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Hiroshi INAGAWA3), Manabu SUGITA2)3)Department of Emergency, Showa General Hospital, Tokyo, Japan1)Department of Anesthesiology, Showa General Hospital, Tokyo, Japan2)Department of Emergency and Disaster Medicine, Juntendo University Graduate School of Medicine, Tokyo, JapanMaki ICHINOSE1, 2, 3), Tomohisa NOMURA2), Yasusei OKADA3), 338Corresponding author: Maki IchinoseDepartment of Anesthesiology, Showa General Hospital, Tokyo, Japan8-1-1 Hanakoganei, Kodaira-shi, Tokyo 187-0002, JapanTEL: +81-42-461-0052　FAX: +81-42-464-7912　E-mail: m-ichi@showa-hp.jp〔Received　Dec. 31, 2020〕〔Accepted　May 6, 2021〕J-STAGE Advance published date: Jul. 2, 2021Copyright © 2021 The Juntendo Medical Society. This is an open access article distributed under the terms of Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original source is properly credited. doi: 10.14789/jmj.JMJ20-OA19Objective: To identify pulmonary thromboembolism (PTE) using the thrombus attenuation value on unenhanced computed tomography (CT).Design: Single-center retrospective study (January 2015-Marcg 2020).Methods: Patients who underwent both unenhanced and enhanced CT for suspected PTE were enrolled. Patients with a hyperdense lumen on unenhanced CT and thrombi in the peripheral pulmonary artery (PA) were excluded. Patients were classified into two groups: thrombi (thrombi detected in PA by enhanced CT) and non-thrombi (attenuations of the main PA evaluated as thrombi). Mean CT attenuation values of the thrombi, main PA, and pulmonary trunk (blood pool) were measured. The attenuation values of the thrombus (T) and the thrombus to blood-pool (T/P), thrombus to hemoglobin (T/Hb), and thrombus to hematocrit (T/Ht) ratios were evaluated. The cut-off attenuation value of the thrombus was calculated by a receiver operating characteristic curve and its accuracy in detecting PTE was determined.Results: Of the 260 patients enrolled, 40 were included, of whom 24 had confirmed PTE. The mean T was 27.25 Hounsfield units(HU) and 36.66HU (p<0.001), and the T/P ratio was 0.74 and 0.99 (p=0.004) in the PTE and non-PTE groups, respectively. The thrombus cut-off value for PTE diagnosis was 30.85 HU. The sensitivity and specificity were 79. 9% and 87.5%.Conclusions: Measuring and evaluating the attenuation value for the central PA and T/P ratio on unenhanced CT improves the diagnostic ability of central PTE in patients suspected to have PTE but cannot tolerate contrast medium. Key words: attenuation value of thrombus, pulmonary thromboembolism, unenhanced computed tomography (CT)Juntendo Medical Journal2021. 67(4), 338-345Original ArticlesQuantitative Evaluation of the Attenuation Value of Pulmonary Abbreviations: PTE, pulmonary thromboembolism; CT, computed tomography; PA, pulmonary artery; HU, Hounsfield unit; T, thrombus attenuation value; T/P, thrombus to blood pool; T/Hb, thrombus to hemo-globin ; T/Ht, thrombus to hematocrit ; ROI, region of interest; ROC, receiver operating characteristic; PPV, positive predictive value; NPV, negative predictive value; PLR, positive likelihood ratio; NLR, negative like-lihood ratio; DVT, deep venous thrombosisPulmonary thromboembolism (PTE) is a life- threatening disease with a mortality rate of 11.9%1). Approximately 350,000 cases of PTE and 85,000 PTE-related deaths occur each year in Japan2). The gold standard for definitive diagnosis of PTE is enhanced computed tomography (CT). However, the use of contrast media may be a concern for Thrombus on Unenhanced Computed TomographyIntroduction