Registry 2025 - X-ray equations

ohm's law equation

This is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit.

Can be done as:

V = I × R

I = V/R

R = V/I

inverse square law formula

The radiation Intensity is inversely proportional to the square of the distance.

The "I" can interchanged to dosage (grays. millagrays, etc) and exposure

Magnification formula

The formula to find the actual size of a part and the magnified size.

Image Size SID

------------- = -------

Object size SOD

unsharpness formula

Measures the unsharpness (blur/penumbra) in the image

Focal spot size x OID

Penumbra = --------------------------

SOD

Reciprocity Law

A law that states any combination of mA and time is able to gives you the same mAs

mA old x Seconds old = mA new x Seconds new

Spatial resolution - pixel size formula

A spatial resolution formula to find the pixel size using FOV (field of view) and Matrix. Pixel size is usually in mm

FOV

Pixel size = -------------

Matrix

Grid conversion factor

a formula to find out the appropriate mAs to use with the grid.

mAs old Grid Conversion factor old

---------- = -----------------------------------

mAs new Grid Conversion Factor new

square law

a formula to figure out when SID is increased, mAs must be increased to maintain constant exposure to IR