**6. Coronary computed tomography angiography**

Coronary computed tomography angiography (CTA) is increasingly used to diagnose CAD and shows potential in predicting the probability of procedural success and clinical benefit in CTO PCI [46, 47]. In contrast to invasive coronary angiography, CTA offers better quantification of anatomical and morphological features in occluded vessels, especially in long lesions with pronounced tortuosity, and usually visualizes distal coronary segments more precisely [48]. There are a number of CTA characteristics in CTO lesions to predict PCI failure. Some report calcifications >50% of the cross-sectional vessel area alone [49] or in combination with an occlusion length of >15 mm to be independent predictors of unsuccessful recanalization [50], while others revealed the ratio of calcification over the cross-sectional vessel area as being predictive for procedural failure [51]. Moreover, marked vessel tortuosity at the occlusion site seems to independently predict unsuccessful guidewire crossing [24] and multiple occlusions might have an adverse effect on revascularization's outcome, as reported from the CT-Registry of Chronic Total Occlusion Revascularization (CT-RECTOR), probably due to reduced feasibility in guiding the wire through the multiple entry and exit points [46].

Conclusively, CTA features in CTO, as described above, may be applicable to assess severity of the occlusion and to predict PCI outcome in order to guide treatment decision, especially in complex lesions.
