02 Imaging
Exposure vs mAs and kVp
Rule of thumb: exposure ∝ mAs × (kVp)^n.
Listen
Listen · English
Simulation
Exposure vs mAs and kVp — Change the numbers; the scene follows.
Where it works
Radiography room
X-ray tube
At the focal spot in the tube housing — spectrum, output, SID geometry, and unsharpness start here.
Open this machineFormula
Variables
Results
X₂
New exposure
6.5306mR
X₂/X₁
Ratio
0.6531
Explanation
What it means
Receptor exposure scales with mAs and approximately with (kVp)^n. n ≈ 2 for air kerma and 4–5 for film optical density. The 15% kVp rule is the n ≈ 5 special case (1.15^5 ≈ 2). This is a working relation in Diagnostic imaging.
Where it is used
Clinically it sits on the Radiography room — X-ray tube. At the focal spot in the tube housing — spectrum, output, SID geometry, and unsharpness start 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.
Radiography room · Open this machineHow to use it
Use this to retake a dark/light radiograph by changing mAs, or to cut dose by raising kVp and dropping mAs. Digital systems still follow n ≈ 2 for detector dose. Change one input and watch the curve and the simulation follow.
Symbols
- X₁Original exposure10 mR
- mAs₁Original mAs20 mAs
- kVp₁Original kVp70 kV
- mAs₂New mAs10 mAs
- kVp₂New kVp80 kV
- nExponent2
Worked example
A typical case from the default values: X₁ = 10 mR (Original exposure); mAs₁ = 20 mAs (Original mAs); kVp₁ = 70 kV (Original kVp); mAs₂ = 10 mAs (New mAs); kVp₂ = 80 kV (New kVp); n = 2 (Exponent). Substituting into the relation gives X₂ = 6.5306 mR; X₂/X₁ = 0.6531. These are teaching numbers — align them with your machine.
Typical values give
- X₂ = 6.5306mR
- X₂/X₁ = 0.6531
Where it comes from
The displayed formula is the working relation. Rule of thumb: exposure ∝ mAs × (kVp)^n. Usual reference: Bushberg / Carlton & Adler. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Bushberg / Carlton & Adler
Assumptions & limits
Ignores AEC, filtration changes, and kVp-dependent scatter. n is empirical — verify on your tube.
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 Do not quote DLP as if it were effective dose without a k-factor. Ignores AEC, filtration changes, and kVp-dependent scatter. n is empirical — verify on your tube.
Keep this
n is 2–3 depending on filtration and detector. 15% kVp rule ≈ 2× exposure.
In this specialty