02 Imaging

Entrance skin dose

Estimate ESD from tube output, kVp, mAs, and FSD.

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Simulation

Entrance skin dose — Change the numbers; the scene follows.

Where it works

Radiography room

Patient

At the patient entrance — skin dose, subject contrast, photoelectric absorption in tissue.

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Formula

\mathrm{ESD} = Y\left(\frac{\mathrm{kVp}}{80}\right)^n \cdot \mathrm{mAs}\cdot\left(\frac{100}{\mathrm{FSD}}\right)^2\cdot\mathrm{BSF}

Variables

Y · Tube outputµGy/mAskVp · Tube voltagekVn · kVp exponentmAs · mAsmAsFSD · Focus-to-skin distancecmBSF · Backscatter factor

Results

ESD
Entrance skin dose
1,300µGy
ESD
Entrance skin dose
1.3mGy

Curve

Explanation

\mathrm{ESD} = Y\left(\frac{\mathrm{kVp}}{80}\right)^n \cdot \mathrm{mAs}\cdot\left(\frac{100}{\mathrm{FSD}}\right)^2\cdot\mathrm{BSF}

What it means

Entrance skin dose estimates the air kerma (with backscatter) at the patient’s skin from tube output Y, kVp, mAs and focus-to-skin distance. It is the quantity compared with deterministic skin thresholds in fluoroscopy. This is a working relation in Diagnostic imaging.

Where it is used

Clinically it sits on the Radiography room — Patient. At the patient entrance — skin dose, subject contrast, photoelectric absorption in tissue. 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.

Radiography room · Open this machine

How to use it

Y is µGy/mAs at 80 kV and 100 cm. n ≈ 2 for air kerma. BSF is ~1.3 at diagnostic energies. Prolonged fluoro: keep ESD in mind versus ~2 Gy temporary erythema. Change one input and watch the curve and the simulation follow.

Symbols

YTube output50 µGy/mAs
kVpTube voltage80 kV
nkVp exponent2
mAsmAs20 mAs
FSDFocus-to-skin distance100 cm
BSFBackscatter factor1.3

Worked example

A typical case from the default values: Y = 50 µGy/mAs (Tube output); kVp = 80 kV (Tube voltage); n = 2 (kVp exponent); mAs = 20 mAs (mAs); FSD = 100 cm (Focus-to-skin distance); BSF = 1.3 (Backscatter factor). Substituting into the relation gives ESD = 1,300 µGy; ESD = 1.3 mGy. These are teaching numbers — align them with your machine.

Typical values give

ESD = 1,300µGy
ESD = 1.3mGy

Where it comes from

The displayed formula is the working relation. Estimate ESD from tube output, kVp, mAs, and FSD. Usual reference: IAEA / Bushberg. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: IAEA / Bushberg

Assumptions & limits

Empirical output model, not a Monte Carlo organ dose. Table attenuation, extra filtration (Cu), and inverse-square inside a C-arm geometry may differ from the simple (100/FSD)² term.

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. Empirical output model, not a Monte Carlo organ dose. Table attenuation, extra filtration (Cu), and inverse-square inside a C-arm geometry may differ from the simple (100/FSD)² term.

Keep this

Y is tube output at 80 kVp, 1 mAs, 100 cm. Exponent n ≈ 2.

In this specialty

Diagnostic imaging