Increased task-relevant fMRI responsiveness in comatose cardiac arrest patients is associated with improved neurologic outcomes

Early prediction of the recovery of consciousness in comatose cardiac arrest patients remains challenging. We prospectively studied task-relevant fMRI responses in 19 comatose cardiac arrest patients and five healthy controls to assess the fMRI's utility for neuroprognostication. Tasks involved instrumental music listening, forward and backward language listening, and motor imagery. Task-specific reference images were created from group-level fMRI responses from the healthy controls. Dice scores measured the overlap of individual subject-level fMRI responses with the reference images. Task-relevant responsiveness index (Rindex) was calculated as the maximum Dice score across the four tasks. Correlation analyses showed that increased Dice scores were significantly associated with arousal recovery (P < 0.05) and emergence from the minimally conscious state (EMCS) by one year (P < 0.001) for all tasks except motor imagery. Greater Rindex was significantly correlated with improved arousal recovery (P = 0.002) and consciousness (P = 0.001). For patients who survived to discharge (n = 6), the Rindex's sensitivity was 75% for predicting EMCS (n = 4). Task-based fMRI holds promise for detecting covert consciousness in comatose cardiac arrest patients, but further studies are needed to confirm these findings. Caution is necessary when interpreting the absence of task-relevant fMRI responses as a surrogate for inevitable poor neurological prognosis.

Severe cerebral edema in substance-related cardiac arrest patients

BACKGROUND

Studies of neurologic outcomes have found conflicting results regarding differences between patients with substance-related cardiac arrests (SRCA) and non-SRCA. We investigate the effects of SRCA on severe cerebral edema development, a neuroimaging intermediate endpoint for neurologic injury.

METHODS

327 out-of-hospital comatose cardiac arrest patients were retrospectively analyzed. Demographics and baseline clinical characteristics were examined. SRCA categorization was based on admission toxicology screens. Severe cerebral edema classification was based on radiology reports. Poor clinical outcomes were defined as discharge Cerebral Performance Category scores > 3.

RESULTS

SRCA patients (N = 86) were younger (P < 0.001), and more likely to have non-shockable rhythms (P < 0.001), be unwitnessed (P < 0.001), lower Glasgow Coma Scale scores (P < 0.001), absent brainstem reflexes (P < 0.05) and develop severe cerebral edema (P < 0.001) than non-SRCA patients (N = 241). Multivariable analyses found younger age (P < 0.001), female sex (P = 0.008), non-shockable rhythm (P = 0.01) and SRCA (P = 0.05) to be predictors of severe cerebral edema development. Older age (P < 0.001), non-shockable rhythm (P = 0.02), severe cerebral edema (P < 0.001), and absent pupillary light reflexes (P = 0.004) were predictors of poor outcomes. SRCA patients had higher proportion of brain deaths (P < 0.001) compared to non-SRCA patients.

CONCLUSIONS

SRCA results in higher rates of severe cerebral edema development and brain death. The absence of statistically significant differences in discharge outcomes or survival between SRCA and non-SRCA patients may be related to the higher rate of withdrawal of life-sustaining treatment (WLST) in the non-SRCA group. Future neuroprognostic studies may opt to include neuroimaging markers as intermediate measures of neurologic injury which are not influenced by WLST decisions.

Ensemble of Convolutional Neural Networks Improves Automated Segmentation of Acute Ischemic Lesions Using Multiparametric Diffusion-Weighted MRI

BACKGROUND AND PURPOSE

Accurate automated infarct segmentation is needed for acute ischemic stroke studies relying on infarct volumes as an imaging phenotype or biomarker that require large numbers of subjects. This study investigated whether an ensemble of convolutional neural networks trained on multiparametric DWI maps outperforms single networks trained on solo DWI parametric maps.

MATERIALS AND METHODS

Convolutional neural networks were trained on combinations of DWI, ADC, and low b-value-weighted images from 116 subjects. The performances of the networks (measured by the Dice score, sensitivity, and precision) were compared with one another and with ensembles of 5 networks. To assess the generalizability of the approach, we applied the best-performing model to an independent Evaluation Cohort of 151 subjects. Agreement between manual and automated segmentations for identifying patients with large lesion volumes was calculated across multiple thresholds (21, 31, 51, and 70 cm³).

RESULTS

An ensemble of convolutional neural networks trained on DWI, ADC, and low b-value-weighted images produced the most accurate acute infarct segmentation over individual networks (P < .001). Automated volumes correlated with manually measured volumes (Spearman ρ = 0.91, P < .001) for the independent cohort. For the task of identifying patients with large lesion volumes, agreement between manual outlines and automated outlines was high (Cohen κ, 0.86-0.90; P < .001).

CONCLUSIONS

Acute infarcts are more accurately segmented using ensembles of convolutional neural networks trained with multiparametric maps than by using a single model trained with a solo map. Automated lesion segmentation has high agreement with manual techniques for identifying patients with large lesion volumes.

Abstract WMP25: In Comatose Survivors of Cardiac Arrest, DWI-based Measurements of Hypoxic-Ischemic Thalamic Injury are Associated With Neurologic Prognosis

Background: Hypoxic-ischemic brain injury caused by cardiac arrest often leaves survivors initially comatose, and with highly variable long-term neurologic outcomes that may be difficult to predict. As the thalamus plays an important role in arousal, we hypothesized that using diffusion-weighted MRI (DWI) to detect acute thalamic ischemic lesions may be helpful in predicting these patients’ long-term neurologic impairment. Because long-term outcome may be influenced by self-fulfilling prophecy bias (SFPB) due to early withdrawal of life sustaining treatment, we also investigated the association of thalamic lesion burden with arousal recovery, which is less influenced by SFPB.

Methods: Images from 79 comatose post-arrest patients were analyzed retrospectively. A neuroradiologist measured lesion burdens as fractions of total thalamic volumes. Group related differences in thalamic lesion burden between patients with good vs. poor outcomes (6-month modified Rankin scale ≤4 or >4, respectively), and between patients who did vs. did not exhibit early arousal recovery (AR+, AR-), as manifested by eye opening in response to a noxious stimulus during the first 7-days post-arrest, were assessed by 2-tailed Wilcoxon exact test.

Results: AR and good outcomes occurred in 38% (30/79) and 18% (14/79) of patients, respectively. Lesion burdens were significantly greater in AR- vs. AR+ patients (mean±SD 37±23% vs. 20±19%, p = 0.001), and greater in patients with poor vs. good outcomes (34±23% vs 11±13%, p < 0.001). All patients with lesion burdens over 56% were AR-, and all patients with lesion burdens over 39% had poor outcomes.

Discussion: In comatose cardiac arrest survivors, the extent of early hypoxic-ischemic damage to the thalamus, as measured on early DWI, is related to long-term neurologic outcome. We also found that more severe thalamic burden was associated with failure to recover arousal in the first week. Our results are consistent with the thalamus’ key role in consciousness as a modulator of peripheral sensory input to the cortex. As comatose cardiac arrest patients’ outcomes are often difficult to predict based on clinical factors alone, the extent of thalamic injury on DWI may be helpful in treatment planning and goals-of-care assessment.

Intravenous thrombolysis in unwitnessed stroke onset: MR WITNESS trial results

OBJECTIVE

Most acute ischemic stroke (AIS) patients with unwitnessed symptom onset are ineligible for intravenous thrombolysis due to timing alone. Lesion evolution on fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) correlates with stroke duration, and quantitative mismatch of diffusion-weighted MRI with FLAIR (qDFM) might indicate stroke duration within guideline-recommended thrombolysis. We tested whether intravenous thrombolysis ≤4.5 hours from the time of symptom discovery is safe in patients with qDFM in an open-label, phase 2a, prospective study (NCT01282242).

METHODS

Patients aged 18 to 85 years with AIS of unwitnessed onset at 4.5 to 24 hours since they were last known to be well, treatable within 4.5 hours of symptom discovery with intravenous alteplase (0.9mg/kg), and presenting with qDFM were screened across 14 hospitals. The primary outcome was the risk of symptomatic intracranial hemorrhage (sICH) with preplanned stopping rules. Secondary outcomes included symptomatic brain edema risk, and functional outcomes of 90-day modified Rankin Scale (mRS).

RESULTS

Eighty subjects were enrolled between January 31, 2011 and October 4, 2015 and treated with alteplase at median 11.2 hours (IQR = 9.5-13.3) from when they were last known to be well. There was 1 sICH (1.3%) and 3 cases of symptomatic edema (3.8%). At 90 days, 39% of subjects achieved mRS = 0-1, as did 48% of subjects who had vessel imaging and were without large vessel occlusions.

INTERPRETATION

Intravenous thrombolysis within 4.5 hours of symptom discovery in patients with unwitnessed stroke selected by qDFM, who are beyond the recommended time windows, is safe. A randomized trial testing efficacy using qDFM appears feasible and is warranted in patients without large vessel occlusions. Ann Neurol 2018;83:980-993.

Abstract WMP16: Elevated Cerebral Neurite Orientation Dispersion and Density Imaging and Diffusion Kurtosis Values Are Associated With Poor Neurologic Outcome in Comatose Cardiac Arrest Patients

Background: For cardiac arrest survivors initially comatose after restoration of spontaneous circulation (ROSC), the extent of brain injury and expected neurologic outcome are crucial for patient management decisions. Advanced diffusion imaging approaches such as neurite orientation dispersion and density imaging (NODDI) or diffusion kurtosis imaging may provide additional insight into tissue integrity and potential for recovery of consciousness complementary to standard diffusion tensor imaging (DTI).

Methods: Multi-shell diffusion imaging was acquired in a prospective study of comatose cardiac arrest patients and in 5 controls. Neurite orientation dispersion (OD), intracellular volume fraction (ICVF), mean kurtosis (MK), axial kurtosis (AK), radial kurtosis (RK), mean diffusivity (MD), axial diffusivity (AD), radial diffusivity (RD) and fractional anisotropy (FA) were calculated. Median whole-brain values in patients with poor outcomes (no arousal recovery [AR] by discharge) were compared with those with AR and to controls (1-way ANOVA, post-hoc 1-sided Wilcoxon exact test).

Results: 18 patients (mean ±SD 48±23 y, 39% men) and 5 controls (37±19 y, 40% men) were analyzed. Median (range) Glasgow Coma Scale was 3 [3-5]. 10 patients exhibited AR, 8 did not. Median [IQR] time-to-MRI was 5 [4-8] days. FA (P=0.009), MK (P=0.017), AK (P=0.026), RK (P=0.014), OD (P=0.018) and ICVF (P=0.0038) were significantly different (see Figure). FA control values were greater than AR and no AR (P<0.05). MK, AK, RK, OD and ICVF values in the no AR group were greater than in the AR and control groups (P<0.05).

Discussion: This is the first report investigating early NODDI and diffusional heterogeneity changes in post-cardiac arrest comatose patients. Patients who failed to recover arousal demonstrated greater values for all kurtosis and NODDI metrics compared to controls. Potential bias from early withdrawal of life sustaining treatment and small cohorts are limitations.

Abstract TP52: Neurite Density and Orientation Dispersion are Decreased in White Matter in Patients With Advanced Leukoariaosis

Background: Alterations to microstructural integrity in white matter hyperintensities (WMH) in patients with severe leukoaraiosis are poorly understood. Neurite density and orientation dispersion imaging (NODDI) produce better estimates of myelin density than diffusion tensor MRI (DTI) and therefore may provide additional in vivo insight into WMH pathophysiology.

Methods: NODDI was acquired in a prospective study of acute ischemic stroke patients with advanced white matter disease (N=36). Neurite density (ND), and orientation dispersion (OD) were calculated along with model-free diffusion parameters: mean kurtosis (MK), axial kurtosis (AK), radial kurtosis (RK), mean diffusivity (MD), axial diffusivity (AD), radial diffusivity (RD) and fractional anisotropy (FA). Median values were measured in in the hemisphere contralateral to the incident stroke in regions of WMH and in normal appearing white matter (NAWM) and compared (paired Wilcoxon-exact test). Linear regression was performed to evaluate univariate predictors of log-transformed WMH volumes (log WMHv).

Results: Patient characteristics were: mean±SD age 69±10 y, time-to-MRI 2.8±1.2 days, median [IQ] normalized WMHv 4.7 [2.3-9.3] and 61% men. MD, AD and RD were greater, while FA, MK, AK, RK, OD and ND were lower in WMH compared to NAWM (P<0.001, see Figure). Increased MD (P=0.004), AD (P<0.0001) and AK (P=0.01) in NAWM and decreased OD in NAWM (P=0.01) were significant predictors of increased log WMHv.

Discussion: Diffusivity, kurtosis and ND and OD in acute stroke patients with moderate to severe leuokoaraiosis differed significantly between NAWM and WMH in the contralateral hemisphere. Reduced neurite density is suggestive of microstructural injury. Reduced OD is typically associated with greater organization, but might also reflect restricted extracellular space diffusivity. The association of MD, AD, AK and OD in NAWM with WMH burden suggests there is ongoing risk for developing future WMH.

Diffuse microvascular dysfunction and loss of white matter integrity predict poor outcomes in patients with acute ischemic stroke

We sought to investigate the relationship between blood-brain barrier (BBB) permeability and microstructural white matter integrity, and their potential impact on long-term functional outcomes in patients with acute ischemic stroke (AIS). We studied 184 AIS subjects with perfusion-weighted MRI (PWI) performed <9 h from last known well time. White matter hyperintensity (WMH), acute infarct, and PWI-derived mean transit time lesion volumes were calculated. Mean BBB leakage rates (K2 coefficient) and mean diffusivity values were measured in contralesional normal-appearing white matter (NAWM). Plasma matrix metalloproteinase-2 (MMP-2) levels were studied at baseline and 48 h. Admission stroke severity was evaluated using the NIH Stroke Scale (NIHSS). Modified Rankin Scale (mRS) was obtained at 90-days post-stroke. We found that higher mean K2 and diffusivity values correlated with age, elevated baseline MMP-2 levels, greater NIHSS and worse 90-day mRS (all p < 0.05). In multivariable analysis, WMH volume was associated with mean K2 ( p = 0.0007) and diffusivity ( p = 0.006) values in contralesional NAWM. In summary, WMH severity measured on brain MRI of AIS patients is associated with metrics of increased BBB permeability and abnormal white matter microstructural integrity. In future studies, these MRI markers of diffuse cerebral microvascular dysfunction may improve prediction of cerebral tissue infarction and functional post-stroke outcomes.

In patients with suspected acute stroke, CT perfusion-based cerebral blood flow maps cannot substitute for DWI in measuring the ischemic core

Background

Neuroimaging may guide acute stroke treatment by measuring the volume of brain tissue in the irreversibly injured “ischemic core.” The most widely accepted core volume measurement technique is diffusion-weighted MRI (DWI). However, some claim that measuring regional cerebral blood flow (CBF) with CT perfusion imaging (CTP), and labeling tissue below some threshold as the core, provides equivalent estimates. We tested whether any threshold allows reliable substitution of CBF for DWI.

Methods

58 patients with suspected stroke underwent DWI and CTP within six hours of symptom onset. A neuroradiologist outlined DWI lesions. In CBF maps, core pixels were defined by thresholds ranging from 0%-100% of normal, in 1% increments. Replicating prior studies, we used receiver operating characteristic (ROC) curves to select thresholds that optimized sensitivity and specificity in predicting DWI-positive pixels, first using only pixels on the side of the brain where infarction was clinically suspected (“unilateral” method), then including both sides (“bilateral”). We quantified each method and threshold’s accuracy in estimating DWI volumes, using sums of squared errors (SSE). For the 23 patients with follow-up studies, we assessed whether CBF-derived volumes inaccurately exceeded follow-up infarct volumes.

Results

The areas under the ROC curves were 0.89 (unilateral) and 0.90 (bilateral). Various metrics selected optimum CBF thresholds ranging from 29%-32%, with sensitivities of 0.79–0.81, and specificities of 0.83–0.85. However, for the unilateral and bilateral methods respectively, volume estimates derived from all CBF thresholds above 28% and 22% were less accurate than disregarding imaging and presuming every patient’s core volume to be zero. The unilateral method with a 30% threshold, which recent clinical trials have employed, produced a mean core overestimation of 65 mL (range: –82–191), and exceeded follow-up volumes for 83% of patients, by up to 191 mL.

Conclusion

CTP-derived CBF maps cannot substitute for DWI in measuring the ischemic core.

Abstract 36: Acute Stroke Evolution is in the Eye of the Beholder: Effects of Interrater Variability on Patient Selection and Outcomes in the Mr Witness (nct01282242) Multicenter Thrombolysis Trial

Background: MR WITNESS was a safety trial giving tPA to acute ischemic stroke (AIS) patients with unwitnessed onset with MRI findings of early stroke. A DWI-positive, FLAIR negative pattern has been shown to identify strokes <4.5 hr duration, but visual inspection alone may be unreliable and insensitive. Signal intensity ratios (SIR) of manual outlines (lesion/contralateral side) increase sensitivity but can lead to variability in patient selection. We investigated the influence of interrater variability on clinical and safety outcomes.

Methods: Core readers blinded to enrollment status and clinical presentation reviewed MRI of screened patients. The MR WITNESS algorithm enrolled subjects with no visible FLAIR or a pre-specified SIR <1.15. Good outcome was defined as 90 Day modified Rankin Scale (mRS) < 2. SIR consistency was measured with intraclass coefficient (ICC) and reader agreement assessed with Fleiss’ Kappa. Statistical analysis included 2-sided Fisher’s Exact test or Wilcoxon Exact test as appropriate.

Results: 201 subjects were screened. 153 baseline MRIs with DWI lesions were reviewed. ICC was 71% (P<.001) and kappa was 67% (P<.0001). Among the 80 subjects enrolled using SIR <1.15 by site reading, 15 (18.8%) subjects would have been excluded based on Core readings. These 15 subjects were younger with worse NIHSS and 90-day mRS (Table). Subset analyses of subjects with pre-stroke mRS<2 showed no statistical difference (P=0.19) between subjects deemed eligible and not eligible by core readers. No difference in hemorrhagic transformation (HT) or good outcome rates was found. Using SIR <1.25 would have only excluded 3 subjects by Core readings, all of whom had HT, and poor 90 day outcomes.

Discussion: Although SIR reads between Core and sites showed good agreement, there was still variability in the absolute values. This demonstrates the potential limitation of subjective lesion volume delineation. Automated approaches should be investigated.

Abstract TP48: Automated Deep Learning-based Measurement of DWI Lesion Volume in Acute Stroke Using Convolutional Neural Networks

Background: In acute ischemic stroke (AIS), therapeutic decisions are increasingly being based upon the volume of likely-unsalvageable brain tissue, which is often estimated using DWI. Deep learning algorithms, e.g. convolutional neural networks (CNN), have been employed for chronic stroke lesion segmentation. Here we investigate the applicability of CNN for DWI lesion measurement in acute stroke.

Methods: 50 AIS patients underwent DWI < 12h from last known well. Apparent diffusion coefficient maps, T2WI, and DWI were used as covariates in a 2D CNN (5-fold cross validation). Including convolutional, inception and fully connected dense layers, a CNN of 15 layers was trained using manually outlined DWI lesions. To avoid overfitting, statistical dropout, L1- and L2-regularization and batch-normalization were used. Automatically segmented lesion volumes (ALV) using a 50% risk threshold were compared to the manual lesion volumes (MLV) using Dice similarity index (DSI, a measure of overlap) and Spearman’s correlation coefficient. Subset analysis was performed evaluating results between small (<10 ml) and large lesions (Wilcoxon rank sums).

Results: The figure shows examples of CNN segmentation. The median [IQR] measured lesion volume and DSI were 25 [13-46] mL and 66% [35-75%], respectively. The correlation of MLV with ALV was 86% (P<0.001). 21 subjects (42%) had lesion volumes less than 10 ml. DSI for small lesions (28% [14-46%]) was significantly lower (P<0.001) than large lesions (73% [67-79]%). Correlation of ALV with MLV for small lesions compared to large lesions were 31 and 84 respectively and differed significantly (P=0.001).

Discussion: Automatic DWI lesion segmentation for large lesions is feasible using CNN. CNN tended to overestimate the volumes of small lesions. Prior methods have used a priori heuristics or morphometric operations to remove artifacts. CNN methods show promise for “learning” to discriminate artifacts from real lesions.

Abstract WP37: Evidence of Late Hemorrhage in Ischemic Stroke Patients Treated With Intravenous Alteplase: A Post Hoc Analysis of the Multicenter MR WITNESS Trial (NCT01282242)

Introduction: Although early hemorrhagic transformation (HT) in acute ischemic stroke has been studied, less is known about patients who develop hemorrhage after the acute phase. We hypothesized that patients with late hemorrhage (LH) would have more severe strokes than those without, and tested this hypothesis in a cohort of thrombolysed patients from the MR WITNESS trial.

Methods: Subjects were recruited from 10 sites between Jan 2011-Oct 2015. MR WITNESS enrolled subjects if they were last seen between 4.5 and 24 hours prior to evaluation, but otherwise qualified for IV tPA in the 3-4.5 hr window per AHA guidelines, and if their brain MRI findings indicated very early infarction: either no FLAIR hyperintensity or subtle hyperintensity, ie signal increase <15% compared to the contralateral hemisphere. Patients with early (≤48 hours) HT (defined per ECASS criteria) were excluded from analysis. Late hemorrhage was defined as imaging manifestation of hemorrhage on 30 days MRI in patient without manifestation of HT at 48h. Good outcome was pre-specified as modified Rankin Scale (mRS) 0-1 at 90 d. Univariate comparisons utilized Fisher’s exact test and Wilcoxon Rank Sums 2-sample exact test for categorical and continuous variables, respectively.

Results: Among the 80 patients included in the MR WITNESS cohort, 53 met our inclusion criteria and were analysed. When compared to those with no HT, patients with LH had larger baseline infarct volumes and perfusion defects, as well as more frequent proximal vessel occlusion at baseline (all p <0.01, See Table). Patients with LH also demonstrated worse functional outcome at 90 days (mRS, Median [IQR], 3 [1.75-4] vs 1 [0-2], p=0.006, Table).

Conclusion: Patients with LH demonstrate a more severe imaging profile at baseline and a worse functional outcome at 90 days when compared to patients without hemorrhage. Understanding the underlying pathophysiology of LH may shed light on to the mechanisms of acute and subacute brain injury.

Longitudinal Diffusion Tensor Imaging Detects Recovery of Fractional Anisotropy Within Traumatic Axonal Injury Lesions

BACKGROUND

Traumatic axonal injury (TAI) may be reversible, yet there are currently no clinical imaging tools to detect axonal recovery in patients with traumatic brain injury (TBI). We used diffusion tensor imaging (DTI) to characterize serial changes in fractional anisotropy (FA) within TAI lesions of the corpus callosum (CC). We hypothesized that recovery of FA within a TAI lesion correlates with better functional outcome.

METHODS

Patients who underwent both an acute DTI scan (≤day 7) and a subacute DTI scan (day 14 to inpatient rehabilitation discharge) at a single institution were retrospectively analyzed. TAI lesions were manually traced on the acute diffusion-weighted images. Fractional anisotropy (FA), apparent diffusion coefficient (ADC), axial diffusivity (AD), and radial diffusivity (RD) were measured within the TAI lesions at each time point. FA recovery was defined by a longitudinal increase in CC FA that exceeded the coefficient of variation for FA based on values from healthy controls. Acute FA, ADC, AD, and RD were compared in lesions with and without FA recovery, and correlations were tested between lesional FA recovery and functional recovery, as determined by disability rating scale score at discharge from inpatient rehabilitation.

RESULTS

Eleven TAI lesions were identified in 7 patients. DTI detected FA recovery within 2 of 11 TAI lesions. Acute FA, ADC, AD, and RD did not differ between lesions with and without FA recovery. Lesional FA recovery did not correlate with disability rating scale scores.

CONCLUSIONS

In this retrospective longitudinal study, we provide initial evidence that FA can recover within TAI lesions. However, FA recovery did not correlate with improved functional outcomes. Prospective histopathological and clinical studies are needed to further elucidate whether lesional FA recovery indicates axonal healing and has prognostic significance.

Abstract WMP14: Increased Diffusion Heterogeneity After Acute Ischemic Stroke is Associated With Salvageable Tissue

Background: Diffusion kurtosis MRI (DKI) may be more sensitive to microstructural changes in acute ischemic stroke (AIS), compared to diffusion weighted MRI (DWI). We investigated differences in diffusion kurtosis metrics related to time and tissue outcome.

Methods: DKI from AIS patients enrolled in a prospective serial MRI study were analyzed (N=18). Mean kurtosis (MK), axial kurtosis (AK), radial kurtosis (RK), mean diffusivity (MD), axial diffusivity (AD), radial diffusivity (RD) and fractional anisotropy (FA) maps were calculated. Follow-up (FU) FLAIR infarct volumes were defined as “final.” Abnormal perfusion was defined as tissue with Tmax values greater than 6 seconds. All maps were co-registered to one another. DWI and DKI were compared (nonparametric Spearman’s correlation analysis) in the following regions: Core (abnormal acute DWI and FU), Growth (normal acute DWI, abnormal FU), and Salvaged (normal acute DWI, abnormal acute perfusion, normal FU).

Results: Patient characteristics were: mean±SD age 66±10 years, median [IQR] admission NIHSS score 6 [3-11], time-to-acute MRI 6.2±2.1 h, time-to-FU MRI 3.0±1.3 days, acute DWI lesion 4.9 [0.8-21.1] cm3 and FU lesion 12.9 [1.8-54.7] cm3. Significant correlations were found between time-to-MRI and diffusivity and kurtosis metrics, but differed depending on tissue outcome (Table).

Discussion: The significant inverse correlation between FA and RK and time-to-MRI in salvaged tissue suggests renormalization of transient ischemia-induced increases in FA and RK tissue with otherwise preserved cytoarchitecture. One possible mechanism underlying this could be that hyperacute ischemia-induced cellular swelling increases tortuosity of water diffusion paths, imposing direction-dependent restrictions upon diffusion. Coupled with changes in DWI, DKI may provide further insight into tissue evolution after AIS and therefore improve identification of potentially salvageable tissue.

Abstract WP38: Lower Fluid Attenuated Inversion Recovery Magnetic Resonance Imaging Signal Intensity After Acute Ischemic Stroke is Associated With Better Discharge Outcomes in Thrombolysed Patients

Background: Increases in FLAIR MRI signal intensity (SI) after acute ischemic stroke (AIS) have been proposed as “tissue clocks”, reflecting the degree of ischemic injury and potential for recovery in brain tissue. We hypothesize that lower SI increases will be a biomarker for less severe tissue injury and hence is potentially salvageable. To test this, we investigated patients who were treated with tPA to determine whether better outcomes were associated with lower FLAIR SI.

Methods: Using our Get with the Guidelines database, we retrospectively analyzed AIS patients admitted between 2011 to 2013 who received full-dose tPA based on institutional protocols. Patients were included if they received a CT scan at our hospital before tPA therapy, and MRI performed < 1 h post CT included a usable FLAIR scan and evidence of stroke on acute DWI. SI ratio (SIR) was calculated on the FLAIR by selecting region of interests in hyperintense FLAIR areas that coincided with the acute DWI lesion and matching contralateral regions. Logistic regression analysis was performed using forward stepwise analysis to combine age, sex, admission NIHSS, SIR and onset-to-treatment (OTT) time to predict discharge outcome. Good outcome was defined as discharge to home or in-patient rehabilitation hospital.

Results: There were 129 AIS patients who received tPA, and 57 met our imaging criteria. Patient characteristics were: mean±SD age 70±15 years, median [IQR] NIHSS 13 [7-18], OTT 2.1±1.1 h, time-to-MRI 2.1±1.0 h, 58% female and median SIR 1.13 [1.05-1.26]. On a univariate basis, only age (P<0.0001) and NIHSS (P=0.005) were significant predictors of outcome. However, multivariate analysis showed that combining age (P=0.019), NIHSS (P=0.022), and SIR (P=0.018) was able to predict good discharge outcome with 100% [95% CI 63-100%] specificity and 90% [77-96%] sensitivity. Using age and NIHSS alone, resulted in the same specificity but only 71% [56-83%] sensitivity.

Discussion: Controlling for age and admission NIHSS, we found that lower FLAIR SIR was a significant predictor of discharge outcome while OTT was not. This suggests that “tissue clocks” may be more accurate than “time clocks” for predicting tissue outcome, and ultimately functional outcome.

Comparing prognostic strength of acute corticospinal tract injury measured by a new diffusion tensor imaging based template approach versus common approaches

BACKGROUND

Long-term motor outcome of acute stroke patients with severe motor impairment is difficult to predict. While measure of corticospinal tract (CST) injury based on diffusion tensor imaging (DTI) in subacute stroke patients strongly predicts motor outcome, its predictive value in acute stroke patients is unclear. Using a new DTI-based, density-weighted CST template approach, we demonstrated recently that CST injury measured in acute stroke patients with moderately-severe to severe motor impairment of the upper limb strongly predicts motor outcome of the limb at 6 months.

NEW METHOD

The current study compared the prognostic strength of CST injury measured in 10 acute stroke patients with moderately-severe to severe motor impairment of the upper limb by the new density-weighted CST template approach versus several variants of commonly used DTI-based approaches.

RESULTS AND COMPARISON WITH EXISTING METHODS

Use of the density-weighted CST template approach yielded measurements of acute CST injury that correlated most strongly, in absolute magnitude, with 6-month upper limb strength (rs=0.93), grip (rs=0.94) and dexterity (rs=0.89) compared to all other 11 approaches. Formal statistical comparison of correlation coefficients revealed that acute CST injury measured by the density-weighted CST template approach correlated significantly more strongly with 6-month upper limb strength, grip and dexterity than 9, 10 and 6 of the 11 alternative measurements, respectively.

CONCLUSIONS

Measurements of CST injury in acute stroke patients with substantial motor impairment by the density-weighted CST template approach may have clinical utility for anticipating healthcare needs and improving clinical trial design.

Diffusion tensor imaging in acute-to-subacute traumatic brain injury: a longitudinal analysis

BACKGROUND

Diffusion tensor imaging (DTI) may have prognostic utility in patients with traumatic brain injury (TBI), but the optimal timing of DTI data acquisition is unknown because of dynamic changes in white matter water diffusion during the acute and subacute stages of TBI. We aimed to characterize the direction and magnitude of early longitudinal changes in white matter fractional anisotropy (FA) and to determine whether acute or subacute FA values correlate more reliably with functional outcomes after TBI.

METHODS

From a prospective TBI outcomes database, 11 patients who underwent acute (≤7 days) and subacute (8 days to rehabilitation discharge) DTI were retrospectively analyzed. Longitudinal changes in FA were measured in 11 white matter regions susceptible to traumatic axonal injury. Correlations were assessed between acute FA, subacute FA and the disability rating scale (DRS) score, which was ascertained at discharge from inpatient rehabilitation.

RESULTS

FA declined from the acute-to-subacute period in the genu of the corpus callosum (0.70 ± 0.02 vs. 0.55 ± 0.11, p < 0.05) and inferior longitudinal fasciculus (0.54+/-0.07 vs. 0.49+/-0.07, p < 0.01). Acute correlations between FA and DRS score were variable: higher FA in the body (R = -0.78, p = 0.02) and splenium (R = -0.83, p = 0.003) of the corpus callosum was associated with better outcomes (i.e. lower DRS scores), whereas higher FA in the genu of the corpus callosum (R = 0.83, p = 0.02) corresponded with worse outcomes (i.e. higher DRS scores). In contrast, in the subacute period higher FA in the splenium correlated with better outcomes (R = -0.63, p < 0.05) and no inverse correlations were observed.

CONCLUSIONS

White matter FA declined during the acute-to-subacute stages of TBI. Variability in acute FA correlations with outcome suggests that the optimal timing of DTI for TBI prognostication may be in the subacute period.

Brain perfusion: computed tomography and magnetic resonance techniques

Cerebral perfusion imaging provides assessment of regional microvascular hemodynamics in the living brain, enabling in vivo measurement of a variety of different hemodynamic parameters. Perfusion imaging techniques that are used in the clinical setting usually rely upon X-ray computed tomography (CT) or magnetic resonance imaging (MRI). This chapter reviews CT- and MRI-based perfusion imaging techniques, with attention to image acquisition, clinically relevant aspects of image postprocessing, and fundamental differences between CT- and MRI-based techniques. Correlations with cerebrovascular physiology and potential clinical applications of perfusion imaging are reviewed, focusing upon the two major classes of neurologic disease in which perfusion imaging is most often performed: primary perfusion disorders (including ischemic stroke, transient ischemic attack, and reperfusion syndrome), and brain tumors.

Role of Acute Lesion Topography in Initial Ischemic Stroke Severity and Long-Term Functional Outcomes

BACKGROUND AND PURPOSE

Acute infarct volume, often proposed as a biomarker for evaluating novel interventions for acute ischemic stroke, correlates only moderately with traditional clinical end points, such as the modified Rankin Scale. We hypothesized that the topography of acute stroke lesions on diffusion-weighted magnetic resonance imaging may provide further information with regard to presenting stroke severity and long-term functional outcomes.

METHODS

Data from a prospective stroke repository were limited to acute ischemic stroke subjects with magnetic resonance imaging completed within 48 hours from last known well, admission NIH Stroke Scale (NIHSS), and 3-to-6 months modified Rankin Scale scores. Using voxel-based lesion symptom mapping techniques, including age, sex, and diffusion-weighted magnetic resonance imaging lesion volume as covariates, statistical maps were calculated to determine the significance of lesion location for clinical outcome and admission stroke severity.

RESULTS

Four hundred ninety subjects were analyzed. Acute stroke lesions in the left hemisphere were associated with more severe NIHSS at admission and poor modified Rankin Scale at 3 to 6 months. Specifically, injury to white matter (corona radiata, internal and external capsules, superior longitudinal fasciculus, and uncinate fasciculus), postcentral gyrus, putamen, and operculum were implicated in poor modified Rankin Scale. More severe NIHSS involved these regions, as well as the amygdala, caudate, pallidum, inferior frontal gyrus, insula, and precentral gyrus.

CONCLUSIONS

Acute lesion topography provides important insights into anatomic correlates of admission stroke severity and poststroke outcomes. Future models that account for infarct location in addition to diffusion-weighted magnetic resonance imaging volume may improve stroke outcome prediction and identify patients likely to benefit from aggressive acute intervention and personalized rehabilitation strategies.

In Acute Stroke, Can CT Perfusion-Derived Cerebral Blood Volume Maps Substitute for Diffusion-Weighted Imaging in Identifying the Ischemic Core?

Background and Purpose

In the treatment of patients with suspected acute ischemic stroke, increasing evidence suggests the importance of measuring the volume of the irreversibly injured “ischemic core.” The gold standard method for doing this in the clinical setting is diffusion-weighted magnetic resonance imaging (DWI), but many authors suggest that maps of regional cerebral blood volume (CBV) derived from computed tomography perfusion imaging (CTP) can substitute for DWI. We sought to determine whether DWI and CTP-derived CBV maps are equivalent in measuring core volume.

Methods

58 patients with suspected stroke underwent CTP and DWI within 6 hours of symptom onset. We measured low-CBV lesion volumes using three methods: “objective absolute,” i.e. the volume of tissue with CBV below each of six published absolute thresholds (0.9–2.5 mL/100 g), “objective relative,” whose six thresholds (51%-60%) were fractions of mean contralateral CBV, and “subjective,” in which two radiologists (R1, R2) outlined lesions subjectively. We assessed the sensitivity and specificity of each method, threshold, and radiologist in detecting infarction, and the degree to which each over- or underestimated the DWI core volume. Additionally, in the subset of 32 patients for whom follow-up CT or MRI was available, we measured the proportion of CBV- or DWI-defined core lesions that exceeded the follow-up infarct volume, and the maximum amount by which this occurred.

Results

DWI was positive in 72% (42/58) of patients. CBV maps’ sensitivity/specificity in identifying DWI-positive patients were 100%/0% for both objective methods with all thresholds, 43%/94% for R1, and 83%/44% for R2. Mean core overestimation was 156–699 mL for objective absolute thresholds, and 127–200 mL for objective relative thresholds. For R1 and R2, respectively, mean±SD subjective overestimation were -11±26 mL and -11±23 mL, but subjective volumes differed from DWI volumes by up to 117 and 124 mL in individual patients. Inter-rater agreement regarding the presence of infarction on CBV maps was poor (kappa = 0.21). Core lesions defined by the six objective absolute CBV thresholds exceeded follow-up infarct volumes for 81%-100% of patients, by up to 430–1002 mL. Core estimates produced by objective relative thresholds exceeded follow-up volumes in 91% of patients, by up to 210-280 mL. Subjective lesions defined by R1 and R2 exceeded follow-up volumes in 18% and 26% of cases, by up to 71 and 15 mL, respectively. Only 1 of 23 DWI lesions (4%) exceeded final infarct volume, by 3 mL.

Conclusion

CTP-derived CBV maps cannot reliably substitute for DWI in measuring core volume, or even establish which patients have DWI lesions.

Multimodal imaging in acute ischemic stroke

OPINION STATEMENT

Recent years have seen the development of novel neuroimaging techniques whose roles in the management of acute stroke are sometimes confusing and controversial. This may be attributable in part to a focus on establishing simplified algorithms and terminology that omit consideration of the basic pathophysiology of cerebral ischemia and, consequently, of the full potential for optimizing patients' care based upon their individual imaging findings. This review begins by discussing cerebral hemodynamic physiology and of the effects of hemodynamic disturbances upon the brain. Particular attention will be paid to the hemodynamic measurements and markers of tissue injury that are provided by common clinical imaging techniques, with the goal of enabling greater confidence and flexibility in understanding the potential uses of these techniques in various clinical roles, which will be discussed in the remainder of the review.

Combining MRI with NIHSS thresholds to predict outcome in acute ischemic stroke: value for patient selection

BACKGROUND AND PURPOSE

Selecting acute ischemic stroke patients for reperfusion therapy on the basis of a diffusion-perfusion mismatch has not been uniformly proved to predict a beneficial treatment response. In a prior study, we have shown that combining clinical with MR imaging thresholds can predict clinical outcome with high positive predictive value. In this study, we sought to validate this predictive model in a larger patient cohort and evaluate the effects of reperfusion therapy and stroke side.

MATERIALS AND METHODS

One hundred twenty-three consecutive patients with anterior circulation acute ischemic stroke underwent MR imaging within 6 hours of stroke onset. DWI and PWI volumes were measured. Lesion volume and NIHSS score thresholds were used in models predicting good 3-month clinical outcome (mRS 0-2). Patients were stratified by treatment and stroke side.

RESULTS

Receiver operating characteristic analysis demonstrated 95.6% and 100% specificity for DWI > 70 mL and NIHSS score > 20 to predict poor outcome, and 92.7% and 91.3% specificity for PWI (mean transit time) < 50 mL and NIHSS score < 8 to predict good outcome. Combining clinical and imaging thresholds led to an 88.8% (71/80) positive predictive value with a 65.0% (80/123) prognostic yield. One hundred percent specific thresholds for DWI (103 versus 31 mL) and NIHSS score (20 versus 17) to predict poor outcome were significantly higher in treated (intravenous and/or intra-arterial) versus untreated patients. Prognostic yield was lower in right- versus left-sided strokes for all thresholds (10.4%-20.7% versus 16.9%-40.0%). Patients with right-sided strokes had higher 100% specific DWI (103.1 versus 74.8 mL) thresholds for poor outcome, and the positive predictive value was lower.

CONCLUSIONS

Our predictive model is validated in a much larger patient cohort. Outcome may be predicted in up to two-thirds of patients, and thresholds are affected by stroke side and reperfusion therapy.

Abstract W P25: Anatomically-Weighted Predictive Algorithms Improve MRI-Based Tissue Outcome Predictions After Acute Ischemic Stroke

Background: In acute ischemic stroke (AIS) patients, multi-parametric MRI-based predictive algorithms have shown promise in identifying tissue at risk of infarction, but do not consider the intrinsic variations of normal or pathological tissue. We hypothesized that extending MRI-based algorithms to take into consideration tissue type will improve predictions of tissue outcome.

Methods: We retrospectively analyzed AIS patients who received neither revascularization nor experimental interventional treatment, who underwent MRI within 12 h from the time since they were last known well and who had follow-up imaging >4 days. Perfusion- and diffusion-parametric maps were combined to predict tissue outcome using 2 models: 1) a generalized linear model (GLM) trained with data from the whole ipsilateral hemisphere (sGLM), irrespective of tissue type, or 2) an anatomically-weighted GLM (aGLM) that was calculated using a weighted average to combine results from models generated using entire white or gray matter regions only. Both methods were evaluated using jack-knifing and predicted and follow-up regions were compared in terms of accuracy (measured as area under the receiver operator characteristic curve, AUC), Dice similarity index (DSI) and root mean square error (RMSE).

Results: Results from 109 patients (65% male, median 68 y IQR [55-77], NIHSS 14 [9-25]) showed that, compared to sGLM, aGLM’s predictions had higher DSI (0.48 [0.19-0.59], P<0.001), and AUC (0.89 [0.86-0.94], P=0.001) and lower RMSE (0.32 [0.29-0.35], P<0.001), all demonstrating improved performance.

Discussion: We showed that anatomically-weighted algorithms may better capture differences in tissue vulnerability in acute ischemic stroke, contributing to improved MRI-based tissue outcome predictions.

Exposing hidden truncation-related errors in acute stroke perfusion imaging

BACKGROUND AND PURPOSE

The durations of acute ischemic stroke patients' CT or MR perfusion scans may be too short to fully sample the passage of the injected contrast agent through the brain. We tested the potential magnitude of hidden errors related to the truncation of data by short perfusion scans.

MATERIALS AND METHODS

Fifty-seven patients with acute ischemic stroke underwent perfusion MR imaging within 12 hours of symptom onset, using a relatively long scan duration (110 seconds). Shorter scan durations (39.5-108.5 seconds) were simulated by progressively deleting the last-acquired images. CBV, CBF, MTT, and time to response function maximum (Tmax) were measured within DWI-identified acute infarcts, with commonly used postprocessing algorithms. All measurements except Tmax were normalized by dividing by the contralateral hemisphere values. The effects of the scan duration on these hemodynamic measurements and on the volumes of lesions with Tmax of >6 seconds were tested using regression.

RESULTS

Decreasing scan duration from 110 seconds to 40 seconds falsely reduced perfusion estimates by 47.6%-64.2% of normal for CBV, 1.96%-4.10% for CBF, 133%-205% for MTT, and 6.2-8.0 seconds for Tmax, depending on the postprocessing method. This truncation falsely reduced estimated Tmax lesion volume by 71.5 or 93.8 mL, depending on the deconvolution method. "Lesion reversal" (ie, change from above-normal to apparently normal, or from >6 seconds to ≤6 seconds for the time to response function maximum) with increasing truncation occurred in 37%-46% of lesions for CBV, 2%-4% for CBF, 28%-54% for MTT, and 42%-44% for Tmax, depending on the postprocessing method.

CONCLUSIONS

Hidden truncation-related errors in perfusion images may be large enough to alter patient management or affect outcomes of clinical trials.

Optimal brain MRI protocol for new neurological complaint

Background/Purpose

Patients with neurologic complaints are imaged with MRI protocols that may include many pulse sequences. It has not been documented which sequences are essential. We assessed the diagnostic accuracy of a limited number of sequences in patients with new neurologic complaints.

Methods

996 consecutive brain MRI studies from patients with new neurological complaints were divided into 2 groups. In group 1, reviewers used a 3-sequence set that included sagittal T1-weighted, axial T2-weighted fluid-attenuated inversion recovery, and axial diffusion-weighted images. Subsequently, another group of studies were reviewed using axial susceptibility-weighted images in addition to the 3 sequences. The reference standard was the study's official report. Discrepancies between the limited sequence review and the reference standard including Level I findings (that may require immediate change in patient management) were identified.

Results

There were 84 major findings in 497 studies in group 1 with 21 not identified in the limited sequence evaluations: 12 enhancing lesions and 3 vascular abnormalities identified on MR angiography. The 3-sequence set did not reveal microhemorrhagic foci in 15 of 19 studies. There were 117 major findings in 499 studies in group 2 with 19 not identified on the 4-sequence set: 17 enhancing lesions and 2 vascular lesions identified on angiography. All 87 Level I findings were identified using limited sequence (56 acute infarcts, 16 hemorrhages, and 15 mass lesions).

Conclusion

A 4-pulse sequence brain MRI study is sufficient to evaluate patients with a new neurological complaint except when contrast or angiography is indicated.

Corticospinal tract diffusion abnormalities early after stroke predict motor outcome

BACKGROUND

Prognosis of long-term motor outcome of acute stroke patients with severe motor impairment is difficult to determine.

OBJECTIVE

Our primary goal was to evaluate the prognostic value of corticospinal tract (CST) injury on motor outcome of the upper limb compared with motor impairment level and lesion volume.

METHODS

In all, 10 acute stroke patients with moderately severe to severe motor impairment of the upper limb underwent diffusion tensor imaging (DTI) and testing of upper limb strength and dexterity at acute, subacute, and chronic poststroke time points. A density-weighted CST atlas was constructed using DTI tractography data from normal participants. This CST atlas was applied, using a largely automated process, to DTI data from patients to quantify CST injury at each time point. Differences in axial diffusivity (AD), radial diffusivity (RD), and fractional anisotropy (FA) of the ipsilesional CST relative to the contralesional CST were measured.

RESULTS

Acute loss in CST AD correlated most strongly and significantly with subacute and chronic strength and dexterity and remained significant after adjusting for acute motor impairment or lesion volume. Subacute loss in CST FA correlated most strongly with chronic dexterity, whereas subacute behavioral measures of limb strength correlated most strongly with chronic strength measures.

CONCLUSIONS

Loss in acute CST AD and subacute CST FA are strong prognostic indicators of future motor functions of the upper limb for stroke patients with substantial initial motor impairment. DTI-derived measure of CST injury early after stroke may have utility in health care planning and in design of acute stroke clinical trials.

Time and diffusion lesion size in major anterior circulation ischemic strokes

BACKGROUND AND PURPOSE

Major anterior circulation ischemic strokes caused by occlusion of the distal internal carotid artery or proximal middle cerebral artery or both account for about one third of ischemic strokes with mostly poor outcomes. These strokes are treatable by intravenous tissue-type plasminogen activator and endovascular methods. However, dynamics of infarct growth in these strokes are poorly documented. The purpose was to help understand infarct growth dynamics by measuring acute infarct size with diffusion-weighted imaging (DWI) at known times after stroke onset in patients with documented internal carotid artery/middle cerebral artery occlusions.

METHODS

Retrospectively, we included 47 consecutive patients with documented internal carotid artery/middle cerebral artery occlusions who underwent DWI within 30 hours of stroke onset. Prospectively, 139 patients were identified using the same inclusion criteria. DWI lesion volumes were measured and correlated to time since stroke onset. Perfusion data were reviewed in those who underwent perfusion imaging.

RESULTS

Acute infarct volumes ranged from 0.41 to 318.3 mL. Infarct size and time did not correlate (R2=0.001). The majority of patients had DWI lesions that were <25% the territory at risk (<70 mL) whether they were imaged <8 or >8 hours after stroke onset. DWI lesions corresponded to areas of greatly reduced perfusion.

CONCLUSIONS

Poor correlation between infarct volume and time after stroke onset suggests that there are factors more powerful than time in determining infarct size within the first 30 hours. The observations suggest that highly variable cerebral perfusion via the collateral circulation may primarily determine infarct growth dynamics. If verified, clinical implications include the possibility of treating many patients outside traditional time windows.

Abstract W P33: Functional MRI and CBF Responses to Transient Oxygen Challenges in Acute Ischemic Stroke Patients

Background: There is increasing interest in using oxygen challenges during MRI to image metabolic dysfunction in acute ischemic stroke.

Methods: Subjects enrolled in a prospective MRI study of patients imaged within 48 h of stroke onset underwent arterial spin labeling (ASL) MRI for 10-12 min, while receiving a transient normobaric oxygen (NBO) challenge, according to this protocol: (1) Room air (RA) for 2-5 min; (2) NBO at 10-12 L/min via nonrebreather mask for 4 min; (3) RA for 2-4 min. ASL data were motion corrected and used to calculate CBF. Changes of blood-oxygen level dependent (BOLD) and perfusion MRI signal intensities during NBO were calculated (along with Z-scores) using FEAT. Mean changes within the DWI lesion (Core), ipsilateral normal tissue (IPS) and contralesional hemisphere (CNL) were compared.

Results: Patient characteristics (N=19) were age 62±13 years, median admission NIHSS 12 [IQR 3-12], time-to-MRI 33±13 h, median Core volume 31.2 [IQR 14.3-68] cc. 6 patients were imaged after tPA therapy. One subject’s CBF data were unusable due to artifacts. Both hyperemia (N=8) and hypoperfusion (N=12) were observed in areas in and around the DWI lesion. Baseline CBF in Core was significantly higher than in CNL (P=0.01) and IPS (P=0.03), indicative of hyperemia. No significant difference was found for perfusion change in response to NBO (mean Z-score < 1.1 for all regions). In contrast, strong positive and negative BOLD responses were found both ipsilaterally and contralaterally. Negative BOLD responses were significantly smaller (P<0.01) in the Core (Z-score 1.3±1.6) than in either IPS (2.5±1.6) or CNL (2.5±1.4). Positive BOLD responses did not significantly differ across regions, but had high Z-scores (Core: 5.2±2.6, IPS: 5.8±2.7, CNL:5.8±2.9), indicating strong response to NBO.

Discussion: Our results confirm previous reports in both human and experimental stroke models of strong increases in BOLD signal in response to NBO challenges. In addition, we found negative BOLD responses both ipsilesionally and contralesionally, often in conjunction with hyperemia, perhaps suggesting a steal effect. Future research may further elucidate the complexities of oxygen metabolism in stroke, perhaps encouraging the development of novel therapies.

Improving door-to-needle times: a single center validation of the target stroke hypothesis

BACKGROUND AND PURPOSE

National guidelines recommend imaging within 25 minutes of emergency department arrival and intravenous tissue-type plasminogen activator within 60 minutes of emergency department arrival for patients with acute stroke. In 2007, we implemented a new institutional acute stroke care model to include 10 best practices and evaluated the effect of this intervention on improving door-to-computed tomography (CT) and door-to-needle (DTN) times at our hospital.

METHODS

We compared patients who presented directly to our hospital with acute ischemic stroke in the preintervention (2003-2006) and postintervention (2008-2011) periods. We did not include 2007, the year that the new protocol was established. Predictors of DTN ≤60 minutes before and after the intervention were assessed using χ(2) for categorical variables, and t test and Wilcoxon signed-rank test for continuous variables.

RESULTS

Among 2595 patients with acute stroke, 284 (11%) received intravenous tissue-type plasminogen activator. For patients arriving within an intravenous tissue-type plasminogen activator window, door-to-CT <25 improved from 26.7% pre intervention to 52.3% post intervention (P<0.001). Similarly, the percentage of patients with DTN <60 doubled from 32.4% to 70.3% (P<0.001). Patients with DTN ≤60 did not differ significantly with respect to demographics, comorbidities, or National Institutes of Health Stroke Scale score in comparison with those treated after 60 minutes.

CONCLUSIONS

Door-to-CT and DTN times improved dramatically after applying 10 best practices, all of which were later incorporated into the Target Stroke Guidelines created by the American Heart Association. The only factor that significantly affected DTN60 was the intervention itself, indicating that these best practices can result in improved DTN times.

The Massachusetts General Hospital acute stroke imaging algorithm: an experience and evidence based approach

The Massachusetts General Hospital Neuroradiology Division employed an experience and evidence based approach to develop a neuroimaging algorithm to best select patients with severe ischemic strokes caused by anterior circulation occlusions (ACOs) for intravenous tissue plasminogen activator and endovascular treatment. Methods found to be of value included the National Institutes of Health Stroke Scale (NIHSS), non-contrast CT, CT angiography (CTA) and diffusion MRI. Perfusion imaging by CT and MRI were found to be unnecessary for safe and effective triage of patients with severe ACOs. An algorithm was adopted that includes: non-contrast CT to identify hemorrhage and large hypodensity followed by CTA to identify the ACO; diffusion MRI to estimate the core infarct; and NIHSS in conjunction with diffusion data to estimate the clinical penumbra.

Abstract WP32: Heterogeneity of Oxygen Extraction Fraction Weighted MRI in Acute Ischemic Stroke Patients

Background: Increases in oxygen extraction fraction (OEF) are postulated to be a marker of tissue at risk of infarction in acute ischemic stroke patients. R2’, a parameter that can be measured rapidly using an asymmetric spin echo MRI sequence (ASE), can be considered an OEF-weighted MRI metric, being a combination of both OEF and venous cerebral blood volume. We investigated the feasibility and utility of OEF-weighted MRI in acute stroke.

Methods: Adult ischemic stroke patients were prospectively enrolled in a 3T MRI study of patients imaged within 48 hours of stroke onset. Only the subset of patients whose MRI included ASE and whose DWI was positive for acute infarction was analyzed. Acquisition time for ASE was 1.5 min. R2’ maps was calculated from ASE data on a voxel-wise basis after motion correction and smoothing. DWI and follow-up images (FU), when available, were co-registered to the ASE. Mean R2’ values and coefficient of variance (mean/SD) within the lesion, ipsilateral DWI-normal (IPS) and contralesional hemisphere (CNL) were calculated and compared.

Results: Fifteen subjects met the inclusion criteria with mean±SD age 64±14 years, median admission NIHSS 12 [IQR 7-15] and time from onset to MRI 39±30 h. Eleven patients were imaged after tPA therapy. Both elevated and decreased R2’ were observed within DWI and FU lesions. Mean R2’ values in the DWI lesion (6.0±1.5 Hz) were significantly lower than CNL (7.2±0.6 Hz, P=0.02) and IPS (7.2±0.7 Hz, P=0.01). Coefficient of variance in R2’ values in the DWI lesion were also significantly lower than IPS (P=0.03) or CNL (P=0.05). For the 12 subjects with follow-up imaging, mean R2’ in tissue that eventually infarcted (5.7±1.8 Hz) was significantly lower (P=0.02) than in ipsilateral tissue surrounding the eventually infarcted lesion (7.3±0.6 Hz) but not significantly lower than DWI lesion values (6.1±1.6 Hz).

Discussion: In this study using ASE, decreased OEF-weighted MRI values can potentially reflect tissue that is destined for infarction since OEF is proportional to cerebral oxygen metabolic rate. Increased OEF-weighted MRI values may be a sign of tissue that is at risk of infarction. Studies including measures of CBF and CBV will allow for better discrimination of tissue status.

Comparison of wideband steady-state free precession and T₂-weighted fast spin echo in spine disorder assessment at 1.5 and 3 T

Wideband steady-state free precession (WB-SSFP) is a modification of balanced steady-state free precession utilizing alternating repetition times to reduce susceptibility-induced balanced steady-state free precession limitations, allowing its use for high-resolution myelographic-contrast spinal imaging. Intertissue contrast and spatial resolution of complete-spine-coverage 3D WB-SSFP were compared with those of 2D T₂-weighted fast spin echo, currently the standard for spine T₂-imaging. Six normal subjects were imaged at 1.5 and 3 T. The signal-to-noise ratio efficiency (SNR per unit-time and unit-volume) of several tissues was measured, along with four intertissue contrast-to-noise ratios; nerve-ganglia:fat, intradural-nerves:cerebrospinal fluid, nerve-ganglia:muscle, and muscle:fat. Patients with degenerative and traumatic spine disorders were imaged at both MRI fields to demonstrate WB-SSFP clinical advantages and disadvantages. At 3 T, WB-SSFP provided spinal contrast-to-noise ratios 3.7-5.2 times that of fast spin echo. At 1.5 T, WB-SSFP contrast-to-noise ratio was 3-3.5 times that of fast spin echo, excluding a 1.7 ratio for intradural-nerves:cerebrospinal fluid. WB-SSFP signal-to-noise ratio efficiency was also higher. Three-dimensional WB-SSFP disadvantages relative to 2D fast spin echo are reduced edema hyperintensity, reduced muscle signal, and higher motion sensitivity. WB-SSFP's high resolution and contrast-to-noise ratio improved visualization of intradural nerve bundles, foraminal nerve roots, and extradural nerve bundles, improving detection of nerve compression in radiculopathy and spinal-stenosis. WB-SSFP's high resolution permitted reformatting into orthogonal planes, providing distinct advantages in gauging fine spine pathology.

Reliability of cerebral blood volume maps as a substitute for diffusion-weighted imaging in acute ischemic stroke

PURPOSE

To assess the reliability of cerebral blood volume (CBV) maps as a substitute for diffusion-weighted MRI (DWI) in acute ischemic stroke. In acute stroke, DWI is often used to identify irreversibly injured "core" tissue. Some propose using perfusion imaging, specifically CBV maps, in place of DWI. We examined whether CBV maps can reliably subsitute for DWI, and assessed the effect of scan duration on calculated CBV.

MATERIALS AND METHODS

We retrospectively identified 58 patients who underwent DWI and MR perfusion imaging within 12 h of stroke onset. CBV in each DWI lesion's center was divided by CBV in the normal-appearing contralateral hemisphere to yield relative regional CBV (rrCBV). The proportion of lesions with decreased rrCBV was calculated. After using the full scan duration (110 s after contrast injection), rrCBV was recalculated using simulated shorter scans. The effect of scan duration on rrCBV was tested with linear regression.

RESULTS

Using the full scan duration (110 s), rrCBV was increased in most DWI lesions (62%; 95% confidence interval, 48-74%). rrCBV increased with increasing scan duration (P < 0.001). Even with the shortest duration (39.5 s) rrCBV was increased in 33% of lesions.

CONCLUSION

Because DWI lesions may have elevated or decreased CBV, CBV maps cannot reliably substitute for DWI in identifying the infarct core.

MR perfusion imaging in acute ischemic stroke

Magnetic resonance (MR) perfusion imaging offers the potential for measuring brain perfusion in acute stroke patients, at a time when treatment decisions based on these measurements may affect outcomes dramatically. Rapid advancements in both acute stroke therapy and perfusion imaging techniques have resulted in continuing redefinition of the role that perfusion imaging should play in patient management. This review discusses the basic pathophysiology of acute stroke, the utility of different kinds of perfusion images, and research on the continually evolving role of MR perfusion imaging in acute stroke care.

Abstract P37: A Novel Emergency Stroke Alert System Reduces Door-to-CT And Door-to-Needle Time in Patients With Acute Ischemic Stroke

Background: National guidelines recommend eligible acute stroke patients undergo neuroimaging within 25 min and IV tPA within 60 min. In order to reduce door-to-needle time, we implemented an “ED2CT” virtual group pager which allows ED staff to simultaneously activate the Stroke Team, neuroradiologists, CT technologists, nursing supervisors and pharmacists.

Methods: We performed an IRB approved retrospective review of a prospectively acquired cohort of consecutive patients with ischemic stroke presenting to a single tertiary stroke center using our Get With the Guidelines Stroke (GWTG-S) database. We compared patients who received IV tPA within 3 hours of symptom onset pre- (March 2006-April 2008) to post-intervention (September 2008-December 2009) by Wilcoxon or Fisher's exact as appropriate.

Results: Overall, there were 56 patients in the pre-intervention and 53 in the post-intervention groups. Patients were 50.5% male, median age was 76 [IQR 63, 85] years, median time to presentation was 50 [IQR 33, 87] min, and median initial NIHSS was 14 [IQR 8, 20]. None of these variables were significantly different between the pre- and post-intervention groups. Implementation of the ED2CT alert was associated with a reduction of 31% in door-to-CT time (29 [22, 40] vs. 20 [16, 29] min; p=<0.001) and 13.5% in door-to needle time (59 [42, 78] vs. 51 [35, 62] min; p=0.02). In addition, there was an increase of 55% in the proportion of patients undergoing CT within 25 min (42.9% vs.66.7 % p=0.01) and 39% in door-to needle within 60 min (51.8% vs. 72.0% p=0.03). Symptomatic intracerebral hemorrhage (sICH) was infrequent among patients receiving IV tPA with or without rescue IA reperfusion (n=109, 8.3%) and those with IV tPA only (n=83, 6.0%); there was a trend in reduced sICH rate post intervention (11.6% vs. 0%; p=0.06).

Conclusions: A novel emergency alert system with which the ED attending directly activates multiple members of the acute stroke clinical and imaging team was associated with an improved door-CT time and improved door-tPA time without an increased risk of sICH. This approach aligns acute stroke care activation with trauma and emergency cardiac care and suggests that team-based approaches may be better than specialty -specific responses.

Cerebral blood flow thresholds for tissue infarction in patients with acute ischemic stroke treated with intra-arterial revascularization therapy depend on timing of reperfusion

BACKGROUND AND PURPOSE

MR perfusion CBF values can distinguish hypoperfused penumbral tissue likely to infarct from that which is likely to recover. Our aim was to determine if CBF thresholds for tissue infarction depend on the timing of recanalization in patients with acute stroke treated with IAT.

MATERIALS AND METHODS

Twenty-six patients with acute proximal anterior circulation strokes underwent DWI and PWI before IAT. rCBF was obtained in the following areas: 1) C with abnormal DWI, reduced CBF, follow-up infarction; 2) PI with normal DWI, reduced CBF, follow-up infarction and 3) PNI with normal DWI, reduced CBF, normal follow-up. rCBF in tissue reperfused at <6 hours (early recanalizers), in tissue reperfused at >6 hours (late RC), and in NRC was compared.

RESULTS

For C, mean rCBF was 0.13 (SEM, 0.002), 0.29 (0.007), and 0.21 (0.004) for early recanalizers, late recanalizers, and nonrecanalizers, respectively (P < .001, for all comparisons). For PI, mean rCBF was 0.34 (0.006), 0.38 (0.008), and 0.39 (0.005) for early recanalizers, late recanalizers, and nonrecanalizers, respectively (P < .001 for early-versus-late recanalizers and versus nonrecanalizers; P > .05 for late recanalizers versus nonrecanalizers). For PNI, the mean rCBF was 0.38 (0.002), 0.48 (0.003), and 0.48 (0.004) for early recanalizers, late recanalizers, and nonrecanalizers, respectively (P < .001 for early-versus-late recanalizers and nonrecanalizers; P > .05 for late recanalizers versus nonrecanalizers). ROC analyzis demonstrated optimal rCBF thresholds for tissue infarction of 0.27 (sensitivity, 80%; specificity, 87%), 0.44 (sensitivity, 77%; specificity, 75%), and 0.41 (sensitivity, 78%; specificity, 77%) for early recanalizers, late recanalizers, and nonrecanalizers, respectively.

CONCLUSIONS

CBF thresholds for tissue infarction in patients with acute stroke are lower in tissue that is reperfused at earlier time points. This information may be important in selecting patients who might benefit from reperfusion therapy.

Time-resolved contrast-enhanced magnetic resonance angiography in the investigation of suspected intracranial dural arteriovenous fistula

Cerebral angiography is widely regarded as the gold standard for the evaluation and diagnosis of neurovascular abnormalities. However, recent improvements in the spatial and temporal resolution of time-resolved magnetic resonance angiography (MRA) offer clinicians a non-invasive alternative to cerebral angiography. We explored the utility of this technique in an elderly female patient with a suspected intracranial dural arteriovenous fistula (dAVF). A product pulse sequence available from the scanner's manufacturer (time-resolved imaging of contrast kinetics, TRICKS; GE Healthcare, Milwaukee, WI, USA) was used with the following parameters: TR/TE 2.832/TE 1.072 ms, flip angle 25°, receiver bandwidth 31.25 kHz, 0.75 NEX, acceleration factor (ASSET) of 2, field of view 14 cm, matrix size 96 × 96, phase-encoding left-right. Twenty overlapping 8-mm-thick slices were acquired in an axial orientation, with a slice spacing of 4mm. Images were acquired at 48 time points, with a temporal resolution of 0.3s/image. We found that all intracranial venous structures enhanced synchronously. There was no evidence of arteriovenous shunting. Retrograde venous flow explained the signal abnormality seen on time-of-flight MRA. We concluded that time-resolved MRA is useful in the investigation of suspected intracranial dAVF.

Early experience of translating pH-weighted MRI to image human subjects at 3 Tesla

BACKGROUND AND PURPOSE

In acute stroke, mismatch between lesions seen on diffusion- (DWI) and perfusion-weighted (PWI) MRI has been used to identify ischemic tissue before irreversible damage. Nevertheless, the concept of PWI/DWI mismatch is oversimplified and the ischemic tissue metabolic status and outcome are often heterogeneous. Tissue pH, a well-regulated physiological index that alters on disrupted tissue metabolism, may provide a surrogate metabolic imaging marker that augments the DWI and PWI for penumbra imaging.

METHODS

pH-weighted MRI was obtained by probing the pH-dependent amide proton transfer between endogenous mobile proteins/peptides and tissue water. The technique was validated using animal stroke models, optimized for human use, and preliminarily tested for imaging healthy volunteers.

RESULTS

pH-weighted MRI is sensitive and specific to ischemic tissue acidosis. pH MRI can be optimized for clinical use, and a pilot human study showed it is feasible using a standard 3 Tesla MRI scanner.

CONCLUSIONS

Ischemic acidosis can be imaged via an endogenous pH-weighted MRI technique, which complements conventional PWI and DWI for penumbra imaging. pH-weighted MRI has been optimized and appears feasible and practical in imaging human subjects. Additional study is necessary to elucidate the diagnostic use of pH MRI in stroke patients.

Combining acute diffusion-weighted imaging and mean transmit time lesion volumes with National Institutes of Health Stroke Scale Score improves the prediction of acute stroke outcome

BACKGROUND AND PURPOSE

The purpose of this study was to determine whether acute diffusion-weighted imaging (DWI) and mean transit time (MTT) lesion volumes and presenting National Institutes of Health Stroke Scale (NIHSS) can identify patients with acute ischemic stroke who will have a high probability of good and poor outcomes.

METHODS

Fifty-four patients with acute ischemic stroke who had MRI within 9 hours of symptom onset and 3-month follow-up with modified Rankin scale were evaluated. Acute DWI and MTT lesion volumes and baseline NIHSS scores were calculated. Clinical outcomes were considered good if the modified Rankin Scale was 0 to 2.

RESULTS

The 33 of 54 (61%) patients with good outcomes had significantly smaller DWI lesion volumes (P=0.0001), smaller MTT lesion volumes (P<0.0001), and lower NIHSS scores (P<0.0001) compared with those with poor outcomes. Receiver operating characteristic curves for DWI, MTT, and NIHSS relative to poor outcome had areas under the curve of 0.889, 0.854, and 0.930, respectively, which were not significantly different. DWI and MTT lesion volumes predicted outcome better than mismatch volume or percentage mismatch. All patients with a DWI volume >72 mL (13 of 54) and an NIHSS score >20 (6 of 54) had poor outcomes. All patients with an MTT volume of <47 mL (16 of 54) and an NIHSS score <8 (17 of 54) had good outcomes. Combining clinical and imaging thresholds improved prognostic yield (70%) over clinical (43%) or imaging (54%) thresholds alone (P=0.01).

CONCLUSIONS

Combining quantitative DWI and MTT with NIHSS predicts good and poor outcomes with high probability and is superior to NIHSS alone.

Magnetic resonance perfusion-weighted imaging of acute cerebral infarction: effect of the calculation methods and underlying vasculopathy

BACKGROUND AND PURPOSE

Various calculation methods are available to estimate the transit-time on MR perfusion-weighted imaging (PWI). Each method may affect the results of PWI. Steno-occlusive disease in the parent vessels is another factor that may affect the results of the PWI. The purpose of this study was to elucidate the effect of the calculation methods and underlying vasculopathy on PWI.

METHODS

From a pool of 113 patients who had undergone PWI during the study period, a total of 12 patients with nonlacunar ischemic strokes who were scanned within 24 hours after onset of symptom were selected for the study. The patient population consisted of 6 patients who had extracranial internal carotid artery stenosis (>70%) and 6 individuals without stenosis. Seven different postprocessing methods were evaluated: first moment, ratio of area to peak, time to peak (TTP), relative TTP, arrival time, full-width at half-maximum, and deconvolution methods. Follow-up MR or CT images were used to determine the areas that evolved into infarcts, which served as the gold standard. Sensitivity and specificity of each transit time technique were calculated.

RESULTS

Calculation methods with high sensitivity were the first moment (sensitivity, 74%), TTP (sensitivity, 77%), and deconvolution methods (sensitivity, 81% to 94%). Between the 2 groups with and without internal carotid artery stenosis, the specificity of most of the techniques was lower in the internal carotid artery stenosis group. The first moment and deconvolution methods maintained relatively high specificity even in the stenosis group.

CONCLUSIONS

The calculation technique and presence of underlying vasculopathy have a direct impact on the results of PWI. The methods with high sensitivity even in the presence of steno-occlusive disease were the first moment and deconvolution methods with arterial input function derived from the peri-infarct arteries; the deconvolution method was the superior choice because of higher lesion conspicuity.

Ischemic stroke: effects of etiology and patient age on the time course of the core apparent diffusion coefficient

PURPOSE

To determine whether the evolution of the core apparent diffusion coefficient (ADC) of water in ischemic stroke varies with patient age or infarct etiology.

MATERIALS AND METHODS

One hundred forty-seven patients with stroke underwent 236 diffusion-weighted magnetic resonance imaging examinations. Etiologies of lesions were classified according to predefined criteria; in 224 images, the diagnosis of lacune could be firmly established or excluded. ADC was measured in the center of each lesion and in contralateral normal-appearing brain. A model was used to describe the time course of relative ADC (rADC), which is calculated by dividing the lesion ADC by the contralateral ADC, and to test for age- or etiology-related differences in this time course.

RESULTS

Transition from decreasing to increasing rADC was estimated at 18.5 hours after stroke onset. In subgroup analysis, transition was earlier in nonlacunes than in lacunes (P =.02). There was a trend toward earlier transition in patients older than the median age of 66.0 years, compared with younger patients (P =.06). Pseudonormalization was estimated at 216 hours. Among nonlacunes, the rate of subsequent rADC increase was more rapid in younger patients than in older patients (P =.001). Within the smaller sample of lacunes, however, no significant age-related difference in this rate was found.

CONCLUSION

Differences in ADC depending on the patient's age and infarct etiology suggest differing rates of ADC progression.

Predicting tissue outcome in acute human cerebral ischemia using combined diffusion- and perfusion-weighted MR imaging

BACKGROUND AND PURPOSE

Tissue signatures from acute MR imaging of the brain may be able to categorize physiological status and thereby assist clinical decision making. We designed and analyzed statistical algorithms to evaluate the risk of infarction for each voxel of tissue using acute human functional MRI.

METHODS

Diffusion-weighted MR images (DWI) and perfusion-weighted MR images (PWI) from acute stroke patients scanned within 12 hours of symptom onset were retrospectively studied and used to develop thresholding and generalized linear model (GLM) algorithms predicting tissue outcome as determined by follow-up MRI. The performances of the algorithms were evaluated for each patient by using receiver operating characteristic curves.

RESULTS

At their optimal operating points, thresholding algorithms combining DWI and PWI provided 66% sensitivity and 83% specificity, and GLM algorithms combining DWI and PWI predicted with 66% sensitivity and 84% specificity voxels that proceeded to infarct. Thresholding algorithms that combined DWI and PWI provided significant improvement to algorithms that utilized DWI alone (P=0.02) but no significant improvement over algorithms utilizing PWI alone (P=0.21). GLM algorithms that combined DWI and PWI showed significant improvement over algorithms that used only DWI (P=0.02) or PWI (P=0.04). The performances of thresholding and GLM algorithms were comparable (P>0.2).

CONCLUSIONS

Algorithms that combine acute DWI and PWI can assess the risk of infarction with higher specificity and sensitivity than algorithms that use DWI or PWI individually. Methods for quantitatively assessing the risk of infarction on a voxel-by-voxel basis show promise as techniques for investigating the natural spatial evolution of ischemic damage in humans.