Published on March 3, 2014
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
1) 2) 3) Difference in density between structures of interest within the image . Built up in three main stages Subject contrast Radiographic contrast Subjective contrast
1) 2) 3) 4) Feature of object (subject) under examination Factors that influence The region of body under examination Contrast media Pathology Kilo voltage
Is the difference in optical density on different parts of the processed film or differences in computer screen brightness and depends on 1) Subject contrast 2) Scattered radiation reaching the image receptor 3) Image acquisition device 4) Film fog 5) Exposure 6) Development
The personal appreciation of the contrast in the image Depends on 1) Radiographic contrast 2) The observer 3) Viewing condition 4) Computer screen / monitor
For a given FFD (focus to film distance) the greater the distance between the object and the film the greater will be the magnification of the image . image size FFD Magnification = = object size FOD FOD – focus to object distance
Distorted image will be produced if not all parts of the image are magnified by the same amount . Beam is right angle to object = distorted elongated image is produced Beam is right angle to image receptor than the foreshortened image is produced
The aim is to produce an image that is as sharp as possible in order to resolve fine detail within the image Important for looking subtle fractures Factor lead to UNSHARPNESS 1) Geometry 2) Movement 3) Absorption 4) Photographic / acquistion factors
Degree increases by increased focal spot size and increased object to film distance object to film distance x focal spot size object to focus distance
Sharpness can be increased by using a shorter exposure time (lower mAs with higher KVp , higher mA or greater tube loading time) by small object to film distance & immobilisation .
The choice of factors will depend on the region being examined . Exposure factors to be selected are 1) The mAs 2) The kilovoltage 3) The FFD – focus to film distance
MA = tube current = number of electrons and quantity of x-rays produced Intensity X-ray tube current and exposure time (sec) Density is diectly related General rule – mA should be as high as possible with a short time , to reduce the risk of 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 to increasing 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 high tube loading Most radiographic examinations are carried out at 100 cms
Focus to Film distance : new dist 2 New mAs x original mAs old dist 2
Undesirable fluctuation in optical density of the image . If image acquisition device is used , then an insufficient mAs will manifest as noise or mottle .
Radiographic variables Density Contrast Increase MAS Decrease MAS increase KVP decrease KVP increase FFD decrease FFD increase OID decrease OID no change no change decrease increase no change no change increase decrease increase decrease increase decrease decrease increase decrease increase
increase collimation decrease collimation decrease increase increase decrease
200 cm 150 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 , avoid equipment vibration Use small OFD ; Ensure all cassettes immobilization ; make maintain good film / patient comfortable ; give screen contact clear instructions to patient about keeping still OFD – object to film distance
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