Joint aperture detection for speckle reduction and increased collection efficiency in ophthalmic MHz OCT
Spotlight summary: In the latest paper from Robert Huber’s group, Thomas Klein presents the joined aperture OCT (JA-OCT) detection scheme, a novel parallel angle-resolved OCT technique, and demonstrates its application for speckle reduction by channel compounding. An experimental OCT system operating at 1.68 MHz axial scan rate was used to validate this technique for “speckle free” retinal imaging. Other advantages of proposed detection scheme include an increase of the collection efficiency, which results in improved effective OCT sensitivity, as well as the availability of information about the object angular scattering. Besides the introduction of this technique, the authors provide a review of the main directions in current retinal OCT research (image quality, imaging speed and functional extensions of OCT) as well as of hardware-based speckle reduction techniques (spatial compounding, scatter reconfiguration as well as wavelength, polarization and angle diversity).
Speckle is an integral part of the Optical Coherence Tomography images acquired in scattering media, which includes most of biological samples. Speckles are formed by the coherent superposition of individually scattered photons within the sample, and OCT due to its coherent detection scheme detects this speckle field. Presence of speckle reduces resolution of the OCT systems and encumbers its application for quantitative measurements. This is why methods to reduce or control speckle contrast in OCT has been integral part of the OCT research from its earliest days.
The proposed JA-OCT can be applied to systems where a large NA is available for imaging but the application of one active channel using the whole available NA for delivering and detecting light is not practical, namely the lateral resolution could be traded for JA-OCT detection benefits. Retinal imaging with OCT is a perfect example were a large pupil is available but cannot be effectively used without implementation of adaptive optics for ocular aberration correction. In standard retinal OCT, the imaging aperture occupies only a fraction of the available pupil, leaving plenty of space for several additional passive channels (light detection only) used in the JA-OCT detection scheme. In the system demonstrated in this paper the authors used three passive channels located within a circular aperture of 3mm at the pupil plane. Each of the channels needed to be adjusted for each subject to maximize the OCT signal that was used to calculate a compound tomogram (average of all channels). In the example images presented by the authors relatively small numbers of compound frames ~10 were sufficient to generate very “smoothly appearing images” resulting in a 168 kHz effective A-scan acquisition rate of speckle reduced images (which is several times faster than current clinical systems need to acquire single A-scan).
In summary, I agree with the author’s prediction that in the future JA-OCT may allow for the reconstruction of the full Doppler vector and tissue discrimination by analysis of angular scattering dependence, which might offer new perspectives for functional imaging with OCT.
--Robert J. Zawadzki
Technical Division: Information Acquisition, Processing, and Display
ToC Category: Optical Coherence Tomography
|OCIS Codes:||(030.6140) Coherence and statistical optics : Speckle|
|(120.3890) Instrumentation, measurement, and metrology : Medical optics instrumentation|
|(170.3880) Medical optics and biotechnology : Medical and biological imaging|
|(170.4500) Medical optics and biotechnology : Optical coherence tomography|
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