Exam 4 Flashcards
What does filtration affect?
quantity & quality of the x-ray beam
When filtering out low energy x-ray photons, the quantity of the beam is ___ & the quality is ____.
-quantity- reduced
quality- increased
What are the 2 types of filtration (total filtration)?
-inherent filtration
-added filtration
What does inherent filtration consists of?
glass envelope with insulating oil & exit window
Added filtration-
added after bec total filtration of the X-ray tube must be operating above 70 kVp, at least 2.5 mm of aluminum or its equivelent
Compensating filters used to-
balance exposure in the image when part thickness varies greatly across the image
Types of compensating filters (3)-
wedge, trough, & boomerang filter
How can compensating filters affect receptor exposure?
the part has to be greater than 1cm thick
Half-Value Layer can measure-
quantity of x-ray beam
Half-Value Layer (HVL)
thickness of absorbing material necessary to reduce beam intensity by 1/2
What does the typical beam have an HVL of?
3-5 mm of aluminum
Filtration decreases-
Receptor exposure & contrast
purpose of beam restriction-
limit size of exposure area & dose to pt.
When limiting the size of the exposure area, what happens to contrast?
contrast increases
Beam Limiting devices (5)-
-manual collimation
-automatic collimation (PBL)
-cylinder/cone- circle on image
-aperature diaphragm- square
-secondary aperture- for off focus radiation
off focus radiation-
x-rays created when electrons strike anywhere other than focal spot
Why were PBL’s created?
bec techs weren’t collimating during exams
What is the general rule for PBL’s?
1/2” beyond the part
larger field size/exposure area have more-
interactions with matter, therefore more scatter
scatter primarily affects-
receptor exposure & contrast
maintain receptor exposure when changing to a smaller field size-
-14x17 - 10x12
-only change mAs- increases by 35%-50%
collimation decreases-
receptor exposure, scatter
As contrast increases, what happens to scatter?
scatter decreases
collimation increases-
contrast
Gustof Bucky-
invented radiographic grid in 1913
purpose of a grid-
absorb scatter produced in pt. before it reaches IR
What is inside a grid?
alternating led strips separated by a radiolucent interspace
led absorbs-
scatter in the interspace as x-rays pass through
how are led strips placed?
vertically to the x-ray beam
What kind of grids are used today?
moving grids & stationary grids
purpose of moving grids-
grids that move to get rid of line
2 types of movements of moving grids-
-reciprocating
-oscillating
reciprocating moving grids-
moves back & forth
oscillating moving grids-
moves in a circular motion in only 1 direction
Why aren’t grids used for every exam?
-small parts produce little/no scatter
-thicker parts have more matter & produce more scatter
technical factors affect scatter production bec-
bec the direction it travels (higher kVp more travels to IR
use grids when-
-part thickness is greater than 10 cm
-kVp is greater than 60
What happens to contrast if scatter never reaches the IR?
contrast increases bec scatter decreases with the use of a grid
what is the formula used when using a grid? (CIF)
measured contrast with a grid/measured contrast without a grid
What does the contrast improvement formula (CIF) measure-
difference in contrast with & without a grid
grid ratio-
major factor in determining efficiency of a grid
Grid ratio formula-
grid ratio = H(Height)/D (Distance)
what are the common grid ratios (6)?
-5:1
-6:1
-8:1
-10:1
-12:1
-16:1
higher grid ratio-
more effective in absorbing scatter (16:1 is most efficient)
16:1 grid absorbs ___% of scatter produced by pt
97%
5:1 grid absorbs ___% of scatter, even though it is the least efficient.
85%
Why don’t you always use a 16:1 grid?
it has the least amount of margin of error
types of errors made with a grid-
-centering has to be 100% perfect
-perpendicular (no angle)
-on hard flat surface
Grid frequency-
-# of led strips per inch in a grid
-anywhere from 60-100 led strips per inch
-not something we can control
What is the grid conversion factor for a No Grid ratio?
1
What is the grid conversion factor for a 5:1 grid?
2
What is the grid conversion factor for a 6:1 grid?
3
What is the grid conversion factor for a 8:1 grid?
4
What is the grid conversion factor for a 10:1 grid?
5
What is the grid conversion factor for a 12:1 grid?
5
What is the grid conversion factor for a 16:1 grid?
6
How do you maintain intensity when using a grid conversion?
change mAs by correct Bucky factor
How do you compensate receptor exposure?
increase mAs
Bucky Factor
ratio of exposure (mAs) required with a grid to exposure without a grid
When is there going to be a higher grid conversion factor?
when there’s a higher grid ratio & higher scatter
2 grid patterns-
linear or non-linear
linear pattern grid-
has led strips that all go in the same direction
non-linear grid pattern-
has led strips that go at a right angle to each other
What is another name for a non-linear grid pattern?
-crossed grid
-cross hatch grid
What is a cross hatch grid more efficient at?
absorbing more scatter
what is the only problem when using a cross hatch grid?
everything has to be lined up perfectly, if not you’ll get grid cut off in the image
Grid Cut Off
when the grid absorbs the useful x-ray beam
linear grids have more ______.
margin of error
2 types of grid focus-
parallel & focused
parallel grid-
-all straight up & down
-least likely to get grid cut off
focused grid-
has canted (angled/tilted) led strips
canted led strips-
parallel to divergence of x-ray beam
grid radius-
distance from the grid at which lines draw from canted led strips would converge to a focal point
When will you get grid cut off in an image?
anytime you angle the tube against the grid lines
What will cause grid cut off? (3 things)
-tilting the grid
-X-ray tube pitch
-off centering
When using a grid, what decreases?
receptor exposure
When using a grid, what increases?
contrast
Air gap technique-
reduced amount of scatter on IR without a grid
What is the minimum OID that should be used in the air gap technique?
4”-6” (results are similar to using 8:1 grid, don’t use OID if possible)
positive side of x-ray tube-
anode (left) side
negative side of x-ray tube-
cathode (right) side
Anode-Heel Effect-
variation in x-ray intensity along longitudinal axis of x-ray tube
Intensity is the same as-
quantity
intensity isn’t going to change ____.
penetrability of x-ray beam
intensity of x-ray beam ___ toward anode side of exposure area.
decreases
intensity of x-ray beam ___ toward cathode side of exposure area.
increases
How much intensity is there where the exposure area or central ray enters?
100% intensity
makes anode-heel effect more noticeable- (3)
-combination of short SID & large field size
-anode bevel with more acute/steeper angles
-use of a large focal spot size
ex. of small field size-
40” SID
ex. of large field size-
72” SID
exams that utilize anode-heel effect- (40” SID with collimation open all the way to 14x17) (3)
AP Abdomen, T-Spine, & Femur
cathode side placed over ___ anatomy-
thicker anatomy
anode side placed over ____ anatomy-
less dense anatomy
2 types of shape distortion-
elongation & foreshortening
elongation-
object in an image longer than what it actually is
foreshortening-
object in an image shorter than what it actually is
shape distortion attributes to- (3)
-beam/part alignment
-beam/IR alignment
-part/IR alignment
Why is the x-ray tube angled for some exams?
so beam can be perpendicular to the part
4 things affecting shape distortion-
-shape of an object
-centering of CR in respect to the object
-angle btwn CR & long axis of the object
-angle btwn long axis of the object & IR
CR should be ___ & part should be _____ to IR.
-perpendicular
-parallel
can get shape distortion from- (4)
-off center (CR not centered to IR or part)
-CR not perpendicular to IR or part
-angling the part
-part not parallel to IR
when you angle the x-ray tube you’re _____ & _____.
increasing SID & changing distance
SID increases as the tube is angled, to compensate-
when angling more than 15* divide the tube angle by 5, then subtract that # from SID
purpose of AEC-
determine when enough exposure has reached IR
angling x-ray tube DOES NOT AFFECT-
receptor exposure
different body habitus-
hyposthenic, hypersthenic, sthenic, & asthenic
size of patient affects tech factors-
affects absorption of x-ray beam
calipers-
measurement device to measure thickness of pt. where CR enters to where it exits
additive disease-
adds something to body that shouldn’t be there
destructive disease-
takes away from body that shouldn’t be there
additive disease could add-
fluid, bone, or prosthetic devices
How does additive disease affect receptor exposure?
absorbs more of x-ray beam & causes RE to decrease
if there’s an increase in mineral content (bone formation) in additive diseases, how much should kVp be increased by?
8%
if there’s an increase in fluid in additive diseases, how much should mAs be increased by?
35%-50%
pneumonia-
increased fluids in lungs
paget’s disease-
increased bone formation
destructive disease could add/take away-
-decrease in fluid/mineral content
-or increase in air/fat content
osteoperosis-
decrease in mineral content in bones
emphysema-
more air in lungs than supposed to be, less penetration is required
if normal tech. factors used in destructive disease-
factors would increase receptor exposure bec there is less there to absorb x-ray beam
How much should kVp & mAs be decreased if destructive diseases are present?
-kVp 8%
-mAs- 35%-50%
tech. factors for soft tissue technique should be-
decreased
general rule for soft tissue techniques-
decrease kVp by 15% & mAs is unchanged
typical cast usually requires _____ of exposure-
doubling of exposure
when extra thick/wet cast increase tech. factors by-
factor of 3
inflatable cast/air splints, tech factors-
remain unchanged
contrast media
helps visualize internal structures of body that normally can’t be seen on a radiograph
is called contrast media bec-
it creates the difference in contrast
positive contrast-
iodine/barium
negative contrast-
air
optimal kVp to penetrate through iodinated contrast media-
76-80
optimal kVp for barium only studies-
116-120
optimal kVp for barium & air contrast exams
90-94
patient factors affect-
receptor exposure, contrast, spatial resolution, & distortion
