the rates of positivity across PI-RADS scores for csPCa.

MATERIAL AND METHODS

Consecutive patients who underwent ultrasound (US)-MRI fusion prostate biopsy from August 2018 to July 2019 and who had prospectively reported ADC were compared to control patients who did not. Rates of positivity by PI-RADS category were computed and compared using Chi-square. Multivariable regression was performed.

RESULTS

In total, 126 patients (median age 65 years) with 165 prostate lesions (19, 51, 70, and 25 PI-RADS 2, 3, 4, and 5, respectively) and prospectively reported ADC values were compared to 113 control patients (median age 66 years) with 157 prostate lesions (17, 42, 64, and 34 PI-RADS 2, 3, 4, and 5, respectively). Rates of positivity across PI-RADS scores were similar between the two cohorts; 11%, 25%, 55%, and 76% and 0%, 21%, 56%, and 62% for PI-RADS 2, 3, 4, and 5 in the test and control cohorts, respectively (Chi-square P = 0.78). Multivariate logistic regression showed no significant association between the presence of prospectively measured ADC and csPCa (odds ratio 1.1, 95% confidence interval 0.7-1.7, P = 0.82).

CONCLUSION

Prospective ADC measurement may not impact PI-RADS category assignments or positivity rates for csPCa under current guidelines. Future versions of PI-RADS may need to incorporate ADC into scoring rules to realize their potential.

Reperfusion Into Severely Damaged Brain Tissue Is Associated With Occurrence of Parenchymal Hemorrhage for Acute Ischemic Stroke

Background and Purpose: This study aims to quantify the reperfusion status within severely damaged brain tissue and to evaluate its relationship with high grade of hemorrhagic transformation (HT).

Methods: Pseudo-continuous ASL was performed along with DWI in 102 patients within 24 h post-treatments. The infarction core was identified using ADC values <550 × 10⁻⁶ mm²/s. CBF within the infarction core and its contralateral counterpart were acquired. CBF at the 25th, median, and 75th percentiles of the contralateral counterpart were used as thresholds and the ASL reperfusion volume above the threshold was labeled as vol-25, -50, and -75, respectively. Recanalization was defined according to Thrombolysis in Myocardial Infarction (TIMI) criteria.

Results: Quantified reperfusion within the infarction core differed significantly in patients with complete and incomplete recanalization. In the ROC analysis for the prediction of parenchymal hematoma (PH), ASL reperfusion vol-25 had the highest area under the curve (AUC) when compared with ASL vol-50 and ASL vol-75. ASL reperfusion vol-25 had significantly higher AUC compared with ADC threshold volume in the prediction of PH (0.783 vs. 0.685, P = 0.0036) and PH-2 (0.844 vs. 0.754, P = 0.0035). In stepwise multivariate logistic regression analysis, only ASL reperfusion vol-25 emerged as an independent predictor of PH (OR = 3.51, 95% CI: 1.65–7.45, P < 0.001) and PH-2 (OR = 2.32, 95% CI: 1.13–4.76, P = 0.022).

Conclusions: Increased reperfusion volume within severely damaged brain tissue is associated with the occurrence of higher grade of HT.

Temporal evolution and spatial distribution of quantitative T2 MRI following acute ischemia reperfusion injury

Background

Determining mechanisms of secondary stroke injury related to cerebral blood flow and the severity of microvascular injury contributing to edema and blood-brain barrier breakdown will be critical for the development of adjuvant therapies for revascularization treatment.

Aim

To characterize the heterogeneity of the ischemic lesion using quantitative T2 imaging along with diffusion-weighted magnetic resonance imaging (DWI) within five hours of treatment.

Methods

Quantitative T2 magnetic resonance imaging was acquired within 5 h (baseline) and at 24 h (follow-up) of stroke treatment in 29 patients. Dynamic contrast enhanced permeability imaging was performed at baseline in a subgroup of patients. Absolute volume change and lesion percent change was determined for the quantitative T2, DWI, and absolute volume change sequences. A Gaussian process with RRELIEFF feature selection algorithm was used for prediction of relative quantitative T2 and DWI lesion growth, baseline and follow-up quantitative T2/DWI lesion ratios, and also NIHSS at 24 h and change in NIHSS from admission to 24 h.

Results

In n = 27 patients, median (interquartile range) lesion percent change was 114.8% (48.9%, 259.1%) for quantitative T2, 48.2% (−12.6%, 179.6%) for absolute volume change, and 62.7% (26.3%, 230.9%) for DWI, respectively. Our model, consisting of baseline NIHSS, CT ASPECTS, and systolic blood pressure, showed a strong correlation with quantitative T2 percent change (cross correlation R2 = 0.80). There was a strong predictive ability for quantitative T2/DWI lesion ratio at 24 h using baseline NIHSS and last seen normal to 24 h magnetic resonance imaging time (cross correlation R2 = 0.93). Baseline dynamic contrast enhanced permeability was moderately correlated to the baseline quantitative T2 values (rho = 0.38).

Conclusion

Quantitative T2 imaging provides critical information for development of therapeutic approaches that could ameliorate microvascular damage during ischemia reperfusion.

Statistical Analysis of the Apparent Diffusion Coefficient in Patients with Clinically Mild Encephalitis/Encephalopathy with a Reversible Splenial Lesion Indicates That the Pathology Extends Well beyond the Visible Lesions

Purpose:

To investigate whether the genu of the corpus callosum is involved in patients with clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) type I.

Methods:

Twenty-three cases of clinically confirmed MERS I were analyzed retrospectively, and MRI features of the lesion were observed. The apparent diffusion coefficient (ADC) values of the same region of interests in lesions at the splenium and genu of the corpus callosum were measured before and after treatment (i.e., four groups), and the average ADC values were calculated. Paired t-tests were used to compare the ADC values of lesions in the splenium and genu before and after treatment. Independent sample t-tests were used to compare the values in the splenium and genu after treatment.

Results:

The mean ADC values of the splenium before and after treatment were 0.448 ± 0.124 and 0.790 ± 0.070 × 10⁻³ mm²/s, respectively, showing significant difference (P < 0.01). The mean ADC values in the genu before and after treatment were 0.783 ± 0.067 and 0.829 ± 0.070 × 10⁻³ mm²/s, respectively, also showing significant difference (P < 0.01). There was no significant difference in the ADC values between the splenium and genu after treatment (P > 0.05).

Conclusion:

The genu showed a slight restriction in diffusion in the acute stage of type I MERS. After treatment, this diffusion restriction diminished as it typically does in the splenium. Our results indicate that the pathology in MERS extends well beyond the visible lesions.

MR Perfusion to Determine the Status of Collaterals in Patients with Acute Ischemic Stroke: A Look Beyond Time Maps

BACKGROUND AND PURPOSE

Patients with acute stroke with robust collateral flow have better clinical outcomes and may benefit from endovascular treatment throughout an extended time window. Using a multiparametric approach, we aimed to identify MR perfusion parameters that can represent the extent of collaterals, approximating DSA.

MATERIALS AND METHODS

Patients with anterior circulation proximal arterial occlusion who had baseline MR perfusion and DSA were evaluated. The volume of arterial tissue delay (ATD) at thresholds of 2-6 seconds (ATD^{2-6 seconds}) and >6 seconds (ATD^{>6 seconds}) in addition to corresponding values of normalized CBV and CBF was calculated using VOI analysis. The association of MR perfusion parameters and the status of collaterals on DSA were assessed by multivariate analyses. Receiver operating characteristic analysis was performed.

RESULTS

Of 108 patients reviewed, 39 met our inclusion criteria. On DSA, 22/39 (56%) patients had good collaterals. Patients with good collaterals had significantly smaller baseline and final infarct volumes, smaller volumes of severe hypoperfusion (ATD^{>6 seconds}), larger volumes of moderate hypoperfusion (ATD^{2-6 seconds}), and higher relative CBF and relative CBV values than patients with insufficient collaterals. Combining the 2 parameters into a Perfusion Collateral Index (volume of ATD^{2-6 seconds} × relative CBV^{2-6 seconds}) yielded the highest accuracy for predicting collateral status: At a threshold of 61.7, this index identified 15/17 (88%) patients with insufficient collaterals and 22/22 (100%) patients with good collaterals, for an overall accuracy of 94.1%.

CONCLUSIONS

The Perfusion Collateral Index can predict the baseline collateral status with 94% diagnostic accuracy compared with DSA.

Review : Mapping Brain Pathophysiology and Higher Cortical Function with Magnetic Resonance Imaging

Advances in magnetic resonance imaging (MRI) have moved the technology beyond its application solely as a diagnostic test to become a tool for addressing questions of in vivo pathophysiology and higher cortical function in humans. Diffusion-weighted MRI measures the apparent rate of translational movement of water molecules through brain parenchyma. This measurement can be used to determine axonal orientation within white matter, to define regions of tissue edema, and to permit early identification of ischemic neuronal injury related to impairment of Na+-K +-ATPase activity in experimental and human stroke. Changes in various aspects of cerebral perfusion—blood volume, blood flow, and hemoglobin oxygen saturation—can be mea sured with MRI, and altered cerebrovascular circulation and regional brain activation can thereby be inves tigated. Echo planar imaging is a method of ultrafast data acquisition with MRI—individual images are ac quired on the order of 100 msec. Echo planar imaging makes diffusion and perfusion measurements more practicable for diverse applications and allows for the study of temporal characteristics of regional brain responses to stimuli. Diffusion and perfusion MRI, generally termed functional MRI, are tools for studying in vivo brain physiology with MRI and are being applied to a broad range of questions in neuroscience. The Neuroscientist 1:221-235, 1995

Clinical and Neuroradiological Spectrum of Metronidazole Induced Encephalopathy: Our Experience and the Review of Literature

Metronidazole is an antimicrobial agent mainly used in the treatment of several protozoal and anaerobic infections, additionally, is often used in hepatic encephalopathy and Crohn disease. Apart from peripheral neuropathy, metronidazole can also cause symptoms of central nervous system dysfunction like ataxic gait, dysarthria, seizures, and encephalopathy which may result from both short term and chronic use of this drug and is collectively termed as "metronidazole induced encephalopathy"(MIE). Neuroimaging forms the backbone in clinching the diagnosis of this uncommon entity, especially in cases where there is high index of suspicion of intoxication. Although typical sites of involvement include cerebellum, brain stem and corpus callosum, however, lesions of other sites have also been reported. Once diagnosed, resolution of findings on Magnetic Resonance Imaging (MRI) of the Brain along with clinical improvement remains the mainstay of monitoring. Here we review the key clinical features and MRI findings of MIE as reported in medical literature. We also analyze implication of use of this drug in special situations like hepatic encephalopathy and brain abscess and discuss our experience regarding this entity.

White matter ischemic changes in hyperacute ischemic stroke: voxel-based analysis using diffusion tensor imaging and MR perfusion

Background and Purpose—

The purpose of this study was to evaluate changes in fractional anisotropy (FA), as measured by diffusion tensor imaging, of white matter (WM) infarction and hypoperfusion in patients with acute ischemic stroke using a quantitative voxel-based analysis.

Methods—

In this prospective study, diffusion tensor imaging and dynamic susceptibility contrast perfusion sequences were acquired in 21 patients with acute ischemic stroke who presented within 6 hours of symptom onset. The coregistered FA, apparent diffusion coefficient, and dynamic susceptibility contrast time to maximum (Tmax) maps were used for voxel-based quantification using a region of interest approach in the ipsilateral affected side and in the homologous contralateral WM. The regions of WM infarction versus hypoperfusion were segmented using a threshold method. Data were analyzed by regression and ANOVA.

Results—

There was an overall significant mean difference (P<0.001) for the apparent diffusion coefficient, Tmax, and FA values between the normal, hypoperfused, and infarcted WM. The mean±SD of FA was significantly higher (P<0.001) in hypoperfused WM (0.397±0.019) and lower (P<0.001) in infarcted WM (0.313±0.037) when compared with normal WM (0.360±0.020). Regression tree analysis of hypoperfused WM showed the largest mean FA difference at Tmax above versus below 5.4 s with a mean difference of 0.033 (P=0.0096).

Conclusions—

Diffusion tensor imaging-FA was decreased in regions of WM infarction and increased in hypoperfused WM in patients with hyperacute acute ischemic stroke. The significantly increased FA values in the hypoperfused WM with Tmax≥5.4 s are suggestive of early ischemic microstructural changes.

Infarct volume predicts critical care needs in stroke patients treated with intravenous thrombolysis

INTRODUCTION

Patients receiving intravenous thrombolysis with recombinant tissue plasminogen activator (IVT) for ischemic stroke are monitored in an intensive care unit (ICU) or a comparable unit capable of ICU interventions due to the high frequency of standardized neurological exams and vital sign checks. The present study evaluates quantitative infarct volume on early post-IVT MRI as a predictor of critical care needs and aims to identify patients who may not require resource intense monitoring.

METHODS

We identified 46 patients who underwent MRI within 6 h of IVT. Infarct volume was measured using semiautomated software. Logistic regression and receiver operating characteristics (ROC) analysis were used to determine factors associated with ICU needs.

RESULTS

Infarct volume was an independent predictor of ICU need after adjusting for age, sex, race, systolic blood pressure, NIH Stroke Scale (NIHSS), and coronary artery disease (odds ratio 1.031 per cm(3) increase in volume, 95% confidence interval [CI] 1.004-1.058, p = 0.024). The ROC curve with infarct volume alone achieved an area under the curve (AUC) of 0.766 (95% CI 0.605-0.927), while the AUC was 0.906 (95% CI 0.814-0.998) after adjusting for race, systolic blood pressure, and NIHSS. Maximum Youden index calculations identified an optimal infarct volume cut point of 6.8 cm(3) (sensitivity 75.0%, specificity 76.7%). Infarct volume greater than 3 cm(3) predicted need for critical care interventions with 81.3% sensitivity and 66.7% specificity.

CONCLUSION

Infarct volume may predict needs for ICU monitoring and interventions in stroke patients treated with IVT.

Quantitative analysis of hypoperfusion in acute stroke: arterial spin labeling versus dynamic susceptibility contrast

BACKGROUND AND PURPOSE

This study compares the concordance between arterial spin labeling (ASL) and dynamic susceptibility contrast (DSC) for the identification of regional hypoperfusion and diffusion-perfusion mismatch tissue classification using a quantitative method.

METHODS

The inclusion criteria for this retrospective study were as follows: patients with acute ischemic syndrome with symptom onset <24 hours and acquisition of both ASL and DSC MR perfusion. The volumes of infarction and hypoperfused lesions were calculated on ASL and DSC multi-parametric maps. Patients were classified into reperfused, matched, or mismatch groups using time to maximum >6 sec as the reference. In a subset of patients who were successfully recanalized, the identical analysis was performed and the infarction and hypoperfused lesion volumes were used for paired pre- and posttreatment comparisons.

RESULTS

Forty-one patients met our inclusion criteria. Twenty patients underwent successful endovascular revascularization (TICI>2a), resulting in a total of 61 ASL-DSC data pairs for comparison. The hypoperfusion volume on ASL-cerebral blood flow best approximated the DSC-time to peak volume (r=0.83) in pretreatment group and time to maximum (r=0.46) after recanalization. Both ASL-cerebral blood flow and DSC-TTP overestimated the hypoperfusion volume compared with time to maximum volume in pretreatment (F=27.41, P<0.0001) and recanalized patients (F=8.78, P<0.0001).

CONCLUSIONS

ASL-cerebral blood flow overestimates the DSC time to maximum hypoperfusion volume and mismatch classification in patients with acute ischemic syndrome. Continued overestimation of hypoperfused volume after recanalization suggests flow pattern and velocity changes in addition to arterial transit delay can affects the performance of ASL.

Periprocedural arterial spin labeling and dynamic susceptibility contrast perfusion in detection of cerebral blood flow in patients with acute ischemic syndrome

BACKGROUND AND PURPOSE

To compare the diagnostic performance of arterial spin-labeling (ASL) and dynamic susceptibility contrast (DSC) perfusion in detecting cerebral blood flow (CBF) changes before and after endovascular recanalization in acute ischemic syndrome.

METHODS

The inclusion criteria for this retrospective study were patients with acute ischemic syndrome who underwent endovascular recanalization and acquisition of both ASL and DSC before and after revascularization. ASL-CBF and multiparametric DSC maps were evaluated for image quality, location, and type of perfusion abnormality. Relative CBF (rCBF) was calculated in the infarction core and hypoperfused areas using coregistered ASL and DSC. Core and hypoperfused rCBF were used for paired pretreatment and posttreatment comparisons. Interobserver and intermodality agreement were evaluated by κ test, and t test was calculated for ASL and DSC rCBF values.

RESULTS

Twenty-five patients met our inclusion criteria. Five studies were rated nondiagnostic, resulting in 45 pairs of DSC-ASL available for comparison. ASL and DSC agreed on type and location of the perfusion abnormality in 71% and 80% of cases, respectively. The image quality of ASL was lower than DSC, resulting in interobserver variability for the type (κ=0.45) and location (κ=0.56) of perfusion abnormality. ASL was unable to show any type of perfusion abnormality in 11% of patients. In successfully recanalized patients, hyperperfusion (rCBF >1) was detected in 100% on DSC and 47% on ASL.

CONCLUSIONS

ASL is less sensitive than DSC for detecting rCBF changes in patients with acute ischemic syndrome, particularly with respect to hyperperfusion after successful recanalization.

Sodium MRI and the assessment of irreversible tissue damage during hyper-acute stroke

Sodium MRI (sMRI) has undergone a tremendous amount of technical development during the last two decades that makes it a suitable tool for the study of human pathology in the acute setting within the constraints of a clinical environment. The salient role of the sodium ion during impaired ATP production during the course of brain ischemia makes sMRI an ideal tool for the study of ischemic tissue viability during stroke. In this paper, the current limitations of conventional MRI for the determination of tissue viability during evolving brain ischemia are discussed. This discussion is followed by a summary of the known findings about the dynamics of tissue sodium changes during brain ischemia. A mechanistic model for the explanation of these findings is presented together with the technical requirements for its investigation using clinical MRI scanners. An illustration of the salient features of the technique is also presented using a nonhuman primate model of reversible middle cerebral artery occlusion.

Visualizing cell death in experimental focal cerebral ischemia: promises, problems, and perspectives

One of the hallmarks of stroke pathophysiology is the widespread death of many different types of brain cells. As our understanding of the complex disease that is stroke has grown, it is now generally accepted that various different mechanisms can result in cell damage and eventual death. A plethora of techniques is available to identify various pathological features of cell death in stroke; each has its own drawbacks and pitfalls, and most are unable to distinguish between different types of cell death, which partially explains the widespread misuse of many terms. The purpose of this review is to summarize the standard histopathological and immunohistochemical techniques used to identify various pathological features of stroke. We then discuss how these methods should be properly interpreted on the basis of what they are showing, as well as advantages and disadvantages that require consideration. As there is much interest in the visualization of stroke using noninvasive imaging strategies, we also specifically discuss how these techniques can be interpreted within the context of cell death.

A case of reversible splenial lesions in late postpartum preeclampsia

There have been reports of various etiologies associated with reversible splenial lesions manifested as obviously restricted diffusion. We describe for a 27-year-old woman suffering from late postpartum preeclampsia associated with reversible splenial lesions. This association has not been reported to date. Awareness that profound late postpartum preeclampsia can rarely result in abnormalities of the splenium of the corpus callosum (SCC) may help clinicians avoid unnecessary diagnostic procedures and therapeutic interventions. Moreover, it must be stated that prediction of the clinical course based on DWI and ADC values should be used with caution.

DWI Reversibility after Intra-Arterial Thrombolysis. A Case Report and Literature Review

We report our case and review the literature on reversal DWI lesions, ADC thresholds and correlation between DWI lesion and outcome measured with clinical scales. A 30-years old woman was admitted to our hospital 18 hours after stroke onset. Considering the absence of alterations on CT and the worsening of symptomatology, the patient underwent MRI, which showed a slightly hyperintense signal in FLAIR images in the left portion of the pons and midbrain and a more evident bilateral DWI hyperintensity of the pons. The patient was treated with mechanical and pharmacological intra-arterial thrombolysis. The patient showed a rapid improvement of symptoms. Two weeks after the treatment her clinical conditions were characterized by a residual right hemiparesis and complete recovery of right motility, respiratory and swallowing difficulties. MR examination demonstrated a slight signal alteration of the pons left hemiportion and a disappearance of the mesencephalic signal alteration and of the right portion of the pons. DWI lesions represent irreversibly damaged tissue but new evidence suggests that DWI lesions may be reversible, especially with reperfusion, by now well demonstrated in animal models. Therefore acute DWI lesions probably contain not only irreversibly injured tissue but also parts of the penumbra. The debate on the capability of ADC maps to discriminate irreversibly from reversibly damaged tissue is a matter of controversy. ADC values in human stoke are not an independent indicator of tissue viability. The use of thresholds may improve reproducibility but not validity.

Reversible post-traumatic bilateral extensive restricted diffusion of the brain. A case study and review of the literature

PRIMARY OBJECTIVE

To increase the knowledge about diffuse traumatic brain injury (TBI) by reporting the magnetic resonance imaging (MRI) findings observed in a patient with reversible extensive restricted diffusion of the brain at diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps.

CASE STUDY

An 18-year-old patient was admitted after high-energy closed TBI. Glasgow Coma Scale score was 4. Head computed tomography showed small left frontal and temporal haemorrhagic contusions and a small haemorrhage in the left thalamus. Ten days later, brain MRI showed diffuse high-signal intensity on T2-weighted images and DWI and restricted diffusion in the subcortical white matter of both centri semiovali, genu and splenium of corpus callosum and parietal cortex bilaterally (mean ADC value = 0.434-0.811 x 10(-3) mm(2) s(-1)). Eleven days later, follow-up brain MRI showed gliotic changes in the left splenium of corpus callosum, a clearcut decrease of T2-weighted high-signal intensity and resolution of abnormalities at DWI and ADC maps in all other involved sites. This was confirmed 36 days later. Three months later, the patient did not show neurological, cognitive or neuropsychiatric deficits.

CONCLUSIONS

In the patient reported herein, closed TBI most likely induced diffuse excitotoxic injury of the brain which resulted in mainly reversible cytotoxic or intramyelinic oedema.

Radiological and histological changes following cerebral venous sinus thrombosis in a rat model

Cerebral venous sinus thrombosis (CVST) is an important cause of stroke in young especially after pregnancy. We induced CVST in rat by topical application of ferric chloride over the exposed superior sagittal sinus (SSS) and pathological changes were monitored on days 1, 2 and 7. Thrombus weight was estimated and H&E staining was performed for comparison with MRI data. T2 weighted signal changes were present in all the study group rats. T2 hyper intensity decreased over the time. In study group, the apparent diffusion coefficient (ADC) was decreased by 31.94% on day 1 compared to the controls. There was 15.5% increase in the ADC value on day 2 compared to day 1 which reached almost equal to control value on day 7. On histological evaluation, neuronal necrosis and cellular infiltration were observed in cortical region after thrombosis. The early decrease in ADC could be attributed to cytotoxic edema that precedes vasogenic edema indicated by normalization of ADC and a decreased T2 hyper intensity. In conclusion, ferric chloride induced CVST in the rat produces cytotoxic edema in early stage followed by vasogenic edema which is related to recanalization of the superior sagittal sinus.

Intravenous polyethylene glycol successfully treats severe acceleration-induced brain injury in rats as assessed by magnetic resonance imaging

OBJECTIVE

Polyethylene glycol (PEG) is a nontoxic molecule with known efficacy as a cell membrane sealant, improving histological and behavioral outcomes in trauma models. Diffusion-weighted (DW) magnetic resonance imaging (MRI) is the most sensitive method of detecting in vivo diffuse axonal injury (DAI), where a decreased apparent diffusion coefficient (ADC) of water reflects cytotoxic edema. We use DW-MRI to assess severe DAI in rats treated with a single acute postinjury injection of PEG.

METHODS

Rats were divided into uninjured, injured saline-treated, and injured PEG-treated groups. Injury groups received a severe brain injury using an impact-acceleration weight-drop model. Saline or PEG was administered acutely as a single intravenous dose to injured saline-treated and injured PEG-treated groups, respectively. DW-MRI analysis was performed at postinjury day 7 with a 9.4-T magnet. ADC was calculated for cortex, corpus callosum/hippocampus, and thalamus in each group.

RESULTS

An expected decrease in ADC, representing cytotoxic edema, was observed in the injured saline-treated group. The injured PEG-treated group demonstrated no decrease in ADC relative to the uninjured rats, and the difference between ADC in saline and PEG-treated groups reached significance for all 3 zones of assessed brain. Differences were seen grossly between injured saline-treated and injured PEG-treated groups on representative color-mapped ADC images.

CONCLUSION

A single intravenous dose of PEG dramatically limits sequelae of severe acceleration-induced brain injury--in this case, assessed by cytotoxic edema on DW-MRI--by intervening at the primary injury level of neuronal membrane disruption. This outcome is unprecedented, as no prior treatments for DAI have demonstrated similar efficacy. DAI treatment with intravenous PEG may have future clinical relevance and warrants further investigation.

Fat-saturated diffusion-weighted imaging of the rat pelvis using three-dimensional MP-RAGE MR sequence

In this work we report on the development of a novel technique for fat-saturated three-dimensional (3D) diffusion-weighted (DW) MRI sequence based upon 3D magnetization-prepared rapid gradient-echo (3D-MP-RAGE). In order to saturate fat, two kinds of procedures were competed CHESS-DW-3D-MP-RAGE sequence (CHESS-3D-DWI) and DW-3D-MP-WE-RAGE sequence (WE-3D-DWI) "chemical shift selective: CHESS method vs. water-excitation: WE method". The CHESS-3D-DWI sequence and WE-3D-DWI sequence were compared in terms of their degree of fat suppression. In CHESS-3D-DWI sequence a preparation phase with a "CHESS-90 degrees RF-motion probing gradient: MPG-180 degrees RF-MPG-90 degrees RF" pulse-train was used to sensitize the magnetization to fat-saturated diffusion. In contrast, WE-3D-DWI sequence a RAGE-excitation pulse with a "binominal-pulse 1-1 or 1-2-1" was selected to water-excited (fat-saturated) diffusion imaging. These imaging were done during in vivo studies using an animal experiment. From experimental results obtained with a phantom, the effect of diffusion weighting and the effect of fat-saturation were confirmed. Fat-saturation was much better in the WE-3D-DWI sequence than CHESS-3D-DWI sequence. From rat experimental results, fat-saturated diffusion-weighted image data were obtained. This sequence was useful for in vivo imaging.

A modelling approach to explore some hypotheses of the failure of neuroprotective trials in ischemic stroke patients

Ischemic stroke is the third cause of death in industrialised countries, but no satisfactory treatment is currently available. The hundreds of neuroprotective drugs developed to block the ischemic cascade gave very promising results in animal models but the clinical trials performed with these drugs showed no beneficial effects in stroke patients. Many hypotheses were advanced to explain this discrepancy, among which the morphological and functional differences between human and rodent brains. This discrepancy could be partly due to the differences in white matter and glial cell proportions between human and rodent brains. In order to test this hypothesis, we built a mathematical model of the main early pathophysiological mechanisms of stroke in rodent and in human brains. This model is a two-scale model and relies on a set of ordinary differential equations. We built two versions of this model (for human and rodent brains) differing in their white matter and glial cell proportions. Then, we carried out in silico experiments with various neuroprotective drugs. The simulation results obtained with a sodium channel blocker show that the proportion of penumbra recovery is much higher in rodent than in human brain and the results are similar with some other neuroprotective drugs tested during phase III trials. This in silico investigation suggests that the proportions of glial cells and white matter have an influence on neuroprotective drug efficacy. It reinforces the hypothesis that histological and morphological differences between rodent and human brains can partly explain the failure of these agents in clinical trials.

Neuroprotective Effects of the Ras Inhibitor S-Trans-Trans-Farnesylthiosalicylic Acid, Measured by Diffusion-Weighted Imaging after Traumatic Brain Injury in Rats

Ras proteins play a role in receptor-mediated signaling pathways and are activated after traumatic brain injury. S-trans-trans-farnesylthiosalicylic acid (FTS), a synthetic Ras inhibitor, acts primarily on the active, GTP-bound form of Ras and was shown to improve neurobehavioral outcome after closed head injury (CHI) in mice. To gain a better understanding of the neuroprotective mechanism of FTS, we used diffusion-weighted imaging (DWI) in a rat model of CHI. Apparent diffusion coefficients (ADC) and transverse relaxation times (T2) were measured in injured rat brains after treatment with vehicle or FTS (5 mg/kg). Neuroprotection by FTS was also assessed in terms of the neurological severity score. One week after injury, significantly better recovery was observed in the FTS-treated rats than in the controls (p = 0.0191). T2 analysis of the magnetic resonance images revealed no differences between the two groups. In contrast, they differed significantly in ADC, particularly at 24 h post-CHI (p < 0.05): in the vehicle-treated rats ADC had decreased to approximately 26% below baseline, whereas it had increased to about 10% above baseline in the FTS-treated rats. As the magnitude of ADC reduction is strongly linked to blood perfusion deficit, these results suggest that the neuroprotective mechanism of FTS might be related to an improvement in cerebral perfusion. We propose that FTS, which is currently being tested in humans for anti-cancer indications, should also be considered as a new strategy for the management of head injury.

Clinicoradiological factors influencing the reversibility of posterior reversible encephalopathy syndrome: a multicenter study

PURPOSE

The aim of this retrospective study was to clarify the relation between the reversibility of posterior reversible encephalopathy syndrome (PRES) with three factors: the anatomical region of the brain involved, the background clinical cause, and the diffusion weighted image (DWI) intensity of PRES lesions.

MATERIAL AND METHODS

This multicenter study, conducted by the PRES Study Group of the Neuroradiology Workshop, involved 52 cases from 28 institutions. Initial and follow-up magnetic resonance imaging were compared regarding the reversibility of PRES lesions according to anatomical location and clinical background. Initial DWI and apparent diffusion coefficient (ADC) maps were reviewed in 20 cases.

RESULTS

Reversibility was significantly lower (P < 0.01) in the brain stem (44%) and deep white matter (47%) compared to the other cortical and subcortical areas (76%-91%). The reversibility was greater in the eclampsia subgroup followed by the hypertension and chemotherapy subgroups. DWI, even with ADC maps, had limitations in predicting the outcome of PRES lesions.

CONCLUSION

The typical cortical and subcortical PRES lesions showed reversibility, whereas the brain stem and deep white matter lesions showed less reversibility. PRES due to eclampsia showed maximum reversibility compared to hypertension- and drug-related PRES. DWI, even with ADC maps, had limitations in predicting the course of PRES.

Diffusion-Weighted Imaging in Acute Stroke

Diffusion MR imaging has improved evaluation of acute ischemic stroke vastly. It is highly sensitive and specific in the detection of infarction at early time points when CT and conventional MR sequences are unreliable. The initial DWI lesion is believed to represent infarction core and usually progresses to infarction unless there is early reperfusion. The initial DWI lesion volume and ADC ratios correlate highly with final infarction volume and with acute and chronic neurologic assessment tests. ADC values may be useful in differentiating tissue destined to infarct from that potentially salvageable with reperfusion therapy. ADC values also may be useful for determining tissue at risk of HT after reperfusion therapy. DTI can quantify differences in the responses of gray versus white matter to ischemia. FA may be important in determining stroke onset time, and tractography provides early detection of wallerian degeneration that may be important in determining prognosis. Finally, DWI can determine which patients who have TIA are at risk for subsequent large vessel infarction and can differentiate stroke from stroke mimics. With improvements in MR software and hardware, diffusion MR undoubtedly will continue to improve the management of patients who have acute stroke.

In vivo isotropic 3D diffusion tensor mapping of the rat brain using diffusion-weighted 3D MP-RAGE MRI

The purpose of this study was to examine the potential of diffusion-weighted (DW) three-dimensional (3D) MP-RAGE MRI for diffusion-tensor mapping of the rat brain in vivo. A DW-3D-MP-RAGE (3D-DWI) sequence was implemented at 2.0 T using six gradient orientations and a b value of 1000 s/mm2. In this sequence, the preparation sequence with a "90 degrees RF-motion proving gradient (MPG): MPG-180 degrees RF-MPG-90 degrees RF" pulse train (DW driven equilibrium Fourier transform) was used to sensitize the magnetization to diffusion. A centric k-space acquisition order was necessary to minimize saturation effects (T1 contamination) from tissues with short relaxation time. The image matrix was 128x128x128 (interpolated from 64x64x64 acquisitions), which resulted in small isotropic DW image data (voxel size: 0.273x0.273x0.273 mm3). Moreover, 3D-DWI-derived maps of the fractional anisotropy (FA), relative anisotropy (RA) and main-diffusion direction were completely free of susceptibility-induced signal losses and geometric distortions. Two well-known commissural fibers, the corpus callosum and anterior commissure, were indicated and shown to be in agreement with the locations of these known stereotaxic atlases. The experiment took 45 min, and shorter times should be possible in clinical application. The 3D-DWI sequence allows for in vivo 3D diffusion-tensor mapping of the rat brain without motion artifacts and susceptibility to distortion.

Diffusion weighted imaging of cerebellar lesions in Wernicke’s encephalopathy

We report unusual findings on MR imaging in a 62-year-old woman with Wernicke's encephalopathy (WE). Initial fluid-attenuated inversion recovery (FLAIR) and diffusion weighted MR imaging (DW-MRI) showed hyperintense lesions in the cerebellum and medial thalami, with a decreased apparent diffusion coefficient (ADC) in the cerebellum (reduced by 45%). After thiamine supplementation, the T2 and diffusion hyperintensities disappeared. However, clinical examination at three months showed persistent cerebellar impairment. The importance of the ADC values should be further investigated.

Map-ISODATA demarcates regional response to combination rt-PA and 7E3 F(ab')2 treatment of embolic stroke in the rat

PURPOSE

To investigate the ability of map-ISODATA (Iterative Self-Organizing Data Analysis Technique) to classify the different categories of ischemic damage in the lesion and to evaluate a combined (thrombolysis plus antiplatelet) treatment efficacy in an embolic stroke of rat.

MATERIALS AND METHODS

Rats subjected to embolic stroke with (N=12) and without (N=10) rt-PA and 7E3 F(ab')2 treatment (4 hours after embolization) were followed (at 2, 24, and 48 hours post-MCAO) with magnetic resonance imaging (MRI) using T1, T2, and apparent diffusion coefficient of water (ADCw). ISODATA was computed from T1, T2, and ADCw maps. The signatures characterized by the map-ISODATA were compared with histological quantitative evaluation and were employed to demarcate the specific regions in the lesion.

RESULTS

The signature described by map-ISODATA is highly correlated with the degree of tissue damage in the lesion and can distinguish the severity of ischemic tissue injury. Based upon map-ISODATA, ischemic lesion area can be divided into three specific regions, each characterized by a distinct evolution of injury and treatment response. The combined treatment significantly reduces the lesion size between 24 and 48 hours and improves the outcome 48 hours post-MCAO compared with the control group.

CONCLUSION

Map-ISODATA provides an accurate means to identify lesion area, to distinguish ischemic damage, and to detect treatment response. 7E3 F(ab')2 extends the rt-PA treatment window to at least four hours after the onset of embolic stroke of rat.

Diffusion-Weighted Imaging in Acute Stroke

In magnetic resonance diffusion-weighted imaging (DWI), regions of the brain are depicted not only on the basis of physical properties, such as T2 relaxation and spin density, which influence image contrast in conventional MR imaging, but also by local characteristics of water molecule diffusion. The diffusion of water molecules is altered in a variety of disease processes, including ischemic stroke. The changes that occur in acute infarction enable DWI to detect very early ischemia. Also, because predictable progression of diffusion findings occurs during the evolution of ischemia, DWI enables more precise estimation of the time of stroke onset than does conventional imaging.

Diffusion-weighted three-dimensional MP-RAGE MR imaging

The advantages of three-dimensional (3D) acquisition are that you obtain thinner and more slices with better profiles, and better signal-to-noise ratio for an equivalent slice thickness. Three-dimensional acquisition is preferable for obtaining contiguous thin-slice MR images. However, the acquisition time extends compared with the two-dimensional acquisition because the second phase-encode axis is applied by the 3D acquisition. Therefore, 3D acquisition should be a high-speed imaging method. In this paper, a new diffusion-sensitive 3D magnetization-prepared rapid gradient-echo (3D MP-RAGE) sequence was studied. In this sequence, a preparation phase with a 90 degrees RF-motion proving gradient (MPG): MPG-180 degrees RF-MPG-90 degrees RF pulse train (diffusion-weighted driven-equilibrium Fourier transform) was used to sensitize the magnetization to diffusion. Centric k-space acquisition order is necessary to minimize saturation effects from tissues with short relaxation times. From phantom experimental results, the effect of the diffusion weighting was changed by the centric vs. sequential k-space acquisition order. The effect of centric k-space acquisition order was larger than the effect of sequential k-space acquisition order. The contrast of centric k-space acquisition order became equal to the contrast of conventional diffusion-weighted spin echo. From rat experimental results, small isotropic diffusion-weighted image data (voxel size: 0.625 x 0.625 x 0.625 mm3) were obtained. This sequence was useful in vivo.

Simultaneous phase correction and SENSE reconstruction for navigated multi-shot DWI with non-cartesian k-space sampling

Phase-navigated multi-shot acquisition and parallel imaging are two techniques that have been applied to diffusion-weighted imaging (DWI) to diminish distortions and to enhance spatial resolution. Specifically, sensitivity encoding (SENSE) has been combined with single-shot echo planar imaging (EPI). Thus far, it has been difficult to apply parallel imaging methods, like SENSE, to multi-shot DWI because motion-induced phase error varies from shot to shot and interferes with sensitivity encoding. Although direct phase subtraction methods have been introduced to correct this phase error, they generally are not suitable for SENSE reconstruction, and they cannot remove all the motion artifacts even if the phase error is fully known. Here, an effective algorithm is proposed to correct the motion-induced phase error using an iterative reconstruction. In this proposed conjugate-gradient (CG) algorithm, the phase error is treated as an image encoding function. Given the complex perturbation terms, diffusion-weighted images can be reconstructed using an augmented sensitivity map. The mathematical formulation and image reconstruction procedures of this algorithm are similar to the SENSE reconstruction. By defining a dynamic composite sensitivity, the CG phase correction method can be conveniently incorporated with SENSE reconstruction for the application of multi-shot SENSE DWI. Effective phase correction and multi-shot SENSE DWI (R = 1 to 3) are demonstrated on both simulated and in vivo data acquired with PROPELLER and SNAILS.

The time course of ischemic damage and cerebral perfusion in a rat model of space-occupying cerebral infarction

We aimed to establish a rat model of space-occupying hemispheric infarction to evaluate potential treatment strategies. For adequate timing of therapy in future experiments, we studied the development of tissue damage, edema formation, and perfusion over time with different MRI techniques. Permanent middle cerebral artery (MCA) occlusion was performed in 32 Fisher-344 rats. Forty-six MRI experiments including diffusion weighted (DW), T2-weighted (T2W), flow-sensitive alternating inversion recovery (FAIR) perfusion-weighted, and T1-weighted (T1W) imaging before and after gadolinium were performed at 1, 3, 8, 16, 24, and 48 h of ischemia. MCA occlusion consistently led to infarction of the complete MCA territory. Mortality was 75%. Lesion volumes as derived from apparent diffusion coefficient (ADC) and T2 maps increased to maximum values of 400+/-48 mm3 at 24 h and 420+/-54 mm3 at 48 h of ischemia, respectively. Midline shift peaked at 24 h. The area with diffusion-perfusion deficit decreased to a minimum at 24 h after onset of ischemia and perfusion of the contralateral hemisphere dropped at the same time point. Leakage of gadolinium through the blood-brain barrier in the entire infarct occurred within 3 h of ischemia. Permanent intraluminal MCA occlusion in Fisher-344 rats is an adequate model for space-occupying cerebral infarction. Rats may benefit from intervention aimed at reducing tissue shift and intracranial pressure (ICP), and at improving cerebral blood flow, if initiated before 24 h after MCA occlusion. The value of treatment modalities depending on an intact blood-brain barrier should be questioned.

Nuclear magnetic resonance (NMR) measurement of the apparent diffusion coefficient (ADC) of tissue water and its relationship to cell volume changes in pathological states

Diffusion-weighted nuclear magnetic resonance (NMR) imaging (DWI) is sensitive to the random translational motion of water molecules due to Brownian motion. Although the mechanism is still not completely understood, the cellular swelling that accompanies cell membrane depolarization results in a reduction in the net displacement of diffusing water molecules and thus a concomitant reduction in the apparent diffusion coefficient (ADC) of tissue water. Cerebral regions of reduced ADC appear hyperintense in a DWI and this technique has been used extensively to study acute stroke. In addition to cerebral ischemia, reductions in the ADC of cerebral water have been observed following cortical spreading depression, ischemic depolarizations (IDs), transient ischemic attack (TIA), status epilepticus, and hypoglycemia. Although the mechanism responsible for initiating membrane depolarization varies in each case, the ensuing cell volume changes follow a similar pattern. Water ADC values are also affected by the presence and orientation of barriers to translational motion (such as cell membranes and myelin fibers) and thus NMR measures of anisotropic diffusion are sensitive to more chronic pathological states where the integrity of these structures is modified by disease. Both theoretical prediction and experimental evidence suggest that the ADC of tissue water is related to the volume fraction of the interstitial space via the electrical conductivity of the tissue. The implication is that acute neurological disorders that exhibit electrical conductivity changes should also exhibit ADC changes that are detectable by DWI. A qualitative correlation between electrical conductivity and the ADC of water has been demonstrated in a number of animal model studies and the results indicate that reduced ADC values are associated with reductions in the extracellular volume fraction and increased extracellular tortuosity. The close relationship between ADC changes and cell volume changes in various pathological states suggests that NMR measurements are also sensitive to chemical communication between cells through the extracellular space (i.e., extrasynaptic or volume transmission, VT).

MRI measurements of water diffusion: impact of region of interest selection on ischemic quantification

OBJECTIVE

To investigate the effect of ADC heterogeneity on region of interest (ROI) measurement of isotropic and anisotropic water diffusion in acute (< 12 h) cerebral infarctions.

METHODS AND MATERIALS

Full diffusion tensor images were retrospectively analyzed in 32 patients with acute cerebral infarction. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were measured in ischemic lesions and in the corresponding contralateral, normal appearing brain by using four ROIs for each patient. The 2 x 2 pixel square ROIs were placed in the center, the lateral rim and the medial rim of the infarction. In addition, the whole volume of the infarction was measured using a free hand method. Each ROI value obtained from the ischemic lesion was normalized using contralateral normal ROI values.

RESULTS

The localization of the ROIs in relation to the ischemic lesion significantly affected ADC measurement (P < 0.01, using Friedman test), but not FA measurement (P = 0.25). Significant differences were found between ADC values of the center of the infarction versus whole volume (P < 0.01), and medial rim versus whole volume of infarction (P < 0.001) with variation of relative ADC values up to 11%. The differences of absolute ADC for these groups were 22 and 23%, respectively. The lowest ADC was found in the center, followed by medial rim, lateral rim and whole volume of infarction.

CONCLUSION

ADC quantification may provide variable results depending on ROI method. The ADC and FA values, obtained from the center of infarction tend to be lower compared to the periphery. The researchers who try to compare studies or work on ischemic quantification should be aware of these differences and effects.

Metronidazole-induced encephalopathy

We report the clinical, neuropsychological, and neuroimaging findings of two patients of diffuse encephalopathy associated with the use of metronidazole. Both patients showed characteristic abnormalities on magnetic resonance imaging (MRI) with diffusion weighted imaging (DWI) and recovered incompletely after the discontinuation of metronidazole. We also suggest that MRI with DWI may be useful in the diagnosis of metronidazole-induced encephalopathy, and that they have a role in the prediction of prognosis.

A case of reversible postpartum cytotoxic edema in preeclampsia

We report on a 32-year-old woman who developed reversible cortical blindness and right-sided weakness after cesarean section at 36 weeks of gestation, due to preeclampsia. An initial brain MRI demonstrated high signal intensity lesions in the bilateral occipito-parietal and left frontal lobes on T2-weighted and diffusion-weighted imaging. All of the lesions showed low signal intensity on apparent diffusion coefficient (ADC) map, which were compatible with cytotoxic edema, and MR angiography (MRA) showed diffuse vasospasm of the intracranial vessels. A follow-up brain MRI showed that most of the lesions disappeared and the vasospasm also resolved. This case suggests that the cytotoxic edema in preeclampsia may evolve differently from the pattern in cerebral infarction and explains the relatively benign course of the neurological signs in preeclampsia.

Investigation of techniques to quantify in vivo lesion volume based on comparison of water apparent diffusion coefficient (adc) maps with histology in focal cerebral ischemia of rats

Stroke lesion-volume estimates derived from calculated water apparent diffusion coefficient (ADC) maps provide a quantitative surrogate end-point for investigating the efficacy of drug treatment or studying the temporal evolution of cerebral ischemia. Methodology is described for estimating ischemic lesion volumes in a rat model of permanent middle cerebral artery occlusion (MCAO) based on absolute and percent-reduction threshold values of the water ADC at 3 h post-MCAO. Volume estimates derived from average ADC (ADC(av)) maps were compared with those derived from post-mortem histological sections. Optimum ADC thresholds were established as those that provided the best correlation and one-to-one correspondence between ADC- and histologically derived lesion-volume estimates. At 3 h post-MCAO, an absolute-ADC(av) threshold of 47 x 10(-5) mm(2)/s (corresponding to a 33% reduction in ADC(av) based on a contralateral hemisphere comparison) provided the most accurate estimate of percent hemispheric lesion volume (%HLV). Experimental and data analysis issues for improving and validating the usefulness of DWI as a surrogate endpoint for the quantification of ischemic lesion volume are discussed.

Effects of reperfusion on ADC and CBF pixel-by-pixel dynamics in stroke: characterizing tissue fates using quantitative diffusion and perfusion imaging

The effects of reperfusion on the spatiotemporal dynamics of transient (60 minutes) focal ischemic brain injury in rats were evaluated on a pixel-by-pixel basis using quantitative cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) measurements every 30 minutes for 3 hours and compared to post-mortem histology at 24 hours. Four biologically relevant clusters were classified based on ADC (0.53 +/- 0.02 x 10mm/s, SD) and CBF (0.30 +/- 0.09 ml/g/min) viability thresholds, namely: (1) the "normal" cluster with ADC and CBF > thresholds; (2) the "mismatch" cluster with ADC > threshold but CBF < threshold; (3) the "core" cluster with ADC and CBF < thresholds; and (4) "non-nourishing reperfusion zone" where ADC < threshold but CBF > threshold. The spatio-temporal progression of tissue volumes, ADC and CBF of each cluster were evaluated. Pixels of each cluster on the CBF-ADC space were mapped onto the image space. Following reperfusion, 28% of the "core" pixels and 90% of the "mismatch" (defined at 60 minutes) pixels were salvaged at 180 minutes, which correlated with histology. The ADC and CBF of subsequently salvaged tissues were significantly higher than those became infarcted. Salvaging "core" pixels indicated that reduced ADC was not synonymous with irreversible injury; duration of exposure and severity of reduced ADC and CBF were likely critical. Projection profiles showed a bimodal ADC, but uni-modal CBF, distributions. The ADC bimodal minima, obtained without histological correlation, were similar to the histology-derived ADC and CBF viability thresholds, and could have potential clinical applications. This study demonstrated a simple but powerful approach to evaluate, on a pixel-by-pixel basis, the spatio-temporal evolution of ischemic brain injury, and a potential for statistical prediction of tissue fate.

Magnetic resonance imaging in experimental models of brain disorders

This review gives an overview of the application of magnetic resonance imaging (MRI) in experimental models of brain disorders. MRI is a noninvasive and versatile imaging modality that allows longitudinal and three-dimensional assessment of tissue morphology, metabolism, physiology, and function. MRI can be sensitized to proton density, T1, T2, susceptibility contrast, magnetization transfer, diffusion, perfusion, and flow. The combination of different MRI approaches (e.g., diffusion-weighted MRI, perfusion MRI, functional MRI, cell-specific MRI, and molecular MRI) allows in vivo multiparametric assessment of the pathophysiology, recovery mechanisms, and treatment strategies in experimental models of stroke, brain tumors, multiple sclerosis, neurodegenerative diseases, traumatic brain injury, epilepsy, and other brain disorders. This report reviews established MRI methods as well as promising developments in MRI research that have advanced and continue to improve our understanding of neurologic diseases and that are believed to contribute to the development of recovery improving strategies.