... / ... / ... / Optics, photoacoustics and intravascular... / Optical coherence tomography for vulnerable...

Optical coherence tomography for vulnerable plaque detection

Researchers: Gijs van Soest, Evelyn Regar, Ton van der Steen

OCT is a relatively new light-based technique, providing high-resolution (10 µm) images of the tissue up to a depth of about 1.5 mm. It is a unique modality for intracoronary imaging, unveiling details of human coronary atherosclerosis in vivo that are too small to observe with other techniques, like intravascular ultrasound. The resolution of OCT makes it the first intravascular technology that can visualize structures as small as the thin fibrous cap that covers a vulnerable plaque.

In intravascular OCT, a catheter, consisting of an optical fiber in a sheath, is advanced into the coronary artery of the patient. The distal end of the fiber consists of an integrated lens/mirror that directs the beam sideways. The fiber rotates inside the catheter, acquiring images by sweeping the beam along the vessel wall. By pulling back the fiber inside the sheath, a three-dimensional, tube-like volume is imaged in highly resolved detail. The first-ever OCT recording in a patient was made in 2002, in the Dept. of Interventional Cardiology at the Thorax center. Following a rapid development of the technology in the years since, OCT is now becoming the method of choice for assessing pathologies in the coronary arteries.

OCT Tissue Typing (OC3T)
One thing that OCT cannot do, at present, is characterize tissue properties, such as plaque composition. Interpretation of the OCT image in terms of tissue type is still a complex and often ambiguous task for trained evaluators. We are developing tissue characterization methods for assessment of atherosclerotic plaque vulnerability. Recent results include quantitative imaging of the optical attenuation coefficient. The tissues occurring in vessel wall pathologies have different optical properties, which may be derived from the OCT signal. We found that two markers of plaque vulnerability, necrotic core and macrophage infiltration, exhibit 3—5 times higher attenuation than more stable plaque components (such as fibrous or calcified material) or healthy vessel wall; see figure 1 [1, 2] .

In a collaboration between the BME lab, the Dept. of Interventional Cardiology, and researchers from the Wellman Center for Photomedicine at Massachusetts General Hospital in Boston, we have shown that the OC3T framework can also be applied in vivo. Research on this topic will continue, assessing other optical properties of the tissue, such as birefringence.

OCT Elastography
Elastography is a technique to image tissue stiffness under applied pressure. It was shown by IVUS that plaque elasticity is a highly sensitive and specific marker for vulnerable plaque [3, 4]. Tissue deformation, in response to the natural blood pressure cycle, is quantified by cross-correlation of frames in an IVUS recording. The algorithms developed for IVUS cannot be applied in a straightforward manner for OCT data, because OCT is much more sensitive to motion, leading to loss of correlation between frames.
Recently, we have proposed a different measurement scheme, employing correlation between adjacent lines in the OCT image, in combination with a synchronous force application, to image elasticity with OCT [5]. We will continue this effort to develop an OCT elasticity imaging technology for intracoronary use.

Motion correction in intravascular OCT
The distal rotation of the OCT catheter, driven by a proximal motor unit and transmitted by the fiber or a flexible drive shaft, is not perfectly uniform. As a result of these variations in the angular velocity, the image appears to hover around its center. This is a mild problem for visual interpretation but a serious problem for quantitative analysis. We have developed an effective correction algorithm based on dynamic time warping, for this non-uniform rotation artifact [6], see Figure 2.

 

References
1. van Soest G, TPM Goderie, N Gonzalo, et al., Imaging atherosclerotic plaque composition with intracoronary optical coherence tomography. Neth. Heart J., 2009. 17(11): p. 444-446.
2. van Soest G, TPM Goderie, E Regar, et al., Atherosclerotic tissue characterization in vivo by optical coherence tomography attenuation imaging. J Biomed Opt, 2009: p. in press.
3. de Korte CL, MJ Sierevogel, F Mastik, et al., Identification of atherosclerotic plaque components with intravascular ultrasound elastography in vivo: A Yucatan pig study. Circulation, 2002. 105(14): p. 1627-1630.
4. Schaar JA, CL de Korte, F Mastik, et al., Intravascular palpography for high-risk vulnerable plaque assessment. Herz, 2003. 28(6): p. 488-495.
5. van Soest G, F Mastik, N de Jong, and AF van der Steen, Robust intravascular optical coherence elastography by line correlations. Phys Med Biol, 2007. 52(9): p. 2445-58.
6. van Soest G, JG Bosch, and AFW van der Steen, Azimuthal registration of image sequences affected by nonuniform rotation distortion. IEEE Trans Inf Technol Biomed, 2008. 12(3): p. 348-355.