Association Between the Systemic Immune‐Inflammation Index and Aneurysmal Wall Enhancement on High‐Resolution Magnetic Resonance Vessel Wall Imaging in Unruptured Intracranial Aneurysms

ABSTRACT Background Inflammation is a critical driver of intracranial aneurysm (IA) instability. The systemic immune‐inflammation index (SII), a novel composite inflammatory biomarker, may reflect the local mural changes observed on vessel wall imaging (VWI). Purpose/Hypothesis To investigate the association between the SII and aneurysmal wall enhancement (AWE) on high‐resolution vessel wall imaging (HRVWI), and to evaluate SII as a potential biomarker for aneurysm instability. Study Type Prospective cross‐sectional study. Population Four hundred and eighteen intracranial aneurysms in 311 patients (65.3% female, median age 58.0 years) underwent HRVWI. A sub‐cohort of 67 patients with 84 aneurysms was included for longitudinal analysis of aneurysm growth (median follow‐up 7.0 months). Field Strength/Sequence Field strength/sequence: A 3.0T MR scanner; 3D time‐of‐flight magnetic resonance angiography (TOF‐MRA); 3D black‐blood T1‐weighted volumetric turbo spin echo acquisition (T1‐VISTA); and post‐contrast vessel wall imaging using a 3D fast field echo sequence. Assessment AWE defined as aneurysm‐to‐pituitary stalk contrast ratio (CR stalk ) ≥ 0.60. SII was calculated from admission blood counts as (platelet count * neutrophil count)/(lymphocyte count * 1000). Statistical Tests Multivariable logistic regression and propensity score matching evaluated the association between SII and AWE. Longitudinal analysis was performed using Firth's logistic regression. Subgroup analyses were stratified by clinical and aneurysmal characteristics. p < 0.05 was considered significant. Results AWE was present in 142/418 (34.0%) aneurysms. In multivariable analysis, elevated SII was independently associated with AWE (odds ratio OR = 1.93). This association remained significant after PSM (median SII: 0.60 in AWE group vs. 0.42 in non‐AWE group). SII was also independently associated with aneurysm growth in the longitudinal sub‐cohort (OR = 20.519). Subgroup analysis showed a significant interaction with aneurysm location, with the strongest association in the internal carotid artery (OR = 3.55; 95% CI: 1.63–7.71). Data Conclusion Elevated SII is independently associated with the presence of AWE and aneurysm growth. Evidence Level 3. Technical Efficacy Stage 2.