Optimized 3-D CT scan protocol for longitudinal morphological estimation in craniofacial surgery

Frequent three-dimensional computed tomography scanning may cause deteriora tion 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 optim ized three-dimensional computed tomography scan protocol for longitudinal m orphological 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 param eters of varied values. The radiation doses induced by each scanning were m easured 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 a ccumulated data set. For each scan parameter, the images generated from dat a accumulated using different values were compared. The study showed that l ens radiation dose increased with tube potential and was almost directly pr oportional to tube current and 1/pitch. The slice thickness did not affect lens radiation doses significantly. Images with good contrast resolution an d low artifact level sufficient for estimating morphological changes were o btained 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 m m 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 k ept to a minimum frequency and performed using a low-dose technique, small slice thickness, and large pitch.