Hounsfield Units

Definition

• Water = 0 HU
• Air = -1000 HU
• Everything else = scaled based on density relative to air/water

Air < Fat < Fluid < Soft tissue < Bone < Metal

Fat

• Fat is less dense than water, negative HU
• Fat floats on water – easy way to remember this.

Soft Tissue and Blood

• Soft tissue is mostly intracellular water, but also contains additional substances (connective tissues) that raise the density slightly
• Positive HU, but not far from water (+25 to +60)

Lung

• Lung is a mixture of soft tissue and air – overall density of lung falls between these two values
• Approximately -500 HU

Bone

• Cortical bone – extremely dense – highly positive HU (+1000)
• Medullary bone – mix of dense bone and soft tissue (+300)