**6. Chronic PE**

DECT pulmonary angiography can also allow for the depiction of perfusion defects in patients with chronic PE or patients with chronic thrombembolic pulmonary hypertension (CTEPH). A typical imaging characteristic of chronic PE can be mosaic patterns of lung attenuation, that is, areas of ground-glass attenuation mixed with areas of normal lung attenuation, suggesting a redistribution of blood flow.

These perfusion defects in BFI beyond chronic clots, similar to what is achievable for acute PE, and these changes closely mirror the mosaic attenuation pattern which is very suggestive of blood flow redistribution in CTEPH. Mosaic attenuation can sometimes be subtle, and BFI appears to accentuate regional differences in parenchymal attenuation, which become very conspicuous when displayed as a color map. In CTEPH, DECT can identify matched defects (i.e., occluded pulmonary arteries to lobe and negligible residual blood flow), mismatched defects (i.e., occluded lobar artery and demonstrable residual blood flow), and normal lung regions (i.e., unobstructed pulmonary arteries with demonstrable normal or increased flow). Perhaps of most interest are areas of mismatch where there is blood supply maintained beyond the occluded pulmonary arteries [2].

### **7. Radiation dose**

There are concerns about radiation dose of DECT pulmonary angiography. For a DECT pulmonary angiography variable dose length product (DLP) have been reported (range, 143-302 mGy\*cm) [Schenzle JC, 2010; Zhang LJ, 2009; Thieme SF, 2010] depending on the acquisition parameters, especially on the mAs settings. This value is lower than the published DLP of chest CT for PE (882 mGy\*cm) and similar to the previously published DLP of routine chest CT (411 mGy \* cm). Pontana et al **[Pontana F , 2008]** reported that the mean DLP of DECT pulmonary angiography for PE is 280 mGy \*cm, corresponding to an average effective patient dose of about 5 mSv. Kang et al [Kang MJ, 2010] reported that the mean DLP of DECT with the PE protocol was 376 mGy\*cm. All reported values for pulmonary DECT imaging are substantially lower than the reference value of 650 mGy\*cm from the European guidelines on quality criteria for CT. Thus, even if the dose of DECT of the thorax can be a little bit higher than the dose values reported for a standard, singlesource, single-energy thoracic CT, the above-mentioned benefits of DECT of the lung in patients with suspected PE seem to justify the moderate increase in the overall radiation dose. It is the only technique allowing for a direct comparison of CT angiograms acquired at different energies in the same patient, at the same time point after the injection of the contrast medium, and within strictly similar hemodynamic conditions.
