技术注释：密度校正以改进胸部变形图像配准中的 CT 数映射。

Technical Note: Density correction to improve CT number mapping in thoracic deformable image registration.

机构信息

Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, USA.

Proton Therapy Center, University of Cincinnati Medical Center, Liberty Township, OH, USA.

出版信息

Med Phys. 2019 May;46(5):2330-2336. doi: 10.1002/mp.13502. Epub 2019 Apr 1.

DOI:10.1002/mp.13502

PMID:30896047

原文链接:https://pmc.ncbi.nlm.nih.gov/articles/PMC6510598/

Abstract

PURPOSE

To improve the accuracy of computed tomography (CT) number mapping inside the lung in deformable image registration with large differences in lung volume for applications in vertical CT imaging and adaptive radiotherapy.

METHODS

The deep inspiration breath hold (DIBH) CT image and the end of exhalation (EE) phase image in four-dimensional CT of 14 thoracic cancer patients were used in this study. Lung volumes were manually delineated. A Demons-based deformable registration was first applied to register the EE CT to the DIBH CT for each patient, and the resulting deformation vector field deformed the EE CT image to the DIBH CT space. Given that the mass of the lung remains the same during respiration, we created a mass-preserving model to correlate lung density variations with volumetric changes, which were characterized by the Jacobian derived from the deformation field. The Jacobian determinant was used to correct the lung CT numbers transferred from the EE CT image. The absolute intensity differences created by subtracting the deformed EE CT from the DIBH CT with and without density correction were compared.

RESULTS

The ratio of DIBH CT to EE CT lung volumes was 1.6 on average. The deformable registration registered the lung shape well, but the appearance of voxel intensities inside the lung was different, demonstrating the need for density correction. Without density correction, the mean and standard deviation of the absolute intensity difference between the deformed EE CT and the DIBH CT inside the lung were 54.5 ± 45.5 for all cases. After density correction, these numbers decreased to 18.1 ± 34.9, demonstrating greater accuracy. The cumulative histogram of the intensity difference also showed that density correction improved CT number mapping greatly.

CONCLUSION

Density correction improves CT number mapping inside the lung in deformable image registration for difficult cases with large lung volume differences.

摘要

目的

提高在体积变化较大的肺部情况下进行形变图像配准中肺部 CT 数映射的准确性，以便应用于垂直 CT 成像和自适应放疗。

方法

本研究使用了 14 例胸部癌症患者的四维 CT 中的深吸气屏气（DIBH）CT 图像和呼气末（EE）相图像。手动勾画了肺体积。首先，针对每个患者，将基于 Demons 的形变配准应用于将 EE CT 配准到 DIBH CT，所得到的形变向量场将 EE CT 图像变形到 DIBH CT 空间。考虑到呼吸过程中肺的质量保持不变，我们创建了一个质量保持模型，将肺密度变化与体积变化相关联，这由形变场得到的雅可比行列式来描述。雅可比行列式用于校正从 EE CT 图像传输的肺 CT 数。通过从 DIBH CT 减去有和没有密度校正的变形 EE CT，比较所创建的绝对强度差异。

结果

DIBH CT 与 EE CT 肺体积的比值平均为 1.6。形变配准很好地注册了肺的形状，但肺内体素强度的外观不同，表明需要密度校正。没有密度校正时，所有病例肺内变形 EE CT 和 DIBH CT 之间的绝对强度差异的平均值和标准差为 54.5±45.5。校正后，这些数字下降到 18.1±34.9，显示出更高的准确性。强度差异的累积直方图也表明，密度校正极大地改善了 CT 数映射。