Factors affecting hemodynamic significance of symptomatic intracranial arterial stenosis (ICAS), for instance, collateralization, could alter subsequent stroke risk , so hemodynamic significance of ICAS may yield a good predictor for stroke risk in patients with symptomatic ICAS. Based on the signal contrast mechanism, flow-related enhancement  of time-of-flight (TOF) magnetic resonance angiography (MRA), and hemodynamic features of flowing blood in the case of stenosis, we assumed that changes of signal intensities (SIs) across an arterial stenosis might yield information on its hemodynamic significance. Therefore, in a pilot study, we developed and evaluated a novel index, named SI ratio (SIR), to quantify the hemodynamic significance of ICAS using TOF MRA.
The index SIR was calculated as follows: SIR = (mean poststenotic SI − mean background SI)/(mean prestenotic SI − mean background SI), where mean prestenotic, mean poststenotic (Fig. 1a), and left/right background SIs (Fig. 1b) were measured as shown in Fig. 1. SIRs of 26 arteries (18 middle cerebral arteries and eight intracranial internal carotid arteries) selected from patients with 50–99% symptomatic ICAS identified by three-dimensional TOF MRA (3·0 T) were measured and calculated by the same expert twice, and reproducibility of repeated measures was evaluated.
Mean SIRs were both 0·84 for repeated measures, with ranges of 0·35–1·25 and 0·39–1·25, respectively. Mean absolute and relative differences between repeated measures were 0·037 and 5%, respectively. Intrarater agreement was 88% for the 26 arteries studied, and Pearson's correlation coefficient for repetitive measures was 0·975.
Measurement and calculation of the index SIR on TOF MRA were easy to perform and highly reproducible, which makes it feasible to be carried out in clinical practice. Future studies are warranted to further test this novel method for evaluating hemodynamic significance of ICAS and to find out whether it is related to outcomes of patients with symptomatic ICAS.