02 Imaging
Water-equivalent diameter
WED = 2 √(A_w / π) from the water-equivalent area of the CT slice.
Listen
Listen · English
Simulation
Water-equivalent diameter — Change the numbers; the scene follows.
Where it works
CT scanner
Patient
On the patient in the bore — size-specific dose uses water-equivalent diameter here.
Open this machineFormula
Variables
Results
WED
Water-equivalent diameter
27.6395cm
A_w
Water-equivalent area
600cm²
Explanation
What it means
Size-specific dose estimate needs a patient-size metric. Water-equivalent diameter converts the slice’s mean HU to the diameter of a water cylinder that would attenuate the same. It is more accurate than geometric diameter for a barrel-chested or dense abdomen. SSDE = f(WED) × CTDIvol. This is a working relation in Diagnostic imaging.
Where it is used
Clinically it sits on the CT scanner — Patient. On the patient in the bore — size-specific dose uses water-equivalent diameter here. 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
Enter water-equivalent area A_w (cm²), or mean HU and the bounding geometric area. A 20×30 cm abdomen of mean HU 0 → A_w = 600 cm² → WED = 27.6 cm. Change one input and watch the curve and the simulation follow.
Symbols
- A_wWater-equivalent area600 cm²
- A_geomGeometric area (optional)600 cm²
- HŪMean HU0
Worked example
A typical case from the default values: A_w = 600 cm² (Water-equivalent area); A_geom = 600 cm² (Geometric area (optional)); HŪ = 0 (Mean HU). Substituting into the relation gives WED = 27.6395 cm; A_w = 600 cm². These are teaching numbers — align them with your machine.
Typical values give
- WED = 27.6395cm
- A_w = 600cm²
Where it comes from
The displayed formula is the working relation. WED = 2 √(A_w / π) from the water-equivalent area of the CT slice. Usual reference: AAPM Report 220. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AAPM Report 220
Assumptions & limits
One slice; a study SSDE usually averages WED over the scan range. Mean HU of the reconstructed FOV includes air outside the patient unless you mask. Report 220 ROI methods vary by vendor.
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 Do not quote DLP as if it were effective dose without a k-factor. One slice; a study SSDE usually averages WED over the scan range. Mean HU of the reconstructed FOV includes air outside the patient unless you mask. Report 220 ROI methods vary by vendor.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. One slice; a study SSDE usually averages WED over the scan range. Mean HU of the reconstructed FOV includes air outside the patient unless you mask. Report 220 ROI methods vary by vendor.
In this specialty