Radio Graphic Quality

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

Radio Graphic Quality

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