Radiography Inverse Square Law Calculator

What are you solving for?

What is the original intensity of the beam? What is the original source to image-receptor distance? What is the new intensity of the beam?

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What is the original intensity of the beam? What is the original source to image-receptor distance? What is the new source to image-receptor distance?

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The Inverse Square Law

Inverse Square Law by Borb CC BY-SA 3.0

X-rays are a form of electromagnetic radiation, which can be quantified by the number of photons. As these photons are emitted from a point-source origin, they spread out. This can be visualized with visibile light by turning on a flashlight in a dark room. The beam of light spreads out much like how an x-ray beam will spread out from an x-ray tube assembly. The intensity of the x-ray beam is inversely proportional to the square of the distance from the source.

Solving for Distance uses the formula:

$$ D_2=\sqrt{I_1\cdot D_1^2\over I_2} $$

Solving for Intensity uses the formula:

$$ I_2 = \frac{I_1\cdot D_1^2}{D_2^2} $$

$ I_1 = $ The original Intensity

$ D_1 = $ The original source to image-receptor Distance

$ I_2 = $ The new Intensity

$ D_2 = $ The new source to image-receptor Distance

Visualization of Distance Changes

Let's assume that an average person undergoing a PA chest x-ray exam will receive a dose of approximately 0.1 mSv from a technique of 100 kVp and 2 mAs at a source to image-receptor distance of 72". Assuming this, use the following slider to move the x-ray tube closer to or further from the patient to see how the patient's dose will be affected.

Distance: "

Dose: mSv