Purpose To develop a fast and robust method to resolve phase ambiguity in dual-echo Dixon imaging. Methods A major challenge in dual-echo Dixon imaging is to estimate the phase error resulting from field inhomogeneity. In this work, a binary quadratic optimization program was formulated to resolve the phase ambiguity. A projected power method was developed to efficiently solve the optimization problem. Both the 1-peak fat model and 6-peak fat model were applied to 3D datasets. Additionally, the proposed method was extended to dynamic MRI applications using the 6-peak fat model. With IRB approval and patient consent/assent, the proposed method was evaluated and compared with region growing on 29 consecutive 3D high-resolution patient datasets. Results Fast and robust water/fat separation was achieved by the proposed method in different representative 3D datasets and dynamic 3D datasets. Superior water/fat separation was achieved using the 6-peak fat model compared with the 1-peak fat model. Compared to region growing, the proposed method reduced water/fat swaps from 76% to 7% of the patient cohort. Conclusion The proposed method can achieve fast and robust phase error estimation in dual-echo Dixon imaging.

• Bone marrow oedema in corticosteroid-induced osteonecrosis predicts progression to bone collapse. • Bone marrow oedema is associated with subchondral fractures in corticosteroid-induced osteonecrosis. • Bone marrow oedema can be used to stratify patients to joint-preserving interventions. • Absence of bone marrow oedema can justify a "wait and watch" approach.

2 J. MAGN. RESON. IMAGING 2017;45:1700-1711.

Background: Ileocolonoscopy (IC) detects mucosal inflammation and magnetic resonance enterography (MRE) detects transmural inflammation in Crohn disease (CD). We aimed to evaluate the relationship between the simplified magnetic resonance index of activity (MARIAs) and measures of inflammation by IC in children with newly diagnosed CD. Methods: Retrospective review of 140 patients 6-18 years of age with CD who had baseline IC and MRE within 5 weeks of diagnosis. MARIAs was calculated for each intestinal segment (terminal ileum [TI], ascending colon, transverse colon, descending colon, sigmoid colon, rectum), defined asSensitivity and specificity were derived using receiver operating characteristic (ROC) curves to compare MARIAs to IC findings. Results: Using IC as the reference standard, the cutoff MARIAs !1 identified TI segments with active inflammation with 84% sensitivity, 73% specificity, 85% positive predictive value (PPV), 70% negative predictive value (NPV), and area under the curve (AUC) 0.782 (95% confidence interval [CI] 0.689-0.876). The cutoff MARIAs !2 identified TI segments with severe lesions with 87% sensitivity, 76% specificity, 87% PPV, 76% NPV, and AUC 0.814 (95% CI 0.712-0.916). There was poor sensitivity for all colonic segments. Conclusion: The MARIAs is feasible and accurate in reflecting disease activity in the TI, but not in the colon, in children with newly diagnosed CD. Although the MARIAs may be useful for monitoring TI disease activity over time, full assessment continues to require both IC and MRE.