Overview
This lecture covers the four prime exposure factors in radiography (kVp, mA, exposure time, distance) and their impact on image quality, contrast, and patient dose.
Prime Exposure Factors
- The four main exposure factors are kVp (kilovoltage peak), mA (milliamperage), exposure time (seconds), and distance (SID, OID).
- Inverse Square Law states that x-ray intensity decreases with increased distance from the source.
kVp (Kilovoltage Peak)
- kVp controls the speed of electrons, x-ray quality, penetrability, and subject contrast.
- High kVp increases penetration, reduces patient dose, but increases scatter and produces low contrast (more grays).
- Low kVp gives less penetration, higher patient dose, less scatter, and high contrast (more black and white).
- kVp is selected based on body part thickness using technique charts.
Contrast
- Subject contrast is the difference in density between two image areas; makes features distinguishable.
- Short scale contrast (low kVp): high contrast, fewer grays, often used for bone imaging.
- Long scale contrast (high kVp): low contrast, many grays, used for chest X-rays.
- For digital imaging, contrast is controlled by the lookup table, not by kVp.
mA and Exposure Time (mAs)
- mA measures x-ray tube current (number of electrons) and controls photon quantity and receptor exposure.
- mAs = mA × time; increasing mAs increases both patient dose and receptor exposure.
- Exposure time should be kept as short as possible to minimize motion blur.
- To make a visible density change, alter mAs by at least 30%.
Brightness
- In digital imaging, brightness refers to luminance on the monitor and is controlled by pixel values.
- Brightness and contrast in digital systems are algorithm-based; mAs does not control brightness.
Distance and Inverse Square Law
- Increasing the distance between source and image (SID) decreases x-ray intensity per the inverse square law.
- To compensate for moving from 40 to 72 inches SID, multiply mAs by 4; the reverse divides by 4.
Other Factors
- Small focal spots are used for bony detail; large focal spots for chest and abdomen.
- Best images are produced with small OID and large SID to minimize magnification and maximize detail.
The 15% Rule
- Increasing kVp by 15% doubles image exposure; decreasing by 15% halves exposure.
- To maintain original exposure when increasing kVp by 15%, halve the mAs, and vice versa.
Key Terms & Definitions
- kVp (Kilovoltage Peak) — controls x-ray penetration and image contrast.
- mA (Milliamperage) — controls the number of x-ray photons produced.
- mAs — product of mA and time; controls total exposure.
- SID (Source to Image Distance) — distance from x-ray tube to image receptor.
- OID (Object to Image Distance) — distance from object to image receptor.
- Receptor Exposure — number of x-ray photons hitting the image receptor.
- Inverse Square Law — x-ray intensity is inversely proportional to the square of the distance.
- Subject Contrast — difference in densities between image areas.
- Lookup Table — digital tool controlling contrast and brightness in digital imaging.
Action Items / Next Steps
- Begin making personal notes on subject contrast, kVp, mAs, and digital imaging differences for boards.
- Review technique charts for body part-specific kVp settings.
- Practice using the 15% rule with sample calculations.