###### Abstract  
Chronic thromboembolic pulmonary hypertension (CTEPH) is confirmed by visual analysis of single-photon emission computer tomography (SPECT) ventilation and perfusion (V/Q) images. Defects in the perfusion image discordant with the ventilation image indicate obstructed segments and the positive diagnosis of CTEPH. A quantitative metric and classification algorithm are proposed based on volumetric data from SPECT V/Q images. The difference in ventilation and perfusion volumes (\(V_{V,P}\)) is defined as a quantitative metric to identify discordant defects in the SPECT images. The algorithm was validated with 22 patients grouped according to their diagnosis: (1) CTEPH and (2) respiratory pathology. Volumetric data from SPECT perfusion images was also compared before and after treatment for CTEPH. CTEPH was detected with a sensitivity of 0.67 and specificity of 0.80. The performance of volumetric data from SPECT perfusion images for the evaluation of treatment response was studied for two cases and improvement of pulmonary perfusion was observed in one case. This study uses volumetric data from SPECT V/Q images for the diagnosis of CTEPH and its differentiation from respiratory pathologies. The results indicate that the defined metric is a viable option for a quantitative analysis of SPECT V/Q images. CTEPH; pulmonary hypertension; SPECT V/Q; quantification; segmentation; computer-assisted diagnosis; treatment response +
Footnote †: journal:Chronic thromboembolic pulmonary hypertension (CTEPH) is a severe variant of PH. It is caused by the occlusion of pulmonary arteries and arterioles by organized blood clots. This results in obstructed segments in the pulmonary perfusion [3]. CTEPH occurs due to pulmonary thromboembolism and can appear after acute pulmonary embolism (PE); however, it is considered rare [4; 5]. The initial diagnosis is usually performed with a qualitative scale and confirmed by medical imaging. Single-photon emission computer tomography (SPECT) ventilation and perfusion (V/Q) images are visually analyzed. As CTEPH is defined by the obstruction of the pulmonary perfusion, the corresponding SPECT perfusion image reveals defects of triangular morphology. However, while these defects are visible in the perfusion image, they are absent in the ventilation image. The presence of discordant defects in the perfusion image confirms the diagnosis of CTEPH [6]. The following criteria are defined for the visual analysis [3]:  
* Positive CTEPH diagnosis--presence of at least one segmental defect or two subsegmental perfusion defects not consistent with ventilation in pulmonary SPECT images. * Negative CTEPH diagnosis--normal perfusion in which the edges of the lung are preserved or presence of concordant defects in both the ventilation and perfusion images. * Respiratory pathology--presence of defects in the ventilation image that are not concordant with the perfusion. Moreover, CTEPH is the only cause of severe PH that can be treated surgically without the need for lung transplantation. The most common treatment for severe CTEPH is pulmonary thromboembolism dermaterectomy. This surgical procedure involves the removal of thromboembolic material that occludes the pulmonary arteries, resulting in significant improvements in hemodynamic and right ventricular function [7]. Alternatives to endarterectomy for those patients with recurrent CTEPH, who are inoperable, or those in whom the risk-benefit ratio of pulmonary endarterectomy is unacceptable, are balloon pulmonary angioplasty [8] or anticoagulant and vasodilator pharmacological treatments, which help to dissolve the thrombus and prevent its recurrence [9]. After the treatment, the blood flow to the problematic segments should be regained, resulting in the disappearance of the hypoperfused regions in the SPECT perfusion image. The ventilation and perfusion of the lungs on SPECT V/Q images for the diagnosis and evaluation of treatment response of CTEPH is usually based on a qualitative analysis of the image data.