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lumen sign in our study. The descending aorta showed several calcifications in her unenhanced CT. This may occur with atherosclerotic disease of the PA. We misunderstood a calcification for a hyperdense lumen sign. In conclusion, the T/Hb ratio and T/Ht ratio were not useful for the diagnosis of PTE, but the thrombus attenuation value and the T/P ratio were useful. Patients in whom PTE is clinically suspected, attenuation values should be measured at multiple points in the main and lobar PAs and a few points in the pulmonary trunk. PTE is suspected when the attenuation value is below 30.85 HU or when the T/P ratio is below 0.835, and thus an enhanced CT can confirm the presence of PTE.There are several limitations to this study. This study was retrospective in nature. The thrombus attenuation values in unenhanced CT were measured after the thrombi were detected by enhanced CT and cut-off values were not consid-ered in a healthy population. In future studies, we will assess whether PTE can be found via unen-hanced CT with a fixed window width of 45 and window level of 70, which was obtained from this present study. The minimum and maximum pulmo-nary blood attenuation values were 20.15 HU and 71.55 HU. The minimum and maximum thrombus attenuation values were 11.75 HU and 44.75 HU. In addition, this was a single-center study with a small sample size, and a larger sample size could improve the accuracy of our results. Our results show that the probability of PTE was extremely high in patients who had a central PA attenuation value below 30.85 HU and a T/P ratio below 0.835. This study suggests that assessing the attenuation value for the central PA and T/P ratio on unenhanced CT improves the diagnostic ability of central PTE in patients with suspected to have PTE but who cannot tolerate contrast medium, and whose unenhanced CT does not show hyperdense lumen sign, PA dilatation and wedge-shaped consolidation． AcknowledgmentsWe are grateful to the radiation technologist for help with data collection.The authors received no financial support for the 344research.MI conceived and designed the study, collected and analyzed the data, drafted the manuscript, and revised it critically for important intellectual content. TN interpreted the data and drafted and revised the manuscript. YU interpreted the CT data. MS, HI, and YO revised the manuscript criti-cally and provided important intellectual content. All authors have read and approved the final version of manuscript.The authors declare no conflicting interests.　 1) Sakuma M, Okada O, Nakamura M, et al: Japanese society of pulmonary embolism research. Recent developments in diagnostic imaging techniques and management for acute pulmonary embolism: multi-center registry by the Japanese Society of pulmonary embolism research. Intern Med, 2003; 42: 470-476. 2) Nakamura M, Yamada M, Ito M: Current management of venous thromboembolism in Japan: current epidemi-ology and advances in anticoagulant therapy. J Cardiol, 2015; 66: 451-459. 3) Tatco VR, Piedad HH: The validity of hyperdense lumen sign in non-contrast chest CT scans in the detection of pulmonary thromboembolism. Int J Cardiovasc Imaging, 2011; 27: 433-440. 4) Sun S, Semionov A, Xie X, Kosiuk J, Mesurolle B: Detection of central pulmonary embolism on non-con-trast computed tomography: a case control study. Int J Cardiovasc Imaging, 2014; 30: 639-646. 5) Gotway MB, Webb WR: Acute pulmonary embolism: visualization of high attenuation clot in the pulmonary artery on noncontrast helical chest CT. Emerg Radiol, 2000; 7: 117-119. 6) Okechukwu N, Currna-Melendez MC, Weyer A, Mickus TJ: The hyperdense lumen sign: the tale of the elusive pulmonary embolism. BMJ Case Rep, 2014; 2014: bcr2014205986. 7) Kanne JP, Gotway MB, Thoongsuwan N, Stern EJ: Six cases of acute central pulmonary embolism revealed on unenhanced multidetector CT of the chest. AJR Am J Roentgenol, 2003; 180: 1661-1664. 8) Mohamed ND, Othman MH, Hassan LS, Yousef HA: The accuracy of non-contrast chest computed tomo-graphic scan in the detection of pulmonary thrombo-embolism. J Curr Med Res Pract, 2019; 4: 61-66. 9) Chien CH, Shih FU, Chen CY, Chen CH, Wu WL, Mak CW: Unenhanced multidetector computed tomography findings in acute central pulmonary embolism. BMC Med Imaging, 2019; 19: 65.10) Wegener, OH: Whole-body computerized tomography, second edition. Cambridge: Blackwell Scientific Publi-cation, 1993: 87-88.11) Morita S, Ueno E, Masukawa A, Suzuki K, Machida H, FundingAuthors’ contributionsConflicting interest statementReferences