ABC is used to keep the brightness of the displayed image at a constant level during examinations. It involves the adjustment of the kV and mA automatically depending on the part of the anatomy being examined. This can be achieved using a small photosensor at the XII output, for instance, which monitors the central portion of intensified images and feeds a signal back to the generator to adjust the kVp, the mA (or both) accordingly.
ABC systems are generally designed to operate between minimum (e.g. 70 kVp) and maximum (e.g. 120 kVp) kilovoltages. The minimum kVp can be used at the start of an exposure sequence, for instance, to prevent low energy X-rays exposing the patient unnecessary, and is then increased automatically so that a pre-determined image brightness level is reached. The tube current (mA) can also be adjusted automatically during this process. Such mA adjustment within the kVp range of the ABC is limited by the power rating of the XRT. When the power limit is reached, in fluoroscopy of the lateral abdomen or at steep-angled cardiac projections, for instance, further adjustment of exposure factors is no longer possible and the Automatic Gain Control (AGC) circuitry can come into play to maintain image brightness. However, images with increased electronic noise generally result.
The actual kVp and mA settings used by ABC systems dictate the contrast displayed in fluoroscopic images as well as the dose to the patient. A High Dose ABC mode can be used which lowers the kVp and boosts the mA so that image contrast can be improved at the same image brightness, while a Low Dose mode increases the kVp and lowers the mA to effect a similar outcome. A third intermediate mode can also be selected on many systems. The heat capacity of the XRT imposes the power limit, which is controlled by the product, (kVp x mA), the ABC system automatically applies. This is dictated by the thickness and composition of the anatomy being screened. When this product reaches the power limit, in the case of a very large patient, for example, the ABC generally maintains constant kVp and mA settings. Hence the need for additional control provided by the camera's AGC. In addition, the AGC control of image brightness happens almost instantaneously whereas the High Voltage circuitry can take about a second or so to respond to any ABC detected illuminance changes. The AGC is therefore also of use during this adaptation period so as to maintain a constant brightness in displayed images.