Beyond Brain Parenchyma: A Pictorial Review of Non-Parenchymal and Extracranial Metastases on Cranial MRI.

Brain MRI is the primary imaging technique used to detect, stage, and monitor intracranial metastatic disease. While parenchymal metastases are easily identifiable, metastatic involvement beyond the brain parenchyma is frequently subtle and underreported, despite representing a significant proportion of metastatic findings identified during cranial examinations. Failure to identify these metastases, or identification at a late stage, can lead to incomplete staging, inappropriate treatment selection, and delays in initiating targeted therapies. This pictorial review presents a systematic, compartment-based approach to non-parenchymal intracranial and extracranial metastases encountered on cranial MRI scans. The review covers metastatic involvement of the calvaria and skull base, leptomeninges, dura mater, ventricular system and choroid plexus, pituitary-hypothalamic axis, pineal gland, orbit, parotid gland, head and neck musculature, cutaneous and subcutaneous tissues, and upper cervical spinal cord. For each compartment, we summarize the most relevant clinical presentations, routes of dissemination, characteristic MRI patterns, major differential diagnoses, and practical diagnostic clues. Particular emphasis is placed on findings that may improve recognition in routine practice, including marrow replacement, leptomeningeal and cranial nerve enhancement, dural-based masses, infundibular involvement, rapidly growing pineal lesions, intraventricular enhancing masses, orbital infiltration, soft-tissue nodules, and intramedullary spinal cord lesions with extensive edema. A structured review of compartments beyond the brain parenchyma may improve detection and diagnostic confidence in oncologic cranial MRI. Awareness of common mimics and integration with oncologic history and interval changes are essential to avoid diagnostic pitfalls.