Diffusion‐Weighted Imaging of Intracranial Ring‐Enhancing Lesions

Ring‐enhancing magnetic resonance imaging (MRI) lesions result from various diseases, including infection, neoplasia, inflammation, and vascular etiologies. Differentiation based on standard MRI sequences can be challenging. This study aims to compare the apparent diffusion coefficient (ADC) values of intracranial ring‐enhancing lesions of infectious etiology with ring‐enhancing lesions caused by other etiologies. Records were reviewed for MRI studies with diffusion‐weighted imaging (DWI) and post‐gadolinium T1‐weighted ring‐enhancing lesions with a definitive histopathological diagnosis or a microbiological diagnosis of brain infection. ADC maps were generated, and regions of interest were selected to evaluate ADC values of ring‐enhancing lesions. Normalized ADC values (rADC) were calculated using ADC values from lesional and contralateral brain regions of interest (rADC = ADC lesion /ADC CB ). A total of 69 cases met the inclusion criteria (68 dogs, 1 cat). Median (range) rADC was significantly lower for intraparenchymal bacterial abscesses [0.54 (0.19–0.82)] compared to ring‐enhancing gliomas [1.7 (0.80–3.9); p = 0.0003) and non‐infectious inflammatory lesions [1.7 (0.74–3.3); p = 0.024], but not significantly different compared to intraparenchymal hemorrhage [0.54 (0.33–0.87); p > 0.99]. Extraparenchymal bacterial empyema and intraparenchymal fungal abscesses did not exhibit apparent restricted diffusion, with median rADC (range) of 2.8 (1.3–3.4) and 1.2 (1.1–1.8), respectively. With exclusion of hemorrhagic lesions, an rADC of 0.65 had a specificity/sensitivity of 98%/78% for intraparenchymal bacterial abscess. Apparent restricted diffusion on DWI and ADC is a useful marker for identifying intraparenchymal bacterial abscesses among ring‐enhancing lesions. However, extraparenchymal bacterial empyema and fungal abscesses may not exhibit this feature.