Hypodensity on head CT means that the abnormal area in the brain is darker than the surrounding normal brain tissue. This type of abnormal tissue can have a variety of sizes and causes. Most commonly we see Hypodensity on head CT when there is air, edema, fluid, and fat. Hypodensity on head CT is a descriptive term. This speaks nothing to the significance of the abnormality.
Air within the head can cause hypodensity on head CT. Air can be within the head and brain after neurosurgery. Air can also occur after fractures of the skull and sinuses. Air gets in from the outside after the bones are broken. Most of the time, the air is not treated and resorbs on its own. Rarely, a tension pneumocephalus can occur when air around the brain exerts pressure on the brain. The patient may deteriorate which indicates the need for surgical intervention.
Fluid around or within the the brain can be a Hypodensity. Fluid is hypodense. Fluid around the brain can be caused by prior trauma or from other causes. This can produce pressure on the brain and require surgical drainage. Cysts within the brain can appear hypodense. These are commonly arachnoid cysts and occur between the meninges or coverings of the brain. Cystic tumors of the brain will often have solid components inside of them. Small cysts can occur from infections like neurocysticercosis.
Edema in the brain can cause hypodensity. Edema is swelling or fluid that is located in the body tissues. Tumors in the brain will sometimes have surrounding edema and be hypodense. Bleeds in the brain are often bright or hyperdense but the surrounding edema is hypodense or dark. Abscesses or infections will often have edema surrounding them which is hypodense. In these cases, the edema is an associated finding.
Fat density can be hypodense. There is normal fat underlying the scalp. Normal fat can sometimes be located in the brain, most commonly along the falx. Fat containing lesions can be tumors. Lipomas are probably the most common fat containing tumors. Other tumors are dermoid cysts, teratomas, rarely meningiomas and fatty transformation of other tumors.
Hypodensity on head CT is an area darker than the normal surrounding brain. It is a descriptive term used by radiologists. The term itself does not provide any idea about the cause or urgency of the finding. It can be the primary finding or an associated finding surrounding some other major abnormality, like edema. The radiologist interpreting the scan will try to provide the most accurate diagnosis. In some cases an MRI of the brain will be needed for further evaluation.