chapter 11 xray physics

Question 1

compton predominates at higher what?

Answer 1

kVp settings

Question 2

Compton is associated with that kind of e-?

Answer 2

scatter

Question 3

what are 3 things that can be used to control scatter?

Answer 3

patient positioning, exposure technique, collimation

Question 4

when there is less tissue to penetrate how does that affect the scatter?

Answer 4

it lessens it

Question 5

how does higher kVp/lower mA technique affect scatter?

Answer 5

it will increase scatter

Question 6

collimation does what?

Answer 6

reduces the field size to area of interest, reduces scatter, reduces pt. dose,

Question 7

as the pt, becomes thicker what controlling mechanism of scatter if important?

Answer 7

collimation

Question 8

why would you want to place pt. in recumbent position instead of a standing?

Answer 8

when they are thicker

Question 9

More than___of the exiting (remnant) beam may be scatter

Answer 9

½

Question 10

purpose of the grid is to do what?

Answer 10

reduce scatter “clean up”

Question 11

what does scatter have an effect on?

Answer 11

it will reduce image contrast

Question 12

what is the trade off when using a grid?

Answer 12

the increased exposure to the pt. and the improved dx. quality of image

Question 13

what % can a grid reduce scatter by?

Answer 13

90-80

Question 14

what is the grid ratio?

Answer 14

“height” of Pb strip divided by distance between the Pb strip

Question 15

what does the height of the Pb strip refer too in recumbent xray technique?

Answer 15

the horizontal measurement from front to back

Question 16

grid frequency =

Answer 16

lines/inch

Question 17

grid selectivity is also known as

Answer 17

sigma

Question 18

sigma =

Answer 18

1^0 radiation t’mitted/scatter t’mitted

Question 19

are higher frequency grids generally less or more efficient than lower frequency grids of the same ratio?

Answer 19

they are less efficient

Question 20

the greater the lead in a grid, what happens to the selectivity?

Answer 20

it becomes greater

Question 21

what is the difference of effectiveness when using higher ratio grids vs lower ratio grids?

Answer 21

the effectiveness increases with higher ratio

Question 22

As SID decreases what margins of the grid are cut off?

Answer 22

lateral

Question 23

what are focused grids used for?

Answer 23

to reduce the grid cut off

Question 24

in a focused grid the grid strip are accommodating what?

Answer 24

accommodate the increasingly divergent angle of photons further away from central ray.

Question 25

focal range is defined as?

Answer 25

the limited range of SID that can be used without grid cutoff

Question 26

grids that are used for general radiography are what kind of grids?

Answer 26

focus

Question 27

what is the relationship between grid ratio and focal range?

Answer 27

an inverse relationship meaning that as grid ratio increases the focal range will decrease

Question 28

will the SID need to decrease or increase as you increase the grid ration?

Answer 28

it will need to decrease

Question 29

what are crossed grids ?

Answer 29

sandwhiching two parallel grids together so their grid strips are perpendicular

Question 30

what do cross grids reduce?

Answer 30

scatter photons

Question 31

how do cross grids negatively affect the image in an general radiograpy?

Answer 31

grid cutoff will occur in the superior to inferior angulation of the xray beam

Question 32

potter bucky diaphragm is known as the

Answer 32

moving grid

Question 33

is a moving grid a new technology or old?

Answer 33

old

Question 34

what is the purpose of the moving grid?

Answer 34

to blur the grid strips on the xray image

Question 35

the disadvantages of the moving grid?

Answer 35

1. freezing of image may happen if exposure time is too short 2. moving parts may require service 3. motion of grid might show up on the image 4. require 15% more exposure

Question 36

what may appear on the image screen if grid is not perpendicular to the central ray?

Answer 36

grid lines on the image

Question 37

grid cutoff on image will move towards what?

Answer 37

the displaced central ray

Question 38

when SID in not in the focal range where will the grid cutoffs be?

Answer 38

at the sides

Question 39

what are some common grid errors?

Answer 39

1. grid not perpendicular 2. central ray not in the center 3. SID not in focal range 4. grid upside down

Question 40

Air gap technique

Answer 40

space between the image receptor and object imaged cause scatter photon to diverge and not hit the image receptor.

Question 41

what are the drawbacks of air gap?

Answer 41

image magnification/distortion | Higher exposure is also necessary (about 10% increase per cm of air gap)

Question 42

What projection includes a natural “air gap”?

Answer 42

Looking top down on the pt. the image receptor and cervical spine will have a air gap --> creating scatter photons that do not end up hitting the image receptor