AEC

Question 1

define mA (in terms of current)

Answer 1

mA = the current through the x-ray tube

Question 2

In a general X-ray system which element does the AEC control?

Answer 2

s = the duration of the current (mAs)

Question 3

role of the AEC (Automatic exposure control)

Answer 3

• Controls the length of the exposure
• These devices measure the amount of radiation at the exit side of the patient (and grid)
• When the correct amount is reached the exposure is terminated

Question 4

where is the aec placed

Answer 4

just below the grid

Question 5

how do the AEC work ?(very intial phase)

Answer 5

• Ionisation
• Ionisation is the process of converting an atom into an ion by adding or removing charged particles such as electrons

Question 6

describe the aec process 6 steps

Answer 6

1.x-ray photon hits atom in ionisation chamber

2. releases e-, e- attracted to positive voltage, e- now creates electrical current (ionisation current)
3. this current charges up the capacitor stores electrical charge
4. the rate wat which the capicitor fills up depends on the ionisation current, and the IC is dependent on the radiation which has interacted with the ionisation chamber
5. (refer to the graph) voltage is directly related to radiation, so as the voltage increases to a certain time it stops due to the pre set volatge.

6.essentailly the volatge will respond to a ceratin amount of dose

Question 7

factor which affect the length of exposure ?

and explain

Answer 7

• The time depends on the rate at which the dose is received
• Quickly=short exposure (low attenuation) reached the pre-set voltage quicker
• Slowly=long exposure (high attenuation) reached the pre-set voltage slower

Question 8

is there usually one or or more chambers in an aec system ?

Answer 8

There is usually more than one chamber in an AEC system

Question 9

what can a low kv cause ?

Answer 9

• Attenuation will be high as photons have lower energy
• Exposure time would be long

Question 10

what can a high kv cause ?

Answer 10

• Attenuation will be low as photons have higher energy
• Exposure time would be short

Question 11

what are the 3 considerations when using aec ? (hint anatomy,pathology,artefacts)

Answer 11

-Anatomy
* Region you’re interested in
* Specific anatomy in that region

-Pathology
* What pathology is present or is suspected that may affect attenuation?

-Artefacts
* What artefacts may be present?

Question 12

Considerations when using AEC, in terms of alignment and collimation ?

Answer 12

• Tube, bucky system(sactter grid), image receptor and patient should be aligned
• Region of interest in is in correct position in relation to the chamber
• Accurate collimation:
• Chambers cannot distinguish between scatter and primary beam
• Whole of the chamber should be in the primary beam

Question 13

how should a patient be orientated when considering aec ?

Answer 13

• Head should be towards the lateral chambers end

Question 14

In summary why are chambers used in combination ?

Answer 14

Provides a bigger sample of the area being imaged

Question 15

describe the feedback error system in the aec ?

Answer 15

• Uses the ionisation current from the chamber
• The system checks that There is a current. - The current is large enough that the exposure will not go on too long.

Question 16

describe what the guard timer does ?

Answer 16

• Monitors the duration of the exposure
• Will terminate regardless of the AEC usually after TWO seconds
• This is a very long time for an exposure

Question 17

what is checked under quality control ?

Answer 17

The operation of the AEC and the safety backup systems need to be checked (QC)