Radiographic Equipment Stacy Kopso, M.Ed.,RT(R)(M)

Automatic Exposure Devices AEC 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 Devices AEC or Phototiming Advantage Reproducibility Radiographs 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 Devices AEC 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 Kv Films exhibit an insufficient density due to under penetration Control the contrast of the resultant image mA Adjusts the “rate of exposure” delivered to photoreceptor Adjust the “speed” or “length of time” for a given phototimed exposure Backup timer Determines the “Maximum risk” of exposure for a phototimed exam Density Used to correct/adjust for changes in body habitus for a give exam Photoreceptor Over/under exposures due to wrong selection or misplacement of body part over cell Alignment of CR Over/under exposure for sensitive exams (LS) Alignment of bucky tray Over/under exposure of area of interest

Automatic Exposure Devices AEC 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 Kv Films exhibit an insufficient density due to under penetration Control the contrast of the resultant image mA Adjusts the “rate of exposure” delivered to photoreceptor Adjust the “speed” or “length of time” for a given phototimed exposure Backup timer Determines the “Maximum risk” of exposure for a phototimed exam Density Used to correct/adjust for changes in body habitus for a give exam Photoreceptor Over/under exposures due to wrong selection or misplacement of body part over cell Alignment of CR Over/under exposure for sensitive exams (LS) Alignment of bucky tray Over/under exposure of area of interest

kVp & mA Selector kV selector mA 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 Make be reached when you select table buck instead of upright bucky for a chest

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 Detectors are the AEC devices that measure the amount of radiation transmitted.

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 Contrast increases Film density Digital: Quantum noise Increasing collimation & field size decrease Decreasing collimation &increasing field size increase An increase in field size has the opposite affect on all of the above