Automatic exposure control

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 1 st yr

2. OBJECTIVE EXPOSURE EXSPOSURE TIMER TYPES OF EXPOSURE TIMER AUTOMATIC EXPOSURE TIMER TYPES OF AEC PHOTOMULTIPLIER PHOTOTIMER IONIZATION CHAMBER PHOTOTIMER SOLID STATE PHOTOTIMER ADVANTAGES OF AEC DISADVANTAGES OF AEC VARIABLE Kvp 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:- Mechanical timer Electronic timer Automatic exposure control [photo timer ] Pulse counting timer 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)

9. 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

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

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