SID - Chapter 22

Question 1

What is SID?

Answer 1

The distance between the x-ray source (focal spot) and the image receptor

Question 2

What are the standard distances of most fixed units?

Answer 2

40” and 72”

Question 3

What effect does SID have on sharpness?

Answer 3

Increased SID leads to increased Sharpness

Question 4

Why does sharpness increase with increased SID?

Answer 4

The x-ray beams become more parallel to the IR which reduces penumbra and increases spatial resolution

Question 5

What effect does SID have on magnification?

Answer 5

Increased SID leads to decreased Magnificataion

Question 6

Why does magnification decrease with increased SID?

Answer 6

The umbral portion of the image gets reduced, which reduces its size

Question 7

Why must magnification be considered in chest radiography?

Answer 7

Increased magnification can lead to misdiagnosis of an enlarged heart

Question 8

What effect does insufficient SID’s have on portable exams?

Answer 8

Can result in anatomical structures being cutoff in the image

Question 9

What effect does SID have on shape distortion?

Answer 9

SID will NOT affect image shape, as long as the beam remains centered and perpendicular

Question 10

What effect does SID have on exposure?

Answer 10

Increased SID leads to decreased exposure

Question 11

Why does exposure decrease when SID is increased?

Answer 11

An increased SID, increases the area of radiation striking the object, spreads out the radiation more which reduces the intensity of the beam

Question 12

When an SID is doubled how is the area of radiation striking and intensity affected?

Answer 12

With 2x SID, the area of radiation striking the object increases 4x and the intensity is 1/4 (quartered)

Question 13

What rule defines the effect of SID on exposure?

Answer 13

Inverse square law

Question 14

What is the inverse square law?

Answer 14

States that the intensity of radiation is inversely proportional to the square of the distance from the source

Question 15

What is the inverse square law formula?

Answer 15

I1/I2=(D2)^2/(D1)^2
I1= intensity 1
I2= intensity 2
D1= first distance
D2= new distance

Question 16

How are the effects of the inverse square law reflected on radiographs?

Answer 16

It is reflected in the exposure index (EI)

Question 17

When SID is increased and techniques kept the same, what effect will it have on EI?

Answer 17

EI will go down when SID is increased
EI will go up with SID is decreased

Question 18

What effect does SID have on density?

Answer 18

As SID increases, density decreases

Question 19

Why does density decrease when SID increases?

Answer 19

With a SID increase, the intensity decreases which decreases density

Question 20

How does decreased density appear on radiographs?

Answer 20

Appear lighter

Question 21

What effect does increased SID have on digital images?

Answer 21

SID can lead to visible image noise in the form of quantum mottle and reduced SNR

Question 22

What should be used to compensate for increases in SID?

Answer 22

Increases in mAs

Question 23

What changes in SID would warrent a change in mAs?

Answer 23

15% change in SID

Question 24

What impact does increasing the mAs have on image when the SID is also increased?

Answer 24

Increasing mAs ensures sufficient exposure to the IR so that noise can be avoided

Question 25

What is the Square Law (Direct Square Law)?

Answer 25

Calculates the new mAs required when a SID changes

Question 26

What is the formula for The Square law?

Answer 26

mAs1/mAs2 = (D1)^2/(D2)^2

Question 27

What does the Square law seek to do?

Answer 27

Maintain exposure levels by adjusting the amount of radiation produced based on SID changes

Question 28

When moving from 40 SID to 30 SID, what is the rule of thumb for mAs?

Answer 28

Cut the mAs in half

Question 29

When moving from 40 SID to 50 SID, what is the rule of thumb for mAs?

Answer 29

Increase mAs by 50%

Question 30

When moving from 40 SID to 60 SID, what is the rule of thumb for mAs?

Answer 30

Double the mAs (2x)

Question 31

When moving from 40 SID to 70 SID, what is the rule of thumb for mAs?

Answer 31

Triple the mAs (3x)

Question 32

When moving from 40 SID to 80 SID, what is the rule of thumb for mAs?

Answer 32

Quadruple the mAs (4x)

Question 33

Why is maintaining a constant SID crucial in imaging?

Answer 33

- Allows for reproducibility of images - Allows for an accurate comparison of studies over time - Allows for proper alignment of the tube and IR

Question 34

What other factors does SID work in conjunction with?

Answer 34

OID and SOD

Question 35

What ratio determines magnification?

Answer 35

SID/SOD