Exam 2 Flashcards
Exam:
Collimation _____ patient dose by:
decreases
limiting the volume of tissue exposed to radiation
Exam:
What does collimation do to radiologic contrast?
How?
improves the radiologic contrast
by limiting the volume of tissue that can create scatter
Exam:
What is the aperture diaphragm?
fixed opening between the x-ray tube & collimator box
Exam:
What is the collimator?
adjustable lead shutters
Exam:
What is PBL?
What does it do?
What can you do to manipulate this?
positive beam limitation
automatic collimator based on IR size
override if the desired field size is smaller than the IR
Exam:
Scatter occurs commonly with:
large field sizes
increased tissue volume
Exam:
What are the other beam limitations?
What are they?
aperture diaphragm: immovable opening between x-ray tube & collimator box)
Mask: lead sheet with an opening used to image specific anatomy of interest (skull x-ray with a hole cut through)
Exam:
What does scatter do to exposure?
scatter increases exposure to the IR
Exam:
Light/radiation field can be off by:
+/- 2% of the SID
Exam:
What is scatter also known as?
Secondary radiation
Exam:
What happens to scatter at higher kVp levels?
What happens to compton?
scatter is increased at higher kVp
Compton interactions proportionally increase at higher kVp levels
Exam:
What does scatter do to noise?
how does scatter affect contrast?
what does scatter do to detail, magnification, or distortion?
scatter increases noise
scatter decreases contrast
scatter does not affect detail, magnification, or distortion
Exam:
What is the number one source for of occupational exposure?
scatter radiation
Exam:
How can we reduce scatter?
what is the most effective way to reduce scatter?
second most effective?
increase collimation (most effective way to control)
decrease part volume (compression) (second most effective way)
reduce kVp
grids (affects scatter reaching the IR, not PRODUCTION)
distance (SID < SOD< OID) (no effect on scatter production)
Exam review:
What affects detail?
focal spot size
penumbra
Exam:
What do grids do?
affect scatter reaching the IR, not the PRODUCTION of scatter
Exam:
What is the purpose for grids?
What does it not affect?
restore subject contrast in an image
grids don’t affect the production of scatter radiation
Exam:
How are grids constructed?
alternating strips of lead & interspace material (AL most common but can also be plastic)
Exam:
How do we calculate grid frequency?
number of lead strips per inch (100/inch)
Exam review:
What are focused grids?
Linear?
crosshatched?
grids that follow the divergent beam
run up and down (only can angle one way)
run up/down & side to side
Exam:
Grids can be _____ _____ or ______ (different types of grids)
linear
crosshatched
focused
Exam:
Grids are designed to be used:
at a specific distance from the focal spot
Exam:
Grids allow the ______ ______ to pass through ______ _______ and absorb ____ ______
primary beam
lead strips
scattered x-rays
Exam:
Focused gridlines are directed to:
a convergence point (generally the focal spot)
Exam:
Motion will?
blur the gridlines
Exam:
modern grids attenuate:
70-80% of scattered photons
Exam:
Grids should be used:
part thickness greater than 10cm
kVp greater than 70
large field sizes
Exam:
The effectiveness of the grids is measured by:
also know as?
the ratio of the height of the lead strips to the width of the interspace material
grid ratio
Exam:
What type of grid errors are there?
what is the worst case scenario?
off-center
off-level
off- focus
upside down (worst outcome)
Exam:
What are the grid ratios?
no grid:
5:1
6:1
8:1
10:1
12:1
16:1
1
2
3
4
5
5
6
Exam:
Grid math:
Old
500 mA
1 sec
12:1 Grid
New
50 mA
___ sec
6:1 grid math
Steps:
500 mA x.1= 50mA
12:1 grid (6) to 6:1 (3) (new/old)
3/5= .6
50 x .6= 30mA
30 mA/ 50 mA= .6
Exam: grid math
Old
100 mA
5 secs
1no Grid
New
____ mA
.25 sec
6:1 Grid
Steps:
100 mA x .5= 50 mA
no grid (1/old) to 6:1 (3/new)= 3/1 new/old
50 mA x 3= 150 mA
150 mA/.25 secs= 600 mA
Exam: Grid math
Exam: Grid math: