02 Imaging
Hounsfield unit
CT number from linear attenuation relative to water.
Listen
Listen · English
Simulation
Hounsfield unit — Change the numbers; the scene follows.
Where it works
CT scanner
Gantry
Inside the rotating gantry: tube, bowtie, and detectors that define CTDI and HU.
Open this machineFormula
Typical values
Variables
Results
HU
Hounsfield unit
19.4HU
μ(HU)
Check μ
0.21cm⁻¹
Explanation
What it means
The Hounsfield unit rescales linear attenuation so water is 0 and air is −1000: HU = 1000 (μ−μ_w)/μ_w. Fat is negative (~ −80 to −100), soft tissue ~0–50, contrast-enhanced vessels hundreds, cortical bone hundreds to 1000+. This is a working relation in Diagnostic imaging.
Where it is used
Clinically it sits on the CT scanner — Gantry. Inside the rotating gantry: tube, bowtie, and detectors that define CTDI and HU. Diagnostic equations live on the tube, the detector, and the patient: magnification, air kerma, CTDI, and why bone lights up at 70 kV. They turn a technique chart into physics you can defend.
CT scanner · Open this machineHow to use it
Invert the formula to get μ from a measured HU if you need it for a dose or PET-AC calculation. Water μ depends on beam energy (typically ~0.20 cm⁻¹ at an effective ~60–70 keV). Change one input and watch the curve and the simulation follow.
Symbols
- μMaterial attenuation0.21 cm⁻¹
- μ_wWater attenuation0.206 cm⁻¹
Worked example
A typical case from the default values: μ = 0.21 cm⁻¹ (Material attenuation); μ_w = 0.206 cm⁻¹ (Water attenuation). Substituting into the relation gives HU = 19.4 HU; μ(HU) = 0.21 cm⁻¹. These are teaching numbers — align them with your machine.
Typical values give
- HU = 19.4HU
- μ(HU) = 0.21cm⁻¹
Where it comes from
The displayed formula is the working relation. CT number from linear attenuation relative to water. Usual reference: Bushberg. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Bushberg
Assumptions & limits
Defined for the scanner’s effective energy and reconstruction kernel. Beam hardening, contrast, and metal make HU non-linear. Not a density meter without calibration.
Pitfalls
kVp is not the same as effective energy. CTDI is not patient dose — SSDE and organ dose come after. Do not quote DLP as if it were effective dose without a k-factor. Defined for the scanner’s effective energy and reconstruction kernel. Beam hardening, contrast, and metal make HU non-linear. Not a density meter without calibration.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Defined for the scanner’s effective energy and reconstruction kernel. Beam hardening, contrast, and metal make HU non-linear. Not a density meter without calibration.
In this specialty