Random Neurorads

- Chordomas are almost always midline! A suspected tumor of the skull base that is exclusively lateral (i.e. condyle) is unlikely to be a chordoma. Chordomas also have "soap bubble" edges on scans (also physaliphorus cells on path) and enhance super bright with contrast, most commonly affect clivus/anterior occipital bone.
- A lateral skull base tumor that is erosive/lytic through bone, with popcorn calcifications, big time tissue edema - first on the differential is chondrosarcoma, which can originate from the cartilage of the formen lacerum or of the occipital condyle. These things also enhance like crazy on MRI. Other things: mets (esp breast cancer), chondroblastoma (tend to be younger patients, muted on T1/T2), weird stuff like langerhans cell histiocytosis, gout/CPPD, osteomyelitis.
- Petrous bone non-oncogenic pathology -- middle-ear ENT-type problems (cholesterol granuloma, cholesteatoma, middle ear effusions), aneurysms (petrous carotid), non-cancerous processes that affect the bone like Gout/CPPD or osteomyelitis (esp older, diabetic/immunocompromised people) -- petrous apicitis can present as Gradenigo syndrome, which is 6th nerve palsy from dorello's canal, facial or retro-orbital pain from meckle's cave.

Lymphoma on imaging:
- CT: looks like a solid, highly cellular/dense mass
- MRI: perventricular, more or less homogenously enhancing, lots of edema. However lymphoma typically does not push significantly into ventricles - a tumor that is mostly inside the ventricle, think more like ependymoma (can invade brain), CPC (CPP typically does not invade brain)
- Except for HIV-associated CNS lymphoma which is ring-enhancing and primarily T-cell based

Giant, gross looking tumor with minimal enhancement is not lymphoma. It is GBM or mets until proven otherwise. One thing that may distinguish is CBV -- edema around mets is purely vasogenic and so will have low CBV, while the edema around GBM is often partially or wholly infiltrative tumor and so will have high CBV

Another thing about GBM - typically tracks along the white matter, really likes the thalamus, heterogenous, non-restricting.

Things that restrict on DWI - A lot more than just strokes!
- Very cellular tumors - "small round blue cell tumors" like PNETs (ie medulloblastoma, pineoblastoma), and other cellular tumors like lymphoma, meningoma, chordoma, germinoma, or the solid/cellular parts of GBM (note: GBMs typically do not restrict but they can, so do not use lack of diffusion restriction to rule out a GBM. GBM can look like anything)
- Infection - empyema, abscess
- Epidermoid cysts (vs arachnoid cysts, which do not) - speaking of skull base lesions, bad epidermoids can become giant tumors along the petrous bone in the posterior fossa, extending all the way up into the midbrain.
- Cytotoxic edema/ischemia
- Weird poisoning syndromes - carbon monoxide, methanol, wernicke's, hyperammonemia
- Weird inhered stuff - X-linked adrenoleukodystrophy, Maple syrup urine, canavan
- Weird prion disease - kuru and CJD "cortical ribboning"
- Weird other weird stuff - CADASIL, wilson's

CMV encephalitis usually involves periventricular T2 hyperintensity. Usu CD4<50

Thought process for differential diagnosis in HIV CNS imaging
- Space occupying - toxo, cns lymphoma
- Non space occupying - PML, HIV encephalitis (usually symmetric), CMV encephalitis

MS usually causes incomplete rings of enhancement!

Multiple enhancing brain lesions with extremely high CSF pressures (put in EVD, CSF hits the ceiling) - think cryptococcus. The buzzword is"perivascular gelatinous pseudocyst"

Oligodendroglioma - on the boards, will classically be frontal, subcortical, heavily calcified (90% are calcified).