Inverse Square Law - IPE

Radiation intensity

Strength of xray beam as it reaches the IR

Radiation intensity is important because it effects

Patient dose, IRE, image quality

Radiation intensity is influenced by

MAs, kVp, SID

The inverse square law helps the radiographer

Calculate the intensity of the beam at the IT when the distance between the tube and receptor changes

Exposure maintenance formula

Used to determine what new technique to use to produce an image with proper IRE when the distance between the tube and receptor changes

Increasing SID causes

Decreased intensity and decreased IRE

SID starts

Where the beam is created in the tube

The relationship b/w the xray quantity and distance can be described by

Inverse square law

Increasing SID effect

Increase spatial resolution and decrease distortion

As the xray tube is moved further from the IR the collimator light will

Cover a larger area of the IR

Inverse square law states that

Intensity of radiation at a given distance from the point of source is inversely proportional to the square of the distance

Imcreasing SID inverse square effect

Decreased beam intensity + increased radiation area

Decreasing SID inverse square effect

Increased beam intensity + decreased radiation area

Inverse square formula

I1D1² = I2D2²

Changing distance does not change

Amount of xrays produced

If the SID is doubled

The intensity at the IR will be 4 times less

If SID is halved

Intensity at IR increased 4x

Exposure maintenance formula allows the radiographer to

Calculate the new mAs needed to maintain IRE when the distance changes

The radiation intensity correlates to

How much radiation the IR receives

2 images taken with same factors at different distances have

Different exposures

Increasing SID requires what to maintain IRE

Increased MAs

New MAs =

(Old MAs x new distance²) / old distance²

If SID is doubled

The MAs will need to increase 4 times

SID and IRE relationship

inverse

SID and spatial resolution relationship

direct

SID and distortion relationship

inverse

original inverse square law formula

I1/I2 = D2²/D1²

if intesntiy is 50 mR @ 36’’, what would it be @ 72

12.5 mR

exposure maintenance formula

new mAs = old mAs x new distance² / old distance²

exposure maintenance can also be known as

direct square law