Automatic exposure control Flashcards
what does AEC stand for
automatic exposure control
when do we need AEC
body parts with wide variation in patient size and thickness and or uncertainty in correct exposure
how is the optimal exposure done for AEC in x-ray room
- in Bucky
- x-ray detecting ionisation chamber behind patient
- can be placed between patient and image receptor or behind receptor (either way being grid)
where is the AEC located
in the wall stand, 3 chambers
under the table, 3 chambers
how does an AEC work
in the AEC chamber, each is a switch which turns off the x-ray beam when its received enough radiation
BUT
radiographer must select;
1. chambers used
2. kV
3. appropriate backup time in case the photo-timer fails
what happens inside the ionisation chamber
- xray beam passes thru patient, table and grid until it reaches the chamber(s)
- radiation ionises the air within, creates electrons collected by an electrode
- when enough electrons are collected, triggers the main exposure circuit
- xray exposure terminated
what is a backup timer
timer that will terminate the exposure
why can you not have a backup timer preset on system
the backup time / mAs must fit according to the patient being imaged (can’t be higher than needed)
what is thyratron
dial on control panel of AEC that allows you to adjust the pre-set dose of AEC ( -3 to +3 )
what might happen if you place a chamber in-between the patient and image receptor
it can be visible in the image only when theres low dose high contrast, high resolution imagin
Define sensitivity
ability to detect X-ray photons and turn them into a diagnostic image
what is the sensitivity of a system known as
speed
what does DQE stand for
detective quantum efficiency
what does MTF stand for
modulation transfer function
what 2 inherent factors limit speed
DQE, MTF
what speed do CR and DR imaging operate at
CR= 400 speed
DR= 800 speed
what does EI stand for and what is ti
exposure index
numbers that indicate the relative speed of system as whole OR exposure that you gave compared to what was needed (tells you if u over or under exposed)(+ no = over, - no = under)
in principle, what does the speed or exposure indicator relate to/mean?
amount of radiation required to produce the optimal ‘image’ in mGy
what chambers are chosen for the chest
outer 2 or sometimes all 3
depending on high or low density pathology, think to choose between chambers, chamber settings or going manual
