Department of Medical Physics, University of Patras, Patras, Greece.
Med Phys. 2012 Sep;39(9):5621-34. doi: 10.1118/1.4747268.
This study investigates the image quality of tomosynthesis slices obtained from several acquisition sets with synchrotron radiation using a breast phantom incorporating details that mimic various breast lesions, in a heterogeneous background.
A complex Breast phantom (MAMMAX) with a heterogeneous background and thickness that corresponds to 4.5 cm compressed breast with an average composition of 50% adipose and 50% glandular tissue was assembled using two commercial phantoms. Projection images using acquisition arcs of 24°, 32°, 40°, 48°, and 56° at incident energy of 17 keV were obtained from the phantom with the synchrotron radiation for medical physics beamline at ELETTRA Synchrotron Light Laboratory. The total mean glandular dose was set equal to 2.5 mGy. Tomograms were reconstructed with simple multiple projection algorithm (MPA) and filtered MPA. In the latter case, a median filter, a sinc filter, and a combination of those two filters were applied on the experimental data prior to MPA reconstruction. Visual inspection, contrast to noise ratio, contrast, and artifact spread function were the figures of merit used in the evaluation of the visualisation and detection of low- and high-contrast breast features, as a function of the reconstruction algorithm and acquisition arc. To study the benefits of using monochromatic beams, single projection images at incident energies ranging from 14 to 27 keV were acquired with the same phantom and weighted to synthesize polychromatic images at a typical incident x-ray spectrum with W target.
Filters were optimised to reconstruct features with different attenuation characteristics and dimensions. In the case of 6 mm low-contrast details, improved visual appearance as well as higher contrast to noise ratio and contrast values were observed for the two filtered MPA algorithms that exploit the sinc filter. These features are better visualized at extended arc length, as the acquisition arc of 56° with 15 projection images demonstrates the highest image reconstruction quality. For microcalcifications, filtered MPA implemented with a combination of median and sinc filters indicates better feature appearance due to efficient suppression of background tissue. The image quality of these features is less sensitive to the acquisition arc. Calcifications with size ranging from 170 to 500 μm, like the ones presently studied, are well identified and visualized for all arcs used. The comparison of single projection images obtained under different beam conditions showed that the use of monochromatic beam can produce an image with higher contrast and contrast to noise ratio compared to an image corresponding to a polychromatic beam even when the latter is acquired with double incident exposure.
Filter optimization in respect to the type of feature characteristics is important before the reconstruction. The MPA combined with median and sinc filters results in improved reconstruction of microcalcifications and low-contrast features. The latter are better visualized at extended arc length, while microcalcifications are less sensitive to this acquisition parameter. Use of monochromatic beams may result in tomographic images with higher contrast acquired at lower incident exposures.
本研究使用包含各种乳房病变模拟细节的乳房体模，在异质背景下，调查利用同步辐射从多个采集集获得的断层合成切片的图像质量。
使用两个商业体模组装了一个具有异质背景和厚度的复杂乳房体模（MAMMAX），其对应于平均组成 50%脂肪和 50%腺体组织的 4.5 厘米压缩乳房。使用 24°、32°、40°、48°和 56°的采集弧，在 17keV 的入射能下，从 ELETTRA 同步辐射实验室的医学物理光束线的同步辐射体模中获得投影图像。总平均腺体剂量设置为 2.5mGy。使用简单的多投影算法（MPA）和滤波 MPA 对断层合成进行重建。在后一种情况下，在 MPA 重建之前，在实验数据上应用了中值滤波器、 sinc 滤波器和这两种滤波器的组合。视觉检查、对比噪声比、对比度和伪影扩展函数是评估低对比度和高对比度乳房特征可视化和检测的重要指标，这取决于重建算法和采集弧。为了研究使用单色光束的好处，使用相同的体模获得了从 14 到 27keV 的入射能的单个投影图像，并对其进行加权以合成典型入射 X 射线谱（W 靶）的多色图像。
优化了滤波器，以重建具有不同衰减特性和尺寸的特征。对于 6mm 的低对比度细节，对于利用 sinc 滤波器的两种滤波 MPA 算法，观察到了更好的视觉外观以及更高的对比噪声比和对比度值。在扩展的采集弧长下，这些特征的可视化效果更好，因为具有 15 个投影图像的 56°采集弧展示了最高的图像重建质量。对于微钙化，使用中值和 sinc 滤波器组合实现的滤波 MPA 表明由于对背景组织的有效抑制，特征的外观更好。这些特征的图像质量对采集弧的敏感性较低。对于尺寸范围为 170 至 500μm 的钙化，如本研究中所研究的那样，所有使用的弧都可以很好地识别和可视化。在不同光束条件下获得的单个投影图像的比较表明，与多色光束对应的图像相比，即使后者是在两倍入射曝光下采集的，使用单色光束也可以产生对比度和对比噪声比更高的图像。
在重建之前，根据特征类型进行滤波器优化很重要。MPA 与中值和 sinc 滤波器结合使用，可改善微钙化和低对比度特征的重建。在扩展的采集弧长下，后者的可视化效果更好，而微钙化对该采集参数的敏感性较低。使用单色光束可能会导致在较低入射曝光下获得对比度更高的断层合成图像。
