Blood-brain barrier

The blood brain barrier (BBB) forms a physical resistance to the passage of lipophobic substances form cerebral capillaries into the brain and is a key reason why there is no CSF enhancement following IV contrast media in CT and MR imaging.

It is formed by a combination of endothelial cells, pericytes, and astroglial andperivascular macrophages along the cerebral capillary walls.

In general, capillary walls in the human body can comprise of three different types

• continuous : present in areas which have a BBB
  • continuous inter endothelial tight junctions
  • no pinocytosis
  • no fenestrations
• fenestrated : present in areas which lack the BBB
• sinusoidal : not found in the brain

In the brain, the majority of capillary walls are of the continuous type with tight junctions and a continous basement membrane

Areas which have fenestrated capillaries and thus lack the blood brain barrier are termed circumventricular organs (CVO) and include

• area postrema
• median eminence
• sub forniceal region
• posterior pituitary
• pineal gland
• organum vasoculosum
• lamina terminalis
• choroid plexus

Generally lipophilic solutes can cross the BBB which include

• caffeine
• ethanol
• radiopharmaceuticals used in PET and SPECT imaging
• CO₂
• O₂
• Nicotine

Hydrophilic solutes in general on the other hand are unable to cross it

• e.g. water soluble CT / MR contrast media

There are a multitude of conditions that are associated with disruption of the BBB. They include

• temporal lobe epilepsy (TLE) ¹
• posterior reversible encephalopathy syndrome : PRES
• hypoxia / ischaemia and infarction
• tumours
• inflammatory conditions : e.g meningitis
• trauma
• intracranial irradiation
• multiple sclerosis
• progressive multifocal leukoencephalopathy : the JC virus can cross the BBB