Neurological Localization and Advanced MRI Interpretation in Dogs and Cats

Neurological localization (NL) in veterinary medicine relies on systematic integration of signalment, history, and neuro exam to pinpoint CNS lesion sites: forebrain (prosencephalon), brainstem (mesencephalon, pons, medulla oblongata), cerebellum, spinal cord (SC) segments: C1–C5, C6–T2, T3–L3, L4–S3), and peripheral nervous system (PNS). Accurate NL essential prior to advanced imaging. Forebrain lesions manifest as altered mentation, compulsive behaviors, seizures, visual deficits, contralateral postural reaction (PR) deficits. Brainstem dysfunction presents with cranial nerve (CN) deficits, altered consciousness, nystagmus, ataxia; midbrain (CN III), pons (CN V, VI, VII), medulla (CN IX–XII). Cerebellar signs: hypermetria, head tremor, intention tremor, ipsilateral PR loss. Vestibular signs: head tilt, nystagmus, circling—central (brainstem) vs. peripheral (CN VIII, vestibular apparatus). Spinal cord lesions: upper motor neuron (UMN) signs (hypertonia, hyperreflexia, crossed extensor) in T3–L3; lower motor neuron (LMN) signs (flaccidity, hyporeflexia) in C6–T2, L4–S3. PR deficits localize to SC segment or brain; conscious proprioception, hopping, wheelbarrowing. NL guides MRI protocol selection. MRI fundamentals: T1WI (anatomy), T2WI (pathology), FLAIR (CSF-suppressed T2), STIR (fat-suppressed T2), DWI (acute ischemia), GRE/SWI (hemorrhage, minerals). 3D sequences (e.g., CISS, SPACE) enhance CSF-tissue contrast for nerve root/foramen evaluation. Contrast (Gd-DTPA) highlights BBB disruption: neoplasia, inflammation, abscess. Avoid contrast in acute trauma (false +). Advanced MRI: DTI (white matter tracts), fMRI (regional activation), MRS (metabolites: NAA=neuronal health, Cr=reference, Cho=membrane turnover, Lac=lactic acidosis). MRS aids tumor grading (↑Cho/NAA), differentiating neoplasia vs. inflammation. Perfusion MRI (rCBV) assesses vascularity—high in gliomas, low in lymphoma. Susceptibility artifacts on GRE/SWI indicate hemorrhage (hemosiderin), calcification, or metal. Key anatomical planes: sagittal (midline structures), transverse (symmetry, CN), dorsal (cerebellar folia, SC symmetry). Species differences: feline brains smaller, more acute angulation; canine breed variations (brachycephalic vs. dolichocephalic). Brachycephalic dogs (e.g., Pugs, Bulldogs) prone to Chiari-like malformation (CLM), syringomyelia (SM)—best visualized on midline sag T2WI. Feline ischemic necrotizing encephalopathy (FITE) shows symmetric T2/FLAIR hyperintensity in thalamus, brainstem. Common pitfalls: misinterpreting flow artifacts (CSF pulsation) as lesion; confusing olfactory bulb asymmetry (normal) with mass; incomplete coverage missing cauda equina. Motion artifacts reduced with sedation (avoid in encephalopathic pts). Protocol: minimum 1.5T; 3T preferred for resolution. Sequences: sag T1, T2, FLAIR; trans T2, T1±Gd; optional DWI, STIR (spine). Spinal MRI: include entire SC, nerve roots, intervertebral foramina. IV disc disease: T2WI hypointensity (disc desiccation), T2 hyperintensity (extrusion), Gd enhancement (inflammation). Neoplasia: meningioma (hyperostosis, dural tail, Gd enhancement), glioma (infiltrative, variable enhancement), ependymoma (ependymal lining). Inflammatory: granulomatous meningoencephalitis (GME)—acute (multifocal T2 lesions), necrotizing (brainstem, thalamus), chronic (meningeal enhancement). Metronidazole toxicity: symmetric T2 hyperintensity in cr. colliculi, dentate nuclei. Toxoplasma/Gas: multifocal ring-enhancing lesions. Epilepsy workup: hippocampal atrophy, T2/FLAIR hyperintensity (mesial temporal sclerosis). Post-op changes: expected Gd enhancement up to 48h; beyond = recurrent/residual tumor. Cerebellar abiotrophy: cerebellar atrophy, normal signal. Always correlate imaging with NL—discordance suggests multifocal disease, artifact, or mislocalization. Use MPR (multiplanar reconstruction) for oblique structures (e.g., cranial cervical spine). Advanced interpretation requires understanding of vascular territories: rostral choroidal, middle cerebral, rostral/cr caudal cerebral arteries. Ischemic stroke: restricted diffusion, vascular territory distribution. Hypoxic-ischemic injury: bilateral cortical DWI changes. Future directions: AI-assisted segmentation, automated lesion detection, quantitative tractography for surgical planning.