Feature Of The Week 7/31/11: Researchers From The Medical University of Vienna Demonstrate Ultra-High Speed Fiber Based PS-OCT
Researchers at the Medical University of Vienna have been doing pioneering work in the field of OCT for over 20 years including some of the first work on low-coherence interferometric imaging of the eye, fundamental work in developing spectral domain OCT, and many other contributions. A snap shot of some of their recent work can be seen Here. Recently they published another interesting paper on ultra-high speed fiber based polarization sensitive OCT. Below is a summary of that work.
Polarization sensitive (PS) OCT is a functional extension of OCT. It takes advantage of the additional polarization information carried by the reflected light, and can therefore add new image contrast compared to intensity based OCT. PS-OCT can reveal important information about biological tissue, such as quantitative distribution of birefringence, which is unavailable in conventional OCT.
A very interesting application field for PS-OCT is retinal imaging. One can distinguish between polarization preserving (e.g. photoreceptor layer), birefringent (e.g. retinal nerve fiber layer (RNFL), Henle’s fiber layer) and depolarizing layers (e. g. retinal pigment epithelium (RPE)).
One of the main limiting factors of PS-OCT image quality and quantitative data is speckle noise. A very successful method to reduce speckle noise in conventional, intensity based OCT imaging, is averaging of several B-scans that are successively recorded at almost the same position. This method is implemented in a commercially available OCT retinal scanner (Spectralis, Heidelberg Engineering). By employing a retinal tracker, the instrument is able to record several successive B-scans almost on the same retinal location. Typically 20 B-scans are averaged to provide a high-quality nearly speckle free image of the retina.
We demonstrate a related method for generation of high quality speckle reduced PS-OCT images. Instead of using a hardware eye-tracker, we take advantage of new high-speed CMOS line scan cameras. We record images at a scanning speed of 70-128 kAlines/s, up to more than 6 times faster than with our previous systems for retinal imaging. This reduces motion artifacts considerably. Residual motion artifacts are corrected by a software algorithm. Up to 60 successively recorded and motion corrected B-scan data sets are averaged. We record images at nominally the same position (i.e. at similar settings of the x-y galvo scanner). Due to eye motion, the speckle pattern of each B-scan changes slightly and therefore after averaging one can obtain speckle reduced PS-OCT images of intensity, retardation, and optic axis orientation. Different averaging schemes are compared, the best ones providing a ~5 fold noise reduction in polarization sensitive images. Furthermore, a novel scheme of calculating images of degree of polarization uniformity (DOPU) is presented. The DOPU parameter can be used to selectively image the retinal pigment epithelium (RPE), a tissue who’s integrity is decisive for the vitality of the adjacent photoreceptor layer. Instead of spatial averaging of Stokes vector data, temporal averaging over successive images is used, which provides a considerably improved spatial resolution of DOPU images. These results will improve imaging and diagnostics of retinal layers that are affected by major diseases like glaucoma and age related macular degeneration.
For more information see recent Article. Courtesy Erich Götzinger.