Abstract Body: Purpose: In ischemic stroke, MRI and CT are both utilized to estimate lesion progression distinguishing reversible from irreversible tissue injury and are therefore used for acute treatment decision-making. On MRI, relative increase of fluid-attenuated inversion recovery signal (rFLAIR) is thought to represent vasogenic edema indicating irreversible tissue injury. On CT, progressing hypoattenuation is the imaging correlate of ischemic net water uptake (NWU), which can be quantified by densitometry and may reflect both ionic edema, which is potentially reversible, and irreversible vasogenic edema following blood-brain barrier breakdown. The relationship of rFLAIR and NWU has not been compared directly. We hypothesized that the degree of early CT-based NWU is directly linked to rFLAIR in patients undergoing both modalities on admission.

Methods: Observational retrospective study of patients with acute anterior circulation ischemic stroke undergoing both CT and MRI on admission with visible core lesion on diffusion-weighted imaging. DWI-FLAIR mismatch was rated on MRI and quantitative NWU was measured on admission-CT. Multiparametric MRI signal changes were measured including rFLAIR and correlated to CT-based NWU with respect to time from symptom onset-admission.

Results: 53 patients were included. The median NWU was 7.7% (IQR: 4.9-13.0) and the median rFLAIR 1.38 (IQR: 1.21-1.72). There was no correlation between NWU and rFLAIR (r=0.15, p=0.48); however, NWU was significantly higher in FLAIR-positive versus FLAIR-negative cases (11.2% versus 5.5%, p<0.0001) and NWU significantly correlated with time (r=0.62, p<0.0001), while rFLAIR did not (r=0.1, p=0.73). In logistic regression analysis, higher NWU was significantly associated with FLAIR-positivity (OR: 1.31, 95%CI:1.11-1.56,p=0.001), and the optimal cut-off for NWU to classify FLAIR-positivity was 7.4% (AUC: 0.82, 95%CI: 0.70-0.94).

Conclusion: CT-based quantitative NWU distinguished FLAIR-positive from FLAIR-negative cases, significantly correlating with progressing time from onset while rFLAIR showed no correlation to time. Hence, NWU and rFLAIR might depict a different early lesion pathophysiology. A low degree of NWU without a correlate on FLAIR might represent early ionic edema, which is potentially reversible and could therefore explain cases of reversible lesion hypodensity.