The first secret is that we describe CT scan findings as ‘densities’,
of which there are three common easily identifiable ones to
learn. ‘In general the higher the density the whiter the appearance
on the CT scan and the lower the density the darker the
appearance on the brain CT scan.’ The reference density (the
one you compare with) is the brain, usually the largest component
inside the skull. Anything of the same density as the brain
is called ISODENSE, and it is characterised by a dull greyish
white appearance (Fig). Thus the brain is the reference density.
Anything of higher density (whiter) than the brain is called
HYPERDENSE, and the skull is the best example of a hyperdense
structure that is seen in a normal brain CT scan. The skull is easily
identified as the thick complete white ring surrounding the brain.
Similarly, anything of lower density (darker tone) than brain is
described as HYPODENSE.
The cerebrospinal fluid (CSF) is the typical example of a hypodense
structure in the brain CT scan (Fig). Air is also hypodense
and surrounds the regular outline of the skull in CT, just
as the air surrounds the head in life. Between the pitch formless
blackness of air and the greyish white appearance of the
brain, the cerebrospinal fluid presents a faint granular hypodense
appearance, which may vary slightly but is identified by its usual
locations. You will come to realise later that ‘appreciating the
usual locations of CSF is the key to understanding brain pathology
on CT scan’ (Igbaseimokumo 2005). We will come back to
this idea later, but for now suffice it to say that the skull is highly
whitish in appearance (Fig) and is clearly identified as an oval
white ring surrounding the brain. The brain is greyish white, and
the CSF is dark and faintly granular on close inspection (but not
as dark as air) and has specific normal locations.