Study Of 3D Visualization Techniques In 3D Conformal Radiotherapy Treatment Planning

When 3D conformal radiotherapy (3D CRT) is employed for tumor treatment, the 3D visual information about tumor, including the volume and the positioning of the tumor, and as well as the relative position between the tumor and its adjacent tissues, should be obtained using 3D visualization techniques, so that the target is determined for virtual radiotherapy and program evaluation, thus accurate planning can be realized.In the thesis, the characteristics of the visual techniques are systematically analyzed, and the reconstruction of 3D medical image with high quality that can be applied for 3D CRT planning is emphasized, which can be run on a general-purpose PC with high efficiency. On this purpose, some improvements for current used methods and innovative algorithms are proposed, and a 3D visual system is developed for 3D CRT by using those proposed techniques.Firstly, based on the aim of 3D visualization for 3D CRT planning, volume visualization methods for medicine designed in modularization are studied, they are: the segmentation module, which is based on hybrid-link region growing method and cluster analysis algorithm for the purpose of the segment of the tissue to be reconstructed; the registration module, which is based on maximum mutual information method to realize the registration of anatomical image (CT) and functional image (PET); the fusion module, which is based on wavelet theory with fusion rule of combining the local standard deviation and energy to realize the fusion of PET and CT images; the volume visualization module, which is based on the improved Shear-Warp algorithm on the basis of ray absorption and emission model to realize volume rending of the series of fused images. Those works establish the foundation for further research.Secondly, for the solution of optimum transfer function, an improved Shear-Warp algorithm(ISWA) is proposed. An optimal transfer function can be conveniently found by combining boundary model and interactive operation when pre-specifing transfer function. In re-sampling process, the parameters of the transfer function is modified to guarantee the reconstruction effect of the image.Thirdly, as to the shortcoming of perspective Shear-Warp algorithm, an improved perspective volume rending algorithm (IPVRA) is developed. In which rapid classification coding for volume data is implemented by making use of the correlation of opacity transfer function and min-max Octree data structure. On this basis, the new run-length encoded(RLE) data structure can be established rapidly so that fast reconstruction can be done when opacity transfer function and view angle change. Moreover, the aliasing artifacts resulted from under-sampling can be eliminated by pre-integrated volume rendering technique incorporated the shading calculation of Phong lighting model, thus improving the reconstruction effect.Fourthly, in order to enhance the quality and efficiency of multi-resolution volume rendering method, a fast multi-resolution volume rending method (FMVRM) is proposed by combining volume rendering and data compress and decompress technique. The technique is as follows: in the beginning, data compress is carried using wavelet transformation, then according to the chosen resolution, data decompression guided by opacity transfer function (OTF) is implemented. After that, based on Shear-Warp algorithm, and by making use of block-based run-length encoded data structure,the correlation of opacity transfer function and pre-integrated volume rendering technique incorporated the shading calculation of Phong lighting model, a fast volume rending with high quality can be realized.In order to improve the reconstruction efficiency of medical image, and sufficiently display the internal hidden interfaces and the details of internal information, an improved volume rendering optical model(IVROM) for translucent volume rendering is developed, which can be rapidly executed on a general-purpose PC. In the model, the lighting absorption and emission model are employed, moreover, the factors of volumetric shadowing, direct and indirect emission are also considered. In addition, the realization of translucent volume rendering method combining the IVROM and Shear-Warp algorithm is presented in detail. In order to enhance the visual effect and reconstruction efficiency of hybrid volume rendering of polygonal models and different-resolution medical volume data running on a general-purpose PC, a hybrid volume rendering algorithm is proposed based on Shear-Warp algorithm. In which, various data for reconstruction are first pre-processed by employing Z-Buffer algorithm and RLE data structure, then a dual re-sampling method is chosen according to accuracy requirement for compositing intermediate image and the intermediate slice interpolation method is employed to solve the problem of over-sparse of re-sampling. Finally, the reconstructed image is rendered using OpenGL texture-mapping technique.Finally, a 3D visualization system for radiotherapy treatment planning is established based on the above technique. Experiments demonstrate that the 3D visual information of the tumor and target volume can be precisely determined using the proposed strategies, thus providing a good means for radiotherapy treatment and program evaluation.