Chapter 13 Flashcards
How do AEC detectors function?
Radiation is transmitted through the patient and converted into an electrical signal that terminates the exposure
A system that controls the duration of the exposure and the amount of radiation used
AEC
2 types of AEC systems
Phototimers
Ionization chambers
Phototimers used a fluorescent screen coupled with a:
Photomultiplier tube or a photodiode
What is the function of photomultiplier tubes and photodiodes?
Covert light into electric signal
Phototimer AECs are _____ devices
Exit-type
What is an exit-type device?
Radiation has to pass through the IR before it can be measured and used to terminate the exposure
Currently, most AEC devices use this type of detector:
Ionization chambers
What is an ionization chamber?
Hollow cell filled with air, connected to a timer circuit via electrical wire
Ion chamber devices are _______ devices:
Enterance-type
What are enterance-type devices?
Positioned in front of the IR
What do ion chambers measure?
DAP
How do ion chambers function?
Once the correct amount of DAP has been detected, an electric signal is sent from the chambers that is used to terminate the exposure
Why are ionization chambers the most common form of AEC?
Less sophisticated and accurate than phototimers but much less prone for failure
AEC controls only the _____ of radiation that reaches the IR
Quantity
Does AEC affect contrast?
No
Using higher kVp with AEC will result in a _______ exposure time
Shorter
Lower mA will result in a _____ exposure time
Longer
Minimum response time is the ________ exposure time a system can produce
Shortest
Minimum response time is usually _______ with AEC systems
Longer
Backup time is the ______ length of time an exposure can go when using AEC
Maximum
The backup timer should be set to terminate the exposure at ____%
150-200
Exposure adjustment is also known as
Density controls
What is the function of density controls?
Increase or decrease the preset exposure levels by a preset amount
What AEC cells should be selected?
That cells that will collect radiation that has passed through the patient first
Is detector size and placement in an AEC system fixed or unfixed?
Fixed
AEC is designed to compensate for changes in:
Patient thickness
Destructive pathologies can cause ____ of the area of interest
Underexposure
Additive pathologies can cause _____ of the area of interest
Overexposure
True/False - the size of the field can also affect AEC exposure
True
Wide open collimation is going to result in _____ scatter
More
More scatter allows the AEC cells to be irradiated quicker than intended, resulting in an _________ image
Underexposed
Collimation that is too tight may result in an ________ image
Overexposed
Exposures need to be able to reproduce using the same technical factors that provide mR reading within __% of each other, and with pixel brightness levels within ____% of each other
5%
30%
Exposure technique charts are based on:
Part thickness
Two primary types of exposure technique charts
Variable kVp/fixed mAs
Fixed kVp/Variable mAs
What are variable kVp charts based on?
kVp can be increased as part thickness is increased
How is kVp adjusted as part thickness increases
kVp is increased by 2 for every 1cm increase in part thickness
Why is variable kVp not ideal?
Because kVp changing kVp impacts contrast
How is mAs adjusted as part thickness increases
For every 4-5cm increase in part thickness, mAs is adjusted by a factor of 2
What types of casts require increased exposure factors?
Plaster
When do splints require an increase in technique
When the splint overlies the anatomy of interest
If the disease is an additive pathology, you may need to _________ factors
increase
If the disease is a destructive pathology, you may need to __________ factors
Decrease
Should mAs or kVp be adjusted for additive/destructive diseases?
kVp
Positive contrast media shows up ________ than the surrounding tissue
Lighter
Negative contrast media shows up ___________ than the surrounding tissue
Darker
What exposure factors should be adjust when using positive contrast?
kVp
Additive conditons in the abdomen
Aortic aneurysm
Ascities
Cirrhosis
Hypertrophy
Additive conditions in the chest
Atelectasis
Congestive heart failure
Malignancy
Pleural effusion
Pnemonia
Additive conditions in the skeleton
Hydrocephalus
Metastases (osteoblastic)
Osteochondroma
Paget disease
Additive conditions in nonspecific sites
Abscess
Edema
Sclerosis
Destructive conditions in the abdomen
Bowel obstruction
Free air
Destructive conditions in the chest
Emphysema
Pneumothorax
Destructive conditions in the skeleton
Gout
Metastases (osteolytic)
Multiple myeloma
Paget disease
Destructive conditions in nonspecific sites
Atrophy
Emaciation
Malnutrition
