Distance

Source to Image Receptor Distance

Distance from the actual focal spot to the image receptor in radiography, which affects image quality and magnification.

What are the other names for source to image receptor?

Target to film distance- TFD

Focal film distance -FFD

Source to object distance (SOD)

Distance between actual focal spot and object

Object to IR distance (OID)

distance between subject/object and IR

SID =

SOD + OID

Beam intensity is measured in 

mR= milliroentgen

1 mR =

1000 (R) Roentgen

Roentgen is

unit of exposure in air

The beam intensity is

Quantity of x-rays required to produce a given amount of ionization

Beam diverges from point of origin (target)

-actual # doesn’t change, only distribution

the rays diverge as the exit from the target and tube window

Beam divergence and intensity:

The farther from the source,

the fewer photons in a given area

SID and Intensity

when the distance is double, the area exposed increases by a factor of 4

SID and Intensity:

The same amount of radiation is distributed over an area

4 times larger

As distance increases

the beam divergence increases from the tube IR

Doubling the distance

decreases beam intensity by 1/4

Halving the distance

increases beam intensity by 4

More beam divergence included at a shorter SID -

36 in

Using more straight-line portion of the beam - 

72 in

Inverse square law

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

What describes the relationship between distance and the intensity of radiation?

inverse square law

As SID increases, the intensity of the xray beam

decreases

As the SID is increased the radiographic exposure intensity will

decrease

The direct square law, or exposure maintenance formula for distance, states that the SID is decreased, mAs must _
__ in order to produce an image with the correct exposure intensity

decrease

Which of the following terms have been used to describe the distance from the x-ray focal spot to the image receptor?

1. focal film distance

2. target film distance

3. source to image receptor

1,2 and 3

as distance increases (from tube to IR), beam divergence

increases

List 3 prime factors of xray emission:

mAs

kVp

distance