Definition
The Hounsfield unit (HU) is a dimensionless value defined as 1,000 × (μ − μ_water) / μ_water, where μ is the linear attenuation coefficient. By definition, water is 0 HU and air is −1000 HU. Dense cortical bone reaches +2,000 HU or higher.
Medical CT scanners are regularly calibrated with water and air phantoms so HU is reliable across exams. This is what makes quantitative bone density assessment possible on CT.
Typical HU ranges
Air: −1000. Fat: −100 to −50. Water and most fluids: 0. Muscle: +35 to +55. Organs like liver and spleen: +40 to +70. Trabecular bone: +150 to +850. Cortical bone: +1,000 to +3,000+. Metallic implants and dental fillings: often saturate the scale above +3,000, which is why metal artefacts appear as streaks.
Why CBCT HU is not really HU
CBCT reconstruction is geometrically different from fan-beam CT and most scanners do not perform water calibration on every exam. The gray value of a voxel in CBCT depends on the scanner model, FOV, exposure settings, and even the position of the object in the FOV because of scatter and beam hardening.
Studies show CBCT values can drift by hundreds of units compared to medical CT for the same tissue. That is why societies like the AAOMR advise against using CBCT gray values as a calibrated measure of bone density. They are useful as a relative signal — for example, the same implant site compared before and after grafting on the same machine.
Using HU in implant planning
A common clinical shorthand is Misch's classification: D1 > 1250 HU (dense cortical), D2 850–1250 (thick cortical with dense trabecular), D3 350–850 (thin cortical with sparse trabecular), D4 150–350 (fine trabecular only). These categories help choose implant shape, surface and protocol.
On CBCT, you can still compare the voxel values of two candidate sites to rank them, but the cutoffs above may under- or over-estimate real density. If exact D1–D4 classification matters for medicolegal reasons, pair the CBCT with a clinical tactile assessment during surgery.