T2 signal ratio enhances the diagnostic performance of apparent diffusion coefficient in differentiating orbital lymphoma from inflammatory mimickers.

Abstract

Purpose: Orbital lymphomas share overlapping clinical and radiological features with immunoglobulin G4-related orbital disease (IgG4-ROD) and granulomatous orbital diseases, which may lead to diagnostic delays. This study aims to evaluate the added diagnostic value of the transverse relaxation time (T2) signal ratio when combined with apparent diffusion coefficient (ADC) measurements in distinguishing orbital lymphoma from inflammatory mimickers.

Methods: In this retrospective study, two blinded radiologists independently measured T2 signal ratios (lesion to cerebral cortex), ADC values, and ADC ratios on pretreatment orbital magnetic resonance imaging scans of 58 patients (21 lymphomas, 21 IgG4-ROD, 16 granulomatous inflammation). Measurements were performed on axial images at the lesion's maximal diameter. Regions of interest were manually drawn to cover the entire lesion, avoiding necrosis and edges. Diagnostic performance was assessed using receiver operating characteristic curve analysis, with optimal cut-off values determined by Youden's index. The ADC ratio (≤1) and T2 signal ratio (>0.88) were combined using OR (either positive) and AND (both positive) rules. Interobserver agreement was evaluated using intraclass correlation coefficients (ICC).

Results: All measurements showed statistically significant differences between the two cohorts. Reviewer 1's ADC ratio measurements demonstrated excellent diagnostic performance, with an area under the curve (AUC) of 0.920 (85.7% sensitivity and 86.5% specificity at the optimal cut-off of ≤1). In comparison, T2 signal ratios showed moderate diagnostic value (AUC: 0.726; 80.95% sensitivity and 64.86% specificity at a cut-off >0.88). The combination of both parameters significantly improved diagnostic accuracy: the OR rule (ADC ≤1 or T2 >0.88) increased sensitivity to 95.2%, whereas the AND rule (ADC ≤1 and T2 >0.88) increased specificity to 94.6%. Interobserver reliability was excellent, with ICC values ranging from 0.969 to 0.985.

Conclusion: Although diffusion imaging remains the primary discriminator for orbital lymphoma, the T2 signal ratio considerably enhances diagnostic confidence, particularly in borderline ADC cases.

Clinical significance: Incorporating T2 signal ratio measurements adds minimal workflow burden in routine clinical practice and provides a practical tool for differentiating lymphoma from IgG4-ROD and granulomatous inflammatory mimics.