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8/4/2019 Radio Graphic Quality
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Radiographic Quality
Radiographic Quality refers to the fidelitywith which the anatomic structures being
examined are images on the film. Three main factors:
Film Factors
Geometric Factors
Subject Factors
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Radiographic Quality
Characteristic of radiographic quality:
Spatial Resolution (Recorded Detail)
Contrast Resolution (Visibility of Detail)
Noise (Visibility of Detail)
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Spatial Resolution
Spatial Resolution is the ability to imagesmall structures that have high subject
contrast such as bone-soft tissueinterface.
When all of the factors are correct,
conventional radiography has excellentspatial resolution.
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Contrast Resolution
Contrast resolution is the ability todistinguish structures with similar subject
contrast such as liver-spleen, fat-muscle. Computed tomography and MRI have
excellent contrast resolution. Convention
radiology is fair to poor.
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Noise
Noise is an undesirable fluctuation inoptical density of the image. Two major
types: Film Graininess- no control over
Structure mottle- no control over
Quantum Mottle- some control over
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Film Graininess
Film graininess refers to the distribution in sizeand space of the silver halide grains in the filmemulsion.
Similar to structure mottle that refers to thesize and shape of the phosphors in the
intensifying screens. They aret under the control of the technologist,
but they contribute little to radiographic noise.
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Quantum Mottle(small number of x-ray photons)
Quantum mottle refers to the randomnature of how the x-rays interact with the
image receptor. It is the primary form of radiographic
noise.
The use of high mAs and low kVp reducedquantum mottle.
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Quantum Mottle
Very fast screens have higher quantummottle because it takes fewer x-rays to
make the image.
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Speed
Resolution and noise are intimatelyconnected with speed.
While the speed of the images receptor isnot apparent on the image, it influencesboth resolution and noise.
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Radiographic Quality Rules
Fast Image receptors have high noise andlow spatial and contrast resolution.
High spatial and contrast resolutionrequire low noise and slow imagereceptors.
Low noise accompanies slow imagereceptors with high spatial and contrastresolution.
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Film Factors of Quality
Characteristic curve
Density
Contrast
Latitude
Processing
Time
Temperature
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Sensitometry
Sensitometry is the study of therelationship between the intensity of
exposure of the film and the blacknessafter the film is processed.
Unexposed film is clear with a blue tint
after processing. Exposed film is black after processing.
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Sensitometry
Two principles involved.
Exposure of the film
Amount of light transmitted through theprocessed film of optical density.
Used to describe the relationship of
radiation exposure and blackness ordensity on the film.
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Characteristic Curve
This relationship iscalled the
characteristic curve orH & D curve of thefilm.
H & D stands for
Hurter and Driffield.
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Parts of the Characteristic
Curve Toe and shoulder
where large changes
in exposure results insmall changes in OD.
Very high and verylow variations of
exposure make verysmall changes indensity.
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Parts of the Characteristic
Curve The straight line or
intermediate area is
where very smallchanges in exposureresults in largechanges in density.
This is the importantpart of the curve inradiography.
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Log Relative Exposure (LRE)
X-ray films responds to awide range of exposurefrom 1 mR to 1000 mR.
Exposure is representedon logarithmic manner.
An increase in log relativeexposure of 0.3 results
from doubling theexposure
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Optical Density Range The optical density range
is from 0.0 for no densityto 4.0 for absolute black.
Density of 4 means only 1in 10,000 light photons iscapable of penetrating
the x-ray film Useful range in general
radiography is from 0.25to 2.5.
75% of radiographs show
image pattern in therange of 0.5 to 1.25 OD OD=Log10(I0/It)
I0: incident photonsIt: transmitted photons
OD always > zero (never equalzero),due to base fog density
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Base fog or base density
The tint of the baseof the film and the
inadvertent exposureof the duringprocessing.
Range is from 0.1 to
0.3. Should be neverabove 0.30 most is.21 OD
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Items that Impact Base Fog
Film storage
Film exposure to wrong spectrum of light
or light intensity.
Chemical contamination.
Improper processing.
High Base fog levels reduce contrast.
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Contrast
Radiographic Contrast is the combinedresult ofimage receptor contrast and
subject contrast. Image receptor contrast refers to the
contrast inherent in the film and
influenced by the processing of the film.
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Contrast
Subject contrast is determined by the size,shape and x-ray attenuating
characteristics of the subject beingexamined and the energy (kVp) of the x-ray beam.
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Average Gradient The average gradient is a
straight line drawn between0.25 OD and 2.0 OD abovebase plus fog.
This is the normal range of
density in a radiograph Average gradient range of
2.5 to 3.5
Average gradient=(2+base fog) (0.25+base fog)
LRE2 LRE1
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Speed Speed is the ability of
the receptor torespond to low x-ray
exposure. The H & D curse is
useful in comparingspeed when selecting
film or screens. Image receptor A is
higher speed thanimage receptor B
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LATITUDE
Latitude can beobserved on the H &
D curve. Latitude refers to the
range of exposurethat will produce a
diagnostic range OD(acceptable image)
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Latitude
Latitude and Contrastare inversely
proportional. Wide latitude has a
long scale contrast orlow contrast. (B)
Narrow latitude has ashort scale contrast orhigh contrast. (A)
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Film Processing
Radiographic Qualityis impacted by film
processingparameters.
The developer mustbe at the proper
concentration and atthe correcttemperature.
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Film Processing
The film must alsospend the correct
amount of time in thedeveloper.
This is the time &temperature
relationship.
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Processing
Speed and base fog increase with thetemperature.
Contrast will increase to a point and then dropwith the base fog increase.
Manufactures set processing parameters tooptimize speed, contrast and low base fog.
The fog level increases with increasing temp, asdoes x-ray film speed
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Distortion
Distortion is the misrepresentation of thetrue size and shape of the object being
radiographed (unequal magnification ofdifferent portions of the same object)
The amount of distortion depends upon
-the size (thickness) and shape-and the position of the object
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Thickness
Thick objects aremore distorted thanthin objects becauseof the greater changein Object ImageDistance (OID)
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Thickness
The position of theobject relative tothe central axis willcause greaterdistortion with thickand/or irregular
shaped objects.
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Object Position
If the object plane and image plane areparallel the image will not be distorted.
If the object plane and image plane are notparallel, distortion will occur.
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End of Lecture