02 Imaging
AEC mAs versus thickness
mAs₂ / mAs₁ = exp(μ Δx) at fixed kVp, or the 4–5 cm doubling rule.
Listen
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Simulation
AEC mAs versus thickness — Change the numbers; the scene follows.
Where it works
Radiography room
Bucky / detector
In the Bucky / detector: grid, AEC, DQE, and the pixel that samples the image.
Open this machineFormula
Variables
Results
mAs₂
New mAs
43.1953
ratio
Ratio
2.1598
x_double
Doubling thickness
4.501cm
Curve
Explanation
What it means
Automatic exposure control holds detector dose constant, so mAs tracks e^{μ x} as the patient thickens. A teaching rule of thumb: double mAs for every extra 4–5 cm of soft tissue at fixed kVp (μ ≈ 0.15–0.2 cm⁻¹ in the 80 kVp range). Prefer raising kVp (15% rule) when the mAs would otherwise become huge — that is the thorax vs abdomen technique split. This is a working relation in Diagnostic imaging.
Where it is used
Clinically it sits on the Radiography room — Bucky / detector. In the Bucky / detector: grid, AEC, DQE, and the pixel that samples the image. 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 machineHow to use it
Enter reference mAs, extra thickness Δx and μ (or leave μ so that doubling distance is 4.5 cm). Read the new mAs. A 20 cm abdomen at 20 mAs becomes ~40 mAs at 25 cm. Change one input and watch the curve and the simulation follow.
Symbols
- mAs₁Reference mAs20
- ΔxExtra thickness5 cm
- μEffective attenuation0.154 cm⁻¹
Worked example
A typical case from the default values: mAs₁ = 20 (Reference mAs); Δx = 5 cm (Extra thickness); μ = 0.154 cm⁻¹ (Effective attenuation). Substituting into the relation gives mAs₂ = 43.1953; ratio = 2.1598; x_double = 4.501 cm. These are teaching numbers — align them with your machine.
Typical values give
- mAs₂ = 43.1953
- ratio = 2.1598
- x_double = 4.501cm
Where it comes from
The displayed formula is the working relation. mAs₂ / mAs₁ = exp(μ Δx) at fixed kVp, or the 4–5 cm doubling rule. Usual reference: Bushberg / Carlton & Adler. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Bushberg / Carlton & Adler
Assumptions & limits
Homogeneous slab, fixed kVp and filtration, no k-edge contrast, no AEC chamber position error. Real AEC also sees the lung / mediastinum weighting of the selected chamber.
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. Homogeneous slab, fixed kVp and filtration, no k-edge contrast, no AEC chamber position error. Real AEC also sees the lung / mediastinum weighting of the selected chamber.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Homogeneous slab, fixed kVp and filtration, no k-edge contrast, no AEC chamber position error. Real AEC also sees the lung / mediastinum weighting of the selected chamber.
In this specialty