imaging test 3 Exposure technique selection

types of technique selection

• Manual 

• Automatic exposure control

• Automatically programmed techniques 

• Technique chart

Automatic exposure control

• Ensure quality radiographic image is produced

• Control consistently the amount of radiation reaching the IR by terminating the length of exposure

  • Predetermined amount of radiation reaches detector

  • Exposure terminates 

•  measure the amount of x-rays exiting the patient with some type of detector; and

• end the x-ray exposure as soon as a pre-set amount of radiation has been measured;

•  produces a radiograph with a pre-set amount of exposure in the area of the image where the detector was located

Manual technique selection technologist determines

• Kvp 

• mA

• Time

AEC technologist determines

• Kvp

• mA (sometimes)

• Detector selection

Two types of ARC devices

• Phototimers

• Ionization chamber system

Phototimers

• Light paddles, coated with a fluorescent material act as the detector

• Radiation interacts with the paddles, producing visible light

• This light is transmitted to photomultiplier tubes or photodiodes that convert this light into electricity

• The electric signal trips the timer

• The radiographic exposure is terminated when a sufficiently large charge has been received 

• Aka: light paddles, together with photomultiplier tubes, measure radiation exposure AFTER it passes through the image receptor

Ionization chamber system

• A hollow cell that contains air and is connected to the timer circuit via an electrical wire

• Ionization chamber is exposed to exit radiation

• The air inside the chamber becomes ionized, creating an electrical charge 

• 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 

• AKA: ionization chamber measures radiation exposure before it reaches the image receptor

AEC system features

• mAs readout

• Minimum response time

• Backup time 

• Density controls

mAs readout

• Actual mAs used for the exam will be displayed following the exposure 

  • Provide critical information needed before attempting any repeat exposures or using a manual technique for other projections

Minimum response time

• Shortest exposure time a system can produce 

• You can’t set this

Backup time

• mAs 

• Maximum allowed time that x-ray can stay on before exposure is terminated

  • Law requires max of 600 mAs when kVp is >50

  • May be able to set this depending on the department and machines

Density adjustment

• Density settings (example -1,0,+1)

• Adjusts the exposure time by some predetermined amount of increment expressed as a percentage

Alignment and positioning considerations

• Select the detector that will be superimposed by the anatomic structures area of interest

• The anatomic area of interest must be centered properly over the selected detectors 

• Patient anatomic area of interest must adequate cover the detector combination

• Improper collimation may affect termination of the exposure time

Collimation

• Too little collimation can cause excessive scatter to terminate exposure too quickly 

• Too much collimation can cause the detector to extend the length of exposure

Patient considerations

The composition of anatomic area centered over the detector will influence the exposure time

Increasing mA

Exposure time decreased

Decreasing kvp

Exposure time increased

Increasing OID

Exposure time increased

Decreasing SID

Exposure time decreased

Anatomic part has excessive gas

Exposure time decreased

Excessive collimation

Exposure time increased

Increased density control

Exposure time increased

changing mA and/or kVp

maintains exposure to IR at pre-set level

changing the SID

maintains exposure to IR pre-set level

patient thickness

maintains exposure to IR at pre-set level

selecting different detectors

maintains exposure to IR at pre-set level

what des ARC provide

• AEC devices provide sufficient exposure only for structures positioned directly above the ionization chamber

how can Photocell(s) selection can affect image quality

Area of interest must cover selected photocell adequately

• Underexposure and Quantum Noise

• Overexposure and Loss of Detail

• Contrast and Dynamic Range Shifts

• False Triggering and Shadow Artifacts

Photocell vs. Phototransistor Characteristics:

• Photocell (LDR) uses variable resistance and a slower response time, making it suitable for slow ambient tracking but prone to lag or blurring if light shifts rapidly.

• Phototransistor offers amplified current output and faster response, delivering tighter precision for dynamic or high-speed optical capture

back up mAs should be what to produce sufficient exposure

• Back-up mAs should be slightly greater than the mAs required to produce sufficient exposure

•  The actual mAs used for an exam will provide critical information needed before attempting an repeat exposures

why Back-up mAs should be slightly greater than the mAs required to produce sufficient exposure

typically 150% to 200% (or about 1.5 to 2 times)

• Allow Normal AEC Operation: Provides enough exposure time for the AEC to reach the required image receptor exposure. Setting the back-up mAs too close to or lower than the required exposure will cause the system to shut off prematurely, resulting in an underexposed, noisy image that must be repeated.

• Protect the Patient and X-Ray Tube: If the AEC ionization chambers fail, or if the wrong chamber/anatomy is selected (e.g., shooting through air or a prosthesis), the AEC will never reach its target density. The back-up mAs forces the X-ray exposure to terminate automatically before the patient receives a massive, harmful overdose or the X-ray tube overheats.

• Account for Anatomical and Pathological Variations: A safety margin of roughly 150% (up to 200% for certain setups) accommodates minor variations in patient size, tissue density, and positioning without triggering a false-positive safety abort.

Anatomically programmed techniques allow techs to do what

• Allows techs to select on the control panel that represents an anatomic area

• Preprogrammed set of exposure factors is displayed and selected for use

• Once an anatomic part and projection or position has been selected, the radiographer can adjust the exposure factors that are displayed

Exposure technique charts

Provide a method of consistency in the quality of the image production

• Standardize the selection of exposure factors for the typical patient to improve quality and consistency

• Designed for the average or typical patient

• Accurate measurement of part critical

Variable kVp fixed mAs

• kVp can be increased as the anatomic part size increases

• kVp is increased by 2 for every 1 cm increase in part thickness

• Kvp used for a particular projection is varied depending on the measured body part thickness

Fixed kVp variable mAs

• kVp held constant for a given range of tissues types and thicknesses while the mAs is varied to achieve an appropriate exposure to IR

• General rule: every 4-5 cm (1.6 -2 inch) changes in part thickness requires a change in mAs by a factor of 2

Variable kVp fixed mAs Advantages Disadvantages

• Advantages

  • Easy to construct and simple to use

  • At low kVp levels, small changes in kVp more effective

• Disadvantages

  • Anatomic part penetration not guaranteed

  • Changing kVp less effective for range of procedures

  • Less accurate for part size extremes

Fixed kVp variable mAs Advantages Disadvantages

• Advantage 

  • Part measurement less critical

  • Increased accuracy with range of procedures

  • More consistent image contrast and overall quality

  • Part penetration assured

• Disadvantage

  • Optimal kVp values not standardized

Part thickness affects

• Beam attenuation

• Exposure reaching image receptor

• Scatter production and image contrast

SPECIAL CONSIDERATIONS Geriatric patients

• May require decreased exposure techniques, lower kVp values, and increased mA to decrease exposure time

SPECIAL CONSIDERATIONS Pediatric patients

• Small size may require a reduction in exposure.

• Quicker exposure times may be necessary

SPECIAL CONSIDERATIONS Bariatric patients

• Need increase in kVp and mAs, require a grid, may need to be imaged in quadrants.

SPECIAL CONSIDERATIONS Projections and positions

• May require changes in exposure technique due to changes in tissue thickness.

SPECIAL CONSIDERATIONS Casts and splints

• Plaster casts require increase in exposure factors

• Splints may require adjustment in exposure factors due to type of material if in the path of the x-ray beam

SPECIAL CONSIDERATIONS Contrast media

• Used to increase radiographic contrast when imaging areas of low subject contrast.

• Positive agents versus negative agents.