Background Following reperfusion treatment in ischemic stroke, CT imaging at 24-hours is widely used to assess radiological outcomes. Even without visible hyperattenuation, occult angiographic contrast may persist in the brain and confound Hounsfield Unit-based imaging metrics such as Net Water Uptake (NWU). Aims We aimed to assess the presence and factors associated with retained contrast post-thrombectomy on 24-hour imaging using Dual Energy CT (DECT), and its impact on the accuracy of NWU as a measure of cerebral edema. Methods Consecutive patients with anterior circulation large vessel occlusion who had post-thrombectomy DECT performed 24-hours post-treatment from two thrombectomy stroke centers were retrospectively studied. NWU was calculated by interside comparison of Hounsfield Units of the infarct lesion and its mirror homolog. Retained contrast was quantified by the difference in NWU values with and without adjustment for iodine. Patients with visible hyperdensities from hemorrhagic transformation or visible contrast retention, and bilateral infarcts were excluded. Cerebral edema was measured by relative hemispheric volume (rHV) and midline shift (MLS). Results Of 125 patients analyzed (median age 71 [IQR 61-80], baseline NIHSS 16 [IQR 9.75-21]), reperfusion (defined as extended-Thrombolysis-In-Cerebral-Infarction 2b-3) was achieved in 113 patients (90.4%). Iodine-subtracted NWU was significantly higher than unadjusted NWU (17.1% vs 10.8%, p<0.001). In multivariable median regression analysis, increased age (p=0.024), number of passes (p=0.006), final infarct volume (p=0.023) and study site (p=0.021) were independently associated with amount of retained contrast. Iodine-subtracted NWU correlated with rHV (rho=0.154, p=0.043) and MLS (rho=0.165, p=0.033) but unadjusted NWU did not (rHV rho=-0.035, p=0.35; MLS rho=0.035, p=0.347). Conclusions Angiographic iodine contrast is retained in brain parenchyma 24-hours post-thrombectomy, even without visually obvious hyperdensities on CT, and significantly affects NWU measurements. Adjustment for retained iodine using DECT is required for accurate NWU measurements post-thrombectomy. Future quantitative studies analyzing CT after thrombectomy should consider occult contrast retention.

Introduction: CT performed 24h post-treatment is widely used to assess radiological outcomes in stroke studies. Even without visible hyperattenuation, occult angiographic contrast may persist in the brain and confound Hounsfield Unit-based imaging metrics such as Net Water Uptake (NWU), a measure of cerebral edema based on tissue hypoattenuation. We aimed to assess (1) the presence of retained contrast post-thrombectomy on 24h CT by comparing NWU measurements with and without adjustment for iodine using dual energy CT (DECT), (2) factors associated with amount of retention, and (3) its impact on the accuracy of NWU. Methods: In a prospective study of patients with anterior circulation large vessel occlusion who had post-thrombectomy follow-up DECT performed 24h post-treatment from two Comprehensive Stroke Centres (November 2021 to May 2022), NWU was calculated by interside comparison of Hounsfield Units of the infarct lesion and its mirror homolog. Retained contrast was quantified by the difference in NWU values with and without adjustment for iodine. We tested correlation between NWU and tissue swelling using relative hemispheric volume (rHV) and midline shift (MLS). Patients with visible hyperdensities from hemorrhagic transformation or visible contrast retention, and bilateral infarcts were excluded. Results: Of 125 patients analysed (median age 71 [IQR 61-80], baseline NIHSS 16 [IQR 9.75-21]), reperfusion (eTICI 2b-3) was achieved in 120 patients (96.8%). NWU measured with iodine adjustment was significantly higher than NWU measured without adjustment (17.1% vs 10.8%, p<0.001). In multivariable median regression analysis, age (p=0.031), number of passes (p<0.001) and having CT perfusion at baseline (p=0.008) were independently associated with amount of retained contrast. NWU measured with iodine adjustment correlated with rHV (p=0.043) and MLS (p=0.033), but NWU without adjustment did not (rHV p=0.350; MLS p=0.347). Conclusion: Occult angiographic iodine contrast significantly affects NWU on CT at 24h. Our data suggest adjustment for retained iodine using DECT is required for accurate NWU measures post-thrombectomy. Future studies analysing CT post-thrombectomy should consider the significance of occult contrast retention.