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certain patients, such as those with allergies to contrast media, patients medicated with bigua-nides, or those suffering from renal dysfunction. For these patients, detection of pulmonary thrombi on unenhanced CT could be useful for diagnosis and initiation of therapy for PTE.High-density regions on unenhanced CT scans, or the hyperdense lumen sign, are known indica-tors of recent PTE. Current reports have shown that the CT attenuation value of the hyperdense lumen sign is 50–80 Hounsfield units (HU)3). However, the attenuation of the clots gradually decreases with time and becomes equal to or less than that of the blood pool. Therefore, pulmonary thrombi are rarely detected on unenhanced CT with the excep-tion of the hyperdense lumen sign. We need to find other methods to detect PTE on unenhanced CT and there are no studies that measure the CT attenuation value of thrombi. The purpose of this study was to examine the usefulness of the thrombus attenuation value on unenhanced CT for the diagnosis of PTE. We also assessed the cut-off CT values for PTE diagnosis using unenhanced CT in patients with PTE. This is the first study that quantitively evaluated the CT attenuation value of thrombi.Materials and methodsThis study reviewed outpatients who presented with dyspnea, chest pain, syncope, and dizziness, showed low saturated oxygen in arterial blood and right heart load, and underwent enhanced CT at Figure 1　A: Thrombi (arrow) in pulmonary artery on enhanced computed tomography. B: Hyperdense lumen sign (arrow); the same position of thrombi in Figure 1A.Showa General Hospital from January 2015 to March 2020.All studies were performed using the GE® light-speed VCT 64 detector (GE Healthcare, Hino, Japan) or the Canon Aquilion® One 320 (Canon, Ota city, Tokyo, Japan) detector. The parameters were varied among the unenhanced and enhanced CT examinations, with slice thickness ranging from 3 mm to 5 mm at axial, coronal, and sagittal views. Patients were administered intravenous injections of 2 ml/kg non-ionic contrast medium (Iopromide : Iopromide Injection FRI® 300 mgI/ml, FujiPharma, Toyama, Japan) at a rate of 3 ml/s. Scans were obtained 30 seconds after injection.CT images were evaluated on a picture archiving and communications system workstation. Based on the enhanced CT images, patients were divided into the PTE group (thrombi were detected) (Figure 1A) or the non-PTE group (thrombi were not detected). Patients who underwent both enhanced and unenhanced CT were included. The central type of PTE was defined as a filling defect in the pulmonary trunk or in the main, lobar, and segmental pulmonary arteries (PAs). We excluded peripheral PTE filling defects in the subsegmental and more peripheral arteries. Patients who showed a hyperdense lumen sign on unenhanced CT were also excluded. The hyperdense lumen sign is a clot with hyperdensity in the PA on unenhanced CT (Figure 1B). In the central PTE group, patients who met the inclusion criteria were labeled as the thrombi group. Patients who met the inclusion 339