A Geometric Calibration Method For Cone Beam CT System

This paper’s work is on the base of Medical Molecular Imaging System of Small Animal,which is a project aiming at combining four imaging modules into one integrated imagingsystem. The four modules include CT (Computed Tomography), Micro-PET (PositronEmission Tomography), SPECT (Single-Photon Emission Computed Tomography) and FMT(Fluorescence Molecular Tomography). The focus of this paper is the geometric calibrationmethod of the CT part, a CBCT (Cone Beam CT). CBCT is also known as Micro-CT or smallanimal CT, and it’s an anatomical or structural imaging technique while its imaging principleis based on X-ray attenuation. CBCT has advantages of non-invasive and in-vivo imaging,especially the high3D reconstruction resolution at the micron scale, which make it gainsextensive attention and rapid development since born, and has become an important researchmean in gene therapy, cancer research, drug development and materials science, industrialNon-Destructive Testing area.In this paper, a dedicated calibration phantom was designed to calibrate the geometricmisalignment of the CBCT system. The features of the phantom’s projects were used toestimate the geometric misalignment parameters of the system and then adjust the deviceswhich includes the X-ray source and the flat panel detector to right positions. The purpose ofthis method is to minimum the geometric misalignment of the CBCT system as best aspossible.This paper introduced the high performance of CBCT and its important role in multipleareas, then the generation of geometric misalignment of CBCT system and the degradation itcaused on the quality of reconstruction images. The research status at home and abroad ofCBCT geometric calibration has been presented also.The operating principle, system constitution of CBCT and main performance of devicessuch as X-ray source and flat panel detector has been discussed then, also the theory of FDKreconstruction algorithm and its demanding requirements to CBCT system geometry. Then this paper analyzed the geometric characteristics of CBCT system with a verticalstructure, and discussed the geometric parameters of a system with misaligned geometry.Based on the existing CBCT system, the characteristics of the calibration phantom which hasbeen designed dedicatedly and its calibration principle have been presented, also the details ofexperiment process. In order to prove the feasibility and validity of this method, anotherphantom was used to calibrate the same system as a comparison experiment. The theoreticalanalyses of alignment accuracy indicated that the two methods are comparable and thededicated phantom should bring a better effectiveness. The tomographic reconstructionimages of a copper wire indicate a high calibration accuracy of the two methods, and thecomparison between the images of two methods proves obviously greater accuracy and higherresolution of the method of the dedicated phantom. Furthermore, a QRM micro-CT phantomwas scanned and reconstructed by this CBCT system before and after it was calibrated by themethod of the dedicated phantom, with the purpose of proving the effectiveness of the methodmore quantifiably. The QRM phantom is a standard phantom, which can be used to assess thespatial resolution of CBCT system intuitively. And according to the reconstruction of theQRM phantom’s section, the calibration method of the dedicated phantom has achieved a highspatial resolution which is lower than50μm but higher than100μm, and this is higher than150μm, the expected standard when this CBCT system was designed at the beginning.In the end, a chicken feet was scanned by the CBCT system after it was calibrated toreconstruct the bones. From the reconstructed images, the size and shape of each bone can beobserved clearly, even the minute depressions on the surface of each bone. The reconstructedimages of a conch, a mantis shrimp, and a crucian also show that the spatial resolution of thereconstructed images is relatively high. Experiments prove high calibration accuracy and highreconstruction resolution, and well meets the requirement of practical application.