PURPOSE: To examine the radiological differences between bacterial orbital cellulitis (OC) and diffuse non-specific orbital inflammation (DNSOI) on magnetic resonance imaging (MRI). METHODS: Retrospective study of patients with OC and DNSOI with an MRI orbital scan. Localised orbital inflammation (e.g., idiopathic dacryoadenitis and myositis), quiescent orbital inflammation and pre-septal cellulitis were excluded. RESULTS: Thirty-two patients presenting between 2008 and 2023, including twenty-one OC patients (mean age: 42.5 ± 24.9 years old, male: 6), and eleven DNSOI patients (mean age: 52.3 ± 17.8 years old, male: 16). Both OC and DNSOI demonstrate orbital fat contrast-enhancement. However, whilst OC demonstrated a hyperintense T2 signal (P <
0.001), variable signal was observed in DNSOI, with a hypointense T2 signal more suggestive of DNSOI (P = 0.012). When the lacrimal glands were involved, indistinct margins were more likely in OC (P <
0.001), whilst gross enlargement and contrast-enhancement was observed in DNSOI (P = 0.032 and 0.017, respectively). Peripheral contrast-enhancement of the extraocular muscle (EOM) (P = 0.002) was more common in OC, whilst DNSOI demonstrated variable contrast-enhancement throughout the affected EOM (P <
0.001). The presence of contralateral abnormalities, such as lacrimal gland enlargement and EOM involvement, are more suggestive of DNSOI. CONCLUSION: Several MRI features, beyond overt sinogenic disease, may help to differentiate OC from DNSOI, including the orbital fat signal intensity, EOM and/or lacrimal gland involvement, and contralateral orbital abnormalities. However, these features may not be specific, and thus highlights the ongoing radiological dilemma clinicians are faced when tasked with differentiating between infectious and non-infectious orbital inflammatory disease.