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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 CIRCUITS TIMERS:. MECHANICAL

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Presentation Transcript
timer circuit
TIMER CIRCUIT:
  • IS SEPARATE FROM THE OTHER MAIN CIRCUITS OF THE IMAGING SYSTEM.

TIMER

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 circuits timers
TYPES OF TIMING CIRCUITSTIMERS:
  • MECHANICAL
  • SYNCHRONOUS
  • ELECTRONIC
  • mAs
  • BACKUP
  • AUTOMATIC EXPOSURE CONTROL
machanical timer
MACHANICAL TIMER
  • OPERATE BY CLOCKWORK.
  • SPRING MECHANISMS IS WOUND UP.
  • DURING EXPOSURE THE SPRING UNWINDS AND TERMINATES EXPOSURE.
synchronous timer
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
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
mas timer
mAs TIMER
  • MONITORS THE PRODUCT OF mA & EXPOSURE TIME AND TERMINATES EXPOSURE WHEN THE DESIRED mAs VALUE IS ATTAINED.
backup
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

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

slide13

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.

slide14

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
AUTOMATIC EXPOSURE CONTROL (AEC)
  • FLAT IONIZATION CHAMBER
  • PHOTOTIME
ionization chamber aec
IONIZATION CHAMBER AEC

X-RAY TUBE

X-RAY TABLE

BUCKY

ionization chambers are situated
IONIZATION CHAMBERS ARE SITUATED
  • BETWEEN THE GRID MECHANISM & THE CASSETTE
chamber selection on the console
CHAMBER SELECTION ON THE CONSOLE

DETECTOR SELECTION

Along with proper

ionization chamber aec21
IONIZATION CHAMBER AEC

X-RAY TUBE

X-RAY TABLE

CASSETTE

SCREEN

pm tube
PM TUBE

LIGHT

ELECTRONS

phototime mechanism is situated
PHOTOTIME MECHANISM IS SITUATED
  • BEHIND THE CASSETTE
  • CASSETTE NEEDS SPECIAL DESIGN

IT HAS OPENING IN THE BACK

slide26

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
FACTORS INFLUENCING AEC
  • PATIENT POSITIONING
  • DETECTOR SELECTION
positioning
POSITIONING!!!!
  • AEC USES DETECTORS TO RECEIVE THE RADIATION THAT PASSED THROUGH THE PATIENT WE MUST POSITION THE ANAOMY OF INTEREST DIRECTLY ABOVE THE DETECTORS.
slide30

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.

density setting
+4

+3

+2

+1

0

-1

-2

-3

-4

+100

+75

+50

+25

0

-25

-50

-75

-100

DENSITY SETTING

D. SETTING

% DENSITY CHANGE

minimum response time
MINIMUM RESPONSE TIME
  • SHORTEST EXPOSURE TIME POSSIBLE
effects of ma kvp sid on aec
EFFECTS OF mA, kVp, SID ON AEC
  • mA AEC TIME
  • kVp AEC TIME
  • SID AEC TIME
problems with aec
PROBLEMS WITH AEC
  • PATIENT PATHOLOGY
  • PROSTHETIC DEVICE
  • COLLIMATION
  • STROBOSCOPIC EFFECT
  • WRONG IMAGE RECEPTOR
slide48
NOT ENOUGH COLLIMATION ALLOWS TOO MUCH SCATTER REACHING THE DETECTOR AND PREMATURE TERMINATION OF THE EXPOSURE TAKES PLACE
stroboscopic effect
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.