Exposure Factors

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Exposure Factors

Kv / KvP Potential difference between film and anode:

       The energy (you can consider this the penetrating power) of the x-ray beam is controlled by the voltage adjustment. This control usually is labelled in keV (thousand electron volts) and sometimes the level is referred to as kVp (kilovoltage potential). Do not be confused by the different terminology, just remember there is a control by which the difference in potential between the cathode and anode can be controlled. The higher the voltage setting, the more energetic will be the beam of x-ray. A more penetrating beam will result in a lower contrast radiograph than one made with an x-ray beam having less penetrating power. It is probably obvious that the more energetic the beam, the less effect different levels of tissue density will have in attenuating that beam. The generator waveform is not in the constant potential (medium frequency etc) will affect the effective Kv.
 
mA (Tube Current):


The second control of the output of the x-ray tube is called the mA (milliampere) control. This control determines how much current is allowed to flow through the filament which is the cathode side of the tube. If more current (and therefore more heating) is allowed to pass through the filament, more electrons will be available in the "space charge" for acceleration to the target and this will result in a greater flux of photons when the high voltage circuit is energized. The effect of the mA circuit is quite linear. If you want to double the number of "x" photons produced by the tube, you can do that by simply doubling the mA. Changing the number of photons produced will affect the blackness of the film but will not affect the film contrast.
 
 S (Time):


The third control of the x-ray tube which is used for medical imaging is the exposure timer. This is usually denoted as an "S" (exposure time in seconds) and is combined with the mA control. The combined function is usually referred to as mAs or milliampere seconds so, if you wanted to give an exposure using 10 milliampere seconds you could use a 10 mA current with a 1.0 second exposure or a 20 mA current for a 0.5 second exposure or any combination of the two which would result in the number 10. Both of these factors and their combination affect the film in a linear way. That is, if you want to double film blackness you could just double the mAs. 

Figure (b) A good chest image the mediastinum is penetrated the image is exposed well indicating the bones and soft tissues.



Questions:

1. Penetrating power of the X-ray beam will increase by increasing,

a) mA

b) S

c) KvP

d) All



2. The output of the X-ray machine will be controlled by,

a) KvP

b) mA

c) S

d) All



Answers:

1. c)KvP

2. b)mA



References:

1. http://www.e-radiography.net 

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