Contributions to the improvement of image quality in CBCT and CBμCT and application in the development of a CBμCT system

摘要

During the last years cone-beam x-ray CT (CBCT) has been established as a widespreadimaging technique and a feasible alternative to conventional CT for dedicated imagingtasks for which the limited flexibility offered by conventional CT advises thedevelopment of dedicated designs. CBCT systems are starting to be routinely used inimage guided radiotherapy; image guided surgery using C-arms; scan of body partssuch as the sinuses, the breast or extremities; and, especially, in preclinical small-animalimaging, often coupled to molecular imaging systems.Despite the research efforts advocated to the advance of CBCT, the challengesintroduced by the use of large cone angles and two-dimensional detectors are a field ofvigorous research towards the improvement of CBCT image quality. Moreover, systemsfor small-animal imaging add to the challenges posed by clinical CBCT the need ofhigher resolution to obtain equivalent image quality in much smaller subjects.This thesis contributes to the progress of CBCT imaging by addressing a variety ofissues affecting image quality in CBCT in general and in CBCT for small-animalimaging (CBμCT).As part of this work we have assessed and optimized the performance of CBμCTsystems for different imaging tasks. To this end, we have developed a new CBμCTsystem with variable geometry and all the required software tools for acquisition,calibration and reconstruction. The system served as a tool for the optimization of theimaging process and for the study of image degradation effects in CBμCT, as well as aplatform for biological research using small animals. The set of tools for the accuratestudy of CBCT was completed by developing a fast Monte Carlo simulation enginebased on GPUs, specifically devoted to the realistic estimation of scatter and its effectson image quality in arbitrary CBCT configurations, with arbitrary spectra, detectorresponse, and antiscatter grids. This new Monte Carlo engine outperformed currentsimulation platforms by more than an order of magnitude.Due to the limited options for simulation of spectra in microfocus x-ray sources used inCBμCT, we contributed in this thesis a new spectra generation model based on anempirical model for conventional radiology and mammography sources modified in accordance to experimental data. The new spectral model showed good agreement withexperimental exposure and attenuation data for different materials.The developed tools for CBμCT research were used for the study of detectorperformance in terms of dynamic range. The dynamic range of the detector wascharacterized together with its effect on image quality. As a result, a new simple methodfor the extension of the dynamic range of flat-panel detectors was proposed andevaluated. The method is based on a modified acquisition process and a mathematicaltreatment of the acquired data.Scatter is usually identified as one of the major causes of image quality degradation inCBCT. For this reason the developed Monte Carlo engine was applied to the in-depthstudy of the effects of scatter for a representative range of CBCT embodiments used inthe clinical and preclinical practice. We estimated the amount and spatial distribution ofthe total scatter fluence and the individual components within. The effect of antiscattergrids in improving image quality and in noise was also evaluated. We found a closerelation between scatter and the air gap of the system, in line with previous results in theliterature. We also observed a non-negligible contribution of forward-directed scatterthat is responsible to a great extent for streak artifacts in CBCT. The spatial distributionof scatter was significantly affected by forward scatter, somewhat challenging the usualassumption that the scatter distribution mostly contains low-frequencies. Antiscattergrids showed to be effective for the reduction of cupping, but they showed a muchlower performance when dealing with streaks and a shift toward high frequencies of thescatter distributions. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------