Chapter 11

1. Chapter 11 Prime Factors

2. Prime Factors • Prime factors: affect x-ray emission; under the control of the radiographer. • Miliamperage-second (mAs) • Kilovoltage (kVp) • Distance (d) • Related to tube design and construction. • Tube housing • Target material • Filtration • Voltage waveform

3. X-Ray Quantity • X-ray quantity: measure of the number of x-ray photons in the useful beam. • AKA x-ray output, intensity, or exposure • Unit Roentgen (R) • Directly affected by: • Miliamperage-second (mAs) • Kilovoltage (kVp) • Distance (d)

4. X-ray quality • X-ray quality: measurement of the penetrating ability of the x-ray beam. • Describes the distance an x-ray beam travels in matter. • High energy x-ray photons travel farther in matter- more penetrating • Numerically represented by the Half-value layer (HVL).

5. Half-Value Layer • Half-value layer of an x-ray beam is that thickness of absorbing material needed to reduce the x-ray intensity to half its original value. • Affected by: • Kilovoltage • Filtration • Not controlled from exposure to exposure

6. Milliamperage • Milliamperage (mA): measurement of x-ray tube current. • The number of electrons crossing the tube from cathode to anode per second • Directly proportional to tube current • Ampere: equal to an electrical charge of 1 coulomb flowing through a conductor per second. • 1 Coulomb = 6.3 x 1018 electron charges.

7. Exposure • Exposure time is directly proportional to the number of electrons crossing the tube and is therefore directly proportional to the number of x-rays created. • The number of x-rays that will be created at the target is a product of the number of electrons crossing the tube (tube current) and how long the electrons are allowed to cross (exposure time). • Measured in mAs- primary controller of x-ray quantity.

8. Density Relationship to mAs • Radiographic film density is the degree of blackening of an x-ray film • Created by deposits of black metallic silver on an x-ray film that has been exposed to light or x-ray and then processed • Densities are the result of an x-ray exposure to the film and intensifying screens. • Film density is determined by the amount of silver deposition in the emulsion due to: • Film type • Exposure conditions • Exposure (mR) • Processing

9. Density Relationship to mAs • If the exposure to a film is increased, the density to that film will increase until the point where the film reached its maximum density (Dmax). • mAs is used as the primary controller of radiographic film density. • By maintaining a specific exposure relative to the speed of the image receptor, consistent film density can be achieved.

10. Reciprocity Law • The reaction of a photographic film to light is equal to the product of the intensity of the light and the duration of the exposure. • The density on the x-ray film should remain unchanged as long as the intensity and duration of the x-ray exposure remains unchanged. • Fails for exposures made at extremely short exposure times (less than 1/100 second) or extremely long exposure times (more than a second) • Law failure in not very significant in diagnostic radiology because exposures are seldom at those extremes

11. Kilovoltage • Increasing the kilovoltage on an x-ray control panel will cause an increase in the speed and energy of the electrons applied across the x-ray tube. • Increased energy of the electrons results in the production of x-ray photons with greater energy. • Controls both the quantity and quality of the x-ray beam. • Quantity: more interactions will occur at the target as kVp increases. • Quality: each electron has more energy resulting in a beam with greater penetrability

12. Density Relationship to kVp • Changes in kilovoltage create changes in beam penetrability. • kVp is the primary controller of the differences in radiographic density. • kVp should NOT be used to control radiographic film density (contrast) • As kVp increases, causes increase in penetrability, which will result in less contrast.

13. 15 percent rule • An increase in kVp by 15 percent will cause a doubling in exposure, the same effect as doubling the mA or doubling exposure time. • Hypothetically, if kVp is doubled, the x-ray quantity would increase by a factor of four, but this does not take into account the increased penetrability of the bean with increasing kVp. • As a result, radiographic density is more significantly affected. • To maintain exposure with changes in kVp, the 15 percent rule can be applied, • To maintain density, if the kVp is increased 15 percent, the mAs must be reduced to one-half its original value.

14. Distance • The intensity of x-rays varies greatly with changes in distance. • Measurement of the x-ray intensity is obtained using a dosimeter. • X-ray photons are most concentrated at the target and from there they spread out in all directions. • X-ray intensity (quantity) begins to diminish. • Photons that exit the tube port constitute the primary useful beam.

15. Inverse Square Law • Intensity of radiation at a given distance from the point source is inversely proportional to the square of the distance. I1 / I2 = D22 / D12 I1 =original intensity I2 = new intensity D22 = original distance D12 = new distance

16. Distance Relationship to Distance • As the distance increases, intensity decreases which causes a decrease in exposure to the image receptor. • Since mAs is the primary controller of x-ray intensity and radiographic film density, mAs can be adjusted to compensate for changes in distance.

17. Exposure (film density) maintenance formula • mAs should increase proportionally to the square of the change when distance increases. (direct square law) mAs1 / mAs2 = D12 /D22 mAs1 = original mAs mAs2 = New mAs D22 = original distance D12 = new distance

18. Density Relationship to mAs, kVp & d • The radiographer should select the kVp based on the desired contrast, and adjust mAs to provide the appropriate total exposure to the receptor.