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 if you want to maintain the same exposure intensity

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