**5. Future perspective**

backscatter. The light attenuation, estimated as the depth-resolved attenuation coefficient, indicates how fast the light signal decays. It is the rate of exponential decreasing intensity related to the light propagation depth. The backscatter, estimated as the depth-resolved back-

In ex vivo validation experiments, highly attenuating regions (attenuation coefficient μ<sup>t</sup> ≥ 8 mm−1) have been associated with necrotic core or macrophages. Conversely, low attenuating regions μ<sup>t</sup> < 6 mm−1 were associated with healthy vessel, intimal thickening or calcified plaque [68, 69]. Thus, by using light attenuation analysis, it could detect the lipid-rich plaque component.

Liu and colleagues extended the light property analysis with light intensity and backscatter in addition to the light attenuation analysis [67]. They evaluated the light property values in each tissue type (fibrous, lipid, calcium, calcium with lipid, macrophages and necrotic core) in histology-matched OCT images and found that all tissue types have their own spectrum of light property, suggesting the possibility of automatic tissue characterization with OCT light

**4. Clinical application of multimodality imaging in detecting vulnerable** 

OCT has been used to evaluate the plaque stability by statin therapy, and an increase in fibrous cap thickness in coronary plaques was observed after the treatment [70, 71]. Recently, IVUS has been used to evaluate the effect of proprotein convertase subtilisin kexin type 9 (PCSK9) inhibitors on the progression of coronary atherosclerosis in statin-treated patients. PCSK9 inhibition with evolocumab was associated with a reduction in percent atheroma volume for evolocumab (−0.95%) but not placebo (+0.05%) and a greater percentage of patients

Previously, bioresorbable scaffold (BRS) has been investigated for the capability of sealing superficial plaque [73–75]. The neointima on top of the BRS struts altered the plaque phenotype by covering the calcific spots and TCFA, thus transforming the TCFA to thick-cap fibroatheromas that associate with the plaque stability without compromising the luminal dimensions [73]. It has been speculated that BRS may prevent the cardiac adverse event by invasive sealing of the high risk to rupture plaques. The ongoing PROSPECT ABSORB (NCT02171065) trial will examine the treatment of vulnerable plaques with the ABSORB bioresorbable vascular scaffold (BVS) plus guideline-directed medical therapy (GDMT) in comparison with GDMT alone [76]. All randomized patients will undergo 2-year followup angiography with three-vessel repeat NIRS-IVUS imaging, thus enabling evaluation of plaque regression/progression in intervening vessels and nonintervening vessels [77]. The

**4.1. Assessment of the effect of pharmacotherapy on plaque modulation**

demonstrating plaque regression (64.3 vs. 47.3%) [72].

**4.2. Assessment of the effect of local therapy on vulnerable plaque**

scattering coefficient, is related to the efficiency of light scattering in the tissue [67].

property analysis in the near future.

98 Clinical Trials in Vulnerable Populations

**plaque**

Aside from the technical issues and validity of each imaging modality in detecting vulnerable plaque, and predicting the future events, a real question is whether identification of vulnerable plaque would have any impact on our practice [78–80]. Although it has been demonstrated that VH-IVUS TCFA is able to predict recurrence of events, the positive predictive value was 18% [37] with the risk of catheter-related complication of 0.6–1.6% [33, 37]. Thus, the improvement of imaging technologies is required to provide complete and detailed evaluation of plaque morphology, physiology, and biology to predict the future events [81]. New hybrid catheters have shown their capabilities in demonstrating the plaque vulnerability; however, the clinical benefit needs confirmation by larger studies. Noninvasive technique, especially CCTA combined with PET, may support the detection of vulnerable plaque and tailoring the treatment of those patients.
