radio graphic factor
TRANSCRIPT
8/11/2019 Radio Graphic Factor
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Dr Saket Jain
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Degree of blackening within the image Defined more accurately when type of
image receptor is considered
1- Photographic film
2- Digital image capture
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Difference in density between structures ofinterest within the image .
Built up in three main stages
1) Subject contrast2) Radiographic contrast3) Subjective contrast
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Feature of object (subject) under examination Factors that influence
1) The region of body under examination2) Contrast media3) Pathology4) Kilo voltage
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Is the difference in optical density on differentparts of the processed film or differences incomputer screen brightness and depends on
1) Subject contrast2) Scattered radiation reaching the image
receptor3) Image acquisition device4) Film fog5) Exposure6) Development
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The personal appreciation of the contrast inthe image
Depends on1) Radiographic contrast2) The observer3) Viewing condition4) Computer screen / monitor
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For a given FFD (focus to film distance) thegreater the distance between the object and
the film the greater will be the magnificationof the image .image size FFD
Magnification = =object size FOD
FOD – focus to object distance
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Distorted image will be produced if not allparts of the image are magnified by the same
amount .
Beam is right angle to object = distortedelongated image is produced
Beam is right angle to image receptor thanthe foreshortened image is produced
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The aim is to produce an image that is assharp as possible in order to resolve fine
detail within the image Important for looking subtle fractures Factor lead to UNSHARPNESS1) Geometry2) Movement3) Absorption4) Photographic / acquistion factors
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Degree increases by increased focal spot sizeand increased object to film distance
object to film distancex focal spot size
object to focus distance
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Sharpness can be increased by using ashorter exposure time (lower mAs with
higher KVp , higher mA or greater tubeloading time) by small object to film distance& immobilisation .
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The choice of factors will depend on the regionbeing examined .
Exposure factors to be selected are1) The mAs2) The kilovoltage3) The FFD – focus to film distance
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MA = tube current = number of electrons andquantity of x-rays produced Intensity
X-ray tube current and exposure time (sec) Density is diectly related
General rule – mA should be as high aspossible with a short time , to reduce the riskof movement unsharpness
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KVP = Energy of x-rays = higher penetrability,it moves through tissue Density directly but
not linearly related Contrast depends
Halving or doubling mAs is equivalent toincreasing or decreasing kVp by 10-15%
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X-ray tube not to be too closed Short FFD could give unacceptable geometric
unsharpness FFD not to be excessive otherwise the large
increase in mAs required would mean hightube loading
Most radiographic examinations are carriedout at 100 cms
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Focus to Film distance :new dist 2
New mAs x original mAsold dist 2
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Undesirable fluctuation in optical density ofthe image .
If image acquisition device is used , then aninsufficient mAs will manifest as noise ormottle .
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Radiographic variables Density Contrast
Increase MAS increase no change
Decrease MAS decrease no changeincrease KVP increase decreasedecrease KVP decrease increase
increase FFD decrease no changedecrease FFD increase no changeincrease OID decrease increasedecrease OID increase decrease
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increase collimation decrease increasedecrease collimation increase decrease
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200 cm 1 0 cm 100 cm
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GEOMETRIC MOVEMENT PHOTOGRAPHIC
A Use fine focus,
standardized(large) FFD
Use short exposure time
with high tube loading
Use fine grained screens,
single sided emulsion
B Use small OFD , avoidequipment vibration
Use small OFD ;immobilization ; makepatient comfortable ; giveclear instructions topatient about keepingstill
Ensure all cassettesmaintain good film /screen contact
OFD – object to film distance