exposure timer, automatic exposure control, photomultipler phototimer, ionization chamber phototimer, solid state phototimer, advantages, disadvantages, fixed kvp, variable kvp.

1. AUTOMATIC EXPOSURE
CONTROL/ VARIABLE KVP/
FIXED KVP
BY: NEHA SINGH
REG.NO.: 210513006
MRIT 1ST YR

2. OBJECTIVE
1. EXPOSURE
2. EXSPOSURE TIMER
3. TYPES OF EXPOSURE TIMER
4. AUTOMATIC EXPOSURE TIMER
5. TYPES OF AEC
6. PHOTOMULTIPLIER PHOTOTIMER
7. IONIZATION CHAMBER PHOTOTIMER
8. SOLID STATE PHOTOTIMER
9. ADVANTAGES OF AEC
10.DISADVANTAGES OF AEC
11. VARIABLE KVP
12. FIXED KVP

3. EXPOSURE
• DEFINATION- IT IS THE MEASURE OF IONIZATION OF AIR DUE TO IONIZING RADIATION.
• E=Q/M
• WHERE: Q= QUANTITY OF CHARGE ON IONS
• M= UNIT MASS OF AIR
• SI UNIT=COLUMB/ KG

4. EXPOSURE TIMER:- IT IS THE DEVICE WHICH IS USED TO CONTROL
THE LENGTH OF XRAY EXPOSURE.
• THERE ARE 4 BASICS TYPES OF EXPOSURE TIMER:-
1. MECHANICAL TIMER
2. ELECTRONIC TIMER
3. AUTOMATIC EXPOSURE CONTROL [PHOTO TIMER]
4. PULSE COUNTING TIMER
5. NOTE: MECHANICAL AND ELECTRONIC TIMERS ARE SUBJECT TO HUMAN ERRORS.

5. AUTOMATIC EXPOSURE
CONTROL
• DEFINATION-IT MEASURE THE ACTUAL AMOUNT OF RADIATION INCIDENT ON THE IMAGE RECEPTOR AND
TERMINATE X-RAY PRODUCTION WHEN PROPER RADIATION EXPOSURE IS OBTAINED.
THE ESSENTIAL ELEMENTS IN PHOTOTIMERS IS A DEVICE THAT CAN DETECT RADIATION, IN RESPONSE TO THIS
RADIATION PRODUCE A SMALL ELECTRIC CURRENT.
THERE ARE THREE SUCH DEVICES :-
1) PHOTOMULTIPLIER PHOTOTIMERS
2)IONIZATION CHAMBER AUTOTIMERS
3) SOLID STATE PHOTOTIMER

6. PHOTOMULTIPLIER PHOTOTIMERS
SELECTED MOSTLY BY THE TECHNICIAN.
• THIS IS THE MOST COMMON TYPE OF AEC.
• THE DETECTOR IS MADE UP OF LUCITE(MATERIAL THAT CAN TRANSMIT LIGHT).
• THE LUCITE IS COATED WITH ONE OR MORE ( COMMONLY THREE) AREA OF PHOSPHOR THAT WILL EMIT LIGHT
WHEN IRRADIATED WITH X- RAY.
• THESE LUCITE DETECTORS ARE CALLED LUCITE PADDLE .
• WHEN PHOSPHOR GENERATE LIGHT, THE INTENSITY OF LIGHT IS DIRECTLY PROPORTIONAL TO INTENSITY OF X-
RAY THAT REACHED PHOSPHOR.

7. LUCITE TRANSMIT LIGHT
OUTPUT REGION CALLED LIGHT GATE
DIRECTS LIGHT TO PHOTOMULTIPLIER TUBE [WHERE
LIGHT IS CONVERTED INTO ELECTRICAL CURRENT]
AMPLIFIED TO PRODUCE AN ELECTRICAL SIGNAL
USED TO CHARGE CAPACITOR
WHEN CAPACITOR REACHES A PREDETERMINED
CHARGE THEN EXPOSURE TERMINATE

8. IONIZATION CHAMBER AUTOTIMER
• IT CONSIST OF TWO THIN PARALLEL SHEETS OF ALLUMINIUM OR LEAD FOILS. GAS(USUALLY ROOM GAS) BECOME IONIZED
WHEN STRUCK BY RADIATION .
• IT CONSISTS OF
• ONE OR MORE RADIATION DETECTORS
• AN AMPLIFIER
• A DIGITAL SNR VARIABLE SELECTORS
• A SIGNAL INTEGRATOR CIRCUIT
• A COMPARATOR CIRCUIT
• A TERMINATION SWITCH
• BACKUP TIMER (SAFETY SHUT OFF SWITCH)

10. X- RAY TRANSMITTED THROUGH THE PATIENT
GENERATES ION PAIRS IN ONE
TO THREE SELECTABLE IONIZATION CHAMBER POSITIONED PRIOR TO THE DETECTOR
AMPLIFIER BOOST THE SIGNAL
FED TO VOLTAGE COMPARATOR AND INTEGRAL CIRCUIT
ACCUMULATED SIGNAL IS EQUALS A PRE SELECTED REFERENCE VALUE ,OUTPUT PULSE TERMINATE THE EXPOSURE
1)SNR /FILM DENSITY SELECTOR ON THE GENERATOR CONTROL PANEL INCREASE OR DECREASE THE REFERENCE VOLTAGE

11. • IN GENERAL DIAGNOSTIC RADIOGRAPHY, PHOTOTIMER SENSORS ARE PLACED IN FRONT OF IMAGE
RECEPTOR.
• IF IN CASE THERE IS PHOTOTIMER CIRCUIT FAILURE THAN THE BACKUP TIMER {SAFETY SWITCH}
TERMINATES THE XRAY EXPOSURE AFTER A PRESET “ON” TIME.

12. SOLID-STATE AUTOTIMER
• THESE ARE BECOMING MORE COMMON SOLID-STATE RADIATION DETECTOR.
• THESE DEVICES OPERATE WITH A VERY ACCURATE INTERNAL CLOCK BASED ON A QUARTZ-CRYSTAL
OSCILLATOR.
• THEY CAN MEASURE EXPOSURE TIMES AS SHORT AS 1MS

13. •ADVANTAGES:
• IRRESPECTIVE OF SIZE AND SHAPE AN AEC DEVICE IS VERY USEFUL IN ACHIEVING CONSISTENT X-RAY FILM
DENSITIES WHICH CAN BE DIFFICULT IN MANUAL SYSTEM.
• SPEED – UP EXAMINATION [TAKES LESS TIME TO SET UP TECHNIQUE]
• LOWERS REPEAT RATE.{REDUCE PATIENT DOSE}
•DISADVANTAGES:
• TECHNOLOGIST COME TO DEPEND UPON SYSTEM
• IF YOUR CENTRE IS NOT CORRECT THAN EXPOSURE MAY NOT BE CORRECT.

14. VARIABLE KVP:
DEVELOPED BY JERMAN- 1925.
USED WHEN KVP RANGE WAS LIMITED BY EQUIPMENT GENERATOR.[CAN ADJUST KVP MORE THAN MA OR TIME]
ADVANTAGES:
ALLOWS FOR SMALL CHANGES IN EXPOSURE FOR SMALL CHANGES IN PART THICKNESS.
HIGHER CONTRAST.
DISADVANTAGES:
MORE PATIENT DOSE.
HIGHER REPEAT RATE.

15. FIXED KVP SYSTEM:
DEVELOPED BY FUCHS IN -1943.
ADVANTAGES:
REDUCTION IN RADIATION DOSE.
PROVIDES MORE INFORMATION WITHIN IMAGES.
LENGTHENS EXPOSURE LATITUDE.
ALLOWS FOR SHORTER EXPOSURE TIME, THUS REDUCES PATIENT’S MOTION.
DISADVANTAGES:
HIGHER AMOUNT OF SCATTER RADIATION REDUCE IMAGE CONTRAST.

16. THANK YOU