Quantitative measurement of superparamagnetic iron oxide nanoparticles (SPION)-labeled macrophage signal in the presence and absence of dexamethasone (Dexa) using three-dimensional semi-automated segmentation software with difference ultra-short echo time (dUTE) magnetic resonance (MR) images. (A) T1-weighted MR images of SPION-labeled macrophages in the synovium with varying hypointense signals (i) high (ii) low. (B) Corresponding dUTE MR images of the T1-weighted images seen in A with varying hyperintense (positive contrast) signal in the synovium (i) high (ii) low, that were used for three-dimensional signal segmentation. (C) Box plot graph comparing the SPION signal between the untreated and Dexa-treated groups using dUTE images. Data points ± standard error of the mean; n = 23, day 6; n = 22, day 10; n = 14, day 13. P <0.05 compared to the untreated control group (day 6, P = 0.00035; day 10, P = 0.00004; day 13, P = 0.0033).