Optimizing window settings for improved presentation of virtual monoenergetic images in dual‐energy computed tomography
Purpose: 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. Materials and Methods: 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. Results: 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. Conclusion: The adjusted W‐L values can significantly improve the perception of VMI dataset image quality by improving the actual display CNR.