Radio Graphic Factor

8/11/2019 Radio Graphic Factor

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Dr Saket Jain

Degree of blackening within the image Defined more accurately when type of

image receptor is considered

1- Photographic film

2- Digital image capture

Difference in density between structures ofinterest within the image .

Built up in three main stages

1) Subject contrast2) Radiographic contrast3) Subjective contrast

Feature of object (subject) under examination Factors that influence

1) The region of body under examination2) Contrast media3) Pathology4) Kilo voltage

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

The personal appreciation of the contrast inthe image

Depends on1) Radiographic contrast2) The observer3) Viewing condition4) Computer screen / monitor

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

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

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

Degree increases by increased focal spot sizeand increased object to film distance

object to film distancex focal spot size

object to focus distance

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 .

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

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

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%

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

Focus to Film distance :new dist 2

New mAs x original mAsold dist 2

Undesirable fluctuation in optical density ofthe image .

If image acquisition device is used , then aninsufficient mAs will manifest as noise ormottle .

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

increase collimation decrease increasedecrease collimation increase decrease

200 cm 1 0 cm 100 cm

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

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