Ch 13 Flashcards
System used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure
AEC
what determines the exposure time
AEC
When does AEC terminate
when’s predetermined amount of radiation has been detected
two types of AEC
pho timers and ionization chambers
refers to the use of an AEC device that uses photomultiplier tubes or photodiodes , and aren’t common today
photo timers
hollow cell that contains air and is connected to the timer circuit via electrical wires.
ionization chamber
why are ionization chamber aec devices considered to be entrance type devices ?
because they are positioned in front of the image receptor
AEC devices that measure the amount of radiation transmitted
detectors/ chambers/ cells
what used a fluorescent screen
photimers
what used a fluorescent screen and a device that converted the light to electricity
phototimers
electronic device that converts visible light energy into electrical energy
photomultiplier tube
where are phototimer aec devices placed
behind the image receptor
how do photimers work?
radiation reaches fluorescent light paddles producing visible light and PM tubes convert the light into electricity which trips the timer and exposure is terminated
how do ionization chamber aec work
radiation interacts either detectors and the air inside becomes ionized creating an electrical charge. the charge travels along the timer circuit and the timer is tripped, terminating exposure
which is less accurate
photo timers or ionization chambers
ionization chambers
what does AEC control
quantity of radiation reaching the image receptor
refers to the shortest exposure time that a system can produce
minimum response time
what is the minimum response time for modern AEC systems
1 ms
refers to the maximum amount of time the x ray exposure continues when using an AEC system
backup time
acts as a safety mechanism when AEC fails
back up timer
back up timer should be set at what percentage of the expected exposure time
150%-200%
what are the density control button numbers?
-2, -1, 1, 2, and 3
why shouldn’t toddlers be used for AEC
because they’re probably too small to even cover the detectors
Refers to a radiographic system that allows the radiographer to select a particular button on the control panel that represents an anatomic area for which a preprogram set of exposure factors are displayed and can be selected
Anatomically program technique
what’s an example of ANATOMIC programming
the difference in technique for an abdomen compared to a chest
why is AEC calibration important
to ensure consistent and appropriate exposures to the receptor are being produced
reproducibility for changes in exposure
5%
reproducibility for pixel brightness levels
30%
preestablish guidelines used by the radiographer to set standardized, manual, or AEC exposure factors for each type of radiographic examination
exposure technique chart
Devices that measure part thickness
Calipers
Based on the concept that KVP can be increased as the anatomic part size increases
variable kVp fixed mAs
baseline kVp for variable kVp fixed mAs is increased by ___for every 1 cm increase in part thickness
2
VARIABLE KVP FIXED MÁS
increase from 10 CM to 18 at 63 kVp
what would new kVp be
79
63+2 eight times
VARIABLE MAS FIXED KVP
for every _________ cm change in thickness the mas should be adjusted by a factor of 2
4-5 cm
is fixed kvp or variable kvp better
fixed kvp
what happens when you do not collimate when using AEC
more scatter reaches AEC termination exposure early
pediatric patient consideration
may not be able to cover detectors so manual technique with a shorter exposure time should be used
geriatric patient considerations
may need decreased technique and short exposure time
bariatric considerations
may need increase in technique, use of grid to control contrast
does fiberglass require a change in technique
NO
do plaster casts require an increase in technique
YES
Diseases or conditions that decrease the absorption characteristics of the part, making that part less difficult to penetrate
Destructive diseases
Diseases or conditions that increase the absorption characteristics of the part, making that part more difficult to penetrate
Additive diseases
if the patient has an additive disease, what do we do with the KVP?
increase
Examples of positive contrast agents
Barium and iodine
Examples of negative contrast agent
air
Do positive contrast agents produce more or less brightness
More
