AED's and post processing Flashcards
Purpose of AED (2 points)
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
What does a phototimer AED consist of? 3 things
1) fluorescent screen
2) photomultiplier
3) electric circuit connected to a circuit
What does the most commonly used AED consist of? 3 things
1) ionisation chamber
2) capacitor
3) exposure terminating switch
Where are ionisation chambers situated
between the grid and the cassette (imaging receptor)
How do we stop the ionisation chamber from appearing on the image?
KvP is sufficient to over penetrate the ionisation chambers-preventing artefacts
how are the ionisation chambers connected?
Interconnected by small electrical wires. exposure termination can be accomplished either through ionisation of one chamber or a combination of two or three
What happens in the ionisation chambers? 4 steps
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
what is a capacitor
a device used to store a specific quantity of electricity
what does the capacitor consist of? 3 things
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
what is the insulator made out of in the capacitor
Bakelite
what 3 things effect capacitance
1) area of the plates
2) distance between plates
3) dielectric material
how does the area of the plates effect capacitance
larger plates provide a greater capacity to store electrical charge
how does the distance between plates effect capacitance
it is directly proportional to the electrostatic forces between plates. Field is stronger when they are closer together- decreases with distance
how does the dielectric material effect capacitance
Bakelite can increase capacitance more than air
Factors that effect AEDs- 7 points
1) patient positioning
2) detector selection
3) density setting
4) collimation
5) minimum exposure time
6) back up timer
7) mAs, kVp, SID
importance of patient position when using AED
incorrect patient positioning can lead to over/under exposure of the patient
importance of detector selection when using AED (3 things)
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
why is collimation important when using AEDs
- 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
how does the density setting effect AED use
-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
how does minimum response time (MRT) affect AEDs
- 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
what is the purpose of the back up timer for AEDs- 5 points
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
What is under exposure
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
what is over exposure
computer attempts to adjust density but this decreases contrast and excessive patient exposure will have occurred
Advantages of AEDs- 3 points
1) maintains consistency and quality of images
2) reduced the number of repeats and therefore effectively reduces patient dose
3) increases efficiency
Disadvantages of AEDs- 2 points
1) detector cannot differentiate between transmitted and scattered radiation
2) If unknown pathology present it can lead to images of sub optimal density
Gives examples of types of pathology that can lead to sub optimal density of image
- pleural effusion
- osteoporosis
- pagets disease
