APR And AEC Flashcards
Exposure timers
- 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)
Synchronous timers
- 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
Electronic timers
- 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)
mAs timer
- 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
AEC (automatic exposure control)
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)
Purpose of AEC
-eliminate the ‘guessing’ for technical factors
Types of AEC
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)
Ionization chambers
- 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
AEC selection
Different configurations
- usually have 3 detectors/cells/chambers
- centre
- left and right (slightly higher than centre cell, and off centred to each side)
Choosing the correct configuration
- 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
Back up timer
- 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
Density control
-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)
APR
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
Positioning with AEC
- 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
Collimation and AEC
-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
