BACKGROUND: Few studies have quantified the projection/distortion errors that occur in anteroposterior (AP) radiographs.

OBJECTIVES: To quantify the projection/distortion errors on AP radiographs caused by lateral translation of the vertebrae. To demonstrate the effect of vertebral shape on projection errors.

STUDY DESIGN: Three models of increasing complexity were constructed to document the distortion. Model 1 consisted of three metal pins embedded in a piece of metal tubing that was X-rayed in three positions. Model 2, a simple model of the X-ray beam mounted on a wooden platform, allowed for translation of a vertebral model in a simulated X-ray beam and the measurement of projected points of contact between the model and rays of the simulated beam. Model 3 is a computer simulation of the X-ray beam in which a vertebral model was translated laterally to varying locations. The projected points of contact between the simulated rays and the vertebral body margins and lamina junction were measured. Model 3 also showed that vertebral body shape has a large effect on the projected axial rotation. Two other simple models were created and discussed in relation to shape-dependent projection errors.

RESULTS: X-axis translation results in projected y-axis rotation. Increasing magnitudes of x-axis vertebral translation results in increasing magnitudes of projected y-axis vertebral rotation. The projected rotation is also influenced by vertebral shape.

CONCLUSION: We have shown that three-dimensional lateral translation projects as axial rotation on the AP radiograph. Projection error is largely influenced by the shape of the object and by the increasing obliquity of the rays of the X-ray beam. This would seemingly create confusion and invalidate spinal listings of vertebral position obtained from the AP radiographic image.

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