Acute stroke: automatic perfusion lesion outlining using level sets.

PURPOSE: To develop a user-independent algorithm for the delineation of hypoperfused tissue on perfusion-weighted images and evaluate its performance relative to a standard threshold method in simulated data, as well as in acute stroke patients. MATERIALS AND METHODS: The study was approved by the local ethics committee, and patients gave written informed consent prior to their inclusion in the study. The algorithm identifies hypoperfused tissue in mean transit time maps by simultaneously minimizing the mean square error between individual and mean perfusion values inside and outside a smooth boundary. In 14 acute stroke patients, volumetric agreement between automated outlines and manual outlines determined in consensus among four neuroradiologists was assessed with Bland-Altman analysis, while spatial agreement was quantified by using lesion overlap relative to mean lesion volume (Dice coefficient). Performance improvement relative to a standard threshold approach was tested with the Wilcoxon signed rank test. RESULTS: The mean difference in lesion volume between automated outlines and manual outlines was -9.0 mL ± 44.5 (standard deviation). The lowest mean volume difference for the threshold approach was -25.8 mL ± 88.2. A significantly higher Dice coefficient was observed with the algorithm (0.71; interquartile range [IQR], 0.42-0.75) compared with the threshold approach (0.50; IQR, 0.27- 0.57; P , .001). The corresponding agreement among experts was 0.79 (IQR, 0.69-0.83). CONCLUSION: The perfusion lesions outlined by the automated algorithm agreed well with those defined manually in consensus by four experts and were superior to those obtained by using the standard threshold approach. This user-independent algorithm may improve the assessment of perfusion images as part of acute stroke treatment. SUPPLEMENTAL MATERIAL: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.13121622/-/DC1.

Interrater agreement for final infarct MRI lesion delineation.

BACKGROUND AND PURPOSE: Lesion volume measured on follow-up magnetic resonance imaging (MRI) is commonly used as an outcome parameter in clinical stroke trials. However, few studies have evaluated the optimal sequence choice and the interrater reliability of this outcome measure. The objective of this study was to quantify the geometric interrater agreement for lesion delineation of chronic infarcts on T2-weighted and fluid-attenuated inverse recovery (FLAIR) MRI. METHODS: In a retrospective study of 14 patients, lesions on 90-day follow-up FLAIR and T2 fast spin echo MRI were outlined by 9 independent, blinded, experienced neuroradiologists. Voxel-wise interrater agreement was measured as (1) the volume of the intersection of individual rater's lesion outlines relative to the mean lesion volume (overlap ratio) and (2) the Hausdorff distance between the lesion markings. RESULTS: Mean patient age was 64.4 years (range, 45 to 79). Lesion volumes on FLAIR were, on average, 2.5 mL greater than were T2 volumes (median; P<0.001). We found considerable differences between raters' lesion markings, but interrater agreement was consistently better on FLAIR than on T2 images, as measured by a greater overlap ratio (P<0.0001) and a smaller Hausdorff distance (P<0.0001) on FLAIR than on T2. CONCLUSIONS: FLAIR should be used to quantify follow-up infarct size to minimize interrater variability. Our study suggests that imaging analysis performed by 1 or a few trained readers may be preferred. Future studies should address objective and preferably automated criteria for final lesion delineation.