In pulmonary embolism, DECT iodine distribution maps typically demonstrate peripherally located, wedge-shaped perfusion defects with segmental or lobar distribution (Fig. 7.2) [14]. Data from animal models [21] suggest that DECT iodine distribution maps may improve the sensitivity of CT pulmonary angiography, in particular for peripheral subsegmental emboli, but this has not been conclusively demonstrated in clinical studies. It is not uncommon to encounter patients in whom one or more small peripheral perfusion defects with a pattern suggestive of embolic disease are present but no emboli are visualized on the corresponding anatomical CTPA reconstructions [22]. This scenario represents a clinical dilemma and there is currently no consensus on how to manage such cases [14]. However, it is generally felt that such perfusion defects – if an expert reader cannot explain them by artifacts or other pathology – represent true findings likely due to subsegmental emboli which are too small to be visualized anatomically. These examinations are typically reported to clinicians as suspicious for small, subsegmental emboli. In this context, it is debatable whether emboli that are too small to be anatomically detectable carry any significance for the patient. This broaches the intense, ongoing debate on overdiagnosis and overtreatment of pulmonary embolism [23]. Although a detailed account of this debate is beyond the scope of this book chapter, there are good reasons to question whether a further increase in sensitivity of CT for small subsegmental pulmonary emboli will translate into a true benefit rather than harm for the patient. Therefore, further clinical evaluation of DECT for suspected PE will likely focus not on increasing the detection rate but rather assessing the hemodynamic relevance of emboli. Such research may establish a role of DECT for the risk stratification of patients and the selection of the most appropriate management strategy.

In addition to visualizing the iodine content of the pulmonary parenchyma as a surrogate for pulmonary perfusion, DE-CTPA can also be used to visualize iodine content within the pulmonary vasculature thus increasing the conspicuity of emboli (Fig. 7.2). This approach has been demonstrated to improve the detection of peripheral pulmonary emboli [24].

In chronic-thromboembolic pulmonary hypertension, the inhomogeneous distribution of pulmonary blood flow can frequently be detected as a mosaic attenuation pattern on standard single-energy CT images. However, the perfusion abnormalities are often more clear on DECT perfusion maps, which typically demonstrate peripheral, wedge-shaped perfusion defects (Fig. 7.3) [25, 26]. In this context it has been suggested that DECT may be of value for differentiating ground glass opacities of vascular cause from ground glass opacities caused by air space filling in infiltrative lung diseases [27]. Renard and colleagues further described an association between the severity of pulmonary arterial obstruction, the development of systemic collateral supply, and the perfusion impairment seen on DECT in chronic pulmonary embolism [28].

It has been demonstrated that DECT pulmonary perfusion maps in pulmonary emphysema moderately correlate with the severity and distribution of the anatomical destruction of lung parenchyma along with air trapping and reduced diffusion capacity as demonstrated by pulmonary function tests [3, 29]. These results suggest DECT pulmonary perfusion maps may aid severity assessment and phenotypical characterization of pulmonary emphysema (Fig. 7.4).

Attempts have been made to use automated quantification of pulmonary perfused blood volume in DECT to assess the severity of pulmonary pathologies, acute pulmonary embolism [30–32], pulmonary emphysema [29], and chronic-thromboembolic pulmonary hypertension [33]. Initial studies have suggested the quantification of perfused blood volume or defect volume in DECT carries prognostic significance in patients with pulmonary embolism [31, 34, 35]. For pulmonary emphysema, it is known that lung volume reduction surgery in patients with upper lobe predominant emphysema improves outcome only if their perfusion to the upper lung is decreased on scintigraphy [36]. In this setting, DECT with automated quantification of pulmonary perfused blood volume could potentially obviate the need for an additional scintigraphy or SPECT in the preoperative assessment of surgical candidates [29].

The assessment of pulmonary nodules represents a frequent challenge in clinical routine. The ability of DECT to visualize and quantify the iodine uptake of nodules may help to differentiate benign from malignant pulmonary nodules [53]. This has been assessed in initial studies with mixed results [53–55]. Virtual non-contrast reconstructions may be useful in differentiating iodine enhancement from calcifications in lung nodules without the need for an additional non-enhanced image acquisition (Fig. 7.7) [55].