Influence of Volume and Density of Computerized Tomography Images in Tube Current Automatic Exposure Control

As computerized tomography (CT) is more frequently used, there is a growing interest about the patient dose. To administer an appropriate dose during examinations, most equipment use an automatic exposure control (AEC) system. This study aimed to affirm that volume and density are relevant factor of AEC which is the correct method for dose selection.

With regard to CT equipment, the MX-16 slice technology was used. A large phantom (35×35×10) and a small phantom (30×30×10) were produced using acrylic material. The phantom solution's density was adjusted by diluting water with contrast medium. The study compared dose and image quality by volume and density, by two phantoms with different volume and density in identical penetrance.

With the dose increase, the noise decreased and the signal-to-noise ratio (SNR) rose. When the density increased, both dose and the noise increased. With the volume increased, both dose and noise increased. As for the measurement of phantoms with different volume and density in identical penetrance, dose of large phantom was 139mAs/slice, noise was 36.41HU while dose of small phantom was 104.3mAs/slice, noise was 38.83HU. To match with noise value in phantom with different volume and density, the small phantom showed 16mAs/slice increase.

In an examination that applied automatic exposure control, the system selected another dose in case of identical penetrance with different volume and density levels. The image quality could not be maintained at a certain level. When a dose is chosen by an AEC system, it is appropriate to consider volume and density as factors to calculate the transmission dose.