Unit 2 Beam Restriction, Grids, & Filtration

Question 1

Beam Restriction

Answer 1

-Limits the scatter that is produced inside the patient
-Reduces the size of the exposure area and volume of tissue being irradiated
-Reduces the # of Compton interactions that occur

Question 2

Functions of collimator

Answer 2

-Reduces effects of off focal radiation
-Reduces Penumbra(image unsharpness)

Question 3

What is Positive Beam Limitation (PBL)?
Why is it important?

Answer 3

• Automatically collimates the beam to the size of the IR
• ALARA: Beam size should never exceed size of IR

Question 4

In what 2 instances should a Radiographer consider grid use?

Answer 4

-Body part over 10cm
-kVp above 70

Question 5

T or F: Grid use decreases subject contrast

Answer 5

False, it increases

Question 6

T or F: Grids reduce scatter that REACHES the IR

Answer 6

True

Question 7

Collimation reduces the ________ of scatter of photons

Answer 7

Production

Question 8

Why do parallel grids only work with long SID?

Answer 8

Because more of the beam is going straight up and down and less spread out

Question 9

What kind of grid is used in moving systems?

Answer 9

Parallel grids

Question 10

Focused Grid characteristics

Answer 10

• Grid strips align with divergent beam
• Grid Radius
• Focal Range: range of appropriate SID

Question 11

What is a Potter-Bucky diaphragm? What are the 2 types of grid motion?

Answer 11

• Grid lines run longitudinally
• Designed to move during exposure

• Reciprocating
-Move back and forth
• Oscillating
-Circular motion

Question 12

Which grid motion can withstand faster/shorter exposure times?

Answer 12

Oscillating

Question 13

Grid Ratio

Answer 13

• Height of lead strips: width of interspace
• Higher ratio = Better Clean up

Question 14

Grid Frequency

Answer 14

• # Lead strips per inch
• Higher frequency = thinner strips
• Digital systems require 178-200 lines/in

Question 15

Does a higher or lower grid ratio absorb more scatter?

Answer 15

Higher

Question 16

Higher grid frequency, the strips become _______

Answer 16

Thinner

Question 17

________ grid frequency can show up as an artifact in x-ray images

Answer 17

Lower

Question 18

Digital systems require a GF of ____
Why?

Answer 18

178-200 lines/in;
because strips will show up in picture if not in this range

Question 19

Does Higher grid ratio mean higher or lower contrast?

Answer 19

Higher

Question 20

Lead Content

Answer 20

Total amount of lead in grams/cm

Question 21

Selectivity

Answer 21

% primary beam transmitted vs. % scatter transmitted

Question 22

What is Contrast Improvement Factor (K)? What is the typical range?

Answer 22

The ratio of Subject contrast obtained with the grid vs. without

1.5-3.5

Question 23

Pros of using grids

Answer 23

• Reduction of Scatter that reaches the IR
• Improved Subject contrast and visibility of detail

Question 24

Cons of using grids

Answer 24

• Reduction of IR Exposure
• Requires increase in mAs
• Increased patient dose
• Radiographer error

Question 25

Common Grid Ratios and their GCF

Answer 25

Grid GCF Ratio No grid —> 1 5:1 ———-> 2 6:1 ———-> 3 8:1 ———-> 4 10:1 ———-> 5 12:1 ———-> 5 16:1 ———-> 6

Question 26

No Grid to Grid Formula

Answer 26

GCF = mAs with Grid/mAs without Grid If you’re going from higher grid ratio to no grid you divide, if you’re going from no grid to higher grid ratio you multiply

Question 27

Grid to grid formula

Answer 27

mAs 1 GCF 1 ——- = ——— mAs 2 GCF 2

Question 28

Technologist ERROR can result in

Answer 28

Grid Cut Off

Question 29

Grid cut off

Answer 29

Absorption of the primary beam

Question 30

4 types of grid cut-offs

Answer 30

-Off-Level/Canting -Off-Centering -Off-Focal -Upside Down

Question 31

Which of the 2 grid cut-offs cut off on ENTIRE IR

Answer 31

-Off-Level/Canting -Off-Centering

Question 32

Which 2 grid cut-offs cut off on Periphery

Answer 32

-Off-Focal -Upside Down

Question 33

What is the purpose of a filtration of the x-ray beam?

Answer 33

-Eliminates low energy photons from the beam -Reduces patient dose -Increases mean energy of the beam (Harder) -Negligible(small/unimportant) impact on IR Exposure

Question 34

As filtration increases, the intensity(quantity) of the beam _________ and the quality ________

Answer 34

Decreases; Increases

Question 35

Half-Value Layer

Answer 35

The amount of absorbing material that will reduce the intensity of the beam by half

Question 36

Inherent Filtration

Answer 36

-0.5 mm Al/Eg These make up the Inherent Filtration: • Envelope • Insulating Oil • Port Window

Question 37

Added Filtration

Answer 37

-2.0 mm Al/Eg Mror These make up the Added Filtration: • Aluminum Sheet 1.0mm • Mirror 1.0mm

Question 38

Total Filtration

Answer 38

2.5 mm Al/Eq

Question 39

Compensation Filter

Answer 39

Designed to compensate for varying thicknesses in tissue in order to even out the Exposure to the IR

Question 40

For a unit that operates over 70 kVp, what is the required minimum amount of filtration?

Answer 40

2.5 mm AI/eq

Question 41

Beam restriction devices

Answer 41

-Collimator -Cones and Cylinders