RESUMO
Frequent three-dimensional computed tomography scanning may cause deterioration of the lenses of the eye, which are susceptible to x-rays. The authors performed an experimental study using a phantom head to establish an optimized three-dimensional computed tomography scan protocol for longitudinal morphological estimation in craniofacial surgery. Volume computed tomography scans were performed using a Hi-Speed Advantage SG CT scanner (GE Medical Systems, Milwaukee, WI) in the axial plane with a combination of scan parameters of varied values. The radiation doses induced by each scanning were measured using thermoluminescent dosimeter chips attached to the position of the lenses in the phantom. Two-dimensional images in the coronal plane and three-dimensional images of the osseous surface were generated from each accumulated data set. For each scan parameter, the images generated from data accumulated using different values were compared. The study showed that lens radiation dose increased with tube potential and was almost directly proportional to tube current and 1/pitch. The slice thickness did not affect lens radiation doses significantly. Images with good contrast resolution and low artifact level sufficient for estimating morphological changes were obtained using a low tube potential of 100 kVp and a tube current of 100 mA. In regard to z-axis spatial resolution, a slice thickness of less than 3 mm was required for precisely pointing out bony edges in the two-dimensional reformation images. These results led us to conclude that volume computed tomography for longitudinal examination in craniofacial surgery should be kept to a minimum frequency and performed using a low-dose technique, small slice thickness, and large pitch.
