Objective To explore the value of 3D black bone sequence for measuring femoral head extrusion index and evaluating Herring classification of Legg-Calvé-Perthes disease(LCPD).Methods Data of hip joint MRI of 38 children with unilateral LCPD were retrospectively analyzed.Femoral head extrusion indices derived from coronal 3D black bone sequence and coronal fat suppression T2WI were compared to those from anteroposterior hip X-ray,respectively.Taken hip X-ray film as gold standards,receiver operating characteristic(ROC)curve was drawn,and the area under the curve(AUC)was calculated to evaluate the efficacy of 3D black bone sequence and fat suppression T2WI for evaluating Herring classification of LCPD,and DeLong test was performed for statistical comparison.Results Femoral head extrusion index of LCPD based on X-ray film and 3D black bone sequence simulated X-ray film was(11.31±6.16)％and(11.47±5.83)％on control side,respectively,being not significantly different(P＞0.05),while was(22.45±9.21)％and(22.82±7.92)％on the affected side,respectively,also being not significantly different(P＞0.05)but both higher than that on the control side(both P＜0.001).Femoral head extrusion index of LCPD based on fat suppression T2WI was(28.01±2.01)％on affected side and(27.25±1.92)％on control side,respectively,being not significantly different(P＞0.05)and both higher than those obtained based on X-ray film(both P＜0.001).AUC of 3D black bone sequence and fat suppression T2WI was 0.750-0.938 and 0.625-0.705,respectively.AUC of 3D black bone sequence for evaluating LCPD Herring B type was higher than that of fat suppression T2WI(P＜0.05),while no significant difference was found for evaluating other Herring type of LCPD(all P＞0.05).Conclusion 3D black bone sequences could be used to measure head extrusion index and assess Herring classification of LCPD,which was expected to replace hip X-ray to some extents combining with fat suppression T2WI.