Erica C Henning

Cerebral ischemia-reperfusion injury in rats--a 3 T MRI study on biphasic blood-brain barrier opening and the dynamics of edema formation.

Serial magnetic resonance imaging (MRI) was performed to investigate the temporal and spatial relationship between the biphasic nature of blood–brain barrier (BBB) opening and, in parallel, edema formation after ischemia–reperfusion (I/R) injury in rats. T2-weighted imaging combined with T2-relaxometry, mainly for edema assessment, was performed at 1 h after ischemia, after reperfusion, and at 4, 24 and 48 h after reperfusion. T1-weighted imaging was performed before and after gadolinium contrast at the last three time points to assess BBB integrity. The biphasic course of BBB opening with a significant reduction in BBB permeability at 24 h after reperfusion, associated with a progressive expansion of leaky BBB volume, was accompanied by a peak ipsilateral edema formation. In addition, at 4 h after reperfusion, edema formation could also be detected at the contralateral striatum as determined by the elevated T2-values that persisted to varying degrees, indicative of widespread effects of I/R injury. The observations of this study may indicate a dynamic temporal shift in the mechanisms responsible for biphasic BBB permeability changes, with complex relations to edema formation. Stroke therapy aimed at vasogenic edema and drug delivery for neuroprotection may also be guided according to the functional status of the BBB, and these findings have to be confirmed in human stroke.

Establishing Final Infarct Volume Stroke Lesion Evolution Past 30 Days Is Insignificant

Background and Purpose— Lesion volume measured on MRI has been used as an objective surrogate marker for outcome in clinical trials. However, lesion volumes vary over time because of edema and tissue loss. This study aims to determine if lesion volumes measured at 30 and 90 days after ictus significantly differ. Methods— We performed a retrospective study of 18 patients who had acute (<24 hours) DWI and follow-up fluid-attenuated inversion recovery imaging at 5, 30, and 90 days. Two expert readers segmented lesions and the mean volumes of both reads were used in all statistical analyses. Results— Patient age was 65.8 (SD, 13.7) years and median NIHSS at baseline was 11.5. Inter-rater variability for lesion volume measurements was 3.7 (5.8) mL. Acute DWI volume was 19.3 (17.3) mL. Fluid-attenuated inversion recovery volumes for 5, 30, and 90 days were 34.3 (23.5), 18.6 (14.0), and 15.9 (13.8) mL, respectively. These volumes differed significantly (P<0.001). Linear regression revealed a strong correlation (r=0.96; P<0.001) between lesion volumes at 30 and 90 days with a slope that did not vary significantly from 1.0 (P=0.448). Conclusions— Lesions continue to evolve between 5 and 90 days, but by 30 days lesion volume approaches final infarct volume. While clinical response is the most meaningful outcome measure, our findings suggest that lesion volumes measured at 30 days may provide a sufficient approximation for final infarct volume for use in early phase clinical trials.

Verification of Enhancement of the CSF Space, not Parenchyma, in Acute Stroke Patients with Early Blood—Brain Barrier Disruption

Enhancement on post-contrast fluid-attenuated inversion recovery (FLAIR) images after acute stroke has been attributed to early blood—brain barrier disruption. Using an estimate of parenchymal volume fraction and the apparent diffusion coefficient (ADC), we investigated the relative contributions of cerebral spinal fluid (CSF) and parenchyma to enhancement seen on postcontrast FLAIR. Enhancing regions were found to have low parenchymal volume fractions and high ADC values, approaching that of pure CSF. These findings suggest that contrast enhancement on FLAIR occurs predominately in the CSF space, not parenchyma.

Feridex preloading permits tracking of CNS-resident macrophages after transient middle cerebral artery occlusion

At this time, the pathophysiology of macrophage involvement and their role in stroke progression are poorly understood. Recently, T2- and T2*-weighted magnetic resonance imaging (MRI), after intravenous administration of iron-oxide particles, have been used to understand the inflammatory cascade. Earlier studies report that image enhancement after stroke is from iron-laden macrophages; however, they do not account for potential blood-brain barrier disruption and nonspecific contrast enhancement. In this study, spontaneously hypertensive rats were preloaded with Feridex 7 days before stroke, permitting the labeling of bone-marrow-derived macrophages. Three-dimensional gradient-echo imaging showed average signal decreases of 13% to 23% in preloaded animals, concentrated on the lesion periphery and reaching a maximum on days 2 to 4 after stroke. Immunohistochemistry showed ED-2+, PB+, MHC-II+ and TNF-α+ perivascular macrophages (PVM), meningeal macrophages (MM), and choroid plexus macrophages (CPM). ED-1+ and IBA+ tissue macrophages and/or activated microglia were located throughout the lesion, but were PB−. These findings indicate the following: (1) Feridex preloading permits tracking of the central nervous system (CNS)-resident macrophages (PVM, MM, and CPM) and (2) CNS-resident macrophages likely play an integral role in the inflammatory cascade through antigen presentation and expression of proinflammatory cytokines. Further refinement of this method should permit noninvasive monitoring of inflammation and potential evaluation of antiinflammatory therapies in preclinical models of stroke.

Hypertension-induced Vascular Remodeling Contributes to Reduced Cerebral Perfusion and the Development of Spontaneous Stroke in Aged SHRSP Rats

Stroke in spontaneously-hypertensive, stroke-prone (SHRSP) rats is of particular interest because the pathogenesis is believed to be similar to that in the clinical setting. In this study, we employed multi-modal MRI—ASL, DWI, T2, GRE, T1 (pre/post contrast)—to investigate the natural history of spontaneous cerebral infarction and the specific role of cerebral perfusion in disease development. Twelve female SHRSP rats (age: ∼1 year) were imaged within 1 to 3 days of symptom onset. The distribution of ischemic lesions was the following: 28.1% visual, 21.9% striatal, 18.8% motorsensory, 12.5% thalamic, 12.5% auditory, 3.1% frontal/prelimbic, and 3.1% multiple areas. Ischemic lesions had significantly reduced blood flow in comparison with healthy tissue. Ischemic lesions were characterized by hyperplastic, thrombosed, and compressed vessels. These findings suggest that ischemic lesion development is related to hypertension-induced vascular remodeling and persistent hypoperfusion. This model should be useful for studying the relationship between chronic hypertension and subsequent stroke, both in terms of primary and secondary prevention.

Pseudocontinuous Arterial Spin Labeling Quantifies Relative Cerebral Blood Flow in Acute Stroke

Background and Purpose— The aim of this study was to test whether arterial spin labeling (ASL) can detect significant differences in relative cerebral blood flow (rCBF) in the core, mismatch, and reverse-mismatch regions, and whether rCBF values measured by ASL in those areas differ from values obtained using dynamic susceptibility contrast (DSC) MRI. Methods— Acute stroke patients were imaged with diffusion-weighted imaging (DWI) and perfusion-weighted imaging (ASL and DSC) MRI. An expert reader segmented the ischemic lesion on DWI and the DSC time-to-peak (TTP) maps. Three regions were defined: core (DWI+, TTP+), mismatch (DWI−, TTP+), and reverse-mismatch (DWI+, TTP−). For both ASL and DSC, rCBF maps were created with commercially available software, and the ratio was calculated as the mean signal intensity measured on the side of the lesion to that of the homologous region in the contralateral hemisphere. Values obtained from core, mismatch, and reverse-mismatch were used for paired comparison. Results— Twenty-eight patients were included in the study. The mean age was 65.6 (16.9) years, with a median baseline National Institutes of Health Stroke Scale score of 10 (interquartile range, 4–17). Median time from last known normal to MRI was 5.7 hours (interquartile range, 2.9–22.6). Mean rCBF ratios were significantly higher in the mismatch 0.53 (0.23) versus the core 0.39 (0.33) and reverse-mismatch 0.68 (0.49) versus the core 0.38 (0.35). Differences in rCBF measured with DSC and ASL were not significant. Conclusions— ASL allows for the measurement of rCBF in the core and mismatch regions. Values in the mismatch were significantly higher than in the core, suggesting there is potential salvageable tissue.

Increased plasma and tissue MMP levels are associated with BCSFB and BBB disruption evident on post-contrast FLAIR after experimental stroke.

In this study, we examined the relationship between tissue and blood levels of matrix metalloproteinase (MMP)-2 and MMP-9 through gelatin zymography at multiple time points after experimental stroke. We additionally investigated the association between these levels and the evidence of blood–cerebrospinal fluid (CSF) barrier (BCSFB) and blood–brain barrier (BBB) disruption on post-contrast fluid-attenuated inversion-recovery (FLAIR) imaging. Increased plasma MMP-9 was associated with BCSFB disruption at 1h post-reperfusion. Ventricular enhancement ipsilateral to the stroke was 500±100%, significantly higher than sham, 24, and 48 h groups. Increased tissue MMP-2 and MMP-9 were associated with BBB disruption at 48 h post-reperfusion. Parenchymal enhancement was 60±20% for a volume equivalent to 260±80 mm3. Although the percent enhancement was comparable across groups, the volume of enhancing lesion was significantly higher at 48 h (260±80 mm3, 100%) in comparison to 1 h (8±3 mm3, 3%) and 24 h (51 mm3, 18%). These findings support the use of imaging markers of BCSFB and BBB status as indirect measures of MMP regulation in the blood and brain tissue. The methods presented herein should be useful in understanding the link between MMPs, barrier integrity, and subsequent hemorrhagic transformation.

Reperfusion-Associated Hemorrhagic Transformation in SHR Rats Evidence of Symptomatic Parenchymal Hematoma

Background and Purpose— Symptomatic hemorrhagic transformation (HT) is the most important complicating factor after treatment with intravenous tissue plasminogen activator. In this study, we used multimodal magnetic resonance imaging to investigate the incidence and severity of reperfusion-based HT in spontaneously hypertensive rats after ischemia/reperfusion. Methods— Twenty male spontaneously hypertensive rats were subjected to 30 minutes of middle cerebral artery occlusion via the suture model. Diffusion-weighted, T2-weighted, and gradient-echo imaging were performed on days 1, 2, 3, 4, and 7 for longitudinal evaluation of lesion evolution, vasogenic edema, and HT, respectively. Findings on gradient-echo images were classified according to the severity of hemorrhage: no HT; punctate or small petechial hemorrhage (HI-1); confluent petechial hemorrhage (HI-2); hematoma with absent/mild space-occupying effect (PH-1, ≤30% lesion volume); and hematoma with significant space-occupying effect and potential perihematomal edema (PH-2, >30% lesion volume). Histopathologic evaluation of HT was performed after final imaging for comparison with magnetic resonance imaging results. Results— Final hemorrhage scores based on severity were as follows: HI-1 23.1%, HI-2 30.8%, PH-1 30.8%, and PH-2 15.4%. Similar to clinical observations, only PH-2 was associated with neurologic deterioration and associated weight loss. Conclusions— This model has a high incidence of parenchymal hematomas (46.2%) and therefore is appropriate for the evaluation of novel therapeutics targeting blood-brain barrier integrity and the reduction of symptomatic HT events (PH-2), as well as those potentially “at risk” for neurologic deterioration (PH-1).

Multivoxel MR Spectroscopy in Acute Ischemic Stroke: Comparison to the Stroke Protocol MRI

Background and Purpose— Few patients with stroke have been imaged with MR spectroscopy (MRS) within the first few hours after onset. We compared data from current MRI protocols to MRS in subjects with ischemic stroke. Methods— MRS was incorporated into the standard clinical MRI stroke protocol for subjects <24 hours after onset. MRI and clinical correlates for the metabolic data from MRS were sought. Results— One hundred thirty-six MRS voxels from 32 subjects were analyzed. Lactate preceded the appearance of the lesion on diffusion-weighted imaging in some voxels but in others lagged behind it. Current protocols may predict up to 41% of the variance of MRS metabolites. Serum glucose concentration and time to maximum partially predicted the concentration of all major metabolites. Conclusion— MRS may be helpful in acute stroke, especially for lactate detection when perfusion-weighted imaging is unavailable. Current MRI protocols do provide surrogate markers for some indices of metabolic activity.

Nitrite does not provide additional protection to thrombolysis in a rat model of stroke with delayed reperfusion

An adjuvant therapy to prolong the therapeutic window for stroke patients is urgently needed. This randomized, blinded, placebo-controlled study investigated adjuvant intravenous sodium nitrite with recombinant tissue plasminogen activator (rtPA) in middle cerebral artery occlusion (MCAO) with 6 and 2 h of ischemia followed by reperfusion in Sprague—Dawley rats (n = 59). Quantitative diffusion, T1-, T2-weighted, and semiquantitative perfusion imaging were performed before and after reperfusion and at 48 h after ischemia to determine the spatiotemporal evolution of stroke. After 48 h animals were killed and examined to evaluate infarct size and evidence of hemorrhagic transformation. Factor VIII immunostaining was performed to assess vessel morphology. Nitrite treatment (6 h group: 37.5 μmol for more than 90 mins; 2 h group: 26.25 and 1.75 μmol for more than 60 mins) did not reduce infarct volume 48 h after MCAO compared with saline-treated placebo groups after 6 or 2 h of MCAO. Stroke progression from baseline to 48 h, based on the apparent diffusion coefficient and relative cerebral blood flow deficits before and after reperfusion and T2-weighted hyperintensity at 48 h, did not differ between treated and control animals. These results suggest that nitrite is not a protective adjuvant therapy to delayed rtPA administration after ischemic stroke in rats.