Abstract
Purpose
To allow an accurate diagnosis of coronary artery diseases by enhancing optical coherence tomography (OCT) images of atheromatous plaques using a novel automated attenuation compensation technique.
Background
One of the major drawbacks of coronary OCT imaging is the rapid attenuation of the OCT signal, limiting penetration in tissue to only few millimetres. Visualisation of deeper anatomy is however critical for accurate assessment of plaque burden in-vivo.
Methods
A compensation algorithm, previously developed to correct for light attenuation in soft tissues and to enhance contrast in ophthalmic OCT images, was applied to intracoronary plaque imaging using spectral-domain OCT.
Results
Application of the compensation algorithm significantly increased tissue contrast in the vessel wall and atherosclerotic plaque boundaries. Contrast enhancement allows a better differentiation of plaque morphology, which is particularly important for the identification of lipid rich fibro atheromatous plaques and to guide decision on treatment strategy.
Conclusion
The analysis of arterial vessel structure clinically captured with OCT is improved when used in conjunction with automated attenuation compensation. This approach may improve the OCT-based interpretation of coronary plaque morphology in clinical practice.
1
Introduction
Accurate assessment of plaque morphology in-vivo during interventions is critical to guide decision of treatment on a particular lesion. Despite improved axial resolution, reaching up to 10 microns, use of intracoronary OCT for the assessment of plaque contour and identification of plaque composition is limited by the rapid physical attenuation of OCT signal in tissue . Typically intravascular OCT has a limited field of view because OCT signal is completely attenuated within a few millimetres. In addition, highly attenuating plaque components create shadow artefacts that can distort the OCT visualisation of deep tissues or mask entirely the atherosclerotic plaque inner structural composition and most external contour (i.e. external elastic lamina or EEL).
Absence of signal in the deepest tissue structures may result in clinical misinterpretation and errors on plaque burden measurements. Because OCT often fails to render deepest tissue structures, its use for guidance of Percutaneous Coronary Interventions (PCIs) has been mainly limited to post-PCI assessment of stent apposition and strut endothelialisation and coverage at follow-up . Contrary to OCT, intravascular ultrasound (IVUS) has a lower axial resolution but can penetrate much deeper into tissue, and remains therefore widely used for pre-stenting assessment of plaque composition and measurement of plaque burden ( Fig. 1 ). While the image quality of OCT is significantly improved, it is still greatly hampered by the presence of shadow artefacts and by poor tissue visibility in the deepest layers. This is also due to signal attenuation, whereby signal strength diminishes as a function of tissue depth. This phenomenon is a barrier to clinical applications of intracoronary OCT and limits its use in the diagnosis and risk management of coronary artery diseases.
To address some of the OCT limitations in the case of intravascular imaging, several studies have attempted to quantify signal attenuation from intravascular OCT B-scans to better understand light/plaque interactions and identify plaque content . However such methods are not straightforward and not easily implemented, as they require signal fitting to a predefined model (e.g. exponential decay modelling of the A-Scan pixel intensity across biological layers), matching specific vessel conformations.
Inspired from an image post-processing approach to correct for ultrasound attenuation , Girard et al. have shown that OCT signal attenuation can be modelled into OCT theory, and its effects corrected. This approach was shown to drastically improve the penetration and contrast in ophthalmic OCT images.
In a preliminary study, we suggested that the same post-processing approach could be applied to enhance the quality of normal intravascular OCT images . Here, it is tested in the particular cases of plaque-induced and strut-induced attenuation. Our ultimate goal is to improve the detection of coronary artery plaques using intravascular OCT.
2
Methods and results
OCT pullbacks acquired on a spectral-domain C7 intracoronary OCT system (St Jude Medical, St Paul, MN) were exported in raw format and then imported in Matlab (Mathworks, US) for post-processing as described in Fig. 2 . Raw OCT data were processed with the described contrast enhancement compensation algorithm applied along each OCT line. The effect of the compensation algorithm on a particular OCT line is described in Fig. 3 . Finally, polar reconstructions of the results after compensation (Enhanced) are obtained for comparison with Baseline OCT images ( Fig. 4 ).
Application of the compensation algorithm on different atheromatous plaque examples are shown on Fig. 5 . Rapid attenuation of the OCT signal on the baseline OCT image precludes the correct interpretation of atherosclerotic plaque morphology and plaque burden ( Fig. 5 , A, C, E).
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