**5.1 Acute PE detection**

Perfusion defects that are consistent with acute PE include those that are peripherally located, wedge-shaped, and in a segmental or lobar distribution (Figure 10). All other perfusion defects, such as patchy or band-like defects without segmental distribution, or complete loss of color-coding (indicating lack of air-containing voxels due to consolidation), were considered to be inconsistent with PE. For the Lung Vessels application, color-coded red PAs was regarded as positive for PE (Figure 11), while color-coded red soft tissue around PAs was discarded.

Fig. 7. Contrast defect in the pulmonary blood volume image caused by lung carcinoma A) A coronal MIP image shows a left lung hilar carcinoma invading the left pulmonary lobar arteries (red arrow), resulting in diffuse decreased contrast enhancement of the left lung ( red circle )at the corresponding coronal BFI image fused with the CT angiogram (B)

Fig. 8. Contrast enhancement defect in the pulmonary blood volume image caused by lung

A) Axial BFI image shows a contrast enhancement defect in the left lower lobe (white circle);

Perfusion defects that are consistent with acute PE include those that are peripherally located, wedge-shaped, and in a segmental or lobar distribution (Figure 10). All other perfusion defects, such as patchy or band-like defects without segmental distribution, or complete loss of color-coding (indicating lack of air-containing voxels due to consolidation), were considered to be inconsistent with PE. For the Lung Vessels application, color-coded red PAs was regarded as positive for PE (Figure 11), while color-coded red soft tissue

B) The corresponding axial CT image clearly shows pulmonary consolidation in the

A B

A B

corresponding left lower lung lobe (white circle)

consolidation

**5. Clinical applications 5.1 Acute PE detection** 

around PAs was discarded.

Fig. 9. Negative BFI image in one patient with left lower pulmonary artery embolus A) axial, B) coronal, and C) sagittal fused images show normal findings with non-occlusive PE(white arrow) and result in the false-negatives

A) Coronal and B) Axial BFI images show a wedge-shaped perfusion defect in the left lung lower lobe dorsal segment (white circle). Pseudo-high contrast enhancement is seen in the anterior portion of the right middle lung anterior to the normal pulmonary contrast enhancement seen in the right middle lobe more posteriorly (arrows). C) Axial contrastenhanced CT image shows a corresponding occlusive filling defect representing pulmonary emboli in the left lower lobe segmental pulmonary arteries (arrow), and non-occlusive emboli elsewhere

Several studies have examined DECT for the detection of PE. Fink et al **[Fink C, 2008]**  reported that both sensitivity and specificity of DECT for the assessment of PE were 100% on a per patient basis. On a per segment basis, the sensitivity and specificity ranged from 60%–66.7% and from 99.5%–99.8%; CTPA was used in this study as the standard of reference in 24 patients with suspected PE, 4 of whom actually had PE. With scintigraphy as the standard of reference, Thieme et al **[ Thieme SF ,2008]** reported 75% sensitivity and 80% specificity on a per patient basis and 83% sensitivity and 99% specificity on a per segment basis in a small group of patients with DECT. A group of 117 patients was examined by Pontana et al **[Pontana F, 2008]** to investigate the accuracy of DECT in the depiction of perfusion defects in patients with acute PE, concluding that simultaneous information on the presence of endoluminal thrombus and lung perfusion impairment can be obtained with

Dual Source, Dual Energy Computed Tomography in Pulmonary Embolism 215

small emboli require treatment to prevent chronic PE and pulmonary artery hypertension in several clinical scenarios in patients with a small embolus and inadequate cardiopulmonary reserve; in patients who have a small embolus and coexisting acute deep venous thrombosis; and in patients with recurrent small emboli possibly owing to thrombophilia **[Remy-JardinM, 2007].** Certainly, the significance of small emboli needs further study.

A B C

D E F

A) Axial, B) coronal, and C) left sagittal BFI images show a wedge-shaped perfusion defect in the right lung lower lobe dorsal segment (white circle); D) Axial, E) coronal, F) left sagittal contrast-enhanced CT image shows no filling defect in the corresponding right lower

In patients with acute PE, rapid risk assessment is critical because high-risk patients may benefit from life-saving thrombolytic therapy or invasive therapies, including catheterguided thrombosuction or thrombectomy **[Dogan H, 2007].** Right heart strain (RHS) has been shown to be independently predictive of 30-day mortality. In addition to use as a CT marker of RHS, the ratio between the size of the right ventricle (RV) and left ventricle (LV) has demonstrated a significant positive correlation with severity of PE and mortality **[Ghaye B, 2006]**. Chae et al. **[ Chae EJ , 2010]** and Zhang et al **[Zhang LJ, 2099(Acta Radio)]** reported good correlation between RV/LV diameter ratio with a novel self-defined dual energy perfusion score or the number of pulmonary segments with perfusion defects, respectively.

Fig. 12. Tiny peripheral emboli in right lower pulmonary artery

pulmonary artery (white arrow)

**5.2 Evaluation of PE severity** 

DECT. In an experimental study by Zhang et al **[Zhang LJ ,2009],** conventional CTPA identified pulmonary emboli in only 12 and the absence of emboli in 18 pulmonary lobes, corresponding to a sensitivity and specificity of 67% and 100%. In contrast, DECT and BFI each correctly identified pulmonary emboli in 16 of 18 pulmonary lobes and reported the absence of emboli in 11 of 12 lobes, corresponding to sensitivity and specificity of 89% and 92% for detecting pulmonary emboli. Thus, pulmonary CTA and DECT lung perfusion have complimentary roles in the diagnosis of PE and DECT lung perfusion images increase the sensitivity for detection of PE (Figure 12), particular for tiny peripheral emboli **[**Lu GM **,2010].** It can be presumed that a simultaneous detection of a clot in a pulmonary artery in the pulmonary CTA and of a corresponding perfusion defect in DECT lung perfusion indicate an occlusive PE.

Fig. 11. Acute pulmonary embolism in one 17-year-old man A) axial, B) coronal, and C) right sagittal lung vessel images show the pulmonary emboli in the right lower pulmonary artery color coded as red

Furthermore, pulmonary CTA and DECT lung perfusion could assist in the detection of pulmonary emboli that are not evident by conventional MDCT pulmonary angiography. Thieme **et al [Thieme SF ,2008] found** that corresponding perfusion defects were observed in DECT and scintigraphy in two patients in whom there was no evidence of intravascular clots in angiographic CT images. They proposed that the observed pulmonary perfusion defects probably corresponded to segments of prior embolism with re-perfused, segmental vessels and residual peripheral thrombosed vessels that were too small to visualize in CTPA. The same assumption was also made in the study by Po**ntana et al [Pontana F, 2008], i**n which four subsegmental perfusion defects were depicted by BFI images, whereas endoluminal thrombi were not visualised in the corresponding arteries by CTPA. **Zhang et al [Lu GM ,2010]** also found a similar so-called false-positive DECT result in one patient with chronic PE in the pulmonary images of BFI. In another patient undergoing anticoagulant therapy, the conventional CTPA performed initially did not visualize abnormal findings. However, the magnified view of the targeted pulmonary arteries corresponding to contrast enhancement defect in the BFI images showed a subtle subsegmental filling defect. These findings indicate that CTPA might not be an adequate gold standard to detect all PE, especially for the small peripheral emboli or chronic PE. However, this does not mean to deny the mainstay role of MDCT in the evaluation of pulmonary emboli. The detection of small emboli is of clinical importance because even

DECT. In an experimental study by Zhang et al **[Zhang LJ ,2009],** conventional CTPA identified pulmonary emboli in only 12 and the absence of emboli in 18 pulmonary lobes, corresponding to a sensitivity and specificity of 67% and 100%. In contrast, DECT and BFI each correctly identified pulmonary emboli in 16 of 18 pulmonary lobes and reported the absence of emboli in 11 of 12 lobes, corresponding to sensitivity and specificity of 89% and 92% for detecting pulmonary emboli. Thus, pulmonary CTA and DECT lung perfusion have complimentary roles in the diagnosis of PE and DECT lung perfusion images increase the sensitivity for detection of PE (Figure 12), particular for tiny peripheral emboli **[**Lu GM **,2010].** It can be presumed that a simultaneous detection of a clot in a pulmonary artery in the pulmonary CTA and of a corresponding perfusion defect in DECT lung perfusion

A B C

A) axial, B) coronal, and C) right sagittal lung vessel images show the pulmonary emboli in

Furthermore, pulmonary CTA and DECT lung perfusion could assist in the detection of pulmonary emboli that are not evident by conventional MDCT pulmonary angiography. Thieme **et al [Thieme SF ,2008] found** that corresponding perfusion defects were observed in DECT and scintigraphy in two patients in whom there was no evidence of intravascular clots in angiographic CT images. They proposed that the observed pulmonary perfusion defects probably corresponded to segments of prior embolism with re-perfused, segmental vessels and residual peripheral thrombosed vessels that were too small to visualize in CTPA. The same assumption was also made in the study by Po**ntana et al [Pontana F, 2008], i**n which four subsegmental perfusion defects were depicted by BFI images, whereas endoluminal thrombi were not visualised in the corresponding arteries by CTPA. **Zhang et al [Lu GM ,2010]** also found a similar so-called false-positive DECT result in one patient with chronic PE in the pulmonary images of BFI. In another patient undergoing anticoagulant therapy, the conventional CTPA performed initially did not visualize abnormal findings. However, the magnified view of the targeted pulmonary arteries corresponding to contrast enhancement defect in the BFI images showed a subtle subsegmental filling defect. These findings indicate that CTPA might not be an adequate gold standard to detect all PE, especially for the small peripheral emboli or chronic PE. However, this does not mean to deny the mainstay role of MDCT in the evaluation of pulmonary emboli. The detection of small emboli is of clinical importance because even

Fig. 11. Acute pulmonary embolism in one 17-year-old man

the right lower pulmonary artery color coded as red

indicate an occlusive PE.

small emboli require treatment to prevent chronic PE and pulmonary artery hypertension in several clinical scenarios in patients with a small embolus and inadequate cardiopulmonary reserve; in patients who have a small embolus and coexisting acute deep venous thrombosis; and in patients with recurrent small emboli possibly owing to thrombophilia **[Remy-JardinM, 2007].** Certainly, the significance of small emboli needs further study.

Fig. 12. Tiny peripheral emboli in right lower pulmonary artery A) Axial, B) coronal, and C) left sagittal BFI images show a wedge-shaped perfusion defect in the right lung lower lobe dorsal segment (white circle); D) Axial, E) coronal, F) left sagittal contrast-enhanced CT image shows no filling defect in the corresponding right lower pulmonary artery (white arrow)

#### **5.2 Evaluation of PE severity**

In patients with acute PE, rapid risk assessment is critical because high-risk patients may benefit from life-saving thrombolytic therapy or invasive therapies, including catheterguided thrombosuction or thrombectomy **[Dogan H, 2007].** Right heart strain (RHS) has been shown to be independently predictive of 30-day mortality. In addition to use as a CT marker of RHS, the ratio between the size of the right ventricle (RV) and left ventricle (LV) has demonstrated a significant positive correlation with severity of PE and mortality **[Ghaye B, 2006]**. Chae et al. **[ Chae EJ , 2010]** and Zhang et al **[Zhang LJ, 2099(Acta Radio)]** reported good correlation between RV/LV diameter ratio with a novel self-defined dual energy perfusion score or the number of pulmonary segments with perfusion defects, respectively.

Dual Source, Dual Energy Computed Tomography in Pulmonary Embolism 217

different energies in the same patient, at the same time point after the injection of the

DECT can provide both anatomical and iodine mapping information of the whole lungs in a single contrast-enhanced CT scan. After recognition of some artifacts in DECT pulmonary angiography, this technology has the capacity to improve the detection and severity evaluation of acute and chronic PE through comprehensive analysis of BFI and CT pulmonary angiography obtained during a single contrast-enhanced chest CT scan in a dual-energy mode. DECT pulmonary angiography can be used as a one-stop-shop

Long Jiang Zhang received the grant from the Peak of six major talents of Jiangsu Province

Guang Ming Lu received the grant from the Natural Science Foundation of Jiangsu Province

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Geyer LL, et al. (2011). Imaging of acute pulmonary embolism using a dual energy CT

Ghaye B, et al. (2006). Severe pulmonary embolism: pulmonary artery clot load scores and cardiovascular parameters as predictors of mortality. Radiology ;239(3):884-91. Kang MJ, et al. (2010) Dual-energy CT: clinical applications in various pulmonary diseases.

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system with rapid kVp switching: Initial results. Eur J Radiol Mar 18. [Epub ahead

contrast medium, and within strictly similar hemodynamic conditions.

**8. Conclusion** 

technique for the evaluation of PE.

Grant (No. WSW-122 for L.J. Z.).

68(3):375–84.

of China (No. BK2009316 for G.M.L.).

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Radiographyics 30(3):685-698.

Radiol 83(992):707–718.

Clin North Am 22:961–983.

Radiology 242(1):78-84.

of print]

**9. Acknowledgement** 

**10. References** 

Recently, Bauer RW et al **[**Bauer RW, 2011**]** reported patients with RHS had significantly higher perfusion defect (PD) size than patients without RHS and confirm that PD size can be seen as marker for RHS. Bauer RWet al **[**Bauer RW, 2011**]** also reported that looking at the incidence of readmission and death due to PE demonstrated these major hard endpoints only in patients with a relative PD size of >5% of the total lung volume, whereas no such event was recorded for patients with <5% RelPD (relatively to the total lung volume, RelPD). Median survival time, however, was significantly lower for patients with >5% RelPD at an increased relative hazard ratio for death compared to patients with <5% RelPD or the control group without PE. Thus, PD size might even be an additional instrument for prognostic evaluation in PE itself.
