Automatic exposure control in computed tomography--an evaluation of systems from different manufacturers

Today, practically all computed tomography (CT) systems are delivered with automatic exposure control (AEC) systems operating with tube current modulation in three dimensions. Each of these systems has different specifications and operates somewhat differently.

To evaluate AEC systems from four different CT scanner manufacturers: General Electric (GE), Philips, Siemens, and Toshiba, considering their potential for reducing radiation exposure to the patient while maintaining adequate image quality.

The dynamics (adaptation along the longitudinal axis) of tube current modulation of each AEC system were investigated by scanning an anthropomorphic chest phantom using both 16- and 64-slice CT scanners from each manufacturer with the AEC systems activated and inactivated. The radiation dose was estimated using the parameters in the DICOM image information and image quality was evaluated based on image noise (standard deviation of CT numbers) calculated in 0.5 cm(2) circular regions of interest situated throughout the spine region of the chest phantom.

We found that tube current modulation dynamics were similar among the different AEC systems, especially between GE and Toshiba systems and between Philips and Siemens systems. Furthermore, the magnitude of the reduction in the exposure dose was considerable, in the range of 35-60%. However, in general the image noise increased when the AEC systems were used, especially in regions where the tube current was greatly decreased, such as the lung region. However, the variation in image noise among images obtained along the scanning direction was lower when using the AEC systems compared with fixed mAs.

The AEC systems available in modern CT scanners can contribute to a significant reduction in radiation exposure to the patient and the image noise becomes more uniform within any given scan.