Exposure Technique Selection Flashcards
Once a predetermined amount of radiation is transmitted through a patient, the x-ray exposure is terminated.
This determines the exposure time and therefore the total amount of radiation exposure to the IR
Important Relationship 📌
Principle of Automatic Exposure Control Operation
variable kVp/fixed mAs technique chart
Key term
Chart based on the concept that kVp can be increased as the anatomic part size increases
The kVp is increase by 2 for every 1 cm of part thickness
A baseline kVp is established and kVp is adjusted up or down for part thickness
Photo timer
Key term
Phototimers use a fluorescent (light-producing) screen and a device that converts light into electricity.
Consider “exit-type” devices because the detectors are positioned behind the IR
Light paddle serves as detector – radiation – visible light – photomultiplier (or photodiodes) – convert to electricity – timer tripped – exposure terminates when enough exposure received
Photomultiplier tube
Key term
…an electronic device that converts visible light energy into electrical energy.
A photodiode is a solid-state device that performs the same function.
Optimal kVp
Key term
…kVp value required to penetrate part without compromising radiographic contrast.
have not been standardized
minimum response time
Key term
…refers to the shortest exposure time that the system can produce
1 ms in modern AEC system
Can be a problem with patients who cannot cooperate (ie pediatric or geriatric)
mAs readout display
Key term
On the control panel – the actual amount of mAs used for that image is displayed immediately after the exposure, sometimes for only a few seconds
ionization chamber
Key term
…ion chamber
…a hollow cell that contains air & is connected to the timer circuit via an electrical wire
Entrance-type – detector is in front of the IR
Radiation – air ionized – electrical charge – charge travels via wire to timer circuit – timer tripped – exposure terminates
Less prone to failure
Most commonly used today
Less sophisticated & less accurate
Fixed kVp/Variable mAs Technique Charts
Key term
Uses the same kVp value for an anatomical part – the mAs is changed based on part thickness
Easier to use
Greater consistency in producing quality image
Uniform radiographic contrast
Increased accuracy with extreme variations in part thickness
Assurance that anatomical part has been penetrated appropriately
Extrapolated
Key term
extrapolated (mathematically estimated) for imaging other similar anatomic areas
Dictionary – To estimate (a value of a variable outside a known range) from values within a known range by assuming that the estimated value follows logically from the known values.
Exposure technique charts
Key term
…pre-established guidelines used by the radiographer to select standardized manual – OR – AEC exposure factors for each type of radiographic examination.
Lists other variables - SID – kVP – Central Ray – which detector to use – size (S, M, L or cm) – collimated field size
Detector
Key term
Other terms…sensors, chambers, cells…or detectors
With some equipment the AEC detector is different from the IR detector.
Density control
on the control panel that are numbered −2, −1, +1, and +2
A common increment is 25%, meaning that the predetermined exposure level needed to terminate the timer can be either increased or decreased from normal in one increment (+25% or −25%) or two increments (+50% or −50%)
Contrast medium
Key term
A positive or negative substance that can be injected or ingested
Barium - Z# 56 – positive medium used in the GI tract
Iodine - Z#53 – positive medium used in the GI tract or vascular system
Air - Z#8 (oxygen) – negative medium used in the lungs or GI tract
Used to INCREASE contrast in areas of low subject contrast
Comparative Anatomy
Key term
…helps the radiographer (you) in determining minimal kVp values.
This concept states that different parts of the same size can be radiographed by use of the same exposure factors, provided that the minimal kVp value needed to penetrate the part is used in each case.
Calipers
Key term
…device that measure part thickness
Measures the patient thickness in either cm or inches
…should be readily accessible in every radiographic room
Back up time
Key term
… maximum length of time for which the x-ray exposure will continue when using an AEC system
Safety mechanism in case the AEC fails
Should be set to terminate at – 600 mAs
Automatic exposure control
Key term
…AEC - system used to consistently control the amount of radiation reaching the IR by terminating the length of exposure
Incorrectly referred to as “phototimer”
Anatomically programmed technique
Key term
…a system that allows the radiographer to select a particular button on the control panel that represents an anatomic area; a preprogrammed set of exposure factors is displayed and selected for use
APT or APR (anatomically programmed radiography)
Example: When performing an AP ankle image – choose the AP ankle technique
Not the same thing as AEC
Kilovoltage Selection
Radiation Protection Alert!!
Using a higher kVp with AEC decreases the exposure time and the overall mAs needed to produce a diagnostic image, significantly reducing patient exposure.
The kVp selected for an examination should produce the desired image contrast for the part examined while being as high as possible to minimize the patient’s radiation exposure.
Monitoring Backup Time
Radiation Protection Alert!!
To minimize patient exposure, the backup time should be neither too long nor too short.
Backup time that is too short results in the exposure being stopped prematurely, and the image may need to be repeated because of poor image quality.
Backup time that is too long results in the patient receiving unnecessary radiation if a problem occurs and the exposure does not end until the backup time is reached.
In addition, the image may have to be repeated because of poor image quality.
Patient Variability
Radiation Protection Alert!!
Factors related to the patient affect the time of exposure reaching the IR and ultimately the image quality
Example: pathology, contrast media, foreign objects, and pockets of gas. Increases or decreases in patient thickness result in changes in the time of exposure if the AEC system is functioning properly.
Anatomically Programmed Technique and Patient Exposure
Radiation Protection Alert!!
When using a preprogrammed set of exposure factors, the radiographer must evaluate the appropriateness of the selected exposure technique factors.
Adjustment of the preprogrammed exposure factors may be necessary for that patient or procedure.
Exposure Technique Charts and Digital Imaging
Radiation Protection Alert!!
Exposure technique charts are just as important, if not more so, when using digital IRs.
Underexposure or overexposure of a film-screen IR can result in a radiograph with decreased or increased density.
Because image brightness is controlled by computer processing, the visual cues for overexposure or underexposure are missing.
Exposure technique charts are an effective tool in selecting appropriate exposure techniques for a quality digital image.
Radiation-Measuring Devices
Important Relationship 📌
Radiation-Measuring Devices
Detectors are the AEC devices that measure the amount of radiation transmitted. The radiographer selects the combination of the three detectors to use.
Function of the Ionization Chamber
Important Relationship 📌
Function of the Ionization Chamber
The ionization chamber interacts with exit radiation before it reaches the IR. Air in the chamber is ionized, and an electrical charge proportional to the amount of radiation is created.
Automatic Exposure Control and mAs Readout
Important Relationship 📌
If the radiographic unit has an mAs readout display, the radiographer should take note of the reading after an exposure is made. This information can be invaluable.
kVp and Automatic Exposure Control Response
Important Relationship 📌
The radiographer must set the kVp as needed to ensure adequate penetration while producing the appropriate level of contrast. The kVp selected determines the length of exposure time when using AEC. A low kVp requires more exposure time to reach the predetermined amount of exposure. A high kVp decreases the exposure time to reach the predetermined amount of exposure and reduces the overall radiation exposure to the patient.
mA and Automatic Exposure Control Response
Important Relationship 📌
If the radiographer can set the mA when using AEC, it will affect the time of exposure for a given procedure. Increasing the mA decreases the exposure time to reach the predetermined amount of exposure.
Decreasing the mA increases the exposure time to reach the predetermined amount of exposure.
Setting Backup Time
Important Relationship 📌
Backup time should be set to 150%–200% of the expected exposure time.
This allows the properly used AEC system to appropriately terminate the exposure but protects the patient and tube from excessive exposure if a problem occurs.
Detector Selection
Important Relationship 📌
The combination of detectors affects the amount of exposure reaching the IR. If the area of radiographic interest is not directly over the selected detectors, that area will likely be overexposed or underexposed.
When performing a radiographic study where the IR is located outside the Bucky, the AEC system should be deactivated and a manual technique should be used.
Patient Centering
Important Relationship 📌
“The art of using AEC’s is the art of positioning” ~ Carlton
Accurate centering of the area of interest over the detectors is critical to ensure proper exposure to the IR.
If the area of interest is not properly centered to the detectors, overexposure or underexposure may occur.
Collimation & Automatic Exposure Control Response
Important Relationship 📌
Excessive or insufficient collimation may affect the amount of exposure reaching the IR.
Insufficient collimation may result in excessive scatter reaching the detectors, causing the exposure time to terminate too quickly.
Excessive collimation may result in an extremely long exposure time.
Type of Image Receptor and Automatic Exposure Control Response
Important Relationship 📌
The AEC system is calibrated based on the type of IR used.
If an IR of a different type is used, the detectors will not sense the difference and the exposure time will terminate at the preset value, which may jeopardize image quality.
Digital Image Receptors and the Automatic Exposure Control Response
Important Relationship 📌
Because the visual cues of increased or decreased radiographic density when using film-screen IRs are lacking in digital imaging, the radiographer must be very conscientious about excessive radiation exposure to the patient.
Anatomically Programmed Technique and Patient Exposure
Important Relationship 📌
When using a preprogrammed set of exposure factors, the radiographer must evaluate the appropriateness of the selected exposure technique factors. Adjustment of the preprogrammed exposure factors may be necessary for that patient or procedure.
How would an AEC ion chamber respond….if the SID is increased?
The exposure time would increase
How would an AEC ion chamber respond….if the OID is increased?
Exposure time would increase
You have the two outer chambers of the AEC activated to do a CXR on a patient with a patient experiencing SOB. How would the system respond if you changed the mA from 300 to 600?
The exposure time would DECREASE
What AEC chambers would be activated to do a Left lateral decubitus abdomen on a hypersthenic patient.
center cell
If you are using AEC, what would be the effect on exposure time if the patient size is increased from 25 cm to 33 cm?
exposure time would increase
The radiologist has requested a repeat image on an abdomen due to excessive scatter and poor collimation. You decide to reduce the kVp from 88 to 78. How would the AEC system respond?
The exposure time would increase
Your patient has a history of primary diagnosis of colon cancer and secondary diagnosis of ascites. Would using the AEC assisted in this situation? Explain why or why not?
Ascites is an additive pathological condition, using an AEC is helpful because the exposure time may increase due to the fluid in the peritoneal cavity.
Your patient is malnourished and is anorexic. Would using an AEC assist in this situation? Explain why or why not?
The body habitus of an anorexic patient would be decreased and may require a reduction in exposure time. The AEC would help in determining exposure time.
A request comes through from the OR for mobile knee images after arthroscopy to be done in PACU. Would an AEC assist in this situation? Explain why or why not?
Not - most mobile imaging systems are not equipped with an AEC. Thus manual factors will need to be set.
A request come through for an x-ray of a Right hip S/P hip replacement. Would an AEC assist in this situation? Explain why or why not?
Not - the hip replacement is a metal medical device that can not be penetrated by x-rays - therefore, AEC would not help in this situation - need to set a manual technique.
When different combinations of chambers or cells are activated…what device terminates the exposure?
operational amplifier - signal from cells are sent to the operational amplifier which averages the voltages received from each cell and terminates the exposure when threshold is reached
