1. Introduction
Pulmonary hypertension (PH) describes a group of pathologies that are defined by an increase of the mean pulmonary arterial pressure (PAP) greater than 25 mmHg at rest [
1]. Due to the increase in PAP, right cardiac overload is induced and the cardiac muscle is weakened, resulting in right ventricular pressure dysfunction [
1,
2].
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 thromboemboli 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 thromboendarterectomy. 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. SPECT V/Q is also used as a reference in the evaluation of other diagnostic techniques [
10,
11,
12,
13,
14,
15,
16]. The quantitative analysis of lung perfusion on SPECT images by volumetric data has previously been proposed [
17]. In the study, lung perfusion volume on SPECT images was segmented by thresholding and compared to anatomical lung volume, as well as clinical parameters, showing the feasibility of quantitative analysis of SPECT perfusion scans. Based on the guidelines and criteria defined for the diagnosis of CTEPH in SPECT V/Q images, as well as the qualitative analysis of those images presented in the cited studies, it can be assumed that functional volumetric data could aid in the diagnostic process. Given the presence of reduced perfusion compared to ventilation, the difference between both volumes is evaluated as a quantitative parameter for the diagnosis of CTEPH.
A new quantitative metric based on volumetric data from SPECT V/Q images is proposed in this study. A quantitative scale based on reference values was designed and validated to automatically classify patients into CTEPH or presenting a respiratory pathology. The defined algorithm also takes into account the guidelines for visual inspection of SPECT V/Q images defined by the European Association of Nuclear Medicine (EANM) [
3] and the European Society of Cardiology (ESC)/European Respiratory Society (ERS) [
6]. Lastly, the utility of volumetric measurements from SPECT perfusion images for the evaluation of treatment response in CTEPH was assessed.
4. Discussion
In this study, a novel quantitative metric obtained from SPECT V/Q images is proposed for the diagnosis of CTEPH. The metric is based on the volumetric data obtained from SPECT images that were shown to be more sensitive than planar imaging for detecting obstructed segments due to CTEPH [
19].
To obtain the optimal segmentation thresholds for the ventilation and perfusion volumes of the SPECT images, a reference group was defined composed of cases without discordant defects in the pulmonary SPECT images. It is important to bear in mind that the reference group was composed of eight women and two men. Therefore, the optimal segmentation thresholds and
VV-P reference values were more representative of a female population. This gender imbalance had a great influence on the overall classification results of the patients of the second and third group. This can be explained by the anatomical differences between female and male lungs, as the former are generally smaller [
20]. Concretely, the algorithm overestimated possible defects in men because obstructed segments of the same anatomical extent in both genders tend to be greater in men and thus, the difference between ventilation and perfusion volumes was higher. Moreover, the difference needed to be much higher in women to receive a positive diagnosis. This can be observed in the results and gender distribution after applying the obtained reference values to the patients diagnosed with CTEPH or a respiratory pathology.
In the case of the patients diagnosed with CTEPH, 6 of the 12 cases were classified as “positive” and 2 yielded a “cannot be ruled out” result. Regarding the gender distribution, 5 of the 8 men were correctly classified and the positive diagnosis could not be ruled out for an additional case. It is also noteworthy that only one of the patients classified as positive with certainty was a woman. Out of the three other women, for one a positive diagnosis could not be ruled out and two received a negative diagnosis. As can be seen by this distribution, most correctly classified cases corresponded to the male patients.
The same pattern can be observed when analyzing the results of the cases diagnosed with respiratory pathologies. Both men who were included in this group were correctly diagnosed with a respiratory pathology. All cases that received a negative diagnosis corresponded to female patients. Moreover, in two cases, the algorithm detected a larger ventilation volume than perfusion volume, which indicates a possible CTEPH and not a respiratory pathology.
In addition to the gender imbalance of the reference group, both groups used for the validation were also not composed equally of women and men. This led to the relatively low sensitivity values, due to the aforementioned overestimation of defects in male patients. Therefore, it can be stated that the main limitation of this study was the composition of the study groups. Not only were they heterogeneous regarding the gender of the patients, but the reference group also consisted of only 10 patients, which included cases of normal lung function and concordant defects present in the SPECT images. To improve the preliminary results presented in this study, the algorithm should be applied separately to women and men. Thus, the segmentation thresholds and reference values for the classification would be gender specific and it can be assumed that the results improve. Another limitation, related to the selected study group, was the exclusion of cases with extensively accumulated radiotracer in the SPECT images. However, less extensive accumulations were still present in some studies and would affect the algorithm. Therefore, an image preprocessing algorithm should be implemented to be able to include these cases and improve the overall performance of the algorithm.
Moreover, in this study, only two cases were available for the comparison of the perfusion volumes before and after the treatment. In addition, similar to the previous cases, the segmentation thresholds need to be revaluated. Moreover, volumetric measurements were only partially useful when evaluating treatment response, as it offers no spatial information. While a general improvement could be determined when the perfusion volume increases, the same could not be said when the difference is low. In that case, the obstructed segment could have been resolved by the treatment, but a new one may have appeared. Therefore, no conclusion can be drawn, and the metric needs to be tested with a larger study population.
Regarding the metric and results of the present work, it needs to be noted that quantitative volumetric data of both ventilation and perfusion scans were not evaluated in any of the referenced studies [
10,
11,
12,
13,
14,
15,
16,
17]. The main image analysis method is a qualitative assessment of the perfusion SPECT and segmental or subsegmental defects, as defined in the clinical guidelines of the EANM [
3]. For example, Johns et al. [
10] determined a positive or negative diagnosis for CTEPH based on visual interpretation of perfusion SPECT images. Wang et al. [
15] also classified the images visually as positive or negative for PE, as the clinical guidelines do not differentiate between acute PE and CTEPH. In Renapurkar et al. [
11], the defects in the perfusion SPECT scan are graded on a four-point scale and only planar V/Q scanning is used for quantitative measurements. On the other hand, Derlin et al. [
17] studied the correlation between the perfusion volume and clinical parameters, but did not include the ventilation information nor conduct a classification procedure based on SPECT images. However, they showed that the perfusion volume is capable of identifying patients with mean PAP greater than 50 mmHg, which is a biomarker of PH (sensitivity, 80%; specificity, 64%).
Lastly, the implemented image segmentation algorithm requires further optimization. Thus, preprocessing should be added to remove possible tracer accumulations on the images. Moreover, alternative automatic segmentation algorithms like active contours could improve the detection of the perfused tissue. Computed tomography scans and its anatomical reference are also a useful tool for the quantitative analysis in CTEPH, as proposed in [
17]. However, these were not available for the study group.