Tomosynthesis is a technique of limited-angle tomography, which improves the detectability of pulmonary nodules compared to conventional X-ray radiography and acquires images with lower exposure dose than CT. Conventional tomosynthesis requires 10 seconds to obtain 60 projections because of the mechanical motion, and it is difficult for patients to hold the breath. We have proposed 4-projection tomosynthesis that allows acquisition of images in less than 1 second and significant reduction of radiation exposure. In this study, the optimal total scan angle for 4-projection tomosynthesis was examined. A chest phantom with artificial nodules was imaged every 1 degree, and the tomosynthesis images were reconstructed using various projection angles by Back Projection. To assess adequate parameters for image data acquisition, quantitative measures such as contrast noise ratio (CNR) and artifact spread function (ASF) were used to analyze tomosynthetic images. In 4-projection tomosynthesis, narrower projection angle was associated with greater CNR, while depth resolution tended to improve with wider projection angle. Thus, the CNR and depth resolution are in a trade-off relationship. In 4-projection tomosynthesis, the optimum total scan angle was considered to be less than 24 degrees due to the effect of the slice interval used in clinical systems.
