Radiological imaging plays a pivotal role in the diagnosis and management of brain tumors. Distinguishing between various intracranial neoplasms and their mimics is crucial for accurate diagnosis, appropriate treatment planning, and predicting patient outcomes. This article provides a comprehensive overview of the radiological features that aid in the differential diagnosis of brain tumors, focusing on key imaging characteristics visible on CT and MRI scans. We will explore extra-axial and intra-axial neoplasms, alongside common tumor mimics, emphasizing the role of radiology in narrowing down the differential in clinical practice.
Extra-axial Neoplasms: Meningiomas and Schwannomas
Extra-axial neoplasms originate from the meninges or cranial nerves, lying outside the brain parenchyma itself. Meningiomas and schwannomas are the most prevalent types within this category and represent the most common non-glial tumors.
Meningiomas: Imaging Hallmarks
Meningiomas, arising from the arachnoid cap cells, typically exhibit a broad dural base, resulting in a hemispherical or globose shape. Another presentation is the “en plaque” meningioma, characterized by a flat, sheet-like growth along the inner skull table.
Key Radiological Features of Meningiomas:
- CT: Homogeneous high attenuation.
- MRI: Gray-matter-like signal intensity on T1 and T2 weighted images.
- Enhancement: Typically demonstrate avid and homogeneous contrast enhancement with gadolinium.
- Dural Tail: Frequently associated with an enhancing “dural tail,” representing reactive dural thickening rather than neoplastic infiltration.
- Hyperostosis: Adjacent hyperostosis (thickening of the bone) of the skull is a common finding.
- Peritumoral Edema: Vasogenic edema is observed in approximately 50% of cases, more likely when a clear CSF cleft is absent, T2 signal is high, and pial blood supply is present.
- Perfusion Imaging: Increased relative Cerebral Blood Volume (rCBV) and relative Cerebral Blood Flow (rCBF) with prolonged Mean Transit Time (MTT).
Figure 1: Meningioma. Coronal T1-weighted MR after gadolinium demonstrates a sharply demarcated, hemispheric, extra-axial lesion consistent with meningioma, note the overlying calvarial thickening (hyperostosis) and the enhancing “dural tail”.
Figure 2: Meningioma. Axial T2-weighted MR shows a bi-frontal soft-tissue mass centered on the falx, with associated bi-frontal intra-axial vasogenic edema, a common feature in meningioma radiology.
Schwannomas: Vestibular Schwannoma and Imaging Characteristics
Schwannomas arise from the Schwann cells of cranial nerves, most commonly affecting the vestibular nerve (Vestibular Schwannoma – VS), typically in the posterior fossa.
Key Radiological Features of Schwannomas:
- Location: Ventral to the brainstem, often in the cerebellopontine angle (CPA), originating from the internal auditory canal (IAC).
- Enhancement: Avid contrast enhancement, similar to meningiomas.
- Heterogeneity: Larger schwannomas often become heterogeneous due to cystic degeneration or necrosis.
- IAC Enlargement: Vestibular schwannomas frequently cause enlargement of the internal auditory canal.
Figure 3: Vestibular schwannoma. Axial T1-weighted MR after gadolinium demonstrates a heterogeneously enhancing left-sided mass in the cerebellopontine angle cistern, with extension into an enlarged internal auditory canal, typical of vestibular schwannoma.
Differential Diagnosis Point: While both meningiomas and schwannomas are extra-axial and enhance, schwannomas, particularly vestibular schwannomas, are distinguished by their nerve origin (e.g., IAC) and tendency to become heterogeneous with growth, unlike the typically homogeneous meningiomas.
Intra-axial Neoplasms: Gliomas, Lymphomas, and Intraventricular Tumors
Intra-axial neoplasms originate within the brain parenchyma. Gliomas are the most common, arising from glial cells. Lymphomas and intraventricular tumors also fall within this category.
Gliomas: Astrocytomas, Oligodendrogliomas, and GBMs
The 2021 WHO classification emphasizes molecular features like IDH and 1p/19q mutations in gliomas. Radiological features, however, remain crucial for initial diagnosis and differential considerations.
Key Radiological Features of Glioma Subtypes:
- Astrocytomas (IDH Mutant):
- T2/FLAIR Mismatch: Hyperintense rim on FLAIR with a hypointense center compared to T2, a sensitive but not always present sign.
- Enhancement: Often non-enhancing, especially lower grades.
- rCBV: Typically low rCBV in lower grades.
Figure 4: Low-grade astrocytoma (grade 2). Axial T2W, FLAIR, T1 post-contrast, and rCBV map demonstrate a left temporal mass with T2/FLAIR mismatch, no enhancement, and low rCBV, consistent with a low-grade astrocytoma.
- Oligodendrogliomas (IDH Mutant 1p19q Deleted):
- Calcification: Frequent intratumoral calcification visible on CT and SWI.
- Location: Frontal lobes, often involving cortex and subcortical white matter.
- Enhancement: Faint enhancement may be present.
- rCBV: Mildly elevated rCBV.
Figure 5: Oligodendroglioma. Axial CT, T2-weighted, FLAIR, post-contrast T1-weighted, SWI, and rCBV map showing a left frontal mass with punctate calcification, faint enhancement, and mildly raised rCBV, typical of oligodendroglioma.
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Glioblastomas (GBMs) (IDH Wild-type):
- Enhancement: Irregular and often with a necrotic center.
- Edema: Surrounding white matter infiltration/edema.
- Heterogeneity: Heterogeneous appearance.
- Non-enhancing GBMs: Increasingly recognized, can present as infiltrative non-enhancing tumors.
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Gliomas with H3 K27 Mutations:
- Midline Location: Typically involve midline structures.
Lymphomas: Primary CNS Lymphoma
Primary CNS lymphomas are predominantly diffuse large B-cell lymphomas.
Key Radiological Features of Lymphomas:
- T2 Signal: Hypointense to gray matter on T2W imaging.
- ADC: Low Apparent Diffusion Coefficient (ADC) values, indicating restricted diffusion.
- Enhancement: Homogeneous contrast enhancement.
- rCBV: Mildly elevated rCBV, lower than GBMs.
- T1 Leakage Effect: Marked T1 leakage effect on perfusion imaging.
- EBV+ Lymphomas: Ring enhancement, variable ADC, hemorrhagic components, associated with immune deficiency.
Figure 6: Primary CNS Lymphoma. Axial T2-weighted, FLAIR, T1 post-contrast, and ADC map demonstrate a mass in the splenium with homogeneous enhancement and low ADC, suggestive of primary CNS lymphoma.
Figure 7: Glioblastoma. Axial T2-weighted, FLAIR, T1 post-contrast, and ADC map showing a mass with heterogeneous enhancement and necrotic center, typical features differentiating glioblastoma.
Intraventricular Neoplasms: Neurocytoma and Others
Intraventricular tumors arise within the ventricular system.
Common Intraventricular Neoplasms and Key Locations:
- Colloid Cysts: Foramen of Monro/third ventricle.
- Meningiomas: Trigone of the third ventricle.
- Choroid Tumors & Ependymomas: Lateral ventricle (children), fourth ventricle (adults).
- Central Neurocytoma: Lateral ventricle, septum pellucidum involvement.
- Metastases: Choroidal/ependymal.
Central Neurocytoma Features:
- Diffusion Restriction: Frequently shows restricted diffusion.
- Calcifications: Often contains calcifications.
Figure 8: Central Neurocytoma. Axial T2-weighted, T1 pre-contrast, ADC map, and SWI demonstrate an intraventricular mass involving the septum pellucidum with heterogeneity, low ADC, and hypointensity on SWI, suggestive of central neurocytoma.
Tumor Mimics in Brain Tumor Differential Diagnosis
It is essential to consider tumor mimics in the differential diagnosis of brain lesions. These can be vascular, inflammatory, or infectious in origin.
Vascular Tumor Mimics: Stroke, Aneurysm, Cavernoma
- Subacute Stroke: Peripheral enhancement mimicking tumor, but DWI is crucial to identify diffusion restriction.
Figure 9: Subacute Stroke. Axial FLAIR, DWI, post-contrast T1-weighted, and coronal T2-weighted images demonstrate a cortico-subcortical lesion with peripheral enhancement but diffusion restriction, characteristic of stroke and a tumor mimic.
- Giant Thrombosed Aneurysm: May present as a non-enhancing mass, but angiography confirms the vascular nature.
Figure 10: Giant Thrombosed Aneurysm. Axial T1-weighted images before and after contrast, FLAIR, and angiogram showing a non-enhancing mass with peripheral thrombus, confirmed as a thrombosed aneurysm.
- Giant Cavernoma: Heterogeneous mass with spontaneous T1 hyperintensity and marked hypointensity on gradient echo T2*, due to blood products.
Figure 11: Giant Cavernoma. Axial FLAIR, T1-weighted, and gradient echo T2* images demonstrate a heterogeneous mass with spontaneous T1 hyperintensity and marked hypointensity on T2*, typical of a giant cavernoma.
Inflammatory Tumor Mimics: Behcet’s Disease and Sarcoidosis
- Behcet’s Disease: Inflammatory mass often in basal ganglia, may enhance and show restricted diffusion.
Figure 12: Behcet’s Disease. Coronal FLAIR, axial enhanced T1-weighted, DWI, and ACD images demonstrate a partially enhancing lesion in the basal ganglia with restricted diffusion, consistent with inflammatory mass in Behcet’s disease.
- Sarcoidosis: Dura-based mass mimicking meningioma, but may lack dural tail and hyperostosis, and show T2 hypointensity suggesting chronic inflammation.
Figure 13: Sarcoidosis. Coronal post-contrast T1-weighted and coronal T2 images reveal a dura-based mass lacking typical meningioma features, with T2 hypointensity suggestive of sarcoidosis.
Infectious Tumor Mimics: Bacterial and Toxoplasmosis Abscesses
- Bacterial Abscess: Ring-enhancing lesion with restricted diffusion centrally and a double rim sign on SWI.
Figure 14: Bacterial Abscess. Axial T2-weighted, post-contrast T1-weighted, DWI, ADC map, SWI, and phase images show a ring-enhancing lesion with central restricted diffusion and double rim sign, typical of bacterial abscess.
- Toxoplasmosis Abscess: Ring-enhancing lesion, but typically shows increased water diffusivity (increased ADC) and a “target sign” on T2W.
Figure 15: Toxoplasmosis Abscess. Axial T2-weighted, contrast-enhanced T1-weighted, DWI, and ACD map demonstrate a ring-enhancing mass with increased water diffusivity and target sign, characteristic of toxoplasmosis abscess.
Conclusion: The Role of Radiology in Brain Tumor Differential Diagnosis
Radiological imaging, particularly MRI and CT, is indispensable in the differential diagnosis of brain tumors. Key features such as location (extra-axial vs. intra-axial, intraventricular), signal intensity on different sequences (T1, T2, FLAIR, DWI), enhancement patterns, and perfusion characteristics, along with clinical context, allow for a narrowed differential diagnosis. Recognizing tumor mimics, including vascular, inflammatory, and infectious lesions, is equally crucial to avoid misdiagnosis and ensure appropriate patient management. A comprehensive understanding of these radiological features is essential for radiologists and clinicians involved in neuro-oncology.
