## 4 Discussion
reported that the lung PBV score could be a noninvasive way of estimating the clinical severity of CTEPH, since it is significantly associated with hemodynamic parameters, including mean PAP and PVR. [11]. [PERSON] et al. reported the possibility of using DE-CT not only to quantify pulmonary vascular perfusion with the lung PBV score but also to qualitatively evaluate microvasculopathy using a more sensitive analysis of PSP in three dimensions [12]. Therefore, DE-CT might be useful for evaluating CTEPH severity, pulmonary vascular perfusion, and microvasculopathy. This study was based on the assessment of subpleural perfusion on DE-CT; the existence of microvasculopathy in CTEPH was not verified by histological examination. PSP reflects the diffuse reduction of pulmonary flow in the peripheral micro vessels, which was observed in 58 of the 113 patients in our study. Although a wedge-shaped segmental defect, likely visible due to proximal vessel occlusion, would be the typical DE-CT finding [20], PSP on DE-CT was observed in more than half of the CTEPH patients; therefore, this might be one of the specific imaging findings in CTEPH. PSP on digital subtraction angiography or DE-CT might suggest the presence of microvasculopathy and/or diffuse distal thrombosis [6; 7; 12]. Indeed, patients with PSP showed poor hemodynamics, disproportionate to the degree of pulmonary vascular obstruction. Additionally, not only pulmonary vascular obstruction but also microvasculopathy may contribute to severe hemodynamic instability [3; 4]. However, PSP is rarely observed in patients with PAH, a disease characterized by obliteration and remodeling of the small pulmonary arteries [2; 13]. PSP in CTEPH may not suggest the same kind of microvasculopathy as the one found in PAH. Histopathological examination by biopsy and autopsy in CTEPH patients not only revealed PAH-like lesions (including intimal fibromuscular proliferation or plexiform lesion), but thrombotic lesions in pulmonary arteriole [21], and these lesions were also observed in pulmonary veins and capillaries [22]. Microvasulopathy in CTEPH also comprises diffuse distal thrombosis [4], which might be represented by PSP on DE-CT, as it reflects the diffusely reduced pulmonary flow in peripheral micro vessels. [PERSON] et al. suggested that distal thrombosis in CTEPH could be diffuse when the patency of small pulmonary arterioles distal to complete obstructions are not maintained because bronchial arteries and anastomoses fail to develop [4]. In a report by [PERSON] et al., PSP in digital subtraction angiography was also found to be associated with less developed bronchial arteries, suggesting that bronchial-pulmonary anastomoses have the role of maintaining the patency of the pulmonary capillary bed distal to the obstructed pulmonary artery in CTEPH [7]. In this study, we showed that more than half of the patients with CTEPH also had diffusely reduced pulmonary flow in peripheral micro vessels. The clinical impact of PSP in CTEPH remains unknown. In this study, the lung PBV score, which could represent lung vascular perfusion of the pulmonary artery, showed a strong inverse correlation with PVR in CTEPH without PSP; however, there was no significant correlation between the lung PBV score and PVR in CTEPH with PSP, nor in PAH. This might suggest that not only pulmonary vascular obstruction by thromboembolism, but also diffuse distal thrombosis is involved in the CTEPH hemodynamics. The data might be comparable to those reported by [PERSON] et al., which demonstrated no significant correlation between the percentage of vascular obstruction and total pulmonary resistance in patients with CTEPH [23]. However, this study demonstrated that hemodynamics at rest improved significantly after adequate BPA in patients with or without PSP (poorly perfused group: mean PAP 19.6 \(\pm\) 5.0 mmHg, PVR 270 \(\pm\) 118 dynes-sec/cm\({}^{5}\); normally perfused group: mean PAP 19.2 \(\pm\) 3.3 mmHg, PVR 220 \(\pm\) 88 dynes-sec/cm\({}^{5}\)).