DWI Breakthrough: MRI Metric Accurately Identifies Canine Brain Abscesses Among Ring-Enhancing Lesions

Diffusion-Weighted Imaging of Intracranial Ring-Enhancing Lesions

Adrien M. Dupanloup, Craig S. Brown, Karen M. Vernau, Ehren M. McLarty, Peter J. Dickinson

Background

Intracranial ring-enhancing lesions are a common but diagnostically challenging MRI finding in dogs and cats, arising from diverse causes such as infection, neoplasia, inflammation, or vascular injury. Conventional MRI sequences often fail to clearly distinguish between these etiologies. Diffusion-weighted imaging (DWI), which assesses water molecule movement, can differentiate restricted from facilitated diffusion, providing microstructural insight. This study aimed to determine whether apparent diffusion coefficient (ADC) values from DWI can distinguish infectious intracranial ring-enhancing lesions from non-infectious ones.

Methods

This retrospective study reviewed 69 cases (68 dogs, 1 cat) with MRI-confirmed intracranial ring-enhancing lesions imaged between 2010 and 2024 using a 1.5 T MRI system. Inclusion required histopathological or microbiological confirmation. Lesions were categorized as intraparenchymal bacterial abscess, extraparenchymal bacterial empyema, fungal abscess, glioma, other neoplasia, non-infectious inflammatory, or vascular. DWI sequences (b = 0, 1000 s/mm²) were used to calculate ADC maps, and normalized ADC ratios (rADC = ADC_lesion/ADC_contralateral brain) were derived. Statistical comparisons employed Kruskal–Wallis and Dunn’s tests, with ROC analysis assessing diagnostic thresholds.

Results

Intraparenchymal bacterial abscesses showed markedly reduced diffusion, with median rADC = 0.54 (0.19–0.82), significantly lower than gliomas (1.7 [0.80–3.9]; p = 0.0003) and non-infectious inflammatory lesions (1.7 [0.74–3.3]; p = 0.024). Extraparenchymal empyemas (rADC = 2.8 [1.3–3.4]) and fungal abscesses (1.2 [1.1–1.8]) did not show restricted diffusion. ROC analysis determined that an rADC < 0.65 (excluding hemorrhagic lesions) had specificity 98% and sensitivity 78% for diagnosing intraparenchymal bacterial abscess. Hemorrhagic lesions also exhibited low ADC values but were distinguishable via T2* or SWI sequences.

Limitations

As a retrospective study, it lacked longitudinal data to assess lesion evolution or treatment effects. The relatively small number of cases per category and absence of interobserver variability analysis may limit generalizability. DWI was acquired with only two b-values, potentially limiting diffusivity characterization, and ROI-based analysis introduced subjectivity.

Conclusions

DWI and ADC mapping are valuable adjuncts to standard MRI for differentiating intraparenchymal bacterial abscesses, which consistently demonstrate restricted diffusion, from gliomas and non-infectious inflammatory lesions. However, extraparenchymal empyemas and fungal abscesses may lack diffusion restriction. Incorporating DWI into MRI protocols enhances diagnostic specificity for ring-enhancing lesions and aids in clinical decision-making regarding infectious versus non-infectious etiologies.

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