The three-dimensional configuration information of each vertebra is essential for surgery of a scoliotic spine. The authors have developed biplane X-ray photogrammetry for the cofiguration measurement and analysis. XCT (X-ray computer tomography) is generally applied to these patients, and more than two scout images of them from different angulations of the tube are taken to determine which part of the body is suitable for tomography. Two scout images from different angulations are sufficient for the reconstruction of the 3D configuration of the vertebra in the line of biplane X-ray photogrammetry. This new method will be called bidirectional CT scout imaging by the authors. This paper shows its basic equations and its accuracy. It was concluded that this is an effective clinical method for the reconstruction of the configuration of the spine.