1. Secondary causes of headache:
- Bleed: SAH, parenchymal  hemorrhage, sentinel bleed from aneurysm
- Vascular: aneurysm, AVM, CCM, dissection, temporal arteritis, hypertensive encephalopathy
- Intracranial non-vascular: tumor, pseudotumor cerebri, hydrocephalus
- Infection: meningitis, abscess, empyema
- Drugs/withdrawal: alcohol, cocaine, opiates, steroids, caffeine
- Metabolic: hypercapnea, hypoglycemia, anoxia, anemia
- H+N trauma: concussion, injury to the soft tissues and bones of the skull and face.
- H+N pathology: problems with sinuses, eyes (glaucoma), radiculopathy (C2 nerve compression)
- Psych
2. Cerebral aneurysms:
- Acomm most common (30%)
- Pcomm next most common (25%)
- MCA third (20%)
3. Relationship between aneurysm size and likelihood of rupture:
<10 mm: 0.05%/year
>10 mm: 1%
>25 mm: 6% (45% within the next 7.5 years)
4. MR modalities
- DWI: most sensitive for ischemia, will appear as bright. Can detect ischemia within 15-30 minutes of onset of symptoms. Pyogenic abscesses will also demonstrate restricted diffusion (bright on DWI) because of increased viscosity in their cores.
- GRE: di/paramagnetic substances (metal, most blood except hyperacute) will appear dark
- SWI (aka VENBOLD) susceptibility weighted imaging, uses GRE pulse sequence, its like GRE but higher resolution. Exquisitely sensitive for venous blood, hemorrhage, iron storage, which appear dark.
- Spect: Normal: choline and creatine about the same, NAA higher than both. Increased choline/cr ratio and decreased NAA/cr ratio seen in cancer. Lactate/lipid elevation is never normal, suggests anaerobic metabolism-- infection vs necrosis (tumor)
5. Insular ribbon sign: insula is very sensitive to ischema; loss of the grey-white matter distinction due to swelling- sign of acute MCA infarct
6. tPA during ischemic stroke:
- FDA approves use up to 3 hours after ischemic insult
- ECASS-III trial showed it is safe up to 4.5 hours.
7. Venous infarcts:
- Due to dural sinus thromboses.
- Generally cause hemorrhage not in any standard arterial distribution
- Risk factors: neonates- shock/dehydration, older children- local infection (mastoiditis) and coagulopathy, adults- hypercoagulability (70%) and infection (10%). Pregnancy, puerperium, OCP are risk factors.
- Image with GRE or SWI/VENBOLD
- Can use MR venogram (will see filling defects of the sinuses that have thrombosed)
8. Cerebral veins: 
- Labbe drains temporal lobe into transverse sinus.

9. AIDS population- ring-enhancing brain mass, think toxo vs primary CNS lymphoma
- Favors toxo: multiple lesions, involving deep structures (BG, caudate, thalami), bright center on T2 (fluid), hyperintensity on T1 (hemorrhage), tons of surrounding vasogenic edema
- Favors CNS lymphoma: single lesion, sub-ependymal, encasement of ventricles, lights up on DWI (lots of diffusion restriction - due to hypercellularity?), hypointense core on T2 (hypercellularity?)
10. Toxo becomes worrisome once CD4<100; such patients who are toxo+ should be treated even if they don't have sx, because the rate of symptomatic toxo is 30%