Badal Andreu, Clark Matthew, Ghammraoui Bahaa
U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Imaging, Diagnostics and Software Reliability, Silver Spring, Maryland, United States.
University of Maryland College Park, Department of Mechanical Engineering, College Park, Maryland, United States.
J Med Imaging (Bellingham). 2018 Jul;5(3):033501. doi: 10.1117/1.JMI.5.3.033501. Epub 2018 Jul 12.
Mammography is currently the standard imaging modality used to screen women for breast abnormalities, and, as a result, it is a tool of great importance for the early detection of breast cancer. Physical phantoms are commonly used as surrogates of breast tissue to evaluate some aspects of the performance of mammography systems. However, most phantoms do not reproduce the anatomic heterogeneity of real breasts. New fabrication technologies, such as three-dimensional (3-D) printing, have created the opportunity to build more complex, anatomically realistic breast phantoms that could potentially assist in the evaluation of mammography systems. The reproducibility and relative low cost of 3-D printed objects might also enable the development of collections of representative patient models that could be used to assess the effect of anatomical variability on system performance, hence making bench testing studies a step closer to clinical trials. The primary objective of this work is to present a simple, easily reproducible methodology to design and print 3-D objects that replicate the attenuation profile observed in real two-dimensional mammograms. The secondary objective is to evaluate the capabilities and limitations of the competing 3-D printing technologies and characterize the x-ray properties of the different materials they use. Printable phantoms can be created using the open-source code introduced, which processes a raw mammography image to estimate the amount of x-ray attenuation at each pixel, and outputs a triangle mesh object that encodes the observed attenuation map. The conversion from the observed pixel gray value to a column of printed material with equivalent attenuation requires certain assumptions and knowledge of multiple imaging system parameters, such as x-ray energy spectrum, source-to-object distance, compressed breast thickness, and average breast material attenuation. To validate the proposed methodology, x-ray projections of printed phantoms were acquired with a clinical mammography system. The quality of the printing process was evaluated by comparing the mammograms of the printed phantoms and the original mammograms used to create the phantoms. The structural similarity index and the root-mean-square error were used as objective metrics to compare the two images. A detailed description of the software, a characterization of the printed materials using x-ray spectroscopy, and an evaluation of the realism of the sample printed phantoms are presented.
乳房X线摄影术是目前用于筛查女性乳房异常情况的标准成像方式，因此，它是早期发现乳腺癌的一项极为重要的工具。物理体模通常被用作乳房组织的替代物，以评估乳房X线摄影系统性能的某些方面。然而，大多数体模无法再现真实乳房的解剖异质性。诸如三维（3-D）打印等新制造技术为构建更复杂、解剖结构更逼真的乳房体模创造了机会，这些体模可能有助于评估乳房X线摄影系统。3-D打印物体的可重复性和相对较低的成本也可能促使开发具有代表性的患者模型集合，可用于评估解剖变异性对系统性能的影响，从而使台架测试研究更接近临床试验。这项工作的主要目标是提出一种简单、易于复制的方法，用于设计和打印能够复制在真实二维乳房X线照片中观察到的衰减剖面的3-D物体。次要目标是评估相互竞争的3-D打印技术的能力和局限性，并表征它们所使用的不同材料的X射线特性。可以使用引入的开源代码创建可打印体模，该代码处理原始乳房X线照片以估计每个像素处的X射线衰减量，并输出对观察到的衰减图进行编码的三角形网格物体。从观察到的像素灰度值转换为具有等效衰减的一列打印材料需要某些假设以及对多个成像系统参数的了解，例如X射线能谱、源到物体的距离、压缩乳房厚度和平均乳房材料衰减。为了验证所提出的方法，使用临床乳房X线摄影系统获取打印体模的X射线投影。通过比较打印体模的乳房X线照片和用于创建体模的原始乳房X线照片来评估打印过程的质量。结构相似性指数和均方根误差被用作比较这两张图像的客观指标。本文还介绍了软件的详细描述、使用X射线光谱对打印材料的表征以及对打印体模样本逼真度的评估。
