Ch. 18 Image Production

Question 1

What is a grid and how does it work?

Answer 1

It is a device that improves the contrast of the Rad image by absorbing scatter radiation before it reaches the IR.

Question 2

What are the three things that happen in the body when exposed to radiation?

Answer 2

Passes through the body unaffected
Absorbed by the body
Interact and change direction

Question 3

Absorption of the photons occur because of what type of interaction?

Answer 3

Photoelectric interaction

Question 4

What causes scatter in the interaction with tissue?

Answer 4

Compton interaction

Question 5

How do you define a photon as scatter?

Answer 5

When it interacts with the outer shell electron and as a result the scattered electron changes direction which can no longer be used to records exposure

Question 6

What is the relationship between kVp and Compton scatter ?

Answer 6

Directly related

Question 7

As kVp increases what happens to contrast (In film)?

Answer 7

results in low contrast (better contrast)

Question 8

What increases scatter in a patient

Answer 8

Increase patient thickness
Larger field size
Decrease in atomic number of tissue(b/c low z# = less absorption

Question 9

When should we use a grid?

Answer 9

When a body parts thickness exceeds 10cm

When kVp is above 60

Question 10

What does grid consist of ?

Answer 10

Thin flat rectangular device that has a series of radiopaque (absorbed) lead strips side by side seaperating the strips by an interspace material that is radiolucent (passes through)

Question 11

What is the material if the radiolucent interspaces?

Answer 11

Aluminum and plastic fiber

Question 12

What is grid ratio defined as

Answer 12

The ratio of the height of the lead strips to the distance between the strips

Grid ratio formula= h(high of the lead strips)/D(interspace width)

Question 13

If the height of the grid is constant and we were to decrease the distance between the lead strips (h) what would result?

Answer 13

Increase grid ratio

Question 14

What is the relationship between distance of lead strips (D) and grid ratio if highs remains the same?

Answer 14

Inversely related

Question 15

Higher grid ratios allow what ?

Answer 15

Less scatter passing to the IR through the interspace

Question 16

Higher grid ratios are_____ but are prone to_____

Answer 16

Better, grid error

Question 17

How are grid ratios rated?

Answer 17

Are sometimes rated by weight instrear or grid ratio

Question 18

What is the term for expressing a grids weight?

Answer 18

Grams per square centimeter (g/cm^2) or in the US (g/in^2)

Question 19

What is grid frequency?

Answer 19

The # of grid lines per inch or centimeter

Question 20

What is the range of grid frequency in centimeters and inches ?

Answer 20

60-200 lines/inch; 25-80 lines/cm

Question 21

What is the most common grid frequency?

Answer 21

85-103 lines/inch; 33-41 lines/cm

Question 22

What aid the grid frequency of a very-high-frequency?

Answer 22

178 to 200 lines/inch; 70-80 lines/centimeter

Question 23

What is a linear grid?

Answer 23

Grids with lead strips running in one direction

Question 24

Two linear grids that are place one on top of the other so that they run at right angles

Answer 24

Cross-cross or cross hatched

Question 25

When are linear grids commonly used

Answer 25

In clinical practice

Question 26

Grid cutoff

Answer 26

When the primary beam is angled into the lead, which will lead to an unwanted about of primarily beam to be absorbed

Question 27

Parallel grids are made with?

Answer 27

Lead and interspaces running parallel to one another

Question 28

Focused grid

Answer 28

Made so that the center grid strips are parallel but as they move away from the center they become inclined

Question 29

Convergence line is?

Answer 29

Where the focused grid’s inclined lines meet in the middle

Question 30

Grid radius is what

Answer 30

Is the distance from the inclined focused grid lines and the convergence line

Question 31

Short-focal-range grids; which modality are they in?

Answer 31

14-18 inches or 36-46cm | Used in mammography (SID)

Question 32

Long-focal-range grids; which modality uses them?

Answer 32

60-72 inches; 152-183cm | Used for chest x- rays (SID)

Question 33

What is the usual size for a grid placed it the Bucky ?

Answer 33

17x19 in

Question 34

Reciprocating movement of a grid

Answer 34

A motor drives the grid back and forth at about 2-3cm

Question 35

How does the oscillating grid move

Answer 35

An electromagnet pulls the grid to one side and then releases it during exposure

Question 36

How do we calculate the grid conversion factor?

Answer 36

GCF= mAs with grid/mAs without grid

Question 37

What is he relationship between GCF and kVp?

Answer 37

no relation

Question 38

What is the relationship between mAs and GCF ?

Answer 38

Directly related

Question 39

What is the ICRU

Answer 39

The internal commission on radiologic units and measurements

Question 40

What criteria is measured to meant the ICRU guide lines for a grids performance?

Answer 40

Selectively and contrast improvement ability

Question 41

A grids selectivity is measured how ?

Answer 41

The percentage of scatter it eats up is greater than the percentage of primary beam it eats

Question 42

How can we calculate grid selectivity?

Answer 42

Selectivity= % primary radiation transmitted / % is scatter

Question 43

A _____ lead content will have greater selectivity

Answer 43

Higher

Question 44

How do you measure the contrast improvement factor(K)?

Answer 44

K= radiographic contrast with the grid/radiograph contrast without the grid

Question 45

What is the usual contrast improvement factor(k)?

Answer 45

1.5 to 3.5

Question 46

The _____ the K factor, the greater the contrast improvement

Answer 46

Higher

Question 47

Why are focused grids prone to grid error?

Answer 47

B/c the grids are made to go with the separation of the x-ray beam, which means the tube must be centered and the distance aligned

Question 48

What does a off-level error occur?

Answer 48

When the tube is angled across the long axis of the grid strips, which can also result in improper tube or grid positioning

Question 49

Using what grid will let you angle alone the long axis of the table ?

Answer 49

With a linear grid

Question 50

Can you angle with a criss-cross grid/cross-hatch grid ?

Answer 50

No

Question 51

What is an off-center grid error

Answer 51

When the grid is not aligned to the tube causing a decrease in exposure on the ENTIRE IMAGE

Question 52

What is off-focus in a grid?

Answer 52

When the distance is too great for the grid you are using; when it’s not in focal ranges CUT OFF AT THE EDGES

Question 53

Upside down grid is what

Answer 53

When a grid is used backwards and results in radiation absorbed away from the center ONLY THE CENTER IS SEEN; SIDES OF IMAGE IS CUT OFF

Question 54

When will the mortise effect occur in a grid?

Answer 54

When the grid lines must run in the same direction as the laser beam that is scanning then imaging plate (CR)

Question 55

what is photoelectric interaction

Answer 55

result in the complete absorption of the primary photon and the production of the second

Question 56

does Compton interaction add to diagnostic value of the image

Answer 56

it has no diagnostic value

Question 57

as scatter is increased b/c of kVp; what happens to contrast

Answer 57

contrast is impaired (weakened)

Question 58

how does thickness of the patient affect scatter

Answer 58

the thicker the patient the more scatter is produced but a higher atomic number will result in less scatter being produced

Question 59

what cause the amount of scatter to increase

Answer 59

increase in patient thickness, decreased atomic number of the tissue, and LARGE field size/ NO collimation