Background:
Intracranial stenosis carries a high risk of recurrent stroke, but there is no noninvasive method to identify high-risk lesions. Fractional flow, or the pressure gradient across a stenosis, may identify hemodynamic effects and ischemic risk beyond percent stenosis of an artery. TOF-MRA signal intensity correlates with blood flow and may serve as a useful, noninvasive risk marker. We hypothesized that diminished TOF-MRA signal intensity distal to an intracranial stenosis predicts stroke risk.
Methods:
TOF-MRA of the intracranial circulation acquired prospectively in the SONIA-WASID trials was digitized to enable measurement of relative signal intensity immediately distal and proximal to symptomatic intracranial stenoses. The distal/proximal signal intensity ratio (SIR) was calculated from 3 mm regions of interest distal and proximal to symptomatic stenoses, correcting for background intensity and blinded to outcome. Univariate and multivariate analyses included clinical variables, SIR, and invasive angiography measures (luminal stenosis, TICI, collateral grade) to identify predictors for risk of stroke in the territory in this SONIA-WASID cohort.
Results:
189 patients with site-based 50-99% symptomatic intracranial stenosis in SONIA-WASID had TOF-MRA available. In a univariate analysis, the hazard ratio (HR) for stroke in the territory of the symptomatic artery with SIR < 0.9 (i.e. SIR below median) was 5.2 (1.8, 15.3; p=0.001) as compared to SIR ≥ 0.9. In a multivariate analysis correcting for baseline blood pressure, LDL, percent stenosis, recency of symptoms, TICI and downstream collaterals, the HR for SIR < 0.9 was 10.9 (2.0, 58.9; p=0.001). Only collaterals also had a significant independent association with stroke risk, HR 13.8 (3.4, 55.5; p<0.001). In the subset of patients with < 70% stenosis, a SIR < 0.9 maintained a significant association with recurrent stroke in the territory (p=0.006), with a two year event rate of 17.3%.
Conclusions:
Fractional flow assessed by TOF-MRA SIR may be a useful noninvasive tool to identify high-risk intracranial lesions, even in patients with moderate (< 70%) degrees of stenosis, and may be suitable for selection of high-risk patients for clinical trials or aggressive treatments.
Noninvasive Fractional Flow on MRA Predicts Stroke Risk of Intracranial Stenosis