Polarization SeNSItIVE (PS-OCT)

 

PS-OCT will be discussed as an example adjuvant OCT technique both because of promising results with in vitro coronary arteries as well as in vivo data in other organ systems.17, 80, 86, 87  The technique is described in detail in the supplemental information/appendix. We have successfully applied this technique in vitro and in vivo for almost a decade most notably in the musculoskeletal system.10, 86, 87  We now have the experience with single detector PS-OCT in multiple clinical studies where the surgeons read the birefringence directly from the image. 

The theory behind single detector PS-OCT is relatively straightforward, as shown in figure 11.  Most tissue in the body is not birefringent, meaning it does not alter the polarization state of light passing through it significantly.  However, tissue with highly organized structure, particularly collagen type I and II bundles but also smooth muscle to a lesser degree, will lead to a rotation of the polarization as the light as it passes through.  In highly birefringent tissue, this results in an intense banding of the tissue as shown in the meniscus in figure 11.  However, in tissue that has a more modest birefringence, the birefringence can be identified by rotating the polarization state of the incident light (or reference arm).  This results in changing backreflection intensity that would not occur if the tissue was not birefringent.  Normal arterial tissue, due primarily to substantial concentrations of organized type I collagen, is moderately birefringent. We have recently utilized single detector PS-OCT to identify organized collagen in coronary arteries.17 An example single detector PS-OCT image of a coronary artery is figure 12.  A video clip showing of a coronary artery as the polarization is rotated from the same study is included with the supplemental information. In this study, a strong correlation was noted between collagen concentrations in the cap measured by PS-OCT and collagen as assessed by picrosirus staining. The regression plot between PS-OCT changes and measured collagen yielded a correlation coefficient value of 0.475 (p<0.002).  The predictive value of a PS-OCT measurement of negligible birefringence (less than 33% change) for minimal collagen was 93% while the predictive value of high birefringence (greater than 66% change) for high collagen concentrations was 89%.  The ability to measure collagen content in coronary arteries was confirmed by another group using a dual detector system.80  However, it took one second in this study to obtain the data set and it is not stated how long it took to reconstruct the image, other than referring to a reference on the techniques earlier description.94  Dual detection/channel PS-OCT, and its limitations (including catheter bending artifacts), are described in the supplementary information.

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