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X-ray cone-beam CT(CBCT)system is becoming an indispensable modality in the image guidance for radiation therapy and other clinical procedures.Major components of a CBCT scanner include x-ray tube and large-area flat panel detector.The system is designed as open-gantry geometry to facilitate its incorporation into treatment process.The hardware configuration of CBCT enables its on-board capability to detect the two-dimensional x-ray projections passing through a large illuminated volume of an object,and then to reconstruct three-dimensional volumetric images.Thus CBCT system acquires accurate patient geometry during treatment and is applied in routine clinical procedures,such as the patient setup in radiation therapy.Can CBCT be applied in advanced clinical applications,e.g.,diagnosis or treatment evaluation? Current performance of CBCT are hindered by several major bottlenecks,including severe shading artifacts due to,e.g.,scatter and beam hardening effects,mono-energetic x-ray spectrum and high accumulated dose in repeated scans.Quantitative and safe CBCT imaging is on urgent demand for its advanced applications.In this presentation,quantitative CBCT schemes will be introduced using effective shading correction and iterative low-dose CT reconstruction with no relying on the prior knowledge of the object.Image segmentation technique is applied in this scheme to adaptively extract the contours of the objects.Shading correction suppresses the severe artifacts in the CBCT images,and reduces the CT number error around 30 HU.Iterative reconstruction is incorporated for low-dose CT imaging using sparse-view strategy and reduces the projection number by 60%while retaining the high spatial resolution.The work has the great potential to facilitate the advanced use of CBCT and promote the clinical outcomes.