Department of Radiology, University of Michigan, Ann Arbor, MI, USA.
Department of Electrical and Computer Engineering, University of Michigan, Ann Arbor, MI, USA.
Med Phys. 2019 Dec;46(12):5572-5592. doi: 10.1002/mp.13801. Epub 2019 Oct 20.
Most digital breast tomosynthesis (DBT) reconstruction methods neglect the blurring of the projection views caused by the finite size or motion of the x-ray focal spot. This paper studies the effect of source blur on the spatial resolution of reconstructed DBT using analytical calculation and simulation, and compares the influence of source blur over a range of blurred source sizes.
Mathematically derived formulas describe the point spread function (PSF) of source blur on the detector plane as a function of the spatial locations of the finite-sized source and the object. By using the available technical parameters of some clinical DBT systems, we estimated the effective source sizes over a range of exposure time and DBT scan geometries. We used the CatSim simulation tool (GE Global Research, NY) to generate digital phantoms containing line pairs and beads at different locations and imaged with sources of four different sizes covering the range of potential source blur. By analyzing the relative contrasts of the test objects in the reconstructed images, we studied the effect of the source blur on the spatial resolution of DBT. Furthermore, we simulated a detector that rotated in synchrony with the source about the rotation center and calculated the spatial distribution of the blurring distance in the imaged volume to estimate its influence on source blur.
Calculations demonstrate that the PSF is highly shift-variant, making it challenging to accurately implement during reconstruction. The results of the simulated phantoms demonstrated that a typical finite-sized focal spot (~0.3 mm) will not affect the reconstructed image resolution if the x-ray tube is stationary during data acquisition. If the x-ray tube moves during exposure, the extra blur due to the source motion may degrade image resolution, depending on the effective size of the source along the direction of the motion. A detector that rotates in synchrony with the source does not reduce the influence of source blur substantially.
This study demonstrates that the extra source blur due to the motion of the x-ray tube during image acquisition substantially degrades the reconstructed image resolution. This effect cannot be alleviated by rotating the detector in synchrony with the source. The simulation results suggest that there are potential benefits of modeling the source blur in image reconstruction for DBT systems using continuous-motion acquisition mode.
大多数数字乳腺断层合成（DBT）重建方法忽略了由于 X 射线焦点的有限大小或运动而导致的投影视图模糊。本文通过分析计算和模拟研究了源模糊对重建 DBT 空间分辨率的影响，并比较了在一系列模糊源尺寸下的源模糊影响。
数学推导的公式描述了源模糊在探测器平面上的点扩散函数（PSF）作为有限大小源和物体的空间位置的函数。通过使用一些临床 DBT 系统的可用技术参数，我们估计了在不同曝光时间和 DBT 扫描几何形状下的有效源尺寸范围。我们使用 CatSim 模拟工具（GE Global Research，NY）生成包含线对和珠子的数字体模，这些线对和珠子位于不同位置，并使用覆盖潜在源模糊范围的四个不同尺寸的源进行成像。通过分析重建图像中测试对象的相对对比度，我们研究了源模糊对 DBT 空间分辨率的影响。此外，我们模拟了一个与源同步围绕旋转中心旋转的探测器，并计算了成像体积中模糊距离的空间分布，以估计其对源模糊的影响。
计算表明，PSF 高度移位变化，使得在重建过程中难以准确实现。模拟体模的结果表明，如果在数据采集期间 X 射线管保持静止，典型的有限大小焦点（~0.3mm）不会影响重建图像的分辨率。如果在曝光期间 X 射线管移动，则由于源运动引起的额外模糊可能会降低图像分辨率，具体取决于沿运动方向的源的有效尺寸。与源同步旋转的探测器并不能显著降低源模糊的影响。
本研究表明，在图像采集期间由于 X 射线管的运动而导致的额外源模糊会显著降低重建图像的分辨率。通过与源同步旋转探测器无法缓解这种影响。模拟结果表明，对于使用连续运动采集模式的 DBT 系统，在图像重建中对源模糊进行建模可能具有潜在的益处。
