Test 3 Flashcards
factors that affect xray photon emissions (5)
tube housing
target material
filteration
voltage waveform
prime factors
what is a prime factor
factors that directly affect xray quality/quantity of xray emissions
list prime factors (3)
mAs
kVp
distance
how is prime factors controlled
direct control by radiographer
what does xray quantity measure
number of xray photons in useful beam
what is associated with xray quantity (AKAs)
xray output
intensity
exposure
what affects xray quantity and how
mAs –> less mAs = less photons in beam
kVp —> more = increase by 4 factors
distance –> more distance less photons in beam
filtration –> hardens beam = less photons in beam
how is xray quantity is measured (2)
mGya –> milligray in air
milliroentgen (mR)
what does mGya measure
number of ion pairs produced in air by quantity of xrays
if mAs is doubled what is doubled
quantity is doubled
if kVp is doubled = quantity _____
increased by factor of 4
what does xray quality measure
penetrating ability of xray beam
what is penetrability
distance an xray beam travels in matter
what can travel farther away (xray beam)
high energy xrays
what affects xray quality and why?
kilovoltage –> gives energy to penetrate
filteration –> takes away energy of photons
what does NOT affect radiation quality and why
distance
mAs
nothing to deal with energy of beam
importance of penetrability for xray
ability to penetrate deeper in tissues
high energy xrays is ____,___
high quality, hard
low energy xrays has ____,___
low quality, soft
difference between high and low energy xrays
high energy –> penetrates tissues deeply
what affects penetrability and give example
atomic number of tissue irradiated
low atomic number = more penetrability
high = less
what thickness of soft tissue to reduce xray intensity by half?
3-6 cm
how is xray quantity represented
HVL
half value layer
what is HVL
thickness of absorbing material to reduce xray intensity by half of original value
HVL of diagnostic xray
3-6 cm
what does milliamperage measure and direction
xray tube current
number of electrons in tube from cathode to anode
if mA increases = ______ (3)
increases for ALL
number of electrons
mass density of patient
exposure time
directly or inversely proportional?
mA and number of electrons
mA and exposure time
mA and density (mass)
directly for ALL
mA
s
stands for what
mA = tube current
s = exposure time
what is the primary control for image receptor exposure
mAs
how is image receptor exposure measured by
exposure value
ex. I#, E#, EI#, S#
how do you know if image is underexposed or overexposed
under –> value below
over –> value
how do you know if image is underexposed or overexposed
under –> value below
over –> value over
low mAs = ____
high mAs = ____
underexposed & grainy
overexposed
kVp controls what? (2)
quantity and quality of xray beam
signal to digital detector
increase kVp = increase ____
speed and energy of electrons
optimal range for kVp
60-110
proper balance for patient dose
higher kVp and lower mAs
how does kVp affect quality
more energy = greater penetrability
how does kVp affect quantity
increase kVp = more interactions at target
what is subject contrast resolution
visibility of details of clear white through various shades of gray/black
ex. lungs
what affects subject contrast resolution
less kVp = less penetrability = less signals = less gray tones
how can image contrast be fixed
computer processing
what does distance affect
intensity of xrays
IR exposure
_____ distance =
_____ intensity
_____ IR exposure
increase = decrease & decrease
compensator for distance?
mAs
inverse square law
intensity of radiation at a given distance from point source has a inverse relationship
what does inverse square law calculate
relationship between distance and xray intensity
what measures xray intensity
mR
what explains distance’s relationship to intensity and IR exposure
exposure maintenance formula
if mAs is increased –> distance is ____
decreased by the square of the change
what is the exposure maintenance formula used for
to compensate effects for changes in distance will have on IR exposure on image
how is density and contrast primarily controlled
post processing parameters
if changes applied to mAs, kVp, and distance but no visual effects –> how does that occur
exposure latitude of digital imaging
what are the factors for digital image quality
mAs
kVp
distance
exposure latitude controls what?
brightness / contrast
When x-rays strike matter what occurs
attenuate
direct transmission –> passes through unaffected
scatter
Differential absorption is
difference between the x-ray photons that are absorbed photoelectrically and those that penetrate the body
why does differential absorption occur
energy levels in the photons and anatomic parts vary = different absorption of primary beam at varying degrees
what does differential absorption control/create
contrast –> varying grays
creates image formation and darken areas
what % of xray actually create the image?
0.5%
how do we increase differential absorption? why?
decrease kVp = more mAs = more photons to interact with to have varying grays
what is attenuation
how much photons in beam have been reduced after passing through given thickness of material
Differential absorption and attenuation of the x-ray beam depend on:
The atomic number of the atoms in tissue
The mass density of tissue
The x-ray energy
attenuation is greater with _____
thicker patients –> body mass
attenuation reduces xrays how?
in % based off EACH thickness of material
Quality and quantity of photons should be ___________ with ____________ body part thickness
increasing, increasing
high atomic number = _____ attenuation
greater
____ density = ____ attenuation
lower density
lower attenuation
4 major substances for variations in x-ray absorption?
air
fat
muscle
bone
what controls the contrast of an image (variations in the shades of gray)
DIFFERENTIAL ABSORPTION
what substance has:
high actual atomic number
low effective atomic number
low tissue density
air
what substance absorbs the least amount of photons and why?
air
less packed atoms (density) = less interactions = more photons reaching IR (direct transmission)
what does it mean if more photons reach the IR
IR gets a greater exposure
what substance has:
lowest effective atomic number
lower tissue density than muscle
fat
what substance has a tissue density similar to water
fat
why can we see a kidney in a KUB
the organ is surround by fat –> increased tissue density = visibility
what substance has:
high atomic number
high effective atomic number
higher density than fat
muscle
list substances on tissue density from lowest to highest
air –> fat –> muscle –> bone
list substance on atomic number from low to high
fat –> muscle –> bone –> air
what substance is the greatest attenuator
bone
why can Psoas muscles on a KUB be seen
greater amount of absorption
why do bones have a high atomic number
calcium –> very dense
what substance has:
greatest tissue density
high atomic number
high effective atomic number
bone
what substance has the highest absorption rate
bone
which substance allows for the least amount of photons to reach the IR?
highest amount?
bone
air
why is direct transmission important for images
provides image that represents the anatomic part –> passes through anatomic structures = darker areas
what does scatter xray produce (4)
noise
lower image contrast
less image visibility
extra patient dose
scatter xray output
ionization and loss of energy
why is scatter xray dangerous
loss of energy = ion –> energy absorbs into body = molecular damage
how is scatter created
compton interaction
factors that affect scatter
kVp
x-ray field size
body part thickness –> mass tissue density
scatter increases with which factors
ALL FACTORS –> kVp, field size, mass tissue density
____ kVp = ____ compton = ____ photoelectric
increase
increase
decrease
____ xray energy = _____ scatter
increase
increase
thicker patients = ____ kVp
increases
kVp affects what? Increase kVP = ?
image contrast is reduced
larger field size = ?
increased scatter and patient dose
why would a larger field size increase dose?
mAs would have to be increased
more body area exposed
what factor would have a higher photoelectric effect and less compton? ex?
higher atomic number
ex. bone absorption (photoelectric) = less scatter
if thickness of tissue increases what occurs?
increase of :
scatter
field size
angle of scatter
30cm thick exposure would produce how much scatter? ex?
100%
abdomen
what is remnant radiation
remaining xray beam after being attenuated by patient
Remnant radiation is composed of?
transmitted and scatter radiation
remnant radiation is also known as
remnant beam or exiting radiation
approximately ______of the x-rays actually create the image
.5%
what technique would produce the least amount of scatter?
a technique involving a high kVp and low mAs
scatter vs diagnostic rays
scatter travels in divergent paths and more likely grid absorbed
____ atomic number = ____ attenuation = ____ scatter
increase
increase
decrease
part of useful beam and not
useful: transmitted
NOT: scatter
methods to reduce scatter
restricting beam (collimating)
use grid
technique
what is transmitted radiation
radiation (rays) that actually hit the IR from the primary beam
why do we get different types of radiation
beam is heterogenous –> varying energy levels
what is beam restriction dependent on
body part size
beam restriction pro? why?
less scatter = better image
less primary photons = less patient dose/ tech dose
___ collimation = increased _____
increase
spatial resolution
collimator pro?
regulates primary beam field size
light source to aid positioning
purpose of shutters
regulate field size –> cones in beam
bottom: reduces penumbra
upper: reduces off focus radiation
bottom shutters affects what part of beam
periphery of beam
how is a penumbra produced and effects what?
ratios of SOD/OID = fuzzier image (edge unsharpness)
increased penumbra effect =
increase OID
decrease SOD
decrease penumbra effect =
decrease OID
increase SOD
why do we have a penumbra effect
x-ray emitted has dimensions (geometric)
why do we get off focus radiation? what does it produce?
photons not produced at focal spot exits and hits IR = ghosting after image
what is repeatable error penumbra or off focus radiation
penumbra
rules of collimation
field size never larger than cassette
collimate to proper anatomic area
positive beam limitation device (PBLD)?
PRO?
auto collimation to size of cassette
good: reduces overexposure
when to use masking
when not to use masking
off focus radiation occurs
cropping exposure field after image taken
what is aperture diaphragm
simple beam restricting device –> additional filtration
made of lead has hole in center attaches to xray tube
aperture diaphragm pro? ex?
fixes field size
increases spatial resolution
decreases scatter
EX. fractures in extremities
type of aperture diaphragm
PRO?
ex when used?
cone and cylinders
most effective for scatter control
dentist
what is a grid
absorbs scatter before it reaches the IR
grid pro?
improves contrast
reduces scatter
who created the grid
when
issue
Gustav Bucky
1913
grid line superimposed anatomy
who improved the grid
when
how
Hollis Potter
1920
moving grid during exposure = blurs lead grid lines
when to use a grid
not use grid
body part thickness over 13 cm or 10 cm for film
kVp above 70
NO: children
how to know when to use a grid
technique charts and manuals for kVp
calipers for thickness
ASRT recommends usage of grid
use grid based on vendor
body parts over 10 cm
why do we not use a grid on children
exceptions:
grid = increase patient dose
exception: fat ass kid –> over 12 cm
pediatric vs adult radiosensitivity
pediatric 10x more
grid constructed of
radiopaque strips with interspace material that is radiolucent
radiopaque strips made of what? why?
interspace material made of what? why?
lead –> high atomic number = high attenuation rate
aluminum or plastic –> absorbs less photons = produces image
overall hardens beam
grid ratio
height of lead strips to distance between strips
higher grid ratio =
less scatter
increase grid errors –> position has to be perfect
grid frequency
number of grid lines per inch or cm
most common grid frequency
85-103 lines/in
higher grid frequency =
thinner lead strips
higher frequency
15:1 or 6:1
15:1
contrast improvement factor measures?
how well a grid functions to improve contrast
____ scatter = ____ contrast and _____ contrast improvement factor
increase
decrease
decrease
grid conversion factor indicates?
increase of mAs needed when converting no grid to grid
compensations if grid is used and why
increase mAs –> grid decreases quantity
grid conversion factor also known as?
bucky factor
____ grid ratio = ____ bucky factor
higher
higher
___ kVp = ___ bucky factor
increase
increase
____ grid frequency = ____ mAs
increase
increase
grid motion types
stationary or moving
con of stationary grid
grid lines may be seen
rule of grids
CR must be perpendicular to avoid grid lines
dependent on tissue thickness
grid types
parallel
focused
crosshatched
parallel grid directions
xray tube effect
lead and interspace strips run parallel to each other
tube can be angled with length of grid without cut off
focused grid direction
central grid strips parallel but as strips move away they become more inclined –> like central beam
when would a focused grid be used
focal range is wide for low ratio grids
focal range is narrow for high ratio grids
focused grids used for?
specific ranges of SID
focal distance
range
crosshatch grid direction
2 linear grids perpendicular to each other
crosshatch grid pro
con
best scatter clean up
no need for tube angle
CON: MOST grid errors
when would a crosshatch grid be used
biplane cerebral angiography
reasons for grid errors
use of focused grids
off centered tube or incorrect distance tube
how to prevent grid error
proper tube/grid alignment
why is grid error bad
unwanted absorption of primary radiation –> grid cut off
what is grid cut off
the primary beam angled into the lead, absorbing undesirable primary radiation
grid cut off effects
decrease density on periphery film
types of grid errors (5)
off level
off focused
off center
upside down
moire effect
off level occurs when?
fix?
when tube is angled across long axis of grid strips
FIX: grid perpendicular to CR
off focused occurs when?
distance used other than specified for focal range
off center occurs when?
effect on image?
CR does not hit grid at center
cutoff visible more on one side of image
upside down occurs when?
effect on image?
focused grid not aligned with labeled tube side facing xray tube and IR in stationary
effect: uneven exposure
moire effect occurs when? common with?
in digital systems
when grid lines are seen and scanned parallel to scan lines
common: stationary grids in mobile digital system
alternative to using a grid
does what?
air gap technique
reduces scatter by creating gap
how to use air gap technique
increase OID –> patient far from IR
Pro and cons of conventional grid use
PRO: reduce scatter and increase contrast
CON: increase dose, stationary grid errors, and increase potential repeat
when would a stationary grid be used
portable (mobile) imaging
pro and con of digital IR
con: highly sensitive scatter
pro: good scatter correction with digital processing
when to use a grid on a digital IR
tissue thickness greater than 13cm
extreme amounts of scatter produced
when not to use a grid with examples
exceptions
aerated thorax
chest
thoracic spine
sternum
ribs above diaphragm
EXCEPT: if patient is very large
what grid ratio do we use for digital IR
lower grid ratios (6:1 portable)
types of noise
scatter and mottle
how do we get mottle
not enough mAs to get interactions
mottle effect on image
pixelated image
loses sharpness of image
no technique compensation
what effects occur by removing grid
reduces probability of mottle
what would the technique compensation be by removing the grid
mAs decrease by 1/2
or decrease all technique by 1/3
can post processing feature remove scatter?
no just removes effects of scatter
when was the virtual grid software created and by who?
2014 by Fujifilm
virtual grid software steps
scatter xray estimation
grid effect calculations
granularity improvement processing
how does scatter xray estimation work
estimates dose and thickness to estimate the amount of scatter will be produced
how does grid effect calculation work
determines intensity of contrast correction is needed
how does granularity improvement processing work
analyzes contrast-to-noise ratio to apply noise reduction algorithms
what cannot be altered after exposure
grid ratio
amount of scatter
virtual grid vs plain grid
virtual:
reduces dose
reduces possibility for grid errors
flexibility to compensate during exposure
reduces effects of scatter
accepts larger tissue thickness (13cm)
plain grid:
increase dose and grid errors
reduces scatter
cannot be changed after exposure
less tissue thickness (10cm)
when would we use virtual grid software
knee
shoulder
c-spine –> not for swimmers
lat/oblique mandible
most pediatric procedures
is grid the most important way to reduce patient dose? why?
no grid less important
most procedures can be done non grid and over 13 cm
grid ratio is less important –> lower ratios can be commonly used
why is pathology important
diseases can alter body so technique needs to be altered
better patient care
pathology is the study of what
study of disease process of how it affects structural and functional changes
pathological conditions affects what
thickness and composition of tissues
types of pathological conditions
additive and destructive
how does additive condition affect the body
increases thickness
increase effective atomic number
increase tissue density
how does additive conditions affect an image? why?
more attenuation = more white
disease worsens for additive condition =
decrease in IR exposure
____ technique for additive condition? why?
increase –> harder to penetrate tissue due to increase in thickness/atomic number/density
if additive …. how much do we increase kVp
5-15% manually
or AEC system changes mAs automatically but not kVp
how does destructive condition affects the body
decreases tissue thickness
decrease effective atomic number
disease density
how does destructive conditions affect an image? why?
darkens image –> less attenuation so more photons go straight to IR
disease worsens for destructive condition =
increase in IR exposure
____ technique for destructive condition? why?
decrease because the photons go straight to IR already making image too dark
abscess-encapsulated infection affects body how?
increase tissue thickness
can alter composition
edema-swelling affects body how?
increases tissue thickness
can alter composition in lungs
tumor-abnormal new growth affects body how?
increase tissue thickness
can alter composition in lungs and bone
types of pathology that does not need change in technique
ulcers
diverticula
simple fractures
pathology that does not need xray diagnosis
diabetes
anemia
meningitis
kvp and mAs changes to accommodate pathology
5%-15% kVp
25%-50% mAs
15% kVp increase = ___
2x exposure
15% kVp decrease = _____
2x mAs to maintain exposure
