02 Imaging
Air kerma inverse square
Air kerma at a new distance from a measured value.
Listen
Listen · English
Simulation
Air kerma inverse square — 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
K₂
New air kerma
3.5556mGy
K₂/K₁
Ratio
1.7778
Explanation
What it means
In air, kerma follows inverse square from a point focal spot (plus a small extrafocal component). Use it to move a measured air-kerma value from the chamber position to the skin or to another SID. 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
K₂ = K₁ (d₁/d₂)². Example: 2 mGy at 100 cm → 3.56 mGy at 75 cm. Then multiply by BSF if you need entrance surface dose. Change one input and watch the curve and the simulation follow.
Symbols
- K₁Measured air kerma2 mGy
- d₁Measurement distance100 cm
- d₂New distance75 cm
Worked example
A typical case from the default values: K₁ = 2 mGy (Measured air kerma); d₁ = 100 cm (Measurement distance); d₂ = 75 cm (New distance). Substituting into the relation gives K₂ = 3.5556 mGy; K₂/K₁ = 1.7778. These are teaching numbers — align them with your machine.
Typical values give
- K₂ = 3.5556mGy
- K₂/K₁ = 1.7778
Where it comes from
The displayed formula is the working relation. Air kerma at a new distance from a measured value. Usual reference: IAEA TRS-457. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: IAEA TRS-457
Assumptions & limits
Air only. Attenuation of the patient, table, or compression paddle is extra. Heel effect makes the field non-uniform.
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 Do not quote DLP as if it were effective dose without a k-factor. Air only. Attenuation of the patient, table, or compression paddle is extra. Heel effect makes the field non-uniform.
Keep this
Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Air only. Attenuation of the patient, table, or compression paddle is extra. Heel effect makes the field non-uniform.
In this specialty