Exposure Technique Selection

Question 1

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 📌

Answer 1

Principle of Automatic Exposure Control Operation

Question 2

variable kVp/fixed mAs technique chart

Key term

Answer 2

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

Question 3

Photo timer

Key term

Answer 3

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

Question 4

Photomultiplier tube

Key term

Answer 4

…an electronic device that converts visible light energy into electrical energy.

A photodiode is a solid-state device that performs the same function.

Question 5

Optimal kVp

Key term

Answer 5

…kVp value required to penetrate part without compromising radiographic contrast.

have not been standardized

Question 6

minimum response time

Key term

Answer 6

…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)

Question 7

mAs readout display

Key term

Answer 7

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

Question 8

ionization chamber

Key term

Answer 8

…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

Question 9

Fixed kVp/Variable mAs Technique Charts

Key term

Answer 9

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

Question 10

Extrapolated

Key term

Answer 10

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.

Question 11

Exposure technique charts

Key term

Answer 11

…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

Question 12

Detector

Key term

Answer 12

Other terms…sensors, chambers, cells…or detectors

With some equipment the AEC detector is different from the IR detector.

Question 13

Density control

Answer 13

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%)

Question 14

Contrast medium

Key term

Answer 14

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

Question 15

Comparative Anatomy

Key term

Answer 15

…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.

Question 16

Calipers

Key term

Answer 16

…device that measure part thickness

Measures the patient thickness in either cm or inches

…should be readily accessible in every radiographic room

Question 17

Back up time

Key term

Answer 17

… 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

Question 18

Automatic exposure control

Key term

Answer 18

…AEC - system used to consistently control the amount of radiation reaching the IR by terminating the length of exposure

Incorrectly referred to as “phototimer”

Question 19

Anatomically programmed technique

Key term

Answer 19

…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

Question 20

Kilovoltage Selection

Radiation Protection Alert!!

Answer 20

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.

Question 21

Monitoring Backup Time

Radiation Protection Alert!!

Answer 21

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.

Question 22

Patient Variability

Radiation Protection Alert!!

Answer 22

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.

Question 23

Anatomically Programmed Technique and Patient Exposure

Radiation Protection Alert!!

Answer 23

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.

Question 24

Exposure Technique Charts and Digital Imaging

Radiation Protection Alert!!

Answer 24

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.

Question 25

Radiation-Measuring Devices

Important Relationship 📌

Answer 25

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.

Question 26

Function of the Ionization Chamber

Important Relationship 📌

Answer 26

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.

Question 27

Automatic Exposure Control and mAs Readout

Important Relationship 📌

Answer 27

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.

Question 28

kVp and Automatic Exposure Control Response

Important Relationship 📌

Answer 28

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.

Question 29

mA and Automatic Exposure Control Response

Important Relationship 📌

Answer 29

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.

Question 30

Setting Backup Time

Important Relationship 📌

Answer 30

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.

Question 31

Detector Selection

Important Relationship 📌

Answer 31

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.

Question 32

Patient Centering

Important Relationship 📌

“The art of using AEC’s is the art of positioning” ~ Carlton

Answer 32

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.

Question 33

Collimation & Automatic Exposure Control Response

Important Relationship 📌

Answer 33

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.

Question 34

Type of Image Receptor and Automatic Exposure Control Response

Important Relationship 📌

Answer 34

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.

Question 35

Digital Image Receptors and the Automatic Exposure Control Response

Important Relationship 📌

Answer 35

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.

Question 36

Anatomically Programmed Technique and Patient Exposure

Important Relationship 📌

Answer 36

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.

Question 37

How would an AEC ion chamber respond….if the SID is increased?

Answer 37

The exposure time would increase

Question 38

How would an AEC ion chamber respond….if the OID is increased?

Answer 38

Exposure time would increase

Question 39

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?

Answer 39

The exposure time would DECREASE

Question 40

What AEC chambers would be activated to do a Left lateral decubitus abdomen on a hypersthenic patient.

Answer 40

center cell

Question 41

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?

Answer 41

exposure time would increase

Question 42

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?

Answer 42

The exposure time would increase

Question 43

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?

Answer 43

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.

Question 44

Your patient is malnourished and is anorexic. Would using an AEC assist in this situation? Explain why or why not?

Answer 44

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.

Question 45

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?

Answer 45

Not - most mobile imaging systems are not equipped with an AEC. Thus manual factors will need to be set.

Question 46

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?

Answer 46

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.

Question 47

When different combinations of chambers or cells are activated…what device terminates the exposure?

Answer 47

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