Exposure factors

Question 1

What are the 3 prime exposure factors?

Answer 1

• kVp
• mAs
• SID

Question 2

What is mAs?

Answer 2

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

Question 3

How do milliamps affect X-ray beams?

Answer 3

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)

Question 4

What is the exposure time? Why do you want to keep it as short as possible?

Answer 4

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

Question 5

How does mAs affect intensity/exposure?

Answer 5

I α mAs

Question 6

What is a negative of short time and high mAs?

Answer 6

Tube loading- wear out the tube needing a broad focus

Question 7

What is kVp?

Answer 7

•Refers to the maximum voltage applied between the cathode and anode at exposure

Question 8

What effect does kVp have on the x-ray beam?

Answer 8

• 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

Question 9

By changing kVp, will it effect the quality of photons (penetrative ability)?

Answer 9

Yes

Question 10

By changing kVp, will it effect the quantity of photons reaching the IR?

Answer 10

Yes

Question 11

What is the relationship between intensity of radiation and kVp?

Answer 11

•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

Question 12

What will kVp quality influence?

Answer 12

–Ability of photons to penetrate matter

–Image Contrast

–Patient Dose

Question 13

•As photon energy ………………., the probability of photoelectric absorption dramatically …………………

Answer 13

•As photon energy increases, the probability of photoelectric absorption dramatically decreases

Question 14

How does the probability of compton scattering compared to photoelectric effect with increased kVp?

Answer 14

•Probability of Compton scatter also decreases with kVp, but less dramatically

Question 15

How does a higher kVp affect attenuation?

Answer 15

–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

Question 16

Effect of higher kVp on contrast?

Answer 16

• 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.

•

•HIGH kVp LOWERS IMAGE CONTRAST

Question 17

Effect of kVp through scattering on the contrast of an image?

Answer 17

• 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”

Question 18

What happens to the patient dose when the kVp is increased?

Answer 18

• 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

Question 19

What is SID, SOD, OID and how do they relate?

Answer 19

Source to image distance

Source to object distance

Object to image distance

SOD + OID = SID

Question 20

Explain inverse square law?

Answer 20

•Doubling the distance increases the area 4 fold

Question 21

What advantages are there to changing distances in imaging?

Answer 21

•Imaging advantages:

–Increased SID will decrease magnification

•Tibial x-ray, CXR

–Or can offset an air gap

•Lateral C-spine

Question 22

What is the equation used to work out the ratio of image distance to mAs required?

Answer 22

Question 23

What are Automatic Exposure Controls (AEC) and how do they work?

Answer 23

•Variable densities exist within patients.

•

•Automatic Exposure Devices (AED) are ionisation chambers that monitor the beam of x-rays emitted from the patient before hitting the image receptor

•

•Exposure terminates once a sufficient quantity of radiation has been received to produce a radiographic image of required density.

Question 24

Advantages of AEC

Answer 24

1) Consistency and reproducibility of image.
2) Minimise inaccurate exposures
3) Decreased reject rate.
4) Decreased patient exposure.
5) Increased departmental efficiency.

Question 25

Disadvantages of AEC

Answer 25

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

Question 26

What are the technical considerations of using AEC?

Answer 26

• 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

Question 27

Common errors associated with AEC

Answer 27

• Wrong Bucky selection
• Wrong Chamber Selection
• Poor Positioning
• Collimation over chamber
• Implants/Contrast/bowel gas

•

•Maximum Exposure Time Limit exists as a safety precaution

Question 28

Hot and cold tubes

Answer 28

• 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