Unit 2 Beam Restriction, Grids, & Filtration Flashcards
Beam Restriction
-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
Functions of collimator
-Reduces effects of off focal radiation
-Reduces Penumbra(image unsharpness)
What is Positive Beam Limitation (PBL)?
Why is it important?
• Automatically collimates the beam to the size of the IR
• ALARA: Beam size should never exceed size of IR
In what 2 instances should a Radiographer consider grid use?
-Body part over 10cm
-kVp above 70
T or F: Grid use decreases subject contrast
False, it increases
T or F: Grids reduce scatter that REACHES the IR
True
Collimation reduces the ________ of scatter of photons
Production
Why do parallel grids only work with long SID?
Because more of the beam is going straight up and down and less spread out
What kind of grid is used in moving systems?
Parallel grids
Focused Grid characteristics
• Grid strips align with divergent beam
• Grid Radius
• Focal Range: range of appropriate SID
What is a Potter-Bucky diaphragm? What are the 2 types of grid motion?
• Grid lines run longitudinally
• Designed to move during exposure
• Reciprocating
-Move back and forth
• Oscillating
-Circular motion
Which grid motion can withstand faster/shorter exposure times?
Oscillating
Grid Ratio
• Height of lead strips: width of interspace
• Higher ratio = Better Clean up
Grid Frequency
• # Lead strips per inch
• Higher frequency = thinner strips
• Digital systems require 178-200 lines/in
Does a higher or lower grid ratio absorb more scatter?
Higher
Higher grid frequency, the strips become _______
Thinner
________ grid frequency can show up as an artifact in x-ray images
Lower
Digital systems require a GF of ____
Why?
178-200 lines/in;
because strips will show up in picture if not in this range
Does Higher grid ratio mean higher or lower contrast?
Higher
Lead Content
Total amount of lead in grams/cm
Selectivity
% primary beam transmitted vs. % scatter transmitted
What is Contrast Improvement Factor (K)? What is the typical range?
The ratio of Subject contrast obtained with the grid vs. without
1.5-3.5
Pros of using grids
• Reduction of Scatter that reaches the IR
• Improved Subject contrast and visibility of detail
Cons of using grids
• Reduction of IR Exposure
• Requires increase in mAs
• Increased patient dose
• Radiographer error
Common Grid Ratios and their GCF
Grid GCF
Ratio
No grid —> 1
5:1 ———-> 2
6:1 ———-> 3
8:1 ———-> 4
10:1 ———-> 5
12:1 ———-> 5
16:1 ———-> 6
No Grid to Grid Formula
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
Grid to grid formula
mAs 1 GCF 1
——- = ———
mAs 2 GCF 2
Technologist ERROR can result in
Grid Cut Off
Grid cut off
Absorption of the primary beam
4 types of grid cut-offs
-Off-Level/Canting
-Off-Centering
-Off-Focal
-Upside Down
Which of the 2 grid cut-offs cut off on ENTIRE IR
-Off-Level/Canting
-Off-Centering
Which 2 grid cut-offs cut off on Periphery
-Off-Focal
-Upside Down
What is the purpose of a filtration of the x-ray beam?
-Eliminates low energy photons from the beam
-Reduces patient dose
-Increases mean energy of the beam (Harder)
-Negligible(small/unimportant) impact on IR Exposure
As filtration increases, the intensity(quantity) of the beam _________ and the quality ________
Decreases; Increases
Half-Value Layer
The amount of absorbing material that will reduce the intensity of the beam by half
Inherent Filtration
-0.5 mm Al/Eg
These make up the Inherent Filtration:
• Envelope
• Insulating Oil
• Port Window
Added Filtration
-2.0 mm Al/Eg Mror
These make up the Added Filtration:
• Aluminum Sheet 1.0mm
• Mirror 1.0mm
Total Filtration
2.5 mm Al/Eq
Compensation Filter
Designed to compensate for varying thicknesses in tissue in order to even out the Exposure to the IR
For a unit that operates over 70 kVp, what is the required minimum amount of filtration?
2.5 mm AI/eq
Beam restriction devices
-Collimator
-Cones and Cylinders
