Radiography Exposure Settings Flashcards
What does kV determine?
The kilovoltage applied across the tube head during the exposure and therefore the speed at which the electrons bombard the target
What does kV do?
Control quality, alter contrast
Why would you increase the kV?
For tissues which
- have a high atomic number
- have a high density
- are very thick
What does mA control?
The degree of heating of the filament which in turn affects the number of electrons produced
What does mA do?
Controls the quantity of X-rays/intensity of the X-ray beam and alters the blackening of the film
What does time (s) indicate?
Total time that the kV is applied across the tube head
What does time (s) determine?
Number of xrays
Increasing time (s) is equivalent to increasing what?
mA
What does the total quantity of X-rays produce depend on and what term describes this?
Length of exposure and mA. mAs.
How do you calculate mAs?
mA x s
Why should you select the largest mA instead of s even tho you can achieve the same mAs with higher seconds?
Risk of movement blur
What does kV alter?
Contrast of the film
Define contrast.
The difference in radio graphic density between two adjacent areas.
A radiograph with lots of black and white but limited grey is said to have high contrast and called …?
Soot and whitewash
A film with a wide grey scale is described as … ?
Flat
What would happen if kV is too high?
Dark film with few different shades of grey. Flat. Lacks contrast.
What would happen if the kV was too low?
Means that the bone is under penetrated. Soot and whitewash. Contrast too high.
What does increased kV cause in relation to tissues?
Increased penetration of all tissues
Increased mA or s causes…?
Increased blackening of the film
What will happen to the film if mA or S is too high?
Dark image, contrast between tissues still visible
What will happen to the film if mA or s is too low?
Pale image
Is there a relationship between kV and mAs? What is this relationship?
Technically no, but appear to be linked. If you increase kV by 10, you can halve the mAs
What is the film focal distance?
Distance between the effective focal spot and film/digital receptor.
Explain inverse square law
As the primary beam leaves the tube head it spreads out and its intensity decreases
What is inverse square law formula?
Intensity ∝ 1
————————
Distance squared
