Since computed tomography (CT) was introduced about 40 y ago, its use has continuously grown, resulting in the increase of the CT dose. Therefore, an awareness of the CT dose and its potential complications has led to the development of several dose-reduction strategies. One of the strategies is automatic exposure control (AEC), which modulates radiation intensity depending on the patient size, z-axis thickness (Z-DOM) or angular thickness (D-DOM). Another dose-reduction method is the in-plane bismuth shield which attenuates radiation to reduce the CT doses of the tissues underneath the shield. We evaluated and compared the dose reduction and image quality of CT for various dose-reduction techniques. The result showed that both AEC and the in-plane shield reduced the CT dose effectively and the combined method of AEC and in-plane shielding reduced the CT dose more than the single use of AEC or in-plane shields. The dose reduction using Z-DOM was normally higher than that using D-DOM. The image quality of CT dramatically degraded when the in-plane shield was directly attached to the phantom without using AEC. In order to effectively reduce CT dose without the significant degradation of the image quality, the in-plane shield should be placed 1 cm apart from the patient with applying AEC control.