(Siemens)

Automatic exposure control using an ionisation chamber between the patient and the film cassette is used in the majority of x-ray generators. A slim panel containing three ionisation chambers (dose detector) is placed between the grid and the cassette in the bucky assembly. The chamber assembly is constructed to be of very low density so as not to interfere with the image.

The principal of operation is that during an exposure the air in the chamber/s is ionised permitting a current o flow through them, this current is used to charge a capacitor. When the capacitor voltage reaches a pre determined level the voltage is used to terminate the exposure via a thyristor based additional control unit.

Film Screen Control
The system is set up during installation to work with the type of film screen combinations used in the department, the setup is designed to work with the film screen combination and the Kv selected to ensure a consistent film density with various film screen combinations and Kv choices.

Minor density control
There is a small resistor in series with the capacitor to provide the operator with a small degree of control of the resultant film density, to allow for the patient build and the amount of scatter produced by the radiographic technique.

Ionization chamber patterns
The unit manufacturer should provide a Perspex slide to fit the LBD to show where the chambers are sited, in general there are three chambers a central chamber and two outer chambers, care must be taken to ensure that the patient part being imaged lies under the selected chamber or chambers. The chambers are often oblong in at centre and round on the outer ones and about 7cm x 5cm and 5 cm round.

Control Panel Controls

AEC in use
Care must be taken in the use of AEC devices to ensure that the a chamber is in the field of radiation when the exposure is made. It is also important to ensure the correct chamber/s selection is made and that the filed size is restricted to a minimum to reduce scatter but must cover the chamber.
The object density and thickness must be such that the minimum response time of the system is allowed for, using high tube currents may produce exposure times  shorter than the equipment can reliably cope with. There is usually some indication on the control desk when an error occurs such as too short a time or the generator cannot give enough exposure, there is usually a facility for reading the mAS given during the exposure.

AEC and tomography
There are some machines which have AEC on tomography these setups are different as it is not possible to reliably adjust the filament current during exposure, the exposure control is performed on the KV.