Automatic exposure control

Question 1

what does AEC stand for

Answer 1

automatic exposure control

Question 2

when do we need AEC

Answer 2

body parts with wide variation in patient size and thickness and or uncertainty in correct exposure

Question 3

how is the optimal exposure done for AEC in x-ray room

Answer 3

1. in Bucky
   - x-ray detecting ionisation chamber behind patient
   - can be placed between patient and image receptor or behind receptor (either way being grid)

Question 4

where is the AEC located

Answer 4

in the wall stand, 3 chambers

under the table, 3 chambers

Question 5

how does an AEC work

Answer 5

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

Question 6

what happens inside the ionisation chamber

Answer 6

• 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

Question 7

what is a backup timer

Answer 7

timer that will terminate the exposure

Question 8

why can you not have a backup timer preset on system

Answer 8

the backup time / mAs must fit according to the patient being imaged (can’t be higher than needed)

Question 9

what is thyratron

Answer 9

dial on control panel of AEC that allows you to adjust the pre-set dose of AEC ( -3 to +3 )

Question 10

what might happen if you place a chamber in-between the patient and image receptor

Answer 10

it can be visible in the image only when theres low dose high contrast, high resolution imagin

Question 11

Define sensitivity

Answer 11

ability to detect X-ray photons and turn them into a diagnostic image

Question 12

what is the sensitivity of a system known as

Answer 12

speed

Question 13

what does DQE stand for

Answer 13

detective quantum efficiency

Question 14

what does MTF stand for

Answer 14

modulation transfer function

Question 15

what 2 inherent factors limit speed

Answer 15

DQE, MTF

Question 16

what speed do CR and DR imaging operate at

Answer 16

CR= 400 speed

DR= 800 speed

Question 17

what does EI stand for and what is ti

Answer 17

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)

Question 18

in principle, what does the speed or exposure indicator relate to/mean?

Answer 18

amount of radiation required to produce the optimal ‘image’ in mGy

Question 19

what chambers are chosen for the chest

Answer 19

outer 2 or sometimes all 3

Question 20

depending on high or low density pathology, think to choose between chambers, chamber settings or going manual