Chen Lingyun, Shaw Chris C, Altunbas Mustafa C, Lai Chao-Jen, Liu Xinming, Han Tao, Wang Tianpeng, Yang Wei T, Whitman Gary J
Department of Imaging Physics, University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030-4009, USA.
Med Phys. 2008 Aug;35(8):3482-90. doi: 10.1118/1.2948397.
This work is to demonstrate that high quality cone beam CT images can be generated for a volume of interest (VOI) and to investigate the exposure reduction effect, dose saving, and scatter reduction with the VOI scanning technique. The VOI scanning technique involves inserting a filtering mask between the x-ray source and the breast during image acquisition. The mask has an opening to allow full x-ray exposure to be delivered to a preselected VOI and a lower, filtered exposure to the region outside the VOI. To investigate the effects of increased noise due to reduced exposure outside the VOI on the reconstructed VOI image, we directly extracted the projection data inside the VOI from the full-field projection data and added additional data to the projection outside the VOI to simulate the relative noise increase due to reduced exposure. The nonuniform reference images were simulated in an identical manner to normalize the projection images and measure the x-ray attenuation factor for the object. Regular Feldkamp-Davis-Kress filtered backprojection algorithm was used to reconstruct the 3D images. The noise level inside the VOI was evaluated and compared with that of the full-field higher exposure image. Calcifications phantom and low contrast phantom were imaged. Dose reduction was investigated by estimating the dose distribution in a cylindrical water phantom using Monte Carlo simulation based Geant4 package. Scatter reduction at the detector input was also studied. Our results show that with the exposure level reduced by the VOI mask, the dose levels were significantly reduced both inside and outside the VOI without compromising the accuracy of image reconstruction, allowing for the VOI to be imaged with more clarity and helping to reduce the breast dose. The contrast-to-noise ratio inside the VOI was improved. The VOI images were not adversely affected by noisier projection data outside the VOI. Scatter intensities at the detector input were also shown to decrease significantly both inside and outside the VOI in the projection images, indicating potential improvement of image quality inside the VOI and contribution to dose reduction both inside and outside the VOI.
这项工作旨在证明可以针对感兴趣的体积（VOI）生成高质量的锥束CT图像，并研究VOI扫描技术的曝光减少效果、剂量节省和散射减少情况。VOI扫描技术包括在图像采集期间在X射线源和乳房之间插入一个过滤面罩。该面罩有一个开口，可使全剂量X射线照射预先选定的VOI，并使VOI以外的区域接受较低的过滤后的照射。为了研究VOI以外区域因曝光减少而增加的噪声对重建的VOI图像的影响，我们从全场投影数据中直接提取VOI内部的投影数据，并向VOI以外的投影添加额外数据，以模拟因曝光减少而导致的相对噪声增加。以相同方式模拟不均匀的参考图像，以归一化投影图像并测量物体的X射线衰减因子。使用常规的Feldkamp-Davis-Kress滤波反投影算法重建三维图像。评估VOI内部的噪声水平，并与全场高曝光图像的噪声水平进行比较。对钙化体模和低对比度体模进行成像。使用基于蒙特卡罗模拟的Geant4软件包估计圆柱形水体模中的剂量分布，研究剂量减少情况。还研究了探测器输入端的散射减少情况。我们的结果表明，通过VOI面罩降低曝光水平，VOI内外的剂量水平均显著降低，而不会影响图像重建的准确性，使VOI成像更清晰，并有助于降低乳房剂量。VOI内部的对比度噪声比得到改善。VOI以外区域噪声较大的投影数据对VOI图像没有不利影响。投影图像中探测器输入端的散射强度在VOI内外也均显著降低，表明VOI内部图像质量有潜在改善，并有助于VOI内外的剂量减少。
