Although coronary angiography is the gold standard for diagnosing and treating coronary artery disease, it is considered a lumenogram of the coronary arteries because of its limitation for detailed assessment of the vessel wall. 

Intracoronary (IC) imaging has the ability to assess the plaque burden and its type, which helps the interventionists decide if they need to do an intervention or not and how to prepare the lesion. In addition to that, it helps to size the vessel properly and optimize the result after stenting and identifying acute complications such as edge dissection. Furthermore, in cases of stent failure, intravascular imaging is beneficial to determine the cause and to plan the management (1). 

IC imaging has an essential role in the assessment of ambiguous lesions, mainly in acute coronary syndrome (ACS) patients, for example, in the identification of the culprit lesion in cases with multiple lesions in whom not possible to identify the culprit angiographically, in addition to Myocardial Infarction with Non- Obstructed Coronary Arteries (MINOCA) including some cases of spontaneous coronary artery dissection (SCAD). IC imaging, mainly OCT, showed an ability also to identify vulnerable plaques. (2)

Intra Vascular Ultra Sound (IVUS) was initially used in clinical practice in 1988; since then, multiple studies have shown the superiority of IVUS-guided percutaneous coronary intervention (PCI) compared to angiography alone. IVUS has a role in assessing the severity of moderate left main (LM) stenosis and has class IIa recommendation for this purpose according to 2018 ESC/EACTS guidelines on myocardial revascularization, with a cut-off of MLA < 6 mm2 indicating severe stenosis as shown in one study; this figure different from an Asian study which showed MLA < 4.5 mm2 is the cut off indicate severe LM stenosis. We don’t have figures from our population, but I think using < 4.5 mm2 to identify severe stenosis and defer revascularization if LM MLA > 6 mm2 is appropriate. For MLA between 4.5 and 6 mm2, further assessment of ischemia is needed, like using Fractional Flow Reserve (FFR).  

2008 the first in human Optical Frequency Domain Imaging (OFDI) was used (3). OFDI is the second-generation Optical Coherence Tomography (OCT) with a high-frequency rate for image acquisition, allowing imaging without coronary occlusion, which was mandatory with Time Domain OCT.

OCT has a higher axial resolution (~ 15 mm) compared to the standard 20-40 MHz IVUS, and it is even better than High-Definition IVUS (HD-IVUS), which has an axial resolution of about 40-60 mm (4). The higher resolution of OCT is attributed to the shorter wavelength of near-infrared light in OCT compared to the ultrasound wavelength in the IVUS, which on the other hand, decreases OCT depth penetration (1-2 mm Vs. 5-6 mm in IVUS). The near-infrared light wavelength of OCT is about 1.3 mm which is shorter than the Red Blood Cells (RBCs) diameter, leading to backscattering of the light and mandating blood clearance from the coronary artery during imaging acquisition (5).

OCT requires blood clearance from the vessel, which is associated with a higher amount of contrast needed to accomplish the procedure compared to IVUS, but recently, multiple flushing agents have been investigated, such as normal saline which looks to be visible. Furthermore, this requirement makes OCT not useful for aorto-ostial lesions; however, aorto-ostial lesions were imaged by using a guide extension catheter such as Telescope, which has a 2 mm soft polymer tip that can be penetrated by the OCT light and allows imaging of such lesions.   

OCT has a lower penetration depth than IVUS, making it not the suitable modality to assess the plaque burden.

IVUS can not assess calcium thickness, while the infrared light of OCT can penetrate calcium and show it as an area with low backscattering, well-delineated border, and low attenuation. This feature gives OCT the ability to assess the calcium thickness except in cases with thickness beyond the penetration ability of the OCT light.  (figures 1 and 2).

Comparing HD-IVUS to OCT in one prospective study included 29 patients (29 lesions) who underwent PCI in whom both OCT and HD-IVUS were done before and after PCI, there were no significant differences regarding minimal luminal area (MLA), proximal or distal reference luminal areas (LAs), and lesion length. OCT detects lipid-rich plaque more often than HD-IVUS (48% Vs. 38%; P< 0.001), while some lipidic plaques were classified as fibrotic plaque by HD-IVUS. OCT labeled 4 cases with plaque rupture while HD-IVUS detected none of them (14% Vs. 0%; P= 0.0382), and also OCT more frequently detected intraluminal thrombus compared to HD-IVUS (14% vs. 0%; P= 0.0382). In the post-intervention part, both modalities were comparable in the estimation of minimal stent area and stent expansion percentage, but OCT was more frequent than HD-IVUS in the detection of tissue prolapse and mal-apposition in addition to the stent edge dissection which has a significant impact on the clinical outcome was detected more by OCT compared to HD-IVUS (14% Vs. 46%; P=0.0089). (4) 

Data more mature for IVUS and from multiple studies showed that IVUS-guided PCI is associated with better clinical outcomes than angiography alone, such as the ULTIMATE trial, which showed lower target vessel failure (TVF) in the IVUS group. Also, the IVUS-XPL trial showed that IVUS was associated with a lower rate of major adverse cardiac events (MACEs) up to 5 years compared to angiography alone PCI. Furthermore, a meta-analysis of 15 studies, including LM and bifurcation lesions, demonstrates the same findings of the lower rate of MACE in the IVUS group. 

A recent meta-analysis of 5 studies by Saleh et al. showed that IVUS and OCT-guided PCI have a similar rate of MACE, all-cause mortality, myocardial infarction, stent thrombosis, and target lesion revascularization.

The OCTIVUS trial showed that the OCT guided is not inferior to the IVUS-guided PCI, which was a multicenter randomized trial powered for non-inferiority compared to OCT-guided Vs. IVUS guided PCI. The primary endpoint was a composite of death from cardiac causes, target vessel myocardial infarction, or ischemia-driven target vessel revascularization at one year and occurred in (2.5%) of the OCT-guided PCI group and in (3.1%) of the IVUS-guided PCI group (P <0.001).(6)

The OCTOBER study showed that OCT-guided PCI in complex bifurcation PCI improves clinical outcomes compared to angiography alone.(6)

ILUMIEN IV trial compared OCT-guided to angiography-guided PCI in high-risk patients and/or high-risk lesions, although the primary clinical endpoint was similar in both groups, but the OCT-guided PCI was superior regarding the primary imaging endpoint which was the minimal stent area (MSA).(6)

 In conclusion using intra-coronary imaging is not a luxury but an evidence-based practice that is associated with better short and long-term outcomes.