67-4

23/90

NPV (%)PLR73.7 (48.8,90.9)60.9 (38.5,80.3)60.9 (38.5,80.3)63.6 (40.7,82.8)6.3 (1.71,23.5)5.0 (1.3-19.0)5.0 (1.3-19.0)5.3 (1.4-20.1)ratio sensitivity and specificity were 66.7% and 87.5%, respectively. A thrombus in the pulmonary trunk or main PA is a life-threatening disease, and a prompt diag-nosis is critical for a favorable outcome. Recent reports have shown that several signs of PTE have been observed via unenhanced CT, including a hyperdense lumen sign, PA dilatation, and wedge-shaped consolidation3-9). Among these signs, the hyperdense lumen has a high diagnostic perfor-mance, but it only occurs in 50-70% of cases8, 9). In our study, only 9 of 49 patients (18%) showed a hyperdense lumen sign.To date, no study has evaluated the CT attenua-tion value for patients with PTE using quantitative indicators on unenhanced CT images. This study aimed to evaluate the thrombus attenuation cut-off value for PTE diagnosis. The thrombus attenuation value was lower in the thrombi group than in the non-thrombi group. Additionally, the attenuation value of thrombus in the thrombi group was lower than that in the blood pool. The thrombus cut-off value of 30.85 HU calculated from the ROC curve showed good accuracy. Our results indicate poten-tial benefits of using measurements of thrombus attenuation values on unenhanced CT to detect PTE. This study is the first to evaluate the useful-ness and reliability of thrombus attenuation value on unenhanced CT in patients with PTE.The blood pool attenuation value is mainly deter-mined by the protein content of red blood cells and it increases linearly with Ht10). The blood pool attenuation value is 20–60 HU3, 7, 8), whereas the density of the thrombus is determined by the concentration of red blood cells and fibrin. From a pathologic point of view, the different densities of ParameterT≤30.85T/Hb≤2.53T/Ht≤0.83T/P≤0.835Sensitivity (%)Specificity (%)79.2 (57.8,92.9)87.5 (61.7,98.4)83.3 (69.4,97.3)62.5 (40.6,81.2)87.5 (61.7,98.4)72.9 (56.5,89.3)62.5 (40.6,81.2)87.5 (61.7,98.4)72.9 (56.5,89.3)66.7 (44.7,84.0)87.5 (61.7,98.4)77.5 (62.7,92.2)Values (95% confidence interval); P: computed tomography value for pulmonary artery blood pool (Hounsfield unit); T: computed tomography value for thrombus (Hounsfield unit); Hb: hemoglobin (g/dL); H: hematocrit (%); AUC: area under the curve; PPV: positive predictive value; NPV: negative predictive value; PLR: positive likelihood ratio; NLR: negative likelihood ratio Table 3　Cut-off computed tomography values and the accuracy of unenhanced chest computed tomography for detecting pulmonary thromboembolismAUCPPV (%)90.5 (69.6,98.8)88.2 (63.6,98.5)88.2 (63.6,98.5)88.9 (65.3,98.6)the thrombus can be explained by the processes undergone by the clot itself while lodged within the PA system. As a thrombus retracts, its water content decreases, which concentrates the hemo-globin and raises the CT attenuation value of thrombi to 50–80 HU. The attenuation of clots decreases gradually to the same as or lower than that of the blood3, 7, 8, 11-15). Acute thrombi—clinically judged to be <8 days old—have an average atten-uation value of 66 HU, whereas those older than 8 days have a lower value14).　In PTE, thrombi are primarily caused by deep venous thrombosis (DVT). As DVT has varied symptoms (and can be asymptomatic in some cases), it is difficult to accu-rately determine when a DVT had formed. This study included emergency outpatients, but it is possible that several days had passed since the clot had formed. This is probably why the CT images showed a hypoattenuation clot.The attenuation value of thrombosis is related to the age of clots and Hb and Ht levels4, 5). To elimi-nate any bias because of the Hb and Ht factors, we also evaluated the T/Hb, T/Ht, and T/P ratios. Our hypothesis is that T/Hb, T/Ht and T/P ratios are more useful than the attenuation value of T alone. There were significant differences for T/Hb, T/Ht and T/P ratios, but sensitivity, PPV, and PLR of the T/Hb and T/Ht ratios were lower than those of the thrombus. The attenuation of a blood-pool CT value becomes 1.7-2.0 HU lower when the Hb drops 1 g/dl10). This was a very small change, and so there were no noticeable changes in the T/Hb and T/ Ht ratios. Sun et al.4) stratified Hb levels into 4 classes and attempted to determine the effect of the Hb value on the accuracy of PTE identifica-tion. However, patients with anemia were not more likely to be diagnosed as truly positive for PTE.One non-PTE patient showed a hyperdense NLR0.2 (0.1,0.5)0.4 (0.2,0.7)0.4 (0.2,0.7)0.4 (0.2,0.7)343Discussion