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 increasesThe 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
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.
...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 produce1 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 wireEntrance-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 failureMost 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 useGreater consistency in producing quality imageUniform radiographic contrastIncreased accuracy with extreme variations in part thickness Assurance that anatomical part has been penetrated appropriately
extrapolated (mathematically estimated) for imaging other similar anatomic areasDictionary -- 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
Other terms...sensors, chambers, cells...or detectorsWith some equipment the AEC detector is different from the IR detector.
on the control panel that are numbered −2, −1, +1, and +2A 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 ingestedBarium - Z# 56 -- positive medium used in the GI tractIodine - Z#53 -- positive medium used in the GI tract or vascular systemAir - Z#8 (oxygen) -- negative medium used in the lungs or GI tractUsed to INCREASE contrast in areas of low subject contrast
Comparative Anatomy (Key term)
...helps the radiographer (you) in determining minimal kVp values.
...device that measure part thickness Measures the patient thickness in either cm or inches ...should be readily accessible in every radiographic room
... maximum length of time for which the x-ray exposure will continue when using an AEC systemSafety 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 exposureIncorrectly 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 useAPT 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
Function of the Ionization Chamber Important Relationship 📌
Function of the Ionization Chamber
Automatic Exposure Control and mAs Readout Important Relationship 📌
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 CenteringImportant 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 📌
Anatomically Programmed Technique and Patient Exposure Important Relationship 📌
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.
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 |
When using an AEC system what will happen once the predetermined amount of radiation is transmitted?
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