SID - Chapter 22 Flashcards
What is SID?
The distance between the x-ray source (focal spot) and the image receptor
What are the standard distances of most fixed units?
40” and 72”
What effect does SID have on sharpness?
Increased SID leads to increased Sharpness
Why does sharpness increase with increased SID?
The x-ray beams become more parallel to the IR which reduces penumbra and increases spatial resolution
What effect does SID have on magnification?
Increased SID leads to decreased Magnificataion
Why does magnification decrease with increased SID?
The umbral portion of the image gets reduced, which reduces its size
Why must magnification be considered in chest radiography?
Increased magnification can lead to misdiagnosis of an enlarged heart
What effect does insufficient SID’s have on portable exams?
Can result in anatomical structures being cutoff in the image
What effect does SID have on shape distortion?
SID will NOT affect image shape, as long as the beam remains centered and perpendicular
What effect does SID have on exposure?
Increased SID leads to decreased exposure
Why does exposure decrease when SID is increased?
An increased SID, increases the area of radiation striking the object, spreads out the radiation more which reduces the intensity of the beam
When an SID is doubled how is the area of radiation striking and intensity affected?
With 2x SID, the area of radiation striking the object increases 4x and the intensity is 1/4 (quartered)
What rule defines the effect of SID on exposure?
Inverse square law
What is the inverse square law?
States that the intensity of radiation is inversely proportional to the square of the distance from the source
What is the inverse square law formula?
I1/I2=(D2)^2/(D1)^2
I1= intensity 1
I2= intensity 2
D1= first distance
D2= new distance
How are the effects of the inverse square law reflected on radiographs?
It is reflected in the exposure index (EI)
When SID is increased and techniques kept the same, what effect will it have on EI?
EI will go down when SID is increased
EI will go up with SID is decreased
What effect does SID have on density?
As SID increases, density decreases
Why does density decrease when SID increases?
With a SID increase, the intensity decreases which decreases density
How does decreased density appear on radiographs?
Appear lighter
What effect does increased SID have on digital images?
SID can lead to visible image noise in the form of quantum mottle and reduced SNR
What should be used to compensate for increases in SID?
Increases in mAs
What changes in SID would warrent a change in mAs?
15% change in SID
What impact does increasing the mAs have on image when the SID is also increased?
Increasing mAs ensures sufficient exposure to the IR so that noise can be avoided
What is the Square Law (Direct Square Law)?
Calculates the new mAs required when a SID changes
What is the formula for The Square law?
mAs1/mAs2 = (D1)^2/(D2)^2
What does the Square law seek to do?
Maintain exposure levels by adjusting the amount of radiation produced based on SID changes
When moving from 40 SID to 30 SID, what is the rule of thumb for mAs?
Cut the mAs in half
When moving from 40 SID to 50 SID, what is the rule of thumb for mAs?
Increase mAs by 50%
When moving from 40 SID to 60 SID, what is the rule of thumb for mAs?
Double the mAs (2x)
When moving from 40 SID to 70 SID, what is the rule of thumb for mAs?
Triple the mAs (3x)
When moving from 40 SID to 80 SID, what is the rule of thumb for mAs?
Quadruple the mAs (4x)
Why is maintaining a constant SID crucial in imaging?
- Allows for reproducibility of images
- Allows for an accurate comparison of studies over time
- Allows for proper alignment of the tube and IR
What other factors does SID work in conjunction with?
OID and SOD
What ratio determines magnification?
SID/SOD
