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In USA, breast cancer is a most frequent cause of deaths for women. It is important to detect the cancer in its early stage. X-ray three-dimensional (3D) mammography can provide a good image resolution and contrast However, the associated radiation is relatively high. Reduction of the soft X-ray radiation for 3D mammography has been a research focus in the past years. In a typical 3D mammography system, the X-ray source and detector rotate around the object (breast) beneath the table, on which the patient lies in a prone position. In order to sample the data as close as possible to the chest base, a circular orbit with half cone-beam geometry has been investigated. It can provide very good reconstruction if the X-ray source is far away from the object. For a relatively short distance between the source and the object for an improved spatial resolution, the circular orbit may not be an optimal choice. In this case, the portion far away from the circular orbit wouldn't be well reconstructed because of the missing of projection data in that region. In this work, we investigated five possible orbits, attempting to find an optimal orbit that can reconstruct satisfactorily the whole object with least projections (less radiation). The results showed that two near half-circular orbits may be a choice, one near the chest base and the other near the breast tip. The redundant samplings beyond 180° were eliminated by our algorithm, rendering very good reconstructions.