Stacy Kopso, M.Ed.,RT(R)(M). Control the amount of radiation reaching the image receptor by...
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Transcript of Stacy Kopso, M.Ed.,RT(R)(M). Control the amount of radiation reaching the image receptor by...
RADIOGRAPHIC EQUIPMENT
Stacy Kopso, M.Ed.,RT(R)(M)
Automatic Exposure DevicesAEC or Phototiming
Control the amount of radiation reaching the image receptor by terminating the length of exposure
Determines the exposure time (therefore, total exposure)
Reproduce a desired amount of film density on images taken of selected body parts regardless of changes within the patients being examined
Designed to maintain consistent radiographic densities on images taken on wide variety of patients
Automatic Exposure DevicesAEC or Phototiming
Advantage ReproducibilityRadiographs taken on a mixed population of
patients will fall within an acceptable or diagnostic range
Reduces repeats due to technical factors Disadvantage
Improper positioning of central ray or bucky alignment
Incorrect photocell selection
Automatic Exposure DevicesAEC or Phototiming
Items that impact the AEC at the control panel kVp mA Backup timer Density selector Photodetector (cell)
Items that impact the AEC at the x-ray table Alignment of CR to part of interest Alignment of bucky tray to CR
Automatic Exposure DevicesAEC or Phototiming
Items that impact the AEC at the control panel kVp mA Backup timer Density selector Photodetector (cell)
Items that impact the AEC at the x-ray table Alignment of CR to part of interest Alignment of bucky tray to CR
kVp & mA Selector
kV selector Reduce dose (increase kVp, to decrease mAs) Penetration Contrast
mA selector “rate of exposure” Change “speed” or “length of time”
Backup Timer
Maximum length of time the x-ray exposure continues when using AEC
Safety measure for patient and tube Set by radiographer or in unit Maximum termination 600 mAs Should be set at 150% to 200% of
expected exposure time
Density Adjustment
The predetermined exposure level needed to terminate the timer is increased or decreased
Change rate of exposure Increase or decrease the amount of radiation
produced based on size of patient +or-1=25% +or-2=50%
When would we need to use the + or – Pediatric if pt doesn’t extend out to the cells on a cxr Hip xray if pts belly is big and you need to penetrate through
to see head of femur Routinely having to use this setting indicates a
problem (AEC)
Radiation Detectors
Radiation is transmitted through the patient & converted into an electrical signal, terminating the exposure time
Over/under expose patient if wrong photocell is selected or patient is not positioned over cell
Predetermined level of radiation AEC uses Ionization chambers or
Phototimers Sensors, chambers, cells or detectors
Phototimer
Phototimers Earlier generations Use a fluorescent (light-producing) screeen
and a device that converts the light to electricity
Photomultiplier tube Electronic device that converts visible light
energy into electrical energy Photodiode
Solid-state device that converts visible light energy into electrical energy
Phototimer
Exit device Detectors are positioned behind the image
receptor Radiation must exit the image receptor before it
is measured by the detectors Light paddles coated with fluorescent material
serve as detectors The radiation interacts with paddles and
produces light Light is transmitted to PM tubes or photodiodes
that then convert light into electricity The timer is tripped and exposure is terminated
Phototimer
Ionization Chamber
Most commonly used today Cell that contains air and is connected to the
timer circuit via electrical wire Entrance device
Detector positioned in front of the image receptor When chamber is exposed to radiation from a
radiographic exposure, the air inside the chamber becomes ionized, creating an electrical charge
Charge travels along wire to timer circuit Timer is tripped and exposure is terminated
Ionization Chamber
Minimum response time
Minimum response time Shortest exposure time that the system can
produce 1ms Increase the mA to decrease the exposure
time(pediatric) Make sure the minimum response time is not
longer than the amount of time selected (overexposure)
Pediatric manual techniques
Manual Exposure
Exposure factors are determined by the size or thickness of the part under examination
Require the use of carefully constructed technique charts
APR Anatomically programmed radiography Table top x-rays
Quality Control AEC
When unit is first installed and at intervals thereafter
When there is a change in IR (CR to DR) Consistent need to adjust density controls
indicates that the AEC needs to be recalibrated
Use of phantoms to show consistent exposures to the image receptor for variations in technique factors, patient thickness and detector selection
Beam Restriction Devices
Aperture diaphragm Lead plate w/ hole cut in middle that is slightly
smaller than the image receptor size
Beam Restriction Devices
Cylinders and Cones Attaches to the bottom of the collimator
housing Creates a circular shaped collimated field Maximum beam restriction occurs w/an
increased length and decreased diameter of cylinder or cone
Beam Restriction Devices
Variable Aperture Collimator Controlled by radiographer Permits adjustment of length & width of
radiation light field Utilized light-localization device to estimate
the size and shape of field
Beam Restriction
Collimation- decrease in the size of the projected radiation field Pt dose
decreases Scatter radiation
decreases Contrast
increases Film density
decreases Digital: Quantum noise
increases Increasing collimation & field size
decrease Decreasing collimation &increasing field size
increase