Research On Calibration Techniques Of VCT Image

Cone-Beam Volume CT(CBVCT) refers to a technique for imaging all cross-sections of an object using 3D image reconstruction algorithms based on a series of X-ray projections captured by a flat panel detector(FPD) at different angles around the object. Owing to the advantages of faster scanning speed, higher spatial resolution and better utilization of X-ray photons compared with traditional 2D CT, cone-beam CT has a broad application perspective in industrial non-destructive testing and medical applications, and has been a hot research topic in the current international CT communities.It is through reconstructed slice image that industrial CT can obtain inner information including 3D structure and ingredients of objects. Therefore, the quality of slice image is one of the most important factors in ICT application. Aiming at enhance slice quality and basing on CBVCT system, the dissertation discusses several factors that impair slice quality, and gives specialized calibration methods followed by experimental testing. Focused research and achievements are as follows:1. Calibration of orientation parameters: Basing on artifacts in reconstructed slice caused by shifted projection center, rotated and tilted FPD, inevitable errors in distance measuring from x-ray focal to rotation center and distance from x-ray focal to FPD, a new precise orientation parameters tracing technique named Thin-Piano-Wire's Slices Evaluation Based Golden Search technique is proposed, whose input is initial measured parameters. Utilizing CT projection geometry, initial measurement technique of orientation parameters and calibration method is also proposed. Finally, experiment tests the effectiveness of the technique.2. Correction of FPD output: Impact on reconstructed slice caused by the nonuniformity of x-ray field and FPD's inherent drawbacks including Dark current, nonuniformity of gain and bad pixels is analyzed. And basing on analyzing of CT principles, the model of FPD output is proposed, whichleads to acquisition of dark image, x-ray field image, gain image, bad pixel image and bad pattern map. Using these images, calibration method is proposed to eliminate artifacts in reconstructed slices. Following experiment tests the effectiveness of the technique.3. Calibration of beam hardening: Basing on causes of beam hardening and its impact on CT slice, an improved polynomial correction method for unimaterial part is proposed, which is the union of practical projection and simulating projection. Experimentation tests the technique is of effectiveness.