APR And AEC

Question 1

Exposure timers

Answer 1

• timer circuit is a critical component of the x-ray circuit
• purpose: ‘make or break’ the high voltage across the tube
• located on the primary side (lower voltage is easier to control than high voltage)
• four types of timing circuits: synchronous, electronic, mAs, AEC (only one that is automatic, not controlled by technologist)

Question 2

Synchronous timers

Answer 2

• exposure time selected will determine the time it takes for the disk to move from the on switch to the off switch
• cannot be used for serial exposures, needs to be reset in between

Question 3

Electronic timers

Answer 3

• most sophisticated, accurate and expensive
• mainly what is used today
• complex circuitry based on time required to charge a capacitor through a variable resistance
• once the capacitor receives its pre programmed charge, it terminates the exposure
• wide range of time intervals can be selected
• can be used for rapid serial exposures (ideal for interventional radiography)

Question 4

mAs timer

Answer 4

• variation of the electronic timer, but it monitors the current passing through the tube and will terminate the exposure when the set mAs is achieved
• designed to safely provide the highest tube current and shortest exposure for mAs selected
• located on the secondary circuit (high voltage) side because must monitor the actual tube current

Question 5

AEC (automatic exposure control)

Answer 5

Serves the same role as the timer (only factor it controls is time)
-operates differently as it uses the patients body part as the variable to determine when the exposure should be terminated
-measure the quantity of radiation reaching the IR, and will terminate the exposure when the required radiation quantity has been received
(Breaks the timer circuit)

Question 6

Purpose of AEC

Answer 6

-eliminate the ‘guessing’ for technical factors

Question 7

Types of AEC

Answer 7

Two types of AEC:
-PHOTODIODE/ PHOTOTIMER
 🔹positoned as exit type of device
 🔹behind IR
 🔹photodiode converts light to electrical energy
 🔹once certain charge is reached, exposure is terminated
 🔹becoming obsolete
-IONIZATION CHAMBER (we use this one)

Question 8

Ionization chambers

Answer 8

• most commonly used today
• located between patient and the IR
• flat parallel plate ionization chamber (radiolucent)
• entrance type: IR positioned after the ion chamber
• x-ray beam travels through the patient and tabletop before striking the chamber
• chamber is a hollow cell containing air
• air inside the cell becomes ionized as it is struck by radiation: creates an electrical charge
• ionization is in direct proportion to the amount of radiation
• when pre determined charge has been reached, exposure will terminate

Question 9

AEC selection

Answer 9

Different configurations

• usually have 3 detectors/cells/chambers
• centre
• left and right (slightly higher than centre cell, and off centred to each side)

Question 10

Choosing the correct configuration

Answer 10

• various combinations of the AEC chambers can be selected (can select any one cell, combination or all 3)
• APR can also select configuration
• when more than one cell is selected, the average signal from all the cells is used
• this can be over-ridden if needed

Question 11

Back up timer

Answer 11

• protects the patient from overexposure (due to equipment failure, or tech error)
• usually set to 1.5 times the expected length of exposure (max time is 6 seconds)
• often it is automatically done when APR is used

Question 12

Density control

Answer 12

-regulates the radiographic density of the image
-selection of 7 density steps
(wants less exposure) -3, -2, -1, 0, 1, 2, 3, (wants more exposure)
-each one is calibrated to increase/decrease the present OD of the IR by 0.1 (about a 25% change in exposure from step to step)
-can be used to accommodate patient condition or to over-ride AEC sensitivity issues
(Not used to compensate for part thickness)

Question 13

APR

Answer 13

Anatomically programmed radiography

• refers to the radiographic system that allows user to select a body part, and a preprogrammed set of exposure factors will be displayed for it (ex kVp, AEC cell selection, mAs if AEC is not selected, focal spot size)
• controlled by computer circuit/chip that has been programmed with different exposure factors for specific position/projections of an atomic parts
• APR and AEC are not related, but often used in conjunction ex an APR setting may utilize AEC

Question 14

Positioning with AEC

Answer 14

• technical factors are taken care of with AEC
• positioning is critical to utilizing AEC
• specific body part must be placed over the cell for appropriate exposure (if not, can result in over or under exposure)
• improper centering is most common reason for repeats of AEC

Question 15

Collimation and AEC

Answer 15

-AEC cannot tell the difference between useful beam and scatter radiation
-if the field size is too BIG, too much scatter will cause the AEC to shut off prematurely
-if the field size is too SMALL, AEC cell will take longer to receive enough radiation to end exposure (overexposure)
-if the field size is too small to cover the entire cell
🔹AEC should not be used and a manual technique should be set

Question 16

Cell selection

Answer 16

• crucial to the exposure

- Improper cell selection can also result in over or under exposure

Question 17

Optimal use of AEC

Answer 17

-appropriate kVp selected for the area of interest
🔹based on contrast characteristics and penetrability of beam
-precise centering
-approproate selection of cell
-appropriate collimation to area of interest