Transcript Timers

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:
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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.
• LOCATED ON THE SECONDARY SIDE
OF THE H. VOLTAGE TRANSFORMER
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 TUBE
X-RAY TABLE
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 TUBE
X-RAY TABLE
CASSETTE
SCREEN
PM TUBE
LIGHT
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
D. SETTING
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+4
+3
+2
+1
0
-1
-2
-3
-4
% DENSITY CHANGE
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+100
+75
+50
+25
0
-25
-50
-75
-100
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
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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!!!