What Does Hypodensity Mean on CT?

Hypodensity means that a described structure in a report is darker on CT than the surrounding tissue or organ.  It says nothing of the cause.    Density on CT is measured with something called hounsfield units.  Air and fat have negative numbers, water is close to zero, and structures like soft tissue, blood and bone are higher than zero.  The lower the number the darker the tissue.

What Does a hypodensity mean?

Radiologists will often describe abnormalities on CT as hypodense or darker than the organ or tissue it’s in.  It says nothing of the cause of the abnormality.  Measuring the density of the abnormality is helpful because it lets us know if it’s air, fat, water, soft tissue, blood, etc.  Knowing the density of the abnormality let’s us narrow the diagnostic possibilities.   It is often not enough to simply state there is an abnormal hypodensity because this tells us nothing about the cause.

What are some examples of hypodensities on CT?

On a head CT, hypodensity will be seen as areas darker than the brain. The appearance of the dark area will help us narrow the possibilities.  Some abnormalities such as strokes, edema or extra water in the brain, masses, air or fat abnormalities will be hypodense.  The radiologist may describe the abnormality as hypodense but will also provide a diagnosis or some possibilities.

On a chest CT, hypodensities can be found throughout.  The lung is dark because it is mostly air.  Darker areas in the lungs may be cysts or areas of air trapping.  Hypodense areas in the mediastinum may be masses, lymph nodes, or cysts.  Hypodense blood vessels may be occlusions or clots after contrast is given through a vein.  Hypodense areas along the chest wall and ribs may be bone lesions.

CT of the abdomen and pelvis presents similar diagnostic possibilities. Within organs, hypodense areas may be cysts or masses.  Hypodense areas in bowel and colon may be air, masses, stool, fat in addition to other possibilities.  Hypodense areas may also indicate life threatening conditions like free air outside the bowel.  This indicates a bowel perforation in many cases. Hypodense areas may be fat which can be found in fatty abnormalities such as lipomas or other fat containing tumors.

What is Hypodensity in the bone?

Hypodensity on CT of the bones may indicate a bone lesion.  The bone is normally dense.  A lytic lesion which is darker than the bone may be described as a hypodensity.   Often the diagnosis is based on history such as if there is cancer and how aggressive the lesion appears.  Hypodensity in the bones may also represent normal marrow or variation in the structure of the bone.

Hypodense areas are therefore non specific simply stating  that an abnormality is darker than the surrounding tissues or organ it’s in.  It says nothing of the cause.  It can be something benign and ignored all the way to life threatening.  The radiologist in many cases will provide a diagnosis or possibilities.  In some cases, he may provide a recommendation for additional testing.