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Chapter 11 Prime Factors.

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Presentation on theme: "Chapter 11 Prime Factors."— Presentation transcript:

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.

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