KEY FACTS Terminology Collection of pus in subdural or epidural space, or both (15%); subdural much more common Subdural empyema (SDE), epidural empyema (EDE) Imaging Best diagnostic clue: Extraaxial collection with enhancing rim, DWI positive Supratentorial typical EDE: Often adjacent to frontal sinus SDE: Convexity in > 50%, parafalcine in 20% Infratentorial (up to 10%); related to mastoiditis T2 MR: Inwardly displaced dura seen as hypointense…

KEY FACTS Terminology Ventricular ependyma infection related to meningitis, ruptured brain abscess, or ventricular catheter Imaging Best imaging clue: Ventriculomegaly with debris level, abnormal ependyma, periventricular T2/FLAIR hyperintensity DWI Restriction of layering debris with low ADC is characteristic T1WI C+ Marked ependymal enhancement with ventriculomegaly Ultrasound Ventriculomegaly with echogenic ependyma and debris in infant – Can play important role in detection of postinfectious hydrocephalus Top Differential…

KEY FACTS Terminology Focal pyogenic infection of brain parenchyma, typically bacterial; fungal or parasitic less common 4 pathologic stages: Early cerebritis, late cerebritis, early capsule, late capsule Imaging Ring-enhancing lesion with T2 hypointense rim and central diffusion restriction characteristic Imaging varies with stage of abscess development Early cerebritis: Ill-defined T2 hyperintense mass Findings with contrast enhancement Early cerebritis: Patchy enhancement Late cerebritis: Intense, irregular rim enhancement…

KEY FACTS Terminology Acute or chronic inflammatory infiltration of pia, arachnoid, and cerebrospinal fluid (CSF) Classified as acute pyogenic (bacterial), lymphocytic (viral), chronic (tuberculosis or granulomatous) Imaging Imaging best delineates complications: Empyema, ischemia, hydrocephalus, cerebritis/abscess, ventriculitis FLAIR MR: Hyperintense signal in sulci, cisterns T1WI C+: Exudate and brain surface (pia) enhance Delayed C+ FLAIR most sensitive sequence for leptomeningeal disease DWI: Invaluable for detecting complications Basilar…

Overview of CNS Infections General Considerations Classification : Infectious diseases can be classified into congenital/neonatal and acquired infections. They can be further subdivided by etiology, i.e., bacterial, viral, fungal, parasitic, and rickettsial diseases. Infectious diseases can have different manifestations depending on disease acuity. Some diseases like herpes encephalitis are typically acute and fulminant. Others are subacute or chronic [e.g., subacute sclerosing panencephalitis (SSPE) and Rasmussen encephalitis].…

KEY FACTS Terminology Cerebrospinal fluid (CSF)-filled parenchymal cavity Deep, unilateral/bilateral cavity/excavation Usually communicates with ventricle &/or subarachnoid space Lined by reactive gliosis/astrocytic proliferation Congenital (perinatal brain destruction) or acquired (trauma, infection, etc) Imaging Best diagnostic clue: CSF-filled cavities with enlarged adjacent ventricle MR: Smooth-walled cavity; CSF isointense; lined by gliotic white matter Top Differential Diagnoses Consider arachnoid, ependymal, neoplastic, or inflammatory cyst Encephalomalacia Schizencephaly Dandy-Walker malformation…

KEY FACTS Terminology Choroid plexus cysts (CPCs) Nonneoplastic, noninflammatory cysts Contained within choroid plexus; lined by compressed connective tissue Imaging General Typically in atria of lateral ventricle Usually small (2-8 mm) Rare: Large cysts (> 2 cm) Usually multiple, often bilateral CT Isodense or slightly hyperdense to CSF Irregular, peripheral Ca++ common in adults MR Isointense or slightly hyperintense compared with CSF on T1WI 2/3 isointense,…

KEY FACTS Terminology Nonneoplastic intrapineal glial-lined cyst Imaging CT Sharply demarcated, smooth cyst behind 3rd ventricle 80% < 10 mm (can be large; reported up to 4.5 cm) Fluid iso-/slightly hyperdense to cerebrospinal fluid (CSF) 25% Ca++ in cyst wall MR Slightly hyperintense to CSF (55-60%) on most sequences Isointense (40%) 1-2% hemorrhage (heterogeneous signal intensity) Does not suppress on FLAIR Top Differential Diagnoses Normal pineal…

KEY FACTS Terminology Perivascular spaces (PVSs) Also known as Virchow-Robin spaces Pial-lined interstitial fluid-filled structures Accompany penetrating arteries Do not communicate with subarachnoid space Imaging Clusters of variably sized, well-delineated nonenhancing cysts PVSs occur in all locations, at all ages; easily seen in most patients on 3T imaging Most common site for normal PVSs = basal ganglia (clustered around anterior commissure) Midbrain, thalami Deep white matter…

KEY FACTS Terminology Synonyms: Hippocampal remnant cyst, hippocampal sulcal cavities Cyst or string of cysts along residual cavity of primitive hippocampal sulcus Imaging String of cysts along lateral margin of hippocampus Cysts follow cerebrospinal fluid signal on all MR sequences T2: Hyperintense FLAIR: Complete suppression T1WI C+: No enhancement Top Differential Diagnoses Mesial temporal sclerosis Choroid fissure cyst Arachnoid cyst Dysembryoplastic neuroepithelial tumor Pathology Represent partially…

KEY FACTS Terminology Intracranial epidermoids: Congenital inclusion cysts Imaging Cerebrospinal fluid-like mass that insinuates cisterns and encases neurovascular structures Morphology: Lobulated, irregular, cauliflower-like mass with “fronds” FLAIR: Usually does not suppress completely DWI: Diffusion hyperintensity definitively distinguishes from arachnoid cyst Top Differential Diagnoses Arachnoid cyst Inflammatory cyst (i.e., neurocysticercosis) Cystic neoplasm Dermoid cyst Pathology Arise from ectodermal inclusions during neural tube closure, 3rd to 5th week…

KEY FACTS Terminology Benign, ectopic, squamous epithelial cyst containing dermal elements, including hair follicles and sebaceous and sweat glands Imaging Midline unilocular cystic lesion with fat Subarachnoid fatty droplets if ruptured Suprasellar or posterior fossa most common intracranial sites Extracranial sites = spine, orbit May have fistulous connections to skin (dermal sinus tract) CT hypodensity (fat) 20% capsular Ca++ MR: T1 hyperintense Fat-suppression sequence confirms lipid…

KEY FACTS Terminology Unilocular mucin-containing 3rd ventricular cyst Imaging > 99% are wedged into foramen of Monro Pillars of fornix straddle, drape around cyst Majority are hyperdense on NECT Density correlates inversely with hydration state MR signal more variable Generally reflects water content Majority isointense to brain on T2WI (small cysts may be difficult to see) 25% mixed hypo/hyper (“black hole” effect) May show mild rim…

KEY FACTS Terminology Intraarachnoid cerebrospinal fluid (CSF)-filled sac that does not communicate with ventricular system Imaging General findings Sharply demarcated round/ovoid extraaxial cyst Isodense/isointense with CSF Location Middle cranial fossa (50-60%) Cerebellopontine angle (10%) Suprasellar (10%) Miscellaneous (10%) (convexity, quadrigeminal) MR Isointense with CSF on all sequences Suppresses completely with FLAIR No restriction on DWI CISS, FIESTA: Use to delineate cyst wall, adjacent structures 2D cine…

General Approach to Brain Cysts General Considerations Overview : Cysts are common findings on MR and CT brain scans. There are many types of intracranial cysts, some significant, some incidental. In this section, we exclude cystic neoplasms (such as pilocytic astrocytoma and hemangioblastoma), solid neoplasms that commonly have intratumoral cysts (such as ependymoma), and tumors that often display central necrosis (e.g., glioblastoma). We also exclude parasitic…

KEY FACTS Terminology Antiangiogenic agents may substantially reduce contrast enhancement in glioblastoma multiforme related to reduced vascular permeability rather than actual tumor response Bevacizumab (Avastin): Anti-VEGF is main antiangiogenic agent currently used for treatment of recurrent malignant gliomas Cediranib : VEGF receptor tyrosine kinase inhibitor has been tested in recent high-grade glioma treatment trials Imaging Decreased enhancement in patient with malignant glioma treated with anti-VEGF agent…

KEY FACTS Terminology Treatment-related increase in contrast enhancement mimics tumor progression Classically described after treatment with chemoradiation (temozolomide with radiation therapy) Typically occurs within 3-6 months after conclusion of radiation therapy (XRT) Imaging New enhancing lesion + ↑ FLAIR hyperintensity in treated malignant glioma at 3-4 months after XRT completion T2/FLAIR: Increased hyperintensity with mass effect DWI: Higher ADC values in PsP compared with tumor DSC…

KEY FACTS Terminology Radiation-induced injury may be divided into acute, subacute/early delayed, late injury Imaging Radiation injury : Mild vasogenic edema to necrosis Radiation necrosis : Irregular enhancing lesion(s) MRS: Markedly ↓ metabolites (NAA, Cho, Cr), ± lactate/lipid peaks Perfusion MR: ↓ relative cerebral blood volume compared with tumor Leukoencephalopathy : T2 white matter (WM) hyperintensity, spares subcortical U fibers Mineralizing microangiopathy : Basal ganglia, subcortical…

KEY FACTS Terminology Remote neurological effects of cancer, associated with extra-CNS tumors Most common tumor: Small cell lung carcinoma Limbic encephalitis (LE) is most common clinical paraneoplastic syndrome Imaging Limbic encephalitis: Hyperintensity in mesial temporal lobes, limbic system Mimics herpes encephalitis but subacute/chronic Paraneoplastic cerebellar degeneration (PCD): Cerebellar atrophy Brainstem encephalitis: T2 hyperintensity in midbrain, pons, cerebellar peduncles, basal ganglia Most paraneoplastic syndromes do not have…

KEY FACTS Terminology Secondary CNS involvement in patients with systemic lymphoma (SCNSL) Imaging Secondary CNS lymphoma: Skull, dura, leptomeninges > > parenchymal mass Best diagnostic clue: Diffusely enhancing dural mass ± bone involvement May see leptomeningeal enhancement or nonsupression of cerebrospinal fluid on FLAIR; CT hyperdensity Lower relative cerebral blood volume than other tumors Top Differential Diagnoses Meningioma Meningeal metastases Primary CNS lymphoma “Tumefactive” demyelinating disease…