**7. Imaging in CTO**

In 50 patients who successfully underwent CTO recanalization compared to 50 matched non-CTO PCI subjects, patients with CTO and an intermediate donor artery stenosis showed a low FFRD with a high frequency of ischemia in the donor artery territory, which was often normalized by successful CTO treatment, thus suggesting recanalization of CTO as a preferred

It is in general difficult to predict which patient with stable ischemic heart disease will receive interventional or surgical revascularization in the long term, after initially being treated with optimal medical therapy (OMT). In the occluded artery trial (OAT), late opening of infarct-related arteries (IRA) post-MI in stable patients with persistent total occlusion and no severe inducible ischemia showed no difference in rates of reinfarction, death, or severe heart failure compared to OMT [39]. Nevertheless, the results of OAT in terms of CTO have to be interpreted with caution because total occlusions in this trial were subacute (3–28 days, median 8 days) and therefore did not meet the CTO definition of at least a 3-month duration. Furthermore, patients in OAT showed a relatively normal baseline LVEF of 48% and were rather asymptomatic, whereas CTOs considered for PCI should be symptomatic or have

An ischemic burden above 12.5% favors PCI in patients with CTO undergoing pre- and postinterventional myocardial perfusion imaging, whereas subjects with mild pre-procedural ischemia (<6.25% of LV myocardium) tend to have increased ischemic burden after PCI [41]. Another magnetic resonance imaging (MRI) study significantly revealed reduction in inducible perfusion defects and improvement in segmental myocardial viability by successful CTO PCI compared to unsuccessful revascularization [42]. Furthermore, successful CTO PCI increases hyperemic and resting myocardial blood flow with enhanced regional contractility already 24 h after the procedure [43]. Patients with an infarction and a transmural involvement < 25% assessed by MRI show significant improvements in segmental wall thickening and a reduction of mean end-systolic and end-diastolic volumes after CTO PCI [44]. Finally, the diagnostic accuracy of pre-procedural contrast enhanced MRI in patients with CTO to detect myocardial infarction and to predict improvement of myocardial function after revascularization seems to be better by using a combined viability analysis rather than focusing on

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

therapeutic strategy. Reference: CCI 2014.

50 Interventional Cardiology

proof of ischemia and viability [40].

the widely used transmural extent of infarction [45].

**6. Coronary computed tomography angiography**

**5. Myocardial viability and left ventricular function**

CTO interventions are technically challenging due to limitations in visualizing occluded arteries by angiography. As mentioned before, ambiguous proximal CTO cap, side branch at the occlusion site, extended tortuosity, or heavy calcification with limited visibility of distal path are important angiographic features which increase procedural difficulties during CTO recanalization [7]. Multislice computed tomography (MSCT) can provide useful pre-procedural information on the dimension of vessel calcification or tortuosity along the occluded segment but does not offer direct guidance during the procedure. Intravascular ultrasound (IVUS) and optical coherence tomography (OCT) can add visual guidance during CTO PCI to improve procedure time, safety, and efficacy.

#### **7.1. Intravascular ultrasound**

In contrast to OCT, IVUS can be operated in occluded vessels throughout the whole interventional procedure. It is used to identify the best wire entry point for penetration of proximal fibrous cap or to visualize the guidewire to check intraluminal position before balloon angioplasty or stent deployment.

The IVUS probe is usually advanced into a side branch originating proximal to the occlusion to determine the vessel course within the CTO segment [52]. Standard IVUS catheters cannot generate information of the vessel distal of the occlusion, and their use is limited by the diameter and angulation of side branches [53].

Besides its antegrade applications, IVUS is used in retrograde procedures to guide retrograde guidewire crossing and reverse controlled retrograde tracking techniques such as reverse CART to improve success rate and limit complications [52]. Generally, when using the retrograde approach in longer CTO lesions, IVUS guidance can reduce the incidence of subintimal wiring with consecutive side branch loss after stenting, angiographic extravasation, coronary hematoma, and perforation [54, 55].

Furthermore, the incidence of restenosis [56] or stent thrombosis after DES implantation [57] is related to minimum stent area detected by IVUS and malapposition due to aneurysm formation after subintimal DES implantation during CTO PCI, and it can be optimized with the help of IVUS [55].

Although IVUS facilitates CTO PCI and has the potential to reduce periprocedural complications, the clinical benefit of IVUS-guided CTO PCI has not yet been proven, and further studies are needed [52].

#### **7.2. Optical coherence tomography**

OCT is more sensitive than IVUS in detecting coronary dissection during PCI and improves stent deployment or detection of acute complications. Furthermore, resolution of OCT is high enough to visualize microvessels, the different layers of the vessel wall, and even collagen concentration in coronary arteries [58].

In contrast to IVUS, conventional OCT, at the cost of penetration depth, has a 10-fold higher imaging resolution as the main advantage but is unable to generate images in completely occluded vessels and does not allow real-time intracoronary imaging for guidance of wire crossing. However, optical coherence reflectometry used in a combined OCT and radiofrequency ablation device might be able to minimize the risk of perforation and increase the crossing potential of the guidewire in CTO PCI [59].
