Fig. 12.1
Comparision of border detection between optical coherence tomography (OCT) and intravascular ultrasound (IVUS). Normal artery wall shows a 3-layered architecture, comprising a high backscattering, thin intima, a low backscattering media, a heterogeneous and/or high backscattering adventitia in both OCT (a) and IVUS (b). OCT could visualize internal elastic membrane (IEM) and external elastic membrane (EEM) (bold arrow heads) (inset, x3). The OCT-derived EEM or IEM measurement could not be made in cross-sectional image that contains diseased vessel (c) whereas IVUS demonstrate EEM border well (d). * represents wire artifact. CSA cross sectional area; PB plaque burden
Table 12.1
Comparison of major quantitative measurements between optical coherence tomography and intravascular ultrasound
OCT | IVUS | |
|---|---|---|
Lesion | ||
Lumen area | + | + |
Vessel area | −/+ | + |
Plaque burden | −/+ | + |
Area stenosis | + | + |
Stent | ||
Stent area | + | + |
Vessel remodeling | −/+ | + |
12.2 Lesion Assessment
12.2.1 Reference Segment
Reference Assessment
Proximal or distal reference is defined as the sites with the largest lumen proximal or distal to a stenosis within the same segment with no major intervening branches (usually within 10 mm of the stenosis).
Reference Lumen and EEM Assessment
Proximal or distal mean reference lumen diameter is the mean value of the shortest and the longest lumen diameter through the center of mass of the lumen at proximal or distal reference site. Proximal or distal mean reference EEM diameter is the mean value of the shortest and the longest EEM diameter through the center of mass of the lumen at proximal or distal reference site.
Average reference lumen diameter is the average value of mean lumen diameter at the proximal and distal reference sites. Average reference –>EEM diameter is the average value of mean EEM diameter at the proximal and distal reference sites. Both average reference lumen diameter and average reference EEM diameter are useful parameters for stent sizing during PCI.
Average reference EEM CSA , which is a useful parameter for evaluation of lesion severity in terms of stenosis, is the average value of EEM CSA at the proximal and distal reference sites.
Recent in the OPINION study , which had a randomized controlled design to compare the benefit of OCT guidance with IVUS guidance during percutaneous coronary intervention (PCI), OCT reference site was defined as the most normal-looking site with free of lipidic plaque (defined as signal-poor region with diffuse border) at a cross-section adjacent to the target lesion [8]. In other randomized controlled study, the ILUMIEN III: OPTIMIZE PCI study comparing OCT guidance, IVUS guidance, or angiography-guided stent implantation, proximal and distal reference mean EEM diameters and the smaller of these diameters to determine stent diameter or the proximal and distal lumen diameters were used if the EEM could not be visualized [9].
12.2.2 Lesion Segment
Lumen Measurements
Lumen CSA is the area bounded by the luminal border. Minimum lumen diameter is the shortest diameter through the center of mass of the lumen. Maximum lumen diameter is the longest diameter through the center of mass of the lumen. Lumen eccentricity is calculated as (maximum lumen diameter minus minimum lumen diameter) divided by maximum lumen diameter.
OCT-measured lumen CSA is well correlated with IVUS-measured lumen CSA. In both phantom models and in vivo study comparing quantitative coronary analysis (QCA) for angiography vs IVUS vs OCT measurements, OCT was most precise to the real value, and IVUS measurement was 8% larger than OCT measurement [6]. The mean minimum lumen diameter (MLD) measured by QCA was 5% smaller than that measured by OCT, and the minimum lumen diameter measured by IVUS was 9% greater than that measured by FD-OCT [6].
Previously several studies regarding IVUS criteria for defining the functional significance evaluated with fractional flow reserve (FFR) demonstrated that MLD had a good correlation with the FFR values, but the utility of IVUS MLA as an alternative to FFR to guide intervention in intermediate lesions may be limited in accuracy and vessel dependent [10–13]. Anatomical measurements of coronary stenosis obtained by OCT show significant correlation with FFR. OCT-derived parameters were smaller than those reported in previous IVUS studies (Table 12.2) [14, 15]. Recent study assessing computational fractional flow reserve from OCT in patient with intermediate stenosis showed promising approach of it in assessment not only of anatomic information but also of the functional significance of intermediate stenosis [16].
Table 12.2
OCT-derived minimal lumen area predicting for physiologic significance assessed by fractional flow reserve
Study | Patients | FFR value | OCT | IVUS |
|---|---|---|---|---|
Gonzalo et al. [14] | 61 intermediate lesions in 56 patients | FFR < 0.8 | 1.95 mm2 (AUC, 0.74; 95% CI, 0.61–0.84; sensitivity, 82%; specificity, 63%) | 2.36 mm2 (AUC, 0.63; 95% CI, 0.47–0.77, sensitivity, 67%; specificity 65%) |
Shiono et al. [15] | 62 intermediate lesions in 59 patients | FFR < 0.75 | 1.91 mm2 (sensitivity, 94%; specificity, 77%) | NA |
EEM Measurements
EEM CSA is the area bounded by EEM border as a surrogated parameter for vessel area. A discrete interface at the border between the media and the adventitia is almost invariably present within OCT images and corresponds closely to the location of the EEM. Because of low penetration depth of OCT signal and rapid OCT signal attenuation within plaque, EEM circumference and area mostly cannot be measured reliably especially in lesion segment. If low signal involves a relatively small arc (<90°), planimetry of the circumference can be performed by extrapolation from the closest identifiable EEM borders, although measurement accuracy and reproducibility will be reduced.
Plaque (or Atheroma) Measurement
Plaque (or atheroma) CSA is the EEM CSA minus the lumen CSA. Maximum plaque (or atheroma) thickness is the largest distance from the intimal leading edge to the EEM along any line passing through the center of mass of the lumen. Minimum plaque (or atheroma) thickness is the shortest distance from the intimal leading edge to the EEM along any line passing through the center of mass of the lumen. Plaque (or atheroma) eccentricity is calculated as (maximum plaque thickness minus minimum plaque thickness) divided by maximum plaque thickness. If EEM area could not be obtained, plaque measurement is not available.
Plaque Burden
Plaque (or atheroma) burden is assessed as plaque CSA divided by the EEM CSA. This parameter can only be defined when the EEM can be demonstrated. The plaque burden is distinct from the luminal area stenosis. The former represents the area within the EEM occupied by atheroma regardless of lumen compromise. The latter is a measure of luminal compromise relative to a reference lumen analogous to the angiographic diameter stenosis. If EEM area cannot be obtained, plaque burden cannot be assessed.
Lumen Area Stenosis
Lumen area stenosis is assessed as reference lumen CSA minus minimum lumen CSA divided by reference lumen CSA.
Plaque Component and Other Measurements
The presence of specific component within the plaque or over the plaque, such as calcium, lipid, or thrombus, could be assessed as quantitative measurements like angle, depth, thickness, or area. Angle or arc could be measured using the center of mass of the lumen as the angle point. Depth is the distance between the lumen and the leading edge of the plaque feature. Thickness is usually assessed as the thickest distance between the inner and outer surfaces of the plaque component (valid only if the deep boundary can be identified). Area of some component could be described as the CSA of the plaque component (valid only if the deep boundary can be identified).
Fibrous cap thickness can be measured by the thickness of a cap present over OCT-delineated lipid or necrotic core either at the single cross-section where the fibrous cap thickness is considered minimal or from multiple samples (three or more). Although studies have been performed to compare the OCT measurement of fibrous cap thickness with histologic measurements of cap thickness, it was generally considered that this area needs further validation, as the boundary between the cap and the necrotic core is not always straightforward to precisely determine.
Remodeling
An index of remodeling can be assessed as lesion EEM CSA/reference EEM CSA, if the EEM CSA is identified in OCT image.
Because of its limited tissue penetration, OCT does not appear to be suited to study vessel remodeling.
12.3 Stent Measurements
OCT has been considered as an useful intracoronary imaging modality for the lesion assessment, stent sizing, and stent optimization during PCI (Figs. 12.2 and 12.3). The Clinical usefulness of OCT-guided PCI will be discussed in next chapter (Chap. 13).
