02 Imaging
Size-specific dose estimate SSDE
SSDE = f(WED) × CTDIvol. f from AAPM 204 exponential fits.
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Simulation
Size-specific dose estimate SSDE — 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
Typical values
Variables
Results
f
Conversion factor
1.3255
SSDE
Size-specific dose estimate
15.9064mGy
Curve
Explanation
What it means
CTDIvol is reported to a 16-cm (head) or 32-cm (body) phantom, not to the patient. A paediatric or slim adult is under-represented by a 32-cm CTDIvol (true dose is higher); an extra-large patient is over-represented. The conversion f(d) maps phantom CTDIvol onto the dose to a water cylinder of diameter d = WED. 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 CTDIvol (mGy), WED (cm) and phantom (16 or 32). A 16-cm toddler, CTDIvol_32 = 5 mGy, WED = 16 cm → f ≈ 2.1 → SSDE ≈ 10 mGy. Always state which phantom the scanner used. Change one input and watch the curve and the simulation follow.
Symbols
- CTDI_volCTDIvol12 mGy
- WEDWater-equivalent diameter28 cm
- phantomReference phantom 16 or 3232 cm
Worked example
A typical case from the default values: CTDI_vol = 12 mGy (CTDIvol); WED = 28 cm (Water-equivalent diameter); phantom = 32 cm (Reference phantom 16 or 32). Substituting into the relation gives f = 1.3255; SSDE = 15.9064 mGy. These are teaching numbers — align them with your machine.
Typical values give
- f = 1.3255
- SSDE = 15.9064mGy
Where it comes from
The displayed formula is the working relation. SSDE = f(WED) × CTDIvol. f from AAPM 204 exponential fits. Usual reference: AAPM Reports 204 & 220. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AAPM Reports 204 & 220
Assumptions & limits
Exponential fit to TG204 tables, not a bilinear interpolation of the published table. SSDE is still a mean dose to a cylinder, not organ dose. Overranging and overbeaming are already inside CTDIvol to varying degrees 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. Exponential fit to TG204 tables, not a bilinear interpolation of the published table. SSDE is still a mean dose to a cylinder, not organ dose. Overranging and overbeaming are already inside CTDIvol to varying degrees by vendor.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Exponential fit to TG204 tables, not a bilinear interpolation of the published table. SSDE is still a mean dose to a cylinder, not organ dose. Overranging and overbeaming are already inside CTDIvol to varying degrees by vendor.
In this specialty