CT-Derived Endocranial Volume as a Surrogate for MRI-Derived Total Brain Volume in Domestic Cats

Background Magnetic resonance imaging (MRI) is the gold standard for brain volumetry but is often constrained by cost and limited accessibility in veterinary medicine. Computed tomography (CT) offers a faster and more available alternative. To develop a reproducible 3D Slicer workflow for feline endocranial volume (EV) and to quantify the agreement between CT-derived EV and MRI-derived total brain volume (TBV) to justify CT as a clinical surrogate, head CT and MRI scans of fifteen domestic cats (Felis catus) of various breeds, aged 4 months to 13 years, were evaluated in this retrospective observational study. Brain volumes were segmented using 3D Slicer, where MRI-TBV was measured via manual ROI annotation, and CT-EV was calculated using thresholding and seeded region-growing. Results CT-derived EV strongly predicted MRI-derived total brain volume (R2 = 0.925, p < 0.00002). EV was the only significant predictor (β = 1.04, p < 0.001). LOOCV confirmed robust accuracy (RMSE: 1.11 cm ³ ; mean absolute error: 1.04 cm ³ ), representing approximately 3%–4% of mean TBV. Bland-Altman analysis showed negligible mean bias (0.00 cm ³ ) with narrow 95% limits of agreement (95% CI: − 1.54 to + 1.54 cm ³ ). Conclusions CT-derived endocranial volume is a highly accurate surrogate for MRI-derived brain volume in domestic cats. This validated workflow enables cost-effective volumetric assessments in clinical neurology and comparative research where MRI is unavailable.Keywords: Brain volume, Computed tomography, Magnetic resonance imaging, Feline neuroanatomy, 3D Slicer, Endocranial volume (EV).