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Principles of Radiologic Imaging Carlton 6th edition Chapter 12
The Prime Factors
Terms in this set (19)
The 3 principles factors that affect x-ray emulsion are under the direct control of the radiologists.
-these 3 factors are:
1. Milliamperage-second (mAs)
2. Kilovoltage (kVp)
3. Distance (d)
Is a measure of the number of x-ray photons in the useful beat.
-also known as x-ray output, intensity or exposure
-is "approximately" directly proportional to the square of the ratio of the change in kVp. Meaning as kVp is doubled, the amount of X-ray photons increases approximately 4 times
-is affected by mAs, distance, kVp, and filtration.
a measurement of the penetrating ability of the x-ray beam
-is affected by kVp and filtration
-penetrability describes the distance an x-ray beam travels in matter
-high-energy x-ray photons travel farther in matter than low-energy photons and are, therefore, more penetrating.
-highly penetrating x-rays are termed hard x-rays
-low penetrating x-rays are called soft x-rays
-x-ray quality is numerically represented by the half-value layer (HVL)
the HVL of an x-ray bean is that thickness of absorbing material needed to reduce the X-ray intensity (quantity) to half its original value
Factors affecting X-ray emission
Unit used to describe the product of tube current and exposure time
-this simple relationship is described by the equation mA X s = mAs
-primary controller of X-ray quantity
-this means that as mAs doubles, X-ray exposure (measured in roentgens) doubles; as mAs triples, X-ray exposure triples, and so forth
is the primary controlling factor that will affect the X-ray quantity and the resultant IR exposure.
-appropriate mAs must be selected to achieve and acceptable radiographic image.
Image Receptor (IR) Exposure Relationships to mAs
In a digital imaging environment, the digital image receptor (IR) records the image that is then displayed on a monitor for viewing.
-although the term density might be used to describe the level of brightness on the monitor (how light or dark the image is), the relationship of this brightness level does not correspond to the IR exposure.
is the degree of blackening of an x-ray film
Exposure index (IE)
A numeric representation of the quantity of exposure received by a digital image receptor
Kilovoltage affects the quantity of the X-ray beam because MORE interactions occur at the target as kVp increases, and it affects the quality of the X-ray beam because each electron has more energy, resulting in a beam with greater penetrability
Controls both the quantity and quality of the X-ray beam
- because changes in kilovoltage create changes in beam penetrability, kVp is the primary controller of the difference in densities/IR exposures
-an increase in kVp causes an increase in penetrability, which will result in an image with less contrast.
-kVp adjustments should not be used to control IR exposure
15 percent rule
an increase in kVp by 15% will cause a doubling in exposure, the same effect as doubling the mA or doubling exposure time. (gets us a lower contrast)
A decrease of kVp by 15% will reduce exposure by 1/2. (gets a higher contrast)
A smaller change in kVp will have a greater impact on x-ray emission in the lower kVp ranges than in the higher kVp ranges.
The intensity of X-rays varies greatly with changes in the distance
-X-ray intensity will decrease as the distance from the tube is increased.
X-ray intensity (exposure)
Measured in roentgens (R) or, more commonly in diagnostic radiology, in milliroentgens (mR)
Inverse square law
the intensity of radiation at a given distance from the point source is inversely proportional to the square of the distance
-the relationship of X-ray quantity to distance is described in the inverse square law
Mathematical expression of the inverse square law
IR exposure Relationship to Distance
Because distance has an effect on X-ray intensity, it will in turn affect IR exposure.
-as the distance increases, intensity decreases, which causes a decrease in IR exposure.
Exposure maintenance formula
A practical application of the inverse square law is found in a formula that is used to compensate for the IR exposure changes that occur when distance is changed.
-based on the principle of the inverse square law. This formula is a direct square law because mAs should increase proportionately to the square of the change when the distance increases, and mAs should decrease proportionately to the square of the change when distance is decreased.
No relationship between distance and mAs
Changes in distance do not cause changes in mAs.
-the radiographer must adjust the mAs to compensate for the distance changes.
-changes in distance will create changes in the X-ray intensity and the image receptor exposures. The exposure maintenance formula can be used to compensate for the effect that changes in distance will have on the image receptor exposure of the radiographic image.
Setting the appropriate mAs, kVp, and distance
-these prime factors control the exposure to the image receptor
-select kVp based on the desired contrast, and adjust mAs to provide the appropriate total exposure to the receptor
-appropriate kVp selection is just as important with digital systems because the selected kVp will control subject contrast (signal differences to the digital detector).
-distance is generally set based on the desired beam geometry.
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