Technical note: a simple solution to the problems of testing automatic exposure control in diagnostic radiology.

Abstract

The complete testing of the automatic exposure control (AEC) in radiographic systems usually involves measurement of the optical density of films exposed under well controlled conditions. This can mean typically 30 exposures per unit tested. Until recently the technique adopted locally followed that recommended in TGR 32 Part IV (Hospital Physicists' Association, 1980). A slab of tissue-equivalent material 30 × 30 × 20 cm thick is placed in the position normally occupied by the patient. The X-ray beam is centred on the phantom with an incident field size of 25 × 25 cm. The focus–film distance should be set such that the tube and grid foci coincide (Figure 1). The consistency of optical density produced while varying the equipment parameters and chambers is then assessed by exposing a film with an L-shaped 2.5 mm mask placed over the cassette (behind the AEC chambers). The use of the current mask means that approximately eight films may be exposed per Bucky assembly, and since two Buckies are commonly included in a typical X-ray room this can mean 16 films must be exposed. The preferred technique is to use the same cassette for each exposure during the testing of each AEC, to use the same film dispenser and the same processor throughout. This avoids screen speed, processing and film batch variations respectively. However the use of this technique can be tedious and expensive in terms of both time and film.