Chapter 13 Flashcards
exposure factor
the factors that influence and determine the quantity and quality of x-radiation to which the patient is exposed
four prime exposure factors
kVP, mA, exposure time, SID
kVP and mAs
the most important factors principally responsible for x-ray quality and quantity. focal spot size, distance, and filtration are secondary factors that may require manipulation for particular examinations
kVP
affects quality and therefore, beam penetrability,with increasing kVP, more x-rays are emitted, and they have higher energy and greater penetrability. but because they have higher energy, they also interact more by Compton effect and produce more scatter radiation, which results in reduced image contrast.
the kVP selected helps to determines the number of x-rays in the image forming beam, and hence the resulting average optical density (OD).
finally, and most importantly, the kVP controls the scale of contrast on the finished radiograph because as kVp increases, less differential absorption occurs, therefore, high kVP results in reduced image contrast.
Milliamperes
the mA selected determines the number of x-rays produced and therefore the radiation quantity. the unit of electric current is the ampere (A) one ampere is equal to 1 coulomb (c) of electrostatic charge flowing each second in a conducter.
fallng load generator
on an x-ray imaging system in which only mAs can be selected, exposure factors are adjusted automatically to the highest mA at the shortest exposure time allowed by the high voltage generator
Distance
affects exposure of the image receptor according to the inverse square law, the SID largely determines the intensity of the x-ray beam at the image receptor.
direct square law
is derived from the inverse square law, it allows a radiologic technologists to calculate the required change in mAs after a change in SID to maintain constant OD.
focal spot size
for general imaging, the large focal spot is used. this ensures that sufficient mAs can be used to image thick or dense body parts. the large focal spot also provides for a shorter exposure time, which minimizes motion blur.
one difference between large and small focal spots is the capacity to produce x-rays. many more x-rays can be produced with the large focal spot because anode heat capacity is higher. with small focal spot, electron interaction occurs over a much smaller area of the anode, and the resulting heat limits the capacity of x-ray production.
a small focal spot is reserved for fine-detail radiography, in which the quantity of x-rays is relatively low. small focal spots are always used for magnification radiography. these are normally used during extremity radiography and in examination of other thin body parts in which higher x-ray quantity is not necessary.
filtration
three types of x-ray filtration are used: inherent, added, and compensating.
inherent filtration
all x-ray beams are affected by the inherent filtration properties of the glass or metal envelope of the x-ray tube. the value of the inherent filtration is approximately 0.5 mm the required total filtration of 2.5 mm is needed.
compensating filters
are shapes of aluminum mounted onto a transparent panel that slides in grooves beneath the collimator. these filters balance the intensity of the x-ray beam so as to deliver a more uniform exposure to the image receptor. they may be shaped like a wedge for examination of the spine or like a trough for chest examination.
as added filtration is increased, the result is increased x-ray beam quality and penetrability. the result on the image is the same as that for increased kVP, that is, more scatter radiation and reduced image contrast.
three basic types of high voltage generators are available
single phase, three phase, and high frequency
high wave rectified generator
has 100% voltage ripple. during exposure with a half wave rectified generator, x-rays are produced and emitted only half the time. during each negative half cycle, no x-rays are emitted.
full wave rectification
identical to half wave rectification except there is no dead time. during exposure, x-rays are emitted continually as pulses. consequently, the required exposure time for full wave rectification is only half that for half wave rectification.
three phase power
comes in two principal forms: 6 pulse or 12 pulse. the difference is determined by the manner in which the high voltage step up transformer is engineered.
the difference between the two forms is minor but does cause a detectable change in x-ray quantity and quality. three phase power is more efficient than single phase power. more x-rays are produced for a given mAs setting, and the average energy of those x-rays is higher. the x-radiation emitted is nearly constant rather than pulsed.
patient factors
such as anatomical thickness and body compositon
image quality factors
such as OD, contrast, detail, and distortion
exposure technique factors
such as kVP, milliamperage, exposure time, and SID, as well as grids, screens, focal spot size, and filtration.
body habitus
sthenic – meaning “strong, active” patients are average.
hyposthenic–are thin but healthy appearing; these patients require less radiographic technique.
hypersthenic—are big in frame, and usually overweight.
asthenic are small, frail, sometimes emaciated, and often elderly.
calipers
are used to measure the thickness of the anatomy that is being irradiated
composition
when only soft tissue is being imaged, low kVp and high mAs are used. with an extremity, which consists of soft tissue and bone, low kVp is used because the body part is thin.
when imaging the chest, the radiologic technologist takes advantage of the high subject contrast. lung tissue has very low mass density, bony structures have high mass density, andd the mediastinal structures have intermediate mass density. consequently, high kVp and low mAs can be used to good advantage. this results in an image with satisfactory contrast and low patient radiation dose.
radiolucent
attenuates few x-rays and appears black on the radiograph
radiopaque
tissue absorbs x-rays and appears white on the radiograph
pathology (destructive)
causing the tissue to be more radiolucent
pathology (constructively)
increase mass density or composition, causing the tissue to be more radiopaaque
image quality factor
refers to characteristics of the radiographic image; these include OD, contrast, image detail, and distortion. image quality factor are considered the “language” of radiography.
Optical density
is the degree of blackening of the finished radiography. OD has a numeric value and can be present in varying degrees, from completely black, in which no light is transmitted to almost clear. whereas black is numerically equivalent to an OD of 3 or greater, clear is less than 2 a radiograph that is too dark has a high OD caused by overexposure. this situation results when too much x-radiation reaches the image receptor. a radiiograph that is too light has been exposed to too little x-radiation, resulting in underexposure and low OD.
Optical density controller
two major factors mAs and SID
it can be affected by other factors, but the mAs becomes the factor of choice for its control. a change in mAs of approximately 30% is required to produce a visible change in OD. as a general rule, when only the mAs setting is changed, it should be halved or doubled. the simplest method used to increase or decrease OD is to increase or decrease the mAs.
contrast
the function of contrast in the image is to make anatomy more visible. contrast is the difference in OD between adjacent anatomical structures, or the variation in OD on a radiograph. Contrast is perhaps the most important factor in radiographic quality.
penetrability of the x-ray beam is controlled by
kVP
gray scale of contrast
refers to the range of ODs from the whitest to the blackest part of the radiograph
high contrast radiographs produce short gray scale.
they exhibit black to white in just a few apparent steps
low contrast radiographs produce long gray scale
appearance of many shades of gray.
high contrast
” a lot of contrast” or a “short scale of contrast” is obtained by using low kVp exposure techniques
low contrast
is the same as “long scale of contrast” and results from high kVp
detail
describes the sharpness of appearance of small structures on the radiograph. with adequate detail, even the smallest parts of the anatomy are visible.
image detail
evaluated by two means–recorded detail and visibility of image detail
sharpness of image detail
refers to the structural lines or borders of tissues in the image and the amount of blur of the image. factors that usually control the sharpness of image detail are the geometric factors —focal spot size, SID, and OID. To produce the sharpest image detail, one should use the smallest appropriate focal spot and the longest SID and place the anatomical part as close to the image receptor as possible.
visibility of image detail
describes the ability to see the detail on the radiograph and is best measured by contrast resolution.
distortion
the misrepresentation of object size and shape on the radiograph.
elongation
means that the anatomical part of interest appears bigger than normal
foreshortening
means that the anatomical part appears smaller than normal
4 image quality factors
Optical density
contrast
detail
distortion
Optical density
controlled by
mAs
influenced by
kVp distance thickness of part mass density development time or temperature image receptor speed collimation grid ratio
Contrast
controlled by
kVp
influenced by
mAs (toe, shoulder) development time or temperature image receptor used collimation grid ratio
Detail
controlled by
focal spot size
influenced by
SID
OID
Motion
All factors related to density and contrast
Distortion
controlled by
patient positioning
influenced by
Alignment of tube
anatomical part
image receptor
exposure technique charts
kVP
mA
exposure time
SID
types of grids
stationary
moving grid
parallel or non focused grid
lead lines run parallel to one another
used primarily in fluoroscopy and mobile imagine
focused grid
has lead lines that are angled to approximately match the angle of divergence of the primary beam
Advantage
allow more transmitted photon to reach the film than parallel grid
convergent point
if imaginary lines were drawn from each of the lead lines in a linear focused grid, these lines would meet to form an imaginary point.
convergent line
if convergent points were connected along the length of the grid they would form an imaginary li
focal distance
distance between the grid and the convergent line or point
focal range
focal range is the recommended range of SIDs that can be used with focused grid.
the convergent line or point always falls within the focal range.
grid cassette
an image receptor that has a grid permanently mounted to its front surface.
grid cap
a grid cap contains a permanently mounted grid and allows the image receptor to slide in behind it.
disadvantage of stationary grid
causes shadows of the grid lines on the image
moving or reciprocating grid
they are part of the bucky (potter bucky diaphragem)
located directly below the radiograhic table top just above the tray that hold the film
grid motion is controlled electrically by the exposure switch
grid moves back and forth in a lateral direction over the image receptor during the entire exposure.
contrast improvement factor
principal function of a grid is to improve contrast
grid conversion factor (bucky factor)
grid also absorb some of the primary radiation
to compensate for this you need to increase the mAs
GCF = mAs with grid/mAs without grid
bucky factor/grid conversion factor
grid ratio bucky factor/GCF
non grid 1
5: 1 2
6: 1 3
8: 1 4
12: 1 5
16: 1 6
grid cutoff
grid cutoff is defined as a decrease in the number of transmitted photons that reach the image receptor because of some misalignment of the grid.
higher grid ratio results in more grid cutoff.
high ratio grid has less positioning latitude
upside down focused grid
appears radiographically as significant loss of density along the edges of the image.
off level grid cutoff
caused from angling the x-ray tube across the grid lines or angling the grid itself during exposure
appears as an overall decrease in density
off center grid cutoff
also called lateral decentering.
occurs when the central ray of the x-ray beam is not aligned with the center of a focused grid (side to side misalignment)
appears as an overall loss of density
off focused grid cutoff
occurs when using an SID outside of the recommended focal range.
occurs if the SID is less than or greater than the focal range.
appears as a loss of density at the periphery of the film.
grid selection
below 90 kVp 8:1 grid is used
above 90 kVp grid ratio above 8:1 is used
grid selection factors
patient dose increase with increasing grid ratio
high ratio grids are used for high kVp examinations
patient dose at high kVp is less than that of low kVp
air gap technique
image receptor is 10 to 15 cm from the patient
alternate to grid
improve image contrast
10% increase in maS for every cm of gap.
image magnification with associated focal spot blur
film construction
radiographic film has many layers
- supercoat
- emulsion
- adhesive layer
- film base
supercoat (overcoat)
supercoat is a durable protective layer that is intended to prevent damage to the sensitive emulsion layer underneath it.
emulsion layer
emulsion layer is the radiation and light sensitive layer of the film.
the emulsion of the film consists of silver halide crystals suspended in gelatin
silver halide contains 90 to 99% of silver bromide acid 1% to 10% of silver iodide.
adhesive layer
adhesive layer keep the emulsion sticking to the film base.
film base
the final layer of the film is base
base is made of polyester (plastic) which gives the film physical stability
most film base has a blue tint.
historic development of film base
glass: break easily, difficult to store
cellulose nitrate: highly flammable
cellulose acetate: could damage when it is wet
polyester base: most modern. dimensional stability
characteristics to be considered when selecting radiographic film
contrast speed spectral matching anti-crossover layer requirement for safelight
contrast
contrast of an IR is inversely propertional to its exposure latitude
exposure latitude = range of exposure techniques that will produce an acceptable radiograph.
depends on size and distribution of the silver halide crystals.
low, medium, or high
screen film
screen films are available with many contrast levels.
high contrast emulsion: smaller silver halide grains with uniform grain size.
low contrast emulsion: larger grains having wide range of sizes.
speed
sensitivity of the screen film combination to x-rays and light
different film emulsion and different intensifying screen phosphors contribute to different speed.
for direct exposure film, speed is affected by the concentration and total number of silver halide crystals.
factors affecting screen film speed
silver halide grain size
shape of the grains
concentration of the grains
double emulsion films are twice as fast as single emulsion films
covering power of the emulsion
current emulsions contain less silver, yet produce the same density per unit exposure because of its increased covering power.
cross over effect
crossover refers to a phenomena in which light from one intensifying screen cross over the film base and expose the emulsion on the opposite side of the base.
it causes increased blur on the image
crossover is a problem that is unique to double emulsion film used with intensifying screen.
crossover can be reduced by
tabular grain emulsion
anti crossover layer
tabular grain or T grain technology
this technology increases the recorded detail
T-grain film uses flat silver halide crystals that can be dispersed more evenly in the emulsion
Crystal types
irregular grain
tabular grain
cubic grain
spectral matching
matching the spectral sensitivity of the film with the spectral emission of the intensifying screen increases the speed of the image receptor
a film that is sensitive to blue color must be placed with an intensifying screen that produce blue light.
spectral emission
calcium tungstate screens emit blue and blue violet light.
rare earth phosphor emits ultraviolet, blue, green, and red.
ortho-chromatic films are sensitive to blue and green lights (two colors)
pan chromatic films are sensitive to the entire visible light spectrum.
safelights
most safelights are incandescent lamps with color filter
provide enough light to illuminate the darkroom without exposing the film.
a 15 watt bulb should not be closer than 1.5 meter (5 ft) from the work surface.
safelights
blue sensitive film–amber color filter.
transmit wavelengths longer than 550 nanometer (above blue region)
green sensitive film–red color filter. transmit wavelengths longer than 600 nm
red filter is suitable for both blue and green lights.
types of film
direct exposure film
screen film
double emulsion
single emulsion
direct exposure film
film is enclosed in an envelope that will not transmit light no intensifying screen thicker emulsion higher concentration of silver halide exposed by direct x-ray interaction increased patient dose used to image thin body parts
mammography film
originally used direct exposure film with double emulsion.
currently uses single emulsion film with single intensifying screen
green sensitive film
terbium doped gadolinium oxysulfide screens
has antihalation coating
antihalation coating
the surface of the base opposite to the screen is coated with special light absorbing dye to reduce the reflection of screen light, which is transmitted through the emulsion and base.
antihalation layer is present in all single emulsion films.
handling and storage of films
improper handling cause artifiacts on the image
should not bend, crease, or handle roughly
clean hand with no lotions
store in a cool dry places
68 and 40 degrees–60% humidity
effect of temperature and humidity
increased heat causes fog
increased humidity also causes fog and loss of contrast
humidity below 40% causes static artifact.
tree, crown or smudge
handling and storage of films
film must be stored and handled in the dark
well sealed darkroom (light and radiation proof)
proper safelights
light proof storage bin
handling and storage of films
the thickness of the lead barrier is designed to keep the total exposure of unprocessed film below 2 micro Gy (0.2 mR).
radioactive materials used in nuclear medicine can fog the film
formation of latent image
the latent image is the invisible change that is induced in the silver halide crystal.
with proper chemical processing the latent image becomes visible image (manifest image)
latent image formation
the term latent image refers to the image that exists on film after the film has been exposed but before it has been processed.
the term manifest image refers to the image that exists on film after exposure and processing.
sensitivity specks
physical imperfections in the crystal lattice of the emulsion layers occur during the film manufacturing process.
these imperfections are called sensitivity specks.
each sensitivity specks serves as electron trap, trapping electrons lost by the bromide when x-ray or light exposure occurs.
therefore, sensitivity speaks are negatively charged.
sensitivity specks
since sensitivity specks are negatively charged, the positive silver ions that are liberated from bromide are attracted to them.
every silver ion that are attracted to an electron becomes neutralized to metallic silver
the more x-ray or light exposure, the more electrons and silver ions available
latent image centers
several sensitivity specks with many silver ions attracted to them become latent image centers.
these latent image centers appear as radiographic density on the manifest image after processing.
for a latent image center to appear it must contain at least three sensitivity specks that have at least three silver atoms each.
the more exposure to the film, the more metallic silver that is present on the radiograph as radiographic density.
intensifying screen
located inside the cassette
contains phosphors
phosphors convert the x-ray energy to light.
the light exposes the radiographic film
the phosphor is a chemical compound that emits light when struck by radiation.
purpose of intensifying screen
decrease the patient’s radiation exposure
in direct exposure radiography, the film is exposed only by the exit radiation
with screens the total amount of energy to which the film is exposed is divided between x-rays and light.
use of intensifying screen reduces patient dose but also reduces recorded detail.
luminescence
luminescence if the essence of light from the screen when stimulated by radiation.
there are two types of luminescence
1) fluorescence
2) phosphorescence.
fluorescence
refers to the ability of phosphors to emit visible light only while exposed to x-rays.
phosphorescence
occurs when phosphors continue to emit light after the x-ray exposure has stopped.
phosphorescence is also known as screen lag or after glow..
cause fog on the film
screen construction
an intensifying screen has four different layers
protective layer
phosphor layer
reflecting layer or absorbing layer
base
protective layer
the outermost layer
10 to 20 micro meter
resistant to abrasions and damage
protect the fragile phosphor material beneath it.
eliminate the build up of static
provides surface for routine cleaning without damaging the phosphor layer.
phosphor layer
also known as active layer
contains the chemical (phophor) that converts x-ray photon in to visible light.
50 to 300 micro meter
calcium tungstate
zinc sulfide
barium lead sulfate
oxysulfide of rare earth elements
rare earth elements
gadolinium
lanthanum
yttrium
discovery of x-rays
accidental discovery by Roentgen
observed luminescence on barium platinocyanide.
thomas edison developed calcium tungstate
favorable properties of phosphors
high atomic number high x-ray absorption (DQE) emit large amount of light (CE) spectral matching minimum after glow should not be affected by heat and humidity
detective quantum efficiency (DQE)
phosphors with high atomic numbers absorb more x-rays. this is called detective quantum efficiency.
rare earth phosphors have higher DQE
conversion efficiency (CE)
the phosphor should emit large amount of light per x-ray absorption. this is called x-ray conversion efficiency.
higher conversion efficiency results in increased noise.
rare earth phosphors have higher CE
the reflecting layer
25 micrometer
consists of either magnesium oxide or titanium dioxide.
light is emitted isotropically with x-ray interaction of the phosphor
reflective layer intercepts with light headed in other direction and redirects it to the film
the absorbing layer
consists of a light absorbing dye
absorb the light directed towards the base
reflecting and absorbing layers are not present at the same time
base
the bottom layer of the intensifying screen
made of polyester or cardboard
provide support and stability for the phosphor layer.
luminescence
occurs when an outer shell electron is raised to an excited state and returns to its ground state with the emission of a light photon
image noise
increases with higher CE but not with higher DQE
spatial resolution
reduction in spatial resolution is greater when phosphor layers are thick
reduction is also greater when crystal size is large
these same conditions increase screen speed by producing more light photons per incident x-ray.
front screen and back screen
back screen is more faster than the front screen
rare earth elements
most commonly used phosphors
absorb more x-rays
convert x-rays to visible light more efficiently
improved recorded detail
very rare and hard to extract from the earth
spectral matching
spectral sensitivity of the film should match with the spectral emission of the intensifying screen.
blue sensitive film should be used with blue light emitting intensifying screen
failure to match the screen and film results in inappropriate radiographic density. (less density)
screen speed
the capability of a screen to produce visible light is called screen speed
faster screen produce more light
faster screen require less exposure and therefore less patient dose
screen speed and density are directly proportional
disadvantage: reduction in detail
intensification factor
intensification factor is the ratio of the exposure without screen to the exposure with screen
IF=exposure without screen/exposure with screen
relative speed
relative speed results from comparing screen film systems, based on the amount of light produced for a given exposure
the amount of light produced with a par speed (medium) calcium tungstate screen system is used as the standard for comparison and is assigned a relative value of 100.
given the same exposure a 200 system will produce twice as much as light (and density)
a 400 system will produce four times more light and density
mAs conversion formula
the mAs conversion formula is a formula for the radiographers to use in determining how to compensate or adjust mAs when changing intensifying screen system speeds
factors affecting screen speed
1) type of phosphor material
2) absorption efficiency
3) conversion efficiency
4) thickness of the phosphor layer
5) size of the phosphor crytal
6) concentration of the phosphor crystal
7) the presence or absence of a reflecting layer
rare earth screen produce 3-4 times faster than calcium tungstate screen
screen speed and recorded detail
when a phosphor material is energized by a photon light it is emitted from the crystal and spreads out towards the film emulsion
therefore the actual physical area of the film exposed to light is greater than the area of the film that would be exposed by an x-ray photon
this spreading of radiographic information decreases the recorded detail, creating more image un-sharpness.
quantum mottle
commonly called as image noise
quantum mottle is the statistical fluctuation in the quantity of x-ray photon that contribute to image formations per square millimeter.
when a very low number of photons are available the image appears mottled or splotchy.
this appearance can be described as a “salt and pepper” look.
quantum mottle decreases the recorded detail
spatial and contrast resolution
spatial resolution refers to how small an object can be imaged
contrast resolution refers to the ability to image similar tissues such as liver and pancreas or gray matter and white matter
conditions that increase intensification factor by reduces spatial resolution
spatial resolution
expressed by number of line pairs per millimeter
higher the number of line pairs, smaller is the object that can be imaged and better is the spatial resolution
fast screens = 7 lp/mm
fine detail screens = 15 lp/mm
direct exposure film = 50 lp/mm
screen maintenance
two important maintenance procedures
1) regular cleaning
2) Maintaining good screen film contact.
cleaning is accomplished by a commercially available antistatic intensifying screen cleaner fluid and gauze pads.
Screen film contact
poor screen film contact greatly reduces recorded detail
it appears as a localized area of unsharpness on the radiograph
first step in testing for poor screen-film contact is identifying the problem cassettes.
perform a wire mesh test to test the screen film contact
wire mesh test
place a wire mesh tool on the cassette in question and radiograph it using an appropriate technique
the resultant radiograph is viewed from a distance of approximately 6 feet to determine any areas of unsharpness.
areas of poor contact will appear darker than areas of good contact
the screen film contact test should be done every 6 to 12 months.
cassettes
cassette serves as a container for both the film and the intensifying screen
cassette must be light proof
must be of little weight
must be rigid enough so that it will not bend under a patient’s weight
should allow maximum amount of radiation to pass through and reach the screen
must provide a good screen-film contact
cassette continuation
low x-ray absorbing material such as bakelite, magnesium or graphite carbon is used to construct the front of the cassette
the inside back of the cassette may be a thin sheet of lead foil, designed to absorb backscatter.
processing sequence
wetting developing rinsing in stop bath fixing washing drying
processing sequence
wetting:
swells the emulsion to permit subsequent chemical penetration
developing:
the latent image is converted to visible image..
rinsing
terminates development and remove excess chemical from the emulsion
processing sequence
fixing
removes remaining silver halide from emulsion and hardens gelatin
washing
removes excess chemicals
drying
removes water and prepares radiograph for viewing
developiing
developing change the silver ions in the exposed silver halide crystals to metallic silver
silver ion is reduced to metallic silver
the chemical responsible for this is called reducing agent
principal component of developer is hydroquinon
the secondary components are phenidone and metol
synergism
the action of two agents working together is greater than the sum of the action of each agent working independently.
usually hydroquinone and phenidone are combined for rapid processing.
hydroquinone: responsible for the black shades of gray, acts slowly
phenidone: responsible for the lighter shades of gray, acts rapidly
fixing
when developing is complete, the film must be treated so that the image will not fade. this stage of processing is called fixing.
the image is fixed on the film
improves archival quality
archival quality refers to the permanence of the radiograph. the image does not deteriorate with age but remains the original stage.
automatic processing
transport system temperature control circulation system replenishment system dryer system
transport system
begins at the feed tray
the entrance rollers grip the film
a micro switch is activated to control replenishment
the shorter dimension of the film should always be against the side rail, so that proper replenishment rate is maintained.
transport system
rollers
transport racks
drive motor
guide shoes are used to turn the film 180 degrees
cross over rack is used to move film from one rack to the other
temperature control system
developer temperature = 95 f
wash water temperature 5 f
circulation system
continually mix the processing solution (agitation)
maintain constant temperature
aid exposure of the emulsion to chemicals
a filter is used to traps particles dislodged from thee emulsion
flow rate of 12L/min (3 gallons/min)
replenishment system
replenishment system add proper quantity of chemicals into each tank to maintain volume and chemical activity.
60 to 70 ml of developer and 100 to 110 ml of fixer for ever 35 cm(inch) of film.
dryer system
wet or damp radiograph picks up dust particles and cause artifacts
wet films are difficult to handle on view box
when stored it can become sticky and may be destroyed
prime exposure factors
the factors that influence and determine the quanity and quality of x-radiation to which the patient is exposed are called exposure factors.
kVp
mA
exposure time
Distance
kVp
primary control of beam quality
controls penetration
controls radiographic contrast
influence beam quanity
increase in kVp reduces differential absorption and reduce the image contrast.
mA
controls radiation quanity
control the number of x-rays produced
with constant exposure time mA controls x-ray quantity and therefore patient dose.
mA
does not change the kE of ellectrons.
does not increase penetration
x-ray quality remains fixed with a change in mA
exposure time
kept as short as possible
short exposure time reduces motion blur
mAs controls optical density
doubling the mAs doubles the density
mAs
mAs=mA times exposure time
time and mA can be used to compensate for each other in an indirect fashion.
mA1/mA2 =time2/time1
new mA = original mAs/new time
exposure time and imagining system characteristics
single phase imaging system can not produce an exposure time less than 1/2 cycle = 1/120 second =8ms
three phase and high frequency generator can provide an exposure a short as 1 ms.
falling load generator
on an x-ray imaging system in which only mAs can be selected, exposure factors adjusted automatically to the highest mA at the shortage exposure time allowed by the high voltage generator. such a design is called a falling load generator.
distance
distance affects exposure of the image receptor according to the inverse square law.
determines the intensity of the x-ray beam at the image receptor
no effect on radiation quality
inverse square law
i1/i2=d2 square /d1 square
effect on longer SID on radiographs
less magnification
less focal spot
improved spatial resolution
requires more mAs
focal spot size
most x-ray tube has two focal spot sizes
large focus
small focus
focal spot size
changing the focal spot size for a given kVp/mAs setting does not change x-ray quanity or quality.
small focal spot size provide better detail
FSS in conventional x-ray tubes
- 5mm/1.0mm
- 6mm/1.2mm
- 0mm/2.0mm
- 3mm/1.0mm for angio-interventional and magnification radiography
- 1mm/0.3mm for mammography (miro focus tube)
filtration
inherent =0.5mm
added
compensating
compensating filters
provide uniform density on the image when radiographing non-uniform objects
wedge filter for thoracic spine
trough filter for chest radiography
high voltage generator
the quality and quantity of radiation is influenced by the type of generator
half wave VS full wave
same radiation quality
double the quantity on full wave rectified generators
power
three phase power results in higher x-ray quantity and quality
high frequency generator results in even greater x-ray quantity and quality
body habitus
the general size and shape of a patient is called the body habitus
hypersthenic 5%
sthenic 50%
hyposthenic 35%
asthenic 10%
radiographic technique charts are based on the sthenic patient
patient factors
thickness]
composition
pathology
thickness
patient thickness should not be guess
always use a caliper to measure the patient
fixed kVp technique
kVp remains fixed
mAs is changed based on patient thickness
Variable kVp technique
mAs remains the same
kVp is changed based on patient thickness
composition
high kVp for chest
low kVp for abdomen
low kVp for soft tissue
pathology
destructive pathology (radiolucent) constructive pathology (radiopaque)
half wave VS full wave
same radiation quality
double the quantity on full wave rectified generators
power
three phase power results in higher x-ray quantity and quality
high frequency generator results in even greater x-ray quantity and quality
body habitus
the general size and shape of a patient is called the body habitus
hypersthenic 5%
sthenic 50%
hyposthenic 35%
asthenic 10%
radiographic technique charts are based on the sthenic patient
patient factors
thickness]
composition
pathology
thickness
patient thickness should not be guess
always use a caliper to measure the patient
fixed kVp technique
kVp remains fixed
mAs is changed based on patient thickness
Variable kVp technique
mAs remains the same
kVp is changed based on patient thickness
composition
high kVp for chest
low kVp for abdomen
low kVp for soft tissue
pathology
destructive pathology (r
image quality factors
optical density
contrast
detail
distortion
optical density
degree of blackening of the finished radiograph
OD of 3 or greater is considered black
OD of 0.2 or less is considered as clear
At OD 2, only 1% of view box light passes through the film
visibility of the image detail
ability to see the detail on the radiograph
best measured by contrast resolution
optical density
30% change in mAs is required for a perceptible change in OD
4% change in kVp is required for a perceptible change in OD
15% increase in kVp will double the OD (15% rule)
contrast
contrast is the difference in OD between adjacent anatomical structures.
kVp is the major factor used in controlling radiographic contrast
high contrast radiographs
also known as short scale contrast
result from low kVp
radiograph of the ribs
low contrast radiograph
long scale contrast
result from high kVp
radiographs of the chest
factors influencing radiographic contrast
mAs
intensifying screen
collimation
grids
5% rule
an increase of 5% in kVp may be accompanied by a 30% reduction in mAs to produce the same OD at a slightly reduce contrast scale.
detail
detail describes the sharpness of appearance of small structures on radiograph
recorded detail
visibility of image detail
sharpness of image detail is best measured by spatial resolution
factors affecting detail
focal spot size SID OID film speed grain size
visibility of the image detail
abi
factors affecting visibility of detail
any factor that affects OD and contrast affects the visibility of image detail
key factors that provide the best visibility of image detail are collimation, use of grids, and other methods that prevent scatter radiation from reaching the image receptor.
fog reduces visibility of detail
distortion
a misrepresentation of object size and shape on the radiograph
position of the x-ray tube, anatomical part and the image receptor could misrepresent the object
distortion
poor alignment of the image receptor or the x-ray tube can result in elongation of the image.
elongation means that the anatomical part of interest appears bigger/longer than normal
distortion
poor alignment of the anatomical part may result in foreshortening means that the anatomical part appears smaller than normal
distortion
distortion is reduced by positioning the anatomical part of interest in a plane parallel to that of the image receptor.
radiographic technique chart
tables that provide a means for determining the specific technical factors to be used in a given radiographic examination
variable kilovoltage
fixed kilovoltage
high kilovoltage
automatic exposure
variable kVp technique chart
fixed mAs kVp varies according to thickness provide shorter contrast scale (use low kVP) higher patient dose less exposure latitude
variable kVp technique chart
kVp varies with the thickness of the anatomical part by 2kVp/cm
beginning kVp =2 times thickness + 23
fixed kVp radiographic technique
used most often developed by Arthur Fuchs longer scale of contrast select the optimum kVp for penetration mAs is changed according to thickness lower patient dose greater latitude
fixed kVp radiographic technique
establish a base technique
for small anatomical part reduce the mAs by 30%
for large anatomical parts increase the mAs by 30%
for part that is swollen as a result of trauma a 50% increase may be required
high kVp technique chart
greater than 100 kVp
reduced patient dose
barium studies
chest
Automatic expsoure techniques
select an optimum kVp for penetration select the desired mA station select the appropriate sensors position the patient accurately collimate accurately exposure time will be adjusted automatically
anatomically programmed radiography (APR)
select the body habitus
select the anatomy
