AED's and post processing

Question 1

Purpose of AED (2 points)

Answer 1

1) allows the radiographer to consistently reproduce radiographic images that have a diagnostically acceptable image density (exposure index)
2) determines when the exposure is terminated by detecting when the appropriate exposure on the receptor has been reached

Question 2

What does a phototimer AED consist of? 3 things

Answer 2

1) fluorescent screen
2) photomultiplier
3) electric circuit connected to a circuit

Question 3

What does the most commonly used AED consist of? 3 things

Answer 3

1) ionisation chamber
2) capacitor
3) exposure terminating switch

Question 4

Where are ionisation chambers situated

Answer 4

between the grid and the cassette (imaging receptor)

Question 5

How do we stop the ionisation chamber from appearing on the image?

Answer 5

KvP is sufficient to over penetrate the ionisation chambers-preventing artefacts

Question 6

how are the ionisation chambers connected?

Answer 6

Interconnected by small electrical wires. exposure termination can be accomplished either through ionisation of one chamber or a combination of two or three

Question 7

What happens in the ionisation chambers? 4 steps

Answer 7

1) x-ray photons penetrate thin aluminium shell and strike the air molecules
2) electrons released and ionisation occurs
3) electrons are attracted to the positively charged electrode and carried through an electrical circuit to charge a capacitor
4) The exposure is terminated when voltage reaches a pre-determined level

Question 8

what is a capacitor

Answer 8

a device used to store a specific quantity of electricity

Question 9

what does the capacitor consist of? 3 things

Answer 9

1) 2 conductors separated by an insulator (Bakelite)
2) one positively charged plate
3) one negatively charged plate
- negative charges are deposited on the left hand plate, leaving the right hand plate positively charged

Question 10

what is the insulator made out of in the capacitor

Answer 10

Bakelite

Question 11

what 3 things effect capacitance

Answer 11

1) area of the plates
2) distance between plates
3) dielectric material

Question 12

how does the area of the plates effect capacitance

Answer 12

larger plates provide a greater capacity to store electrical charge

Question 13

how does the distance between plates effect capacitance

Answer 13

it is directly proportional to the electrostatic forces between plates. Field is stronger when they are closer together- decreases with distance

Question 14

how does the dielectric material effect capacitance

Answer 14

Bakelite can increase capacitance more than air

Question 15

Factors that effect AEDs- 7 points

Answer 15

1) patient positioning
2) detector selection
3) density setting
4) collimation
5) minimum exposure time
6) back up timer
7) mAs, kVp, SID

Question 16

importance of patient position when using AED

Answer 16

incorrect patient positioning can lead to over/under exposure of the patient

Question 17

importance of detector selection when using AED (3 things)

Answer 17

1) need for correct detector selection to produce the desired radiographic density
2) sound knowledge of anatomy and radiographic positioning for proper detector selection
3) correct cassette/receptor size

Question 18

why is collimation important when using AEDs

Answer 18

• Detectors cannot distinguish between transmitted or scatter radiation and ends the exposure when the pre-set density has been reached
• Detector measures both types of radiation- exposure terminated prematurely when there is excessive scatter

Question 19

how does the density setting effect AED use

Answer 19

-AED installation capacitor is set by the engineer to terminate the exposure when it has reached a specific charge
-Optimal density setting is neutral setting e.g. 0 or average
-Density settings can be used e.g. -1, -2 for small male/female/child
or +1, +2 for large male/female

Question 20

how does minimum response time (MRT) affect AEDs

Answer 20

• shortest exposure time possible for an AED to begin to respond to irradiation
• MRT needs to be constant and regularly checked
• In modern equipment this is as little as 1ms

Question 21

what is the purpose of the back up timer for AEDs- 5 points

Answer 21

1)prevents over exposure of the patient in case of timer switch failure or radiographer error
2)protects x-ray tube
3)maximum length of time the xray exposure will continue using the AED
4)set as exposure time or mAs
-set as 150-200% of expected exposure
AUDIBLE WARNING IF BACK UP TIMER USED

Question 22

What is under exposure

Answer 22

poor image quality due to quantum mottle or noise.
Inadequate photon energy has been converted into electrical energy therefore the computer is limited to what it can do to improve image quality

Question 23

what is over exposure

Answer 23

computer attempts to adjust density but this decreases contrast and excessive patient exposure will have occurred

Question 24

Advantages of AEDs- 3 points

Answer 24

1) maintains consistency and quality of images
2) reduced the number of repeats and therefore effectively reduces patient dose
3) increases efficiency

Question 25

Disadvantages of AEDs- 2 points

Answer 25

1) detector cannot differentiate between transmitted and scattered radiation
2) If unknown pathology present it can lead to images of sub optimal density

Question 26

Gives examples of types of pathology that can lead to sub optimal density of image

Answer 26

• pleural effusion
• osteoporosis
• pagets disease