Exposure factors Flashcards
What are the 3 prime exposure factors?
- kVp
- mAs
- SID
What is mAs?
Is the product of two quantities:
Current (mA) flowing from cathode filament to anode target; and,
Time (s) that this current is allowed to flow to the anode mA x seconds = mAs
Correlates with the current applied across the cathode filament circuitry Indicative of the quantity of photons in the x-ray beam
How do milliamps affect X-ray beams?
More electrons arriving at anode, more x-rays produced.
Effects Intensity: “Total energy per second flowing through a unit area”
Does not effect x-ray quality (photon energy spectrum)
What is the exposure time? Why do you want to keep it as short as possible?
The length of time current is allowed to flow to anode facilitating x-ray production Ideally, kept as short as possible to minimise blur from patient motion
If a diagnostic image requires a certain mAs exposure: Reduced time = Increased mA
How does mAs affect intensity/exposure?
I α mAs
What is a negative of short time and high mAs?
Tube loading- wear out the tube needing a broad focus
What is kVp?
•Refers to the maximum voltage applied between the cathode and anode at exposure
What effect does kVp have on the x-ray beam?
- This effects the speed and energy with which electrons cross from cathode to anode
- Therefore, directly related to the maximum energy of x-ray photons produced
By changing kVp, will it effect the quality of photons (penetrative ability)?
Yes
By changing kVp, will it effect the quantity of photons reaching the IR?
Yes
What is the relationship between intensity of radiation and kVp?
•Raising kVp increases the intensity of radiation emitted from tube
I α (kVp)2
•This means more photons reaching image receptor and increased image density
What will kVp quality influence?
–Ability of photons to penetrate matter
–Image Contrast
–Patient Dose
•As photon energy ………………., the probability of photoelectric absorption dramatically …………………
•As photon energy increases, the probability of photoelectric absorption dramatically decreases
How does the probability of compton scattering compared to photoelectric effect with increased kVp?
•Probability of Compton scatter also decreases with kVp, but less dramatically
How does a higher kVp affect attenuation?
–So, at higher kVp less attenuation will occur overall
–At higher kVp, the relative likelihood of the photoelectric effect is decreased
–At higher kVp, the relative likelihood of Compton scatter or no interaction increases
Effect of higher kVp on contrast?
- A more penetrating beam results in a lower contrast radiograph than one made with an x-ray beam having less penetrating power.
- This is because of the reduced comparative differential attenuation between adjacent tissue
•
•The more energetic the beam, the less effect different levels of tissue density will have in attenuating that beam.
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•HIGH kVp LOWERS IMAGE CONTRAST
Effect of kVp through scattering on the contrast of an image?
- At low kVp the photoelectric effect is more prevalent, this increases contrast
- As kVp increases the proportion of Compton scatter interactions increases leading to more image noise and lower contrast
- However, the increased photon energy also means more x-rays can reach the IR contributing to an image
- HIGH kVp CAN INCREASE IMAGE DETAIL AND “PENETRATION”
What happens to the patient dose when the kVp is increased?
- At a Higher kVp there is less chance of absorption and/or scattering within the patient.
- Therefore a higher kVp will actually reduce dose where the mAs has been adjusted accordingly
- E.g. 70/20 vs 81/10
What is SID, SOD, OID and how do they relate?
Source to image distance
Source to object distance
Object to image distance
SOD + OID = SID
Explain inverse square law?
•Doubling the distance increases the area 4 fold
What advantages are there to changing distances in imaging?
•Imaging advantages:
–Increased SID will decrease magnification
•Tibial x-ray, CXR
–Or can offset an air gap
•Lateral C-spine
What is the equation used to work out the ratio of image distance to mAs required?
What are Automatic Exposure Controls (AEC) and how do they work?
•Variable densities exist within patients.
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•Automatic Exposure Devices (AED) are ionisation chambers that monitor the beam of x-rays emitted from the patient before hitting the image receptor
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•Exposure terminates once a sufficient quantity of radiation has been received to produce a radiographic image of required density.
Advantages of AEC
1) Consistency and reproducibility of image.
2) Minimise inaccurate exposures
3) Decreased reject rate.
4) Decreased patient exposure.
5) Increased departmental efficiency.
Disadvantages of AEC
1) Reliant on skill of radiographer.
2) Not suitable for all patients.
3) Correct positioning is critical.
4) Radiographers less experienced at setting a manual exposure
5) Calibration important
What are the technical considerations of using AEC?
- Collimation: This is important as increased scatter from an under collimated exposure will trigger AEC sooner leading to potential underexposure
- “Density adjustments”: May allow changes to the threshold dose of the AEC chambers if under exposure is diagnostically acceptable
- Highest possible mA usually set
Common errors associated with AEC
- Wrong Bucky selection
- Wrong Chamber Selection
- Poor Positioning
- Collimation over chamber
- Implants/Contrast/bowel gas
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•Maximum Exposure Time Limit exists as a safety precaution
Hot and cold tubes
- Differences in generator phasing, anode material or filtration can change x-ray spectrum of tubes.
- Therefore same kVp can cause different intensities on different equipment
