Control Of Scatter Radiation

Question 1

Define contrast

Answer 1

Arises from areas of light, dark, and shades of gray in the image

Question 2

Define Contrast resolution

Answer 2

Ability to image adjacent similar tissues as different

Question 3

Scatter puts what in the image

Answer 3

A density in the film that is not related to patient anatomy and makes the image less visible=noise

Question 4

Scatter is produced when and destroys what

Answer 4

Produced during Compton interaction
High kV ranges
Scatter destroys contrast

Question 5

Factors affecting scatter PRODUCTION

Answer 5

Volume of material irradiated
Atomic number of material being irradiated
KV

Question 6

Ways to control scatter

Answer 6

1. kVp selection
2. Beam restriction
3. Compression
4. Grid use

Question 7

What is the best way to reduce scatter?

Answer 7

Beam restriction

Question 8

Aperture diaphragm

Answer 8

Flat sheet of metal with a hole in the center 
Attached to tube head
Simple, cheap, easy to use
High penumbra
High off focus radiation
No light field

Question 9

Comes/cylinders

Answer 9

Extended metal structure attached to tube head or collimator
Cheap, easy to use
Less penumbra and off focus than diaphragm

Question 10

Collimator

Answer 10

Lead shutters at right angles to one another regulate field size
More complex, more expensive
Infinite field sizes, provides light source

Question 11

PBL

Answer 11

Positive beam limitation devices

Automatic collimator a were required from 1974-1993

Question 12

First stage shutters

Answer 12

Decrease off focus radiation

Fixed, don’t move

Question 13

2nd stage shutters

Answer 13

Control field size and decrease penumbra

Question 14

Lead strips and lead masks

Answer 14

Attach to collimator head used in oncology in size of tumor

Question 15

Compression

Answer 15

Used for bigger patients to decrease patient thickness

Question 16

Grid use

Answer 16

Doesn’t affect scatter production

Absorbs scatter after it’s been produced

Question 17

What is the purpose of a grid?

Answer 17

Improves contrast of image by absorbing scatter
Use when: body part is greater than 10 cm thick
kV is greater than 60 in film and CR
kV is greater or equal to 90 in digital

Question 18

What is a grid made out of?

Answer 18

Lead strips with Al or plastic fiber interspace

Question 19

What is the Al interspace for?

Answer 19

More durable
Easier for manufacture 
Nonhygroscopic
Acts as a grids grid
Absorbs more low energy primary also

Question 20

Grid ratio

Answer 20

Height/distance

Higher grid ratio absorbs more scatter

Question 21

Grid frequency

Answer 21

Number of grid lines/in
Range from 60-110 lines per inch
High frequency grids show less distinct grid lines

Question 22

Linear parallel grid

Answer 22

Strips in one direction, run parallel to each other.
Some grid cut off bc not canted along divergence of beam
Better for long SID

Question 23

What is grid cutoff?

Answer 23

Absorption of primary beam

Question 24

Linear focused grid

Answer 24

Central strips run parallel as strips move farther away from center they become more and more inclined
Less cut off
SID must be within focal range

Question 25

Convergence line

Answer 25

Imaginary line in space where strips would meet on linear focused grid

Question 26

Grid radius

Answer 26

Distance from face of grid to convergence line

Question 27

Cross cross or cross hatched grid

Answer 27

Lead strips in 2 directions at right angles of each other More efficient than linear Centering is crucial Cannot angle tube against grid

Question 28

Stationary grid uses

Answer 28

Portable procedures Most uprise procedures Grid cassette

Question 29

Moving grid uses

Answer 29

Table top procedures Usually focused grids Blurs grid lines

Question 30

Single stroke mechanism

Answer 30

Spring, requires manual resetting for each exposure

Question 31

Reciprocating mechanism

Answer 31

Motor, back and forth | Most common

Question 32

Oscillating mechanism

Answer 32

Electromagnet circular motion | Vibrating

Question 33

Stroboscopic effect

Answer 33

Grid lines occurring at very short exposure times. With moving grids

Question 34

What grid to use up to 80 kV?

Answer 34

5:1 & 6:1

Question 35

What grid to use up to 100 kV?

Answer 35

8:1 & 10:1

Question 36

What grid to use over 100 kV?

Answer 36

12:1 & 16:1

Question 37

Grid conversion factor

Answer 37

Bucky factor Tells how much to increase mAs with grid B= mAs with grid/mAs without grid B= incident radiation/transmitted radiation

Question 38

Selectivity of grid

Answer 38

Ability of grid to absorb scatter and not primary radiation % primary radiation transmitted/% scatter radiation transmitted Want a higher selectivity

Question 39

Contrast improvement factor

Answer 39

``` K factor Usually 1.5-3.5 If K=1, then no improvement is seen K=radio graphic contrast with grid/without grid Want higher number Average gradient ```

Question 40

Off level

Answer 40

Tube angled across long axis of grid Grid cutoff on entire image: underexposed

Question 41

Off center

Answer 41

Tube not centered along central axis of focused grid Beam divergence my aligned Grid cut off on entire image

Question 42

Off focus

Answer 42

Tube not within focal range o focused grid (SID) | Grid cut off along periphery

Question 43

Upside down

Answer 43

Tube side of focused grid against film Line down the middle of image Severe peripheral grid cut off

Question 44

Off center and off focus

Answer 44

Grid cut off on one side

Question 45

Air gap method

Answer 45

Use greater OID | Increase mAs 10% for every inch of air gap

Question 46

What is the grid conversion equation?

Answer 46

mAs2= mAs1 x GCF2/GCF1

Question 47

Lead content

Answer 47

Measured in g/cm^2 High lead content means high ratio with low frequency High lead content means higher absorption of scatter Lead strips become thinner as you add more