Assessing reperfusion with whole-brain arterial spin labeling: a noninvasive alternative to gadolinium.

BACKGROUND AND PURPOSE: Arterial spin labeling (ASL) is a perfusion imaging technique that does not require gadolinium. The study aimed to assess the reliability of ASL for evaluating reperfusion in acute ischemic stroke in comparison with dynamic susceptibility contrast (DSC) imaging. METHODS: The study included 24 patients with acute ischemic stroke on admission and 24-hour follow-up ASL and DSC scans. Two readers rated images for interpretability and evidence of reperfusion. Cohen unweighted κ was used to assess (1) inter-rater reliability between readers for determining interpretability and the presence of reperfusion, (2) agreement between ASL and DSC for determining reperfusion for individual raters, and (3) agreement between ASL and DSC for determining reperfusion after consensus. RESULTS: Inter-rater reliability for both ASL and DSC was moderate to good (κ of 0.67 versus 0.55, respectively). Reader 1 rated 16 patients as having interpretable ASL and DSC when compared with 15 patients for reader 2. The κ between ASL and DSC for determining reperfusion was 0.50 for reader 1 and 0.595 for reader 2. After consensus, 18 ASL and 17 DSC image sets were rated interpretable for reperfusion and 13 had both interpretable ASL and DSC scans, yielding a κ for assessment of reperfusion of 0.8. CONCLUSIONS: Inter-rater reliability of ASL and DSC was moderate to good. Agreement between ASL and DSC for determining reperfusion was moderate for each individual rater and increased substantially after consensus. ASL is a noninvasive and practical alternative to DSC for reperfusion assessments in patients with confirmed acute ischemic stroke.

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.

A pragmatic approach using magnetic resonance imaging to treat ischemic strokes of unknown onset time in a thrombolytic trial.

BACKGROUND AND PURPOSE: Toward the goal of designing a clinical trial using imaging parameters to treat stroke patients with unknown onset time, we investigated the timing of changes on MRI in patients with well-defined stroke onset. METHODS: Hypothesis-generating (n=85) and confirmatory (n=111) samples were scored by blinded readers for fluid-attenuated inversion recovery (FLAIR) hyperintensity in diffusion-positive regions. Reader-measured signal intensity ratio (SIR) of the lesion to contralateral tissue was compared with SIR measured by coregistration. RESULTS: Lesion conspicuity increased with time on FLAIR (P=0.006). Qualitative assessment of FLAIR-negative vs FLAIR hyperintensity (k=0.7091; 95% CI, 0.61-0.81) showed good interrater agreement. Subtle hyperintensity was less reliably categorized (k=0.59; 95% CI, 0.47-0.71). Reader-measured SIR <1.15 can identify patients within the treatable time window of 4.5 hours (positive predictive value=0.90). The SIR was greater for right hemisphere lesions (P=0.04) for a given reported time from stroke symptom onset. CONCLUSIONS: The SIR on FLAIR provides a quantitative tool to identify early ischemic strokes. In developing SIR thresholds, right hemisphere lesions may confound the accurate estimate of stroke onset time. Image coregistration for thrombolytic trial enrollment is not necessary. A SIR <1.15 on FLAIR yields a practical estimate of stroke onset within 4.5 hours.

Whole-brain arterial spin labeling perfusion MRI in patients with acute stroke.

BACKGROUND AND PURPOSE: Perfusion MRI can be used to identify patients with acute ischemic stroke who may benefit from reperfusion therapies. The risk of nephrogenic systemic fibrosis, however, limits the use of contrast agents. Our objective was to evaluate the ability of arterial spin labeling (ASL), an alternative noninvasive perfusion technique, to detect perfusion deficits compared with dynamic susceptibility contrast (DSC) perfusion imaging. METHODS: Consecutive patients referred for emergency assessment of suspected acute stroke within a 7-month period were imaged with both ASL and DSC perfusion MRI. Images were interpreted in a random order by 2 experts blinded to clinical information for image quality, presence of perfusion deficits, and diffusion-perfusion mismatches. RESULTS: One hundred fifty-six patients were scanned with a median time of 5.6 hours (range, 3.0-17.7 hours) from last seen normal. Stroke diagnosis was clinically confirmed in 78 patients. ASL and DSC imaging were available in 64 of these patients. A perfusion deficit was detected with DSC in 39 of these patients; ASL detected 32 of these index perfusion deficits, missing 7 lesions. The median volume of the perfusion deficits as determined with DSC was smaller in patients who were evaluated as normal with ASL than in those with a deficit (median [interquartile range], 56 [10-116] versus 114 [41-225] mL; P=0.01). CONCLUSIONS: ASL can depict large perfusion deficits and perfusion-diffusion mismatches in correspondence with DSC. Our findings show that a fast 2½-minute ASL perfusion scan may be adequate for screening patients with acute stroke with contraindications to gadolinium-based contrast agents.