DR: Exposure Factor Physics Flashcards
(13 cards)
Components that contribute to exposure factors
- X-ray tube
- kVp
- mAs
- mA & s
What does the x-ray tube consist of?
- Glass cylinder
- Within it, all air removed (vacuum)
- Filament (-ve terminal) cathode
- Anode (+ve charge) and rotates very quickly
What is kVp?
Stands for kilovoltage peak
- Max voltage applied to x-ray tube
- Influences energy and penetrating power of x-ray beam
- Determines contrast, higher kVp generally results in lower contrast, leading to broader greyscale, lower kVp results in higher contrast, leading to narrower range of greyscale
What is mAs?
Stands for milliampere-seconds
- mA = rate at which electrons are released from cathode
- s = amount of time released for
- mAs = total number of electrons released for the duration of the exposure
- Affects density of the resultant image
mAs formula
mA = electrons / time
mAs = mA x s
mAs = (electrons / time) x time
thus mAs = number of electrons
Focal spot size
Size of hole through which x-rays are released
Small focal spot restricts number of electrons that are able to flow through simultaneously.
Related to mA, NOT mAs
- The smaller the focal spot (mA), the greater the sharpness of the image
- Can only be used with low mA settings
- If greater image detail provided by small focal spot is required, a longer exposure time is required to produce same mAs
However
- Longer exposure time = higher chance of motion artefact
When should you avoid using long exposure times?
- Uncooperative patients (children, dementia patients, those affected by drugs/alcohol)
- In areas of involuntary movement (heartbeat, bowel gas in abdomen, Parkinson’s disease)
- Trauma patients
When should high kVp be used?
Small differences in adjacent structures, as well as higher dense areas
When should low kVp be used?
Areas with higher differences in adjacent structures, as well as lower dense areas
Why is 55kVp used for forearm and elbow imaging?
This is used as forearm and elbow are not very dense, therefore not a high beam penetration and a reflectively low kVp is used which would give us high image contrast for an extremity.
Why is 100mA used for forearm and elbow imaging?
It facilitates the use of fine focus and is necessary for sharp image detail
Why is 0.04s used in unison with 100mA in elbow and forearm imaging?
Allows us to achieve request density of 4mAs
Why is no grid required for forearm and elbow imaging?
No grid is required as it is a small anatomical area with little scatter radiation.
