= focal cavity as a result of localized brain destruction

1. AGENETIC PORENCEPHALY
   = SCHIZENCEPHALY = TRUE PORENCEPHALY
2. ENCEPHALOCLASTIC PORENCEPHALY
   Time of injury: during first half of gestation
   Histo: necrotic tissue completely reabsorbed without surrounding glial reaction (= liquefaction necrosis)
   MR:
   • smooth-walled cavity filled with CSF on all pulse sequences (= porencephalic cyst)
   • lined by white matter
3. ENCEPHALOMALACIA
   = PSEUDOPORENCEPHALY = ACQUIRED PORENCEPHALY
   = end result of destructive process with replacement by CSF
   Prevalence: 2.5% of children with perinatal brain injury
   Cause: intraparenchymal hemorrhage, infection, surgery
   Time of injury: after end of 2^nd trimester / postnatally (brain has developed capacity for glial response)
   Location: parasagittal watershed areas with sparing of periventricular region + ventricular wall

CT:

• hypodense regions

MR:

• hypointense on T1WI + hyperintense on T2WI
• surrounding hyperintense rim on T2WI (= gliosis)
• glial septa coursing through cavity identified on T1WI + proton density images

US:

• damaged area hyperechoic (during first days to weeks) following initial hemorrhagic event
• subsequent conversion to area with anechoic center and echogenic border ← clot retraction
• ultimately completely anechoic cystic area ← CSF:
  • septations in cavity well visualized
  • ± discrete wall calcifications