Basics of Brain CT Scan

THREE BASIC DENSITIES OR DIFFERENT SHADES OF GREY
The first secret is that we describe CT scan findings as ‘densities’,of which there are three common easily identifiable ones tolearn. ‘In general the higher the density the whiter the appearanceon the CT scan and the lower the density the darker theappearance on the brain CT scan.’ The reference density (the one you compare with) is the brain, usually the largest componentinside the skull. Anything of the same density as the brain is called ISODENSE, and it is characterised by a dull greyish white appearance.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. below). 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.below) 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.

The different densities of CT scan.

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