Distance (SID) as a Prime Factor

What is SID?

distance from the actual focal spot to the image receptor

What are some older terms for SID?

target-to-film distance (TFD)

focal-film distance (FFD)

SID = ___ + ___

OID + SOD

What does SOD stand for?

source-to-object distance (focal spot to object being imaged

What does OID stand for?

object-to-IR distance

Beam intensity is measured in ___

milliroentgen (mR)

1 mR = ___ R

1/1000

Define Roentgen

unit of exposure in air (quantity of x-rays required to produce a given amount of radiation)

Explain beam divergence

beam diverges from the point of origin (target); the farther from the source, the fewer photons in a given area (the actual # of photons doesn’t change, they are only spread out)

When SID is doubled, the area that is exposed increases by a factor of ___

4 (the same amount of radiation is distributed over an area 4x larger)

As SID increases, beam divergence ___

increases

When SID is doubled, beam intensity is ___

¼

When SID is halved, beam intensity is ___

4 times

What is the inverse square law used for?

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

What is the ISL equation?

I₁/I₂ = (D₂)²/(D₁)²

An increase in SID results in a(n) ___ in dose

decrease

An increase in SID ___ exposure intensity

reduces

SID and exposure intensity have a(n) ___ relationship

indirect

SID and mAs have a(n) ___ relationship

direct

What is the direct square law used for?

used to calculate adjustment in mAs when distance is changed

What is the DSL equation?

mAs₁/mAs₂ = (D₁)²/(D₂)²

A ___% change in distance causes a perceptible change in exposure intensity

20

The SID margin of forgiveness is ___%

10

Why do we need to use the specified SID?

SID affects magnification and recorded detail

Angling the tube without a vertical adjustment results in a(n) ___ SID

increased

For tube angles of 15^o or greater, decrease SID by ___ for every 5^o of tube angulation

1”

SID affects geometric qualities, such as ___

magnification, resolution, distortion

What are the 3 photographic properties (properties that affect visibility) of image quality?

IR exposure (brightness/density), noise, and contrast (gray scale)

How do distance and mAs affect contrast?

THEY DON’T