Chapter 13 Exam Flashcards
Automatic exposure control (AEC) devices work by measuring _____.
a. Attenuation of primary radiation by the patient
b. Radiation that exits the patient
c. Radiation that is absorbed by the patient
d. Radiation exiting the tube
b. Radiation that exits the patient
Which of the following exposure system operates by ionizing air that creates an electrical charge?
a. Anatomically programed technique
b. Phototimer
c. Ionization chamber
d. Non of the above
c. Ionization chamber
During operation of the AEC device, the time of exposure:
a. Is inversely related to the intensity of the exit radiation
b. Is directly related to the intensity of the exit radiation
c. Has an inverse squared relationship to the exit radiation
d. Has no relationship to the intensity of the exit radiation
b. Is directly related to the intensity of the exit radiation
What factors are important when using automatic exposure control (AEC) devices?
1. Detector selection
2. Centering part to detector
3. Back-up mAs, if set
a. 1 and 2 only
b. 1 and 3 only
c. 2 and 3 only
d. 1, 2, and 3
d. 1, 2, and 3
Increasing patient thickness while using an AEC device would result in:
a. Increased exposure time
b. Decreased brightness
c. decreased mAs readout
d. Increased image contrast
a. Increased exposure time
Which detector or combination is best for a right hip?
a. Center
b. Left
c. Right
d. Right and left
a. Center
Which is the primary goal of exposure technique charts?
a. Extend life of x-ray tube
b. Improve radiographer’s accuracy
c. Consistency in image quality
d. Increase patient work flow
c. Consistency in image quality
What type of exposure technique system uses a fixed mAs regardless of patient thickness?
a. Fixed kVp
b. Variable kVp
c. Preprogrammed
d. AEC
b. Variable kVp
Of the following, which is most important when using a technique chart?
a. Same radiographer revises chart
b. Same chart used for all x-ray tubes
c. Accurately measured patient
d. Include a patient history
c. Accurately measured patient
An advantage of the variable kVp technique chart is that it:
a. Produces lower contrast images
b. Reduces patient exposure
c. Makes a 2 kVp change sufficient with any kVp
d. Makes smaller technique changes possible
d. Makes smaller technique changes possible
Which of the following special considerations may require a decrease in mAs?
a. Bariatric patient
b. Soft tissue examination
c. Pediatric patient
d. b and c only
d. b and c only
Which of the following special considerations may require an increase in mAs?
a. Contrast media
b. Bariatric patient
c. Tissue edema
d. All of the above
d. All of the above
a system used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure
automatic exposure control (AEC)
Why do we have an AEC?
to control the amount of radiation reaching the IR
How does the AEC work?
Once a predetermined amount of radiation is transmitted through a patient, the x-ray exposure is terminated. This determines the exposure the exposure time and therefore the total amount of radiation exposure to the image receptor
What is the AEC system good at?
producing consistent levels of exposure when used properly
What are the two types of AEC?
- Phototimers: first generation of AEC systems
- Ionization chambers: more common today
Use a fluorescent screen and a device that converts light to electricity
phototimer
an electronic device that converts visible light energy into electrical energy
Photomultiplier (PM) tube
How many AEC detectors are the norm?
Most AEC systems use a set of three radiation-measuring detectors
What is an Ionization Chamber?
A hollow cell that contains air and is connected to the timer circuit via an electrical wire
- When exposed to radiation, the air in the chambers becomes ionized, creating an electrical charge
- The charge travels along the wire to the timer circuit, which trips the timer
Entrance-type devices AEC
What happens when x-ray photons interact with the ionization chamber?
the air inside the chamber becomes ionized, creating an electrical charge. This charge travels along the wire to the timer circuit. The timer is tripped and the radiographic exposure is terminated when a sufficiently large charge has been received.
- Refers to the shortest exposure time that the system can produce
- This can be a problem with some segments of the patient population, such as pediatric patients and uncooperative patients
Minimum response (exposure) time
what is the minimum response time with modern AEC systems?
1 ms (usually longer with AEC systems than with other types of radiographic timers
- Refers to the maximum length of time the x-ray exposure continues when using an AEC system
- May be set by the radiographer or may be controlled automatically by the radiographic unit
- Acts as a safety mechanism when an AEC system fails or the equipment is not used properly
back-up timer
How do you know the exposure amount when using AEC?
mAs readout
what is mAs readout?
the actual amount of mAs used for that image displayed immediately after the exposure
Allow the radiographer to adjust the amount of preset radiation detection values
density controls (exposure adjustment)
what numbers are the buttons on the control panel for density controls?
-2, -1, +1, +2
how do the numbers (-2, -1, +1, +2) on the density control panel change the exposure time?
they change the exposure time by increments of one increment (-25% or +25%) or two increments (-50% or +50%)
The kVp selected determines:
the length of exposure time when using AEC
The radiographer must be sure to set the kVp value as needed to ensure:
adequate penetration and to produce the appropriate subject contrast
Using a low kVp with AEC requires:
more exposure time to reach the predetermined amount of exposure
Using a higher kVp with AEC:
decreases the exposure time and overall mAs needed, significantly reducing the patient’s exposure
How do you know which detector to use?
Select the detectors that will be superimposed by the anatomic structures of greatest interest
The additional scatter radiation produced by failure to accurately restrict the beam may cause the detector to:
terminate the exposure prematurely
If the x-ray field size is collimated too closely, the detector:
does not receive sufficient exposure initially and may prolong the exposure time, which could result in overexposure
When Anatomically Programmed Technique (APR) is used with AEC, the system:
selects and displays manual exposure factors, selects and displays the AEC detectors to be used for a specific radiographic examination
Reproducibility of exposures for a given set of exposure factors and selected detector should result in milliroentgen (mR) readings within ___ , and pixel brightness levels for areas within the displayed image should be within ____
5%, 30%
During computer processing, image brightness can be adjusted after __________; however , there may be an increase in the visibility of ______________
underexposure, quantum noise
- Pre-established guidelines used to select standardized manual or AEC exposure factors for each type of radiographic examination
- Instrumental in the production of consistent quality radiographs, reduction in repeat radiographic studies, and reduction in patient exposure
Exposure Technique Charts
Exposure technique charts are important for digital imaging due to:
the IR’s wide dynamic range
What are the two types of technique charts?
Variable kVp - fixed mAs
Fixed kVp - variable mAs
- Based on the concept that kVp can be increased as the anatomic part size increases
- 2 kVp for every 1 cm of thickness
Variable kVp - fixed mAs
- Uses the concept of selecting an optimal kVp that is required for the radiographic examination and adjusting the mAs for variation in part thickness
- 4-5 cm thickness = increase mAs x 2
Fixed kVp - variable mAs
How many AEC detectors can be used at one time?
You can use all 3, just 2, or only 1. it depends on the anatomic area you are imaging
Can you collimate when using AEC?
Yes but the x-ray field must be well-collimated to the anatomy of interest to avoid the exposure terminating prematurely or prolonging the exposure time causing overexposure
