X-Ray Exposure Timers: Synchronous vs. Electronic

Exposure Timer

• An exposure timer is a device controls the length of X-ray exposure.
• Located in the primary circuit of the X-ray circuit.
• There are several variations of the exposure timer.

Synchronous Timer

• Based on a synchronous motor.
• A motor is designed to turn a shaft at precisely 60 revolutions per second depending on the frequency of the current.
• A shaft turns a disk, which is connected to on-off switches, through reduction gears.
• The exposure time selected determines the reduction gear used and therefore the time it takes the disk to move from the on switch to the off switch.
• Has a minimum exposure time of 1/60 second.
• The available exposure times are multiples of 1/60.

Electronic Timer

• Its sophisticated and accurate, and is the most widely used timer today.
• It is based on the time takes to charge a capacitor through a variable resistor.
• Once the capacitor receives its preprogrammed charge, it terminates the exposure.
• To achieve a 1-second exposure, the resistance is increased so that it takes longer to charge the capacitor.
• To achieve a 1-millisecond exposure, there is virtually no resistance; the capacitor charges very quickly, and the timer terminates the exposure.

mAs Timer

• It is a variation of the electronic timer, it monitors the current passing through the x-ray tube, and it terminates the exposure when the desired mAs is reached.
• It is located in the secondary circuit instead of the primary circuit.

Automatic Exposure Control (AEC)

• It is available on most modern radiographic units which is used to assist the radiographer.
• A system consistently used to control the amount of radiation reaching the image receptor by terminating the length of exposure.
• It is also called automatic exposure devices, and are sometimes referred to as phototiming.
• Eliminates the need for the radiographer to set an exposure time.
• The radiographer loses control over time and, as a result, mAs when using an AEC.
• Factors are preprogrammed by the anatomically programmed radiography system.
• Systems can be overridden and the mA and kVp can be set manually when the radiographer determines adjustments to the tech. factors.
• kVp needs to be adjusted to increase or decrease the quantity of scatter radiation produced.
• Radiation is transmitted via the patient and concerted into an electrical signal which terminates the exposure time.
• It happens when a predetermined amount of radiation is detected, level of of electrical signal that has been produced.
• Service personnel calibrate the predetermined level of radiation to meet the departmental standards of image quality.

AEC Radiation Detectors

• Two types of AEC systems have been used: phototimers and Ionization chambers
• Regardless of the AEC system, almost all systems use a set of three radiation-measuring detectors in a specific manner.

Phototimers

• Use a fluorescent light producing screen and a device converts the light to electricity.
• A photomultiplier (PM) tube is an electronic device that converts visible light energy into electrical energy.
• A photodiode is a solid-state device that performs the same function.
• Considered exit-type devices
• Light paddles, coated with a fluorescent material, served as the detectors, and the radiation interacted with the paddles, producing visible light.
• Light was transmitted to PM tubes or photodiodes that convert this light into electricity.
• The electrical charge was in proportion to the radiation to which the light paddles have been exposed.
• The timer is tripped, and the radiographic exposure was terminated when a sufficiently large charge had been received.
• Largely been replaced with ionization chamber systems.

Ionization Chamber Systems

• An ionization or ion chamber is a hollow cell that contains air and is connected to the timer circuit the wire.
• Are entrance-type devices.
• When ionization chamber is exposed from a rad. exposure, the air inside the chamber becomes ionized, creating an electrical charge.
• This charge travels via wire to the timer circuit.
• The electrical charge is in proportion to the radiation to which the ionization chamber has been exposed.
• The timer is tripped, and the rad. exposure is terminated when sufficiently large has been received.
• It is less sophisticated and accurate, but they less prone to failure.
• Most of today’s AEC systems use ionization chambers.

AEC Characteristics

• The amount of radiation (mAs) to produce the appropriate exposure is determined by the system.
• AEC has on the image characteristics. Select a kVp level that provides an appropriate subject contrast.
• The higher the kVp value used, the shorter the exposure time needed by the AEC device.
• Higher kVp with AEC decreases the exposure time and overall the mAs.

AEC Alignment and Positioning

• AEC systems with multiple detectors typically allow the radiographer to select any combination of one, two, or three detectors.
• The selected detectors measure radiation during exposure, and the electrical signals are averaged.
• The detector that receives the greatest amount of exposure has a great effect on the total exposure.
• Guideline is to select the detectors superimposed by the anatomic structures of greatest interest for image.
• Failure to use the proper detectors could result in underexposure or overexposure to the image receptor.
• Proper centering of the part examined is important.
• The anatomic area of interest must be centered over the detectors that the radiographer has selected.
• Improper centering may expose or underexpose the image receptor.
• The size of the detectors are fixed and cannot be adjusted.
• The radiographer should determine whether AEC should be used during the radiographic procedure.

AEC Compensating Issues

• Designed to compensate for changes in patient thickness.
• If the area of interest is thicker, the exposure time will lengthen to determine preset exposure to the detectors.
• Systems do not adequately compensate for changes, it may need adjusting
• Abdominal exams may be comprised if the pt has an excessive amount of bowel gas.
• If a detector is superimposed by an area of abdomen with excessive gas, it will terminate the exposure prematurely.
• Pathologic conditions can cause underexposure of area of radiographic interest.
• Positive contrast adds a condition or a prosthetic device that overexpose area of interest.

AEC - II. Collimation

• Failure to accurately restrict the beam may underexpose the area of interest.
• If the x-ray field is collimated too closely, the detector does not initially receive exposure, which could result in overexposure.

AEC Timing Considerations

• The minimum response time is the length of time necessary for the AEC to respond to the ionization and send a signal to terminate the exposure.
• Modern AECs has a minimum response time of 0.001
• High-speed systems for smaller part sizes can cause problems when AECs need less than 0.001 to produce a diagnostic-q.
• mA should be decreased to permit longer AEC time.
• The backup time establishes the exposure time to prevent over.
• The backup time should be set at 150% of the manual exposure time
• U.S law requires gens to terminate at 600mAs and and 50kVp / 2000 mAs below 50kVp
• When time is too short it will terminate the exposure before the AEC signal, producing an underexposed image.

Computerized AEC

• Anatomically programmed radiography (APR) units combine an AEC system with an exposure system.
• It is computerized to correspond to anatomical procedures.
• You select a procedure as the control.
• This choice results in the computer entering the average tech
• The radiographer override the suggested tech when pt has a condition make it desire able

Quality Control

• An AEC device should be calibrated and recalibrated at intervals.
• Failure to maintain regular calibration of the AEC:
  • The lack of consistent exposures and affect image quality
  • Lead to overexposure of the patient
  • Poor department efficiency, and possibility of images being misread
• Should provide consistent exposures to the image receptor for differences in tech., patient, and detector selection.
• AEC performance can be monitored by imaging homogenous.