Midterms Flashcards
Name 4 different ways to control scatter
- Use relatively low kVp
- Use compression
- Collimate
- Use grids
T or F
As we increase kVp we increase compton scatter
True
What is optimum kVp to control compton scatter?
70-90
At what kVp do we increase the probability of compton scatter?
60-125 kVp
T or F
Increased patient thickness improves contrast
False
Decreased thickness improves contrast
What is the advantage of a PA vs. an AP lumbar view?
Decreases shape distortion because the angle of the vertebrae match the angles of the beams
T or F
Decreased collimated area = increased compton scatter?
False
Increase collimated area increase compton scatter.
What is the rule when you are collimating significantly in regards to mAs? Why?
When collimating significantly increase mAs by 50%
- you have to increase at least 30% to see a difference. You are losing scatter when you collimate but you have to make up for it by increasing mAs
T or F
When you have 0 OID you will lose more X-rays and therefore less density and will have to increase mAs
False!
When you have no OID the same scatter on the object will be on the film. (no air gap)
What happens when you have increase OID?
Increased OID = increased divergent rays and therefore lose density and will therefore have to increase mAs by 50%
T or F
Tissue compression is the best device to absorb Compton Scatter
False!!!
Bucky Grids are the best device
What is the base of a Xray film made of?
Polyester plastic
What is the emulsion on the Xray film?
Silver halide crystals
T or F
The faster the “speed” of a film the more sensitive the film is and less x-ray is needed to produce the same radiographic density.
True
What is the purpose of the intensifying screens?
Reduce exposure
What is the most important factor for radiation protection?
Screens
If you go from a 200 speed to a 600 speed film what must you do to the mAs?
Divide by the factor of change. You multiplied the speed by a factor of 3 and will therefore have to reduce the mAs by 1/3
What is quantum mottle?
Graininess on a film
Considered visual noise
Why is quantum mottle or graininess a bad thing?
Interferes with our ability to to interpret info on the film
Why does the high speed systems result in increased quantum mottle?
Fast systems use relatively few X-rays to stimulate the screen. It is almost like you see the pattern of each packet and where it has interacted with the screen. Similar to having fewer pixels.
What are the different types of grids?
Linear Parallel
Linear Focused
Crosshatch cutoff
When should we use a radiographic grid?
When the body part is greater than 10 cm
At what kVp should we consider using grids?
When using greater than 60 kVp
What is the “grid material” made of?
lead
What are the lines of radiolucent “interspace material” made of?
Plastic, aluminum, carbon fibre.
How do grids work?
They transmit only the rays in line from the focal spot to the film. Obliquely angled rays are “cleaned up”
When is a parallel linear grid recommended?
If both 40” and 72” SID are used.
T or F
In a parallel linear grid, they are usually installed with the grid lines running parallel to the floor?
False!
Usually installed vertically in upright position in bucky cabinet.
How are the strips angled in a focused linear grid in terms of the beam?
Angled to coincide with the divergent beam.
T or F
A focused linear grid can be used at both 40” & 72” SID
False
They cannot!! Must be at a very specific SID because you have a very specific divergence.
What grid is very efficient in removing scatter in both directions?
Crosshatch
Why is crosshatch grids rarely used?
CR must be perpendicular to the centre of the grid.
Can’t angle the beam in ANY direction
What is the term used for when you use the wrong grid and an undesirable absorption of the primary beam by a grid.
Grid Cutoff
What is grid ratio?
The height of lead strips / width of the interspace strip
T or F
Increase grid ratio - decrease contrast
False
it will increase contrast
T or F
Increase grid ratio - increase lateral angle tolerance
False!
Decreases lateral angle tolerance
T or F
As grid ratio increases patient exposure decreases
False!
As grid ratio increases patient exposure increases
Define grid frequency
number of lines per inch or cm
What happens when you increase the grid frequency in terms of visibility of lines?
decrease line visibility
T or F
High grid frequency requires a mobile device
False!
Low grid frequency requires a mobile device.
Name the three types of moving grids
Single stroke/manually cocked
Reciprocating
Oscillating
T or F
A single stroke grid moves in two directions
False
Only in one direction
How does a reciprocating grid work?
Motor driven back and forth during the exposure. Movement is achieved by electromagnets
What happens from lateral de-centring of a grid?
CR off centre to the left or right and causes collision of primary x-rays with the lead strips.
With what type of grids is lateral de-centering a problem?
Focused or crosshatch
What happens when a grid is off level and what grids are affected most?
Either the grid is angled or the CR is angled against the direction of the strips. Collision of primary x-rays with grid strips.
- problem for ALL types of grids.
What happens when a focused grid is used at improper SID
You will see the cutoff pattern and density will decrease on both lateral sides of the radiograph.
Name the advantage and disadvantage or using an air gap technique instead of a grid?
A = scatter escapes before it hits the film D = increased magnification and blur on image.
