Note that exposure factors vary between x-ray machines make, model and age. Figures quoted below are illustrative only. However, certain factors may determine what kV and mAs you should use. These are:
a. FFD (film focus distance )to be used
For FFD of 180cm, a higher kV is needed compared to FFD of 100cm.
For example, if normally for PA chest at FFD of 180cm, you may be using 80-90kVp. If you are going to take AP chest where the FFD is 100cm, you will need to reduce it to 60-65kVp.
For extremities, the FFD is 100cm. Thus, your kV should be around 60-65kVp.
b. Patient’ s size and build
The bigger the patient’s size or parts to be x-rayed, the higher will be the attenuation of the x-rays. Thus, the bigger the patient’s size or parts to be x-rayed, the higher should be the mA. If you are doing a chest x-ray of a thin person, you may use 2-4 mAs but if the person has a big chest, you may increase the mAs to between 6-8.
If you are doing extremities, you may use 2 mAs for wrist but only 1 mAs for fingers.
c. Type of film and screen used
Most films nowadays are green sensitive type. These require less exposure factors compared to blue sensitive films. Check what type of films you are using. Newer screens are more sensitive than older screen. Thus, as you screen ages, you will need to adjust the exposure factors accordingly.
d. Processing conditions
Newly prepared chemicals are more sensitive and thus a lower exposure factors are required. However, as the chemicals are becoming oxidised or exhausted, you may need to increase your exposure factors accordingly.