在专用锥形束乳腺 CT 的散射校正中离焦辐射的作用。
The role of off-focus radiation in scatter correction for dedicated cone beam breast CT.
机构信息
Nuclear and Radiological Engineering and Medical Physics Programs, The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA.
Department of Radiology, Stanford University, Palo Alto, CA, 94305, USA.
出版信息
Med Phys. 2018 Jan;45(1):191-201. doi: 10.1002/mp.12686. Epub 2017 Dec 16.
PURPOSE
Dedicated cone beam breast CT (CBBCT) suffers from x-ray scatter contamination. We aim to identify the source of the significant difference between the scatter distributions estimated by two recent methods proposed by our group and to investigate its effect on CBBCT image quality.
METHOD
We recently proposed two novel methods of scatter correction for CBBCT, using a library based (LB) technique and a forward projection (FP) model. Despite similar enhancement on CBBCT image qualities, these two methods obtain very different scatter distributions. We hypothesize that the off-focus radiation (OFR) is the contributor and results in nontrivial signals in x-ray projections, which is ignored in the scatter estimation via the LB method. Experiments using a thin wire test tool are designed to study the effect of OFR on CBBCT spatial resolution by measuring the point spread function (PSF) and the modulation transfer function (MTF). A narrow collimator setting is used to suppress the OFR-induced signals. In addition, "PSFs" and "MTFs" are measured on clinical CBBCT images obtained by the LB and FP methods using small calcifications as point sources. The improvement of spatial resolution achieved by suppressing OFR in the wire experiment as well as in the clinical study is quantified by the improvement ratios of PSFs and spatial frequencies at different MTF values. Our hypothesis that OFR causes the imaging difference between the FP and LB methods is verified if these ratios obtained from experimental and clinical data are consistent.
RESULTS
In the wire experiment, the results show that suppression of OFR increases the maximum signal of the PSF by about 14% and reduces the full-width-at-half-maximum (FWHM) by about 12.0%. Similar improvement on spatial resolution is achieved by the FP method compared with the LB method in the patient study. The improvement ratios of spatial frequencies at different MTF values without OFR match very well in both studies at a level of around 16%, with an average root-mean-square difference of 0.47%.
CONCLUSION
The results of the wire experiment and the clinical study indicate that the main difference between the LB and FP methods is whether the OFR-induced signals are included after scatter correction. Our study further shows that OFR significantly affects the image spatial resolution of CBBCT, indicating that the visualization of micro-calcifications is susceptible to OFR contamination. Our finding is therefore important in further improvement of diagnostic performance of CBBCT.
目的
专用锥形束乳腺 CT（CBBCT）存在 X 射线散射污染。我们旨在确定两种最近由我们小组提出的散射校正新方法所估计的散射分布之间存在显著差异的来源，并研究其对 CBBCT 图像质量的影响。
方法
我们最近提出了两种新的 CBBCT 散射校正方法，使用基于库的（LB）技术和正向投影（FP）模型。尽管这两种方法对 CBBCT 图像质量都有类似的增强效果，但它们得到的散射分布却非常不同。我们假设离焦辐射（OFR）是造成这种差异的原因，并且在 LB 方法的散射估计中忽略了其在 X 射线投影中的非平凡信号。设计了使用细金属丝测试工具的实验，通过测量点扩散函数（PSF）和调制传递函数（MTF）来研究 OFR 对 CBBCT 空间分辨率的影响。使用窄准直器设置来抑制 OFR 引起的信号。此外，使用 LB 和 FP 方法在临床 CBBCT 图像上测量了小钙化点作为点源的“PSF”和“MTF”。通过 wire 实验和临床研究中抑制 OFR 来量化 PSF 和空间频率在不同 MTF 值下的改善程度，以定量评估 OFR 对空间分辨率改善的效果。如果从实验和临床数据中获得的这些比值一致，则验证 OFR 导致 FP 和 LB 方法之间成像差异的假设成立。
结果
在 wire 实验中，结果表明抑制 OFR 可使 PSF 的最大信号增加约 14%，并使半峰全宽（FWHM）减少约 12.0%。在患者研究中，与 LB 方法相比，FP 方法在空间分辨率上也取得了类似的改善。在没有 OFR 的情况下，不同 MTF 值的空间频率的改善比在两项研究中都非常吻合，平均均方根差为 0.47%。
结论
wire 实验和临床研究的结果表明，LB 和 FP 方法之间的主要区别在于散射校正后是否包含了 OFR 引起的信号。我们的研究进一步表明，OFR 会显著影响 CBBCT 的图像空间分辨率，表明微钙化的可视化容易受到 OFR 污染的影响。因此，我们的发现对于进一步提高 CBBCT 的诊断性能非常重要。
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