Ch. 18 Image Production Flashcards
What is a grid and how does it work?
It is a device that improves the contrast of the Rad image by absorbing scatter radiation before it reaches the IR.
What are the three things that happen in the body when exposed to radiation?
Passes through the body unaffected
Absorbed by the body
Interact and change direction
Absorption of the photons occur because of what type of interaction?
Photoelectric interaction
What causes scatter in the interaction with tissue?
Compton interaction
How do you define a photon as scatter?
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
What is the relationship between kVp and Compton scatter ?
Directly related
As kVp increases what happens to contrast (In film)?
results in low contrast (better contrast)
What increases scatter in a patient
Increase patient thickness
Larger field size
Decrease in atomic number of tissue(b/c low z# = less absorption
When should we use a grid?
When a body parts thickness exceeds 10cm
When kVp is above 60
What does grid consist of ?
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)
What is the material if the radiolucent interspaces?
Aluminum and plastic fiber
What is grid ratio defined as
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)
If the height of the grid is constant and we were to decrease the distance between the lead strips (h) what would result?
Increase grid ratio
What is the relationship between distance of lead strips (D) and grid ratio if highs remains the same?
Inversely related
Higher grid ratios allow what ?
Less scatter passing to the IR through the interspace
Higher grid ratios are_____ but are prone to_____
Better, grid error
How are grid ratios rated?
Are sometimes rated by weight instrear or grid ratio
What is the term for expressing a grids weight?
Grams per square centimeter (g/cm^2) or in the US (g/in^2)
What is grid frequency?
The # of grid lines per inch or centimeter
What is the range of grid frequency in centimeters and inches ?
60-200 lines/inch; 25-80 lines/cm
What is the most common grid frequency?
85-103 lines/inch; 33-41 lines/cm
What aid the grid frequency of a very-high-frequency?
178 to 200 lines/inch; 70-80 lines/centimeter
What is a linear grid?
Grids with lead strips running in one direction
Two linear grids that are place one on top of the other so that they run at right angles
Cross-cross or cross hatched
When are linear grids commonly used
In clinical practice
Grid cutoff
When the primary beam is angled into the lead, which will lead to an unwanted about of primarily beam to be absorbed
Parallel grids are made with?
Lead and interspaces running parallel to one another
Focused grid
Made so that the center grid strips are parallel but as they move away from the center they become inclined
Convergence line is?
Where the focused grid’s inclined lines meet in the middle
Grid radius is what
Is the distance from the inclined focused grid lines and the convergence line
Short-focal-range grids; which modality are they in?
14-18 inches or 36-46cm
Used in mammography (SID)
Long-focal-range grids; which modality uses them?
60-72 inches; 152-183cm
Used for chest x- rays (SID)
What is the usual size for a grid placed it the Bucky ?
17x19 in
Reciprocating movement of a grid
A motor drives the grid back and forth at about 2-3cm
How does the oscillating grid move
An electromagnet pulls the grid to one side and then releases it during exposure
How do we calculate the grid conversion factor?
GCF= mAs with grid/mAs without grid
What is he relationship between GCF and kVp?
no relation
What is the relationship between mAs and GCF ?
Directly related
What is the ICRU
The internal commission on radiologic units and measurements
What criteria is measured to meant the ICRU guide lines for a grids performance?
Selectively and contrast improvement ability
A grids selectivity is measured how ?
The percentage of scatter it eats up is greater than the percentage of primary beam it eats
How can we calculate grid selectivity?
Selectivity= % primary radiation transmitted / % is scatter
A _____ lead content will have greater selectivity
Higher
How do you measure the contrast improvement factor(K)?
K= radiographic contrast with the grid/radiograph contrast without the grid
What is the usual contrast improvement factor(k)?
1.5 to 3.5
The _____ the K factor, the greater the contrast improvement
Higher
Why are focused grids prone to grid error?
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
What does a off-level error occur?
When the tube is angled across the long axis of the grid strips, which can also result in improper tube or grid positioning
Using what grid will let you angle alone the long axis of the table ?
With a linear grid
Can you angle with a criss-cross grid/cross-hatch grid ?
No
What is an off-center grid error
When the grid is not aligned to the tube causing a decrease in exposure on the ENTIRE IMAGE
What is off-focus in a grid?
When the distance is too great for the grid you are using; when it’s not in focal ranges CUT OFF AT THE EDGES
Upside down grid is what
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
When will the mortise effect occur in a grid?
When the grid lines must run in the same direction as the laser beam that is scanning then imaging plate (CR)
what is photoelectric interaction
result in the complete absorption of the primary photon and the production of the second
does Compton interaction add to diagnostic value of the image
it has no diagnostic value
as scatter is increased b/c of kVp; what happens to contrast
contrast is impaired (weakened)
how does thickness of the patient affect scatter
the thicker the patient the more scatter is produced but a higher atomic number will result in less scatter being produced
what cause the amount of scatter to increase
increase in patient thickness, decreased atomic number of the tissue, and LARGE field size/ NO collimation
