Department of Electrical and Computer Engineering, and Carl E. Ravin Advanced Imaging Laboratories, Department of Radiology, Duke University, Durham, NC, 27705, USA.
Department of Radiology, Duke University Medical Center, Durham, NC, 27705, USA.
Med Phys. 2017 Nov;44(11):5686-5696. doi: 10.1002/mp.12501. Epub 2017 Sep 22.
Dual-energy computed tomography virtual monoenergetic imaging (VMI) at 40 keV exhibits superior contrast-to-noise ratio (CNR), although practicing radiologists do not consistently prefer it over VMI at 70 keV due to high perceivable noise. We hypothesize that the presentation of 40 keV VMI may be compromised using window settings (i.e., window-and-level values [W-L values]) designed for conventional single-energy CT. This study aimed to devise optimum window settings that reduce the apparent noise and utilize the high CNR of 40 keV VMI, in order to improve the conspicuity of hypervascular liver lesions.
Three W-L value adjustment methods were investigated to alter the presentation of 40 keV VMI. To harness the high CNR of 40 keV VMI, the methods were designed to achieve (a) liver histogram distribution, (b) lesion-to-liver contrast, or (c) liver background noise comparable to those perceived in 70 keV VMI. This IRB-approved study included 18 patient abdominal datasets reconstructed at 40 and 70 keV. For each patient, the W-L values were determined using the three methods. For each of the images with default or adjusted W-L values, the noise, contrast, and CNR were calculated in terms of both display space and native CT number (referred to as HU) space. An observer study was performed to compare the 40 keV images with the three adjusted W-L values, and 40 and 70 keV images with default W-L values in terms of noise, contrast, and diagnostic preference. A comparison was also made in terms of the applicability of using patient-specific or patient-averaged W-L values.
Using the default W-L values, 40 keV VMI exhibited higher HU CNR than 70 keV VMI by 24.6 ± 14.9% (P < 0.001) but lower display CNR by 38.0 ± 16.4% (P < 0.001). Using adjusted W-L values, 40 keV images showed increased display CNR as compared to 70 keV images, by 21.2 ± 13.1%, 17.4 ± 13.6%, and 24.2 ± 15.9% (P < 0.001) for histogram-, noise-, and contrast equalization methods, respectively. The 40 keV images with all three W-L value adjustment methods showed improved perceived conspicuity (CNR) of liver presentation by 103-120% (P < 0.001), as compared to default W-L values. The qualitative observer study revealed that 40 keV images with noise- and histogram-equalized W-L values were the most preferred, followed by 40 keV images with contrast-equalized W-L values and 70 keV images with default W-L values. The 40 keV images with default W-L values were the least preferred. Patient-specific W-L values offered similar results to those of patient-averaged W-L values.
The adjusted W-L values can significantly improve the perception of VMI dataset image quality by improving the actual display CNR.
在 40keV 下进行双能计算机断层扫描虚拟单能量成像（VMI）可以获得更高的对比噪声比（CNR），尽管由于可感知的噪声较高，临床放射科医生并不总是更喜欢 70keV 下的 VMI。我们假设使用专为传统单能量 CT 设计的窗宽设置（即窗宽-窗位[W-L]值）可能会影响 40keV VMI 的呈现。本研究旨在设计最佳的窗宽设置，以降低表观噪声并利用 40keV VMI 的高 CNR，从而提高高血流肝脏病变的显示对比度。
研究了三种 W-L 值调整方法，以改变 40keV VMI 的呈现方式。为了利用 40keV VMI 的高 CNR，这些方法旨在实现（a）肝脏直方图分布，（b）病变与肝脏的对比度，或（c）肝脏背景噪声与在 70keV VMI 中感知到的相似。本研究经机构审查委员会批准，包括 18 例腹部患者的 40keV 和 70keV 重建数据集。对于每个患者，使用三种方法确定 W-L 值。对于具有默认或调整后的 W-L 值的每个图像，在显示空间和原始 CT 数（称为 HU）空间中计算噪声、对比度和 CNR。进行了观察者研究，以比较具有默认 W-L 值的 40keV 图像与三种调整后的 W-L 值的图像，以及具有默认 W-L 值的 40keV 和 70keV 图像在噪声、对比度和诊断偏好方面的差异。还比较了使用患者特定或患者平均 W-L 值的适用性。
使用默认的 W-L 值，40keV VMI 的 HU CNR 比 70keV VMI 高 24.6±14.9%（P<0.001），但显示 CNR 低 38.0±16.4%（P<0.001）。使用调整后的 W-L 值，与 70keV 图像相比，40keV 图像的显示 CNR 分别增加了 21.2±13.1%、17.4±13.6%和 24.2±15.9%（P<0.001），这分别是直方图均衡、噪声均衡和对比度均衡方法的结果。与默认 W-L 值相比，所有三种 W-L 值调整方法的 40keV 图像都能显著提高肝脏显示的感知（CNR），提高了 103-120%（P<0.001）。定性观察者研究表明，噪声和直方图均衡的 W-L 值的 40keV 图像最受欢迎，其次是对比度均衡的 W-L 值的 40keV 图像和默认 W-L 值的 70keV 图像。默认 W-L 值的 40keV 图像最不受欢迎。患者特定的 W-L 值提供的结果与患者平均的 W-L 值相似。
调整后的 W-L 值可以通过提高实际显示 CNR 显著改善 VMI 数据集图像质量的感知。
