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.
