Atypical idiopathic inflammatory demyelinating lesions (IIDL): Conventional and diffusion-weighted MR imaging (DWI) findings in 42 cases

Introduction: The purpose of this study was to evaluate MR imaging characteristics with conventional and advanced MR imaging techniques in patients with IIDL. Methods: MR images of the brain in 42 patients (20 male, 22 female) with suspected or known multiple sclerosis (MS) from four institutions were retrospectively analyzed. Lesions were classified into five different subtypes: (1) ring-like lesions; (2) Balo-like lesions; (3) diffuse infiltrating lesions; (4) megacystic lesions; and (5) unclassified lesions. The location, size, margins, and signal intensities on T1WI, T2WI, and diffusion-weighted images (DWI), and the ADC values/ratios for all lesions, as well as the contrast enhancement pattern, and the presence of edema, were recorded. Results: There were 30 ring-like, 10 Balo-like, 3 megacystic-like and 16 diffuse infiltrating-like lesions were detected. Three lesions were categorized as unclassified lesions. Of the 30 ring-like lesions, 23 were hypointense centrally with a hyperintense rim. The mean ADC, measured centrally, was 1.50 ± 0.41 × 10⁻³ mm²/s. The mean ADC in the non-enhancing layers of the Balo-like lesions was 2.29 ± 0.17 × 10⁻³ mm²/s, and the mean ADC in enhancing layers was 1.03 ± 0.30 × 10⁻³ mm²/s. Megacystic lesions had a mean ADC of 2.14 ± 0.26 × 10⁻³ mm²/s. Peripheral strong enhancement with high signal on DWI was present in all diffuse infiltrating lesions. Unclassified lesions showed a mean ADC of 1.43 ± 0.13 mm²/s. Conclusion: Restriction of diffusion will be seen in the outer layers of active inflammation/demyelination in Balo-like lesions, in the enhancing part of ring-like lesions, and at the periphery of infiltrative-type lesions