Prime Factors Flashcards
What are the three principal factors that affect X-ray quality and quantity?
- Milliamperage per second (mAs)
- Kilovoltage (kVp)
- Distance
These factors are under the direct control of the radiographer.
What does the term ‘Quantity’ refer to in X-ray imaging?
The measure of the number of x-ray photons in the useful beam
Also known as output, intensity, or exposure, measured in roentgen (R).
What is the prime controller of X-ray quantity?
Milliamperage per second (mAs)
It is also affected by kilovoltage (kVp), distance, and filtration.
What does ‘Quality’ measure in X-ray imaging?
The penetrating ability of the x-ray beam
It is numerically represented by the Half Value Layer (HVL).
What is the relationship between mA and exposure time in X-ray imaging?
Both mA and exposure time are directly proportional to the number of electrons crossing the tube and the number of x-ray photons created.
How is mAs calculated?
mAs = mA x s
Where mA is tube current and s is exposure time.
What is the Exposure Index (EI)?
A numeric exposure value used to assess image receptor (IR) exposure
It indicates the amount of exposure received but does not indicate patient dose.
What happens to the EI value if it falls outside the specified range?
It indicates an over or under-exposed image.
What is the effect of increasing kVp on the number of x-ray photons?
A 15% increase in kVp doubles the number of photons
Doubling kVp increases the number of photons about four times.
How does kVp affect image contrast?
Higher kVp results in lower contrast, while lower kVp results in higher contrast.
What is the 15% Rule in X-ray imaging?
An increase or decrease in kVp by 15% will double or halve the exposure.
What is the Inverse Square Law in the context of X-ray distance?
Doubling the distance increases the area by 4 times, decreasing the intensity by ¼.
What is the Direct Square Law used for?
To compensate for IR exposure changes when the distance changes.
How does increasing distance affect X-ray exposure?
Increasing distance causes a decrease in exposure.
What should the radiographer consider when setting kVp?
The desired contrast for the image.
