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1 University of Parma Parma 43100, Italy
2 Ospedale Maggiore Parma Parma 43100, Italy
3 University of Parma Parma 43100, Italy
We read with interest the article by J. Kanne et al. [1] regarding the detection of acute pulmonary embolism on unenhanced CT of the chest. We agree with the authors about the possibility of identifying hyperattenuating clots on unenhanced CT under particular conditions. We describe a similar finding that we observed in two patients who were referred to our hospital to be assessed for possible acute pulmonary embolism. We performed unenhanced CT of the thorax, which showed hypoattenuating clots (Fig. 1A, 1B).
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We believe that hyperattenuating clots are the sign of current acute pulmonary embolism, but hypoattenuating clots probably are the remaining sign of a previous episode of acute pulmonary embolism that has not yet completely resolved. The intravascular clots are mainly composed of RBCs and fibrin, so their hematocrit level is greater than that of the circulating blood.
This difference also explains the possibility of distinguishing intravascular hyperattenuating clots on unenhanced CT. Nevertheless, other factors affect the visualization of a clot, such as the age of the clot, the patient's hematocrit level at the time of imaging [1], and probably the patient's hematocrit level at the time of formation of the clot in the venous system.
Acute thrombi less than 8 days old have an attenuation value of approximately 66 H, and older thrombi have a lower attenuation of 55 H [2]. Moreover, the clot remains visible in the vessel for some months in approximately 50% of episodes of acute pulmonary embolism.
The decrease in density could be explained by the evolution of the thrombus in the vessel; in fact, the predominant attenuation level of the blood is produced by the protein fraction of hemoglobin. Therefore, the attenuation changes are consistent with the progressive breakdown of RBCs and the removal by phagocytic activity of those cell elements, predominantly proteins, that contribute most heavily to the high density of the clot [3].
The clot eventually epithelizes and is finally incorporated into the wall of the vessel; in this way, it appears on CT as peripheral, not completely obstructing the lumen, and adhering to the vessel wall.
In conclusion, we think that our observation could be important. First, it shows that acute pulmonary embolism can be diagnosed using unenhanced CT even in cases in which the clot is more hypoattenuated than the blood density. Second, the same findings make it possible to diagnose chronic pulmonary embolism. Further study would be useful to assess the true accuracy of unenhanced CT in detecting acute or chronic pulmonary embolism.
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