TIMERS

BASIC X-RAY SCHEMATIC

TIMER CIRCUIT:

• IS SEPARATE FROM THE OTHER MAIN CIRCUITS OF THE IMAGING SYSTEM.

TIMER

TIMER

• A MECHANICAL OR ELECTRONIC DEVICE WHOSE ACTION IS TO MAKE OR BRAKE THE HIGH VOLTAGE ACROSS THE X-RAY TUBE

TYPES OF TIMING CIRCUITSTIMERS:

• MECHANICAL

• SYNCHRONOUS

• ELECTRONIC

• mAs

• BACKUP

• AUTOMATIC EXPOSURE CONTROL

MACHANICAL TIMER

• OPERATE BY CLOCKWORK.• SPRING MECHANISMS IS WOUND UP.• DURING EXPOSURE THE SPRING UNWINDS

AND TERMINATES EXPOSURE.

SYNCHRONOUS TIMER• IN SYNCH WITH FREQUENCY OF THE A/C

CURRENT (60 Hz in the US).• MINIMUM EXPOSURE TIME IS 1/60 s.• TIMING INTERVALS INCREASE BY 1/60 s.• MUST BE RESET AFTER EACH EXPOSURE

ELECTRONIC TIMER• THE MOST SOPHISTICATED.• THE MOST ACCURATE• IT’S CIRCUITRY CONSISTS OF

THYRATRON TUBE OR THYRISTOR.

• ALLOW WIDE RANGE OF TIME INTERVALS.

• ALLOW SERIAL EXPOSURES.• NOWADAYS CONTROLLED BY

MICROPROCESSOR

THYRATRON CIRCUIT

mAs TIMER

• MONITORS THE PRODUCT OF mA & EXPOSURE TIME AND TERMINATES EXPOSURE WHEN THE DESIRED mAs VALUE IS ATTAINED.

BACKUP

• SET TO TERMINATE EXPOSURE IN THE EVENT OF FAILURE IN THE AEC CIRCUIT.

• MAX. mAs ALLOWABLE – 600 mAs

UNLESS TUBE POTENTIAL IS LESS THAN 50 kVp – 2,000 mAs IS THE LIMIT.

AUTOMATIC EXPOSURE CONTROL

Automatic exposure control devices can assist the radiographer in producing consistent radiographic images from patient to patient, regardless of size or presence of pathology. The advantages of this consistency are numerous and include: decreased repeat rate; decreased patient exposure; and increased department efficiency. The most important benefit being decreased repeat rate

When an AEC device is used to terminate an exposure, the technologist sets the kVp and mA, but the time of the exposure is automatically determined by the machine. The AEC device differs from a manual timer because the AEC does not stop the exposure until the film has reached an appropriate density. Unlike manual timers, which simply stop the exposure after the preset time has elapsed.

A major benefit of the AEC device is its ability to consistently obtain accurately exposed radiographs, even in the presence of pathology. While manual timers terminate the exposure at the preset time, regardless of pathology or achievement of proper film density. The following example demonstrates the difference

AUTOMATIC EXPOSURE CONTROL (AEC)

• FLAT IONIZATION CHAMBER

• PHOTOTIME

IONIZATION CHAMBER AEC

X-RAY TABLE

X-RAY TUBE

BUCKY

IONIZATION CHAMBERS ARE SITUATED

• BETWEEN THE GRID MECHANISM & THE CASSETTE

CHAMBERS

CHAMBER SELECTION ON THE CONSOLE

DETECTOR SELECTION

    Along with proper

AEC

IONIZATION CHAMBER AEC

X-RAY TABLE

X-RAY TUBE

CASSETTE

SCREEN

PM TUBELIGHT

ELECTRONS

PHOTOTIME MECHANISM IS SITUATED

• BEHIND THE CASSETTE

• CASSETTE NEEDS SPECIAL DESIGN

IT HAS OPENING IN THE BACK

PHOTOTIME CASSETTE

PHOTOTIME SCREEN

X

Even though the ionization chamber and the phototimer operate differently, they both have the same function: convert radiation into an electrical signal which will be used to automatically stop the exposure when the film has reached the proper density

FACTORS INFLUENCING AEC

• PATIENT POSITIONING• DETECTOR SELECTION

POSITIONING!!!!

• AEC USES DETECTORS TO RECEIVE THE RADIATION THAT PASSED THROUGH THE PATIENT WE MUST POSITION THE ANAOMY OF INTEREST DIRECTLY ABOVE THE DETECTORS.

Without a technologist who is very knowledgeable about anatomy and positioning, automatic timers are worthless. In fact, they may actually decrease department efficiency because of the increased amount of repeat radiographs that will result if used improperly.

DETECTOR SELECTION

LAT. L-SPINE

C-SPINE

C-SPINE -AP

SHOULDER

KNEE

HIP

PELVIS

ABDOMEN

CHEST PA

LAT. CHEST

DENSITY SETTING

• +4

• +3

• +2

• +1

• 0

• -1

• -2

• -3

• -4

• +100

• +75

• +50

• +25

• 0

• -25

• -50

• -75

• -100

D. SETTING % DENSITY CHANGE

MINIMUM RESPONSE TIME

• SHORTEST EXPOSURE TIME POSSIBLE

EFFECTS OF mA, kVp, SID ON AEC

• mA AEC TIME

• kVp AEC TIME

• SID AEC TIME

PROBLEMS WITH AEC

• PATIENT PATHOLOGY

• PROSTHETIC DEVICE

• COLLIMATION

• STROBOSCOPIC EFFECT

• WRONG IMAGE RECEPTOR

CERTAIN PATHOLOGIES ALTER AEC READING

PROSTHESIS GIVES FALSE AEC READING

NOT ENOUGH COLLIMATION ALLOWS TOO MUCH

SCATTER REACHING THE DETECTOR AND PREMATURE

TERMINATION OF THE EXPOSURE TAKES PLACE

STROBOSCOPIC EFFECT

• TOO SHORT OF THE EXPOSURE TIME IS SELECTED BY THE AE, SHORTER THAN THE SPEED OF THE OSCILLATION OF THE GRID AND IT FREEZES THE GRID MOVEMENT.

WRONG SPEED OF IMAGE RECEPTOR!!!