Feature Of The Week 7/21/13: VU University Amsterdam Demonstrates Focus-Extension by Depth-Encoded Synthetic Aperture in Optical Coherence Tomography
Optical coherence tomography (OCT) is an interferometric technique that provides cross-sectional images of biological tissue. It has become an essential tool for clinical diagnosis and disease monitoring in ophthalmology and it shows great potential in other clinical areas such as dermatology, cardiology and gastroenterology. The axial resolution in OCT is provided by the coherence gate and is invariant over the full image depth. The lateral resolution is determined by the beam parameters such as wavelength and numerical aperture. The Rayleigh range determines the depth range over which the lateral resolution can be maintained. The depth of focus is generally defined as twice the Rayleigh range. Current broadband laser sources permit axial resolutions below 10 μm over several millimeters of image depth. However, it is impossible to achieve ~10 μm transverse resolution over a depth range of several millimeters using Gaussian beams and normal objectives. The relatively short focal-depth of imaging optics limits the application of high lateral resolution OCT to thin slices.
To date, various methods have been proposed to address the limited depth-of-focus of OCT imaging. Examples are dynamic focus, multi-focus and focal-depth extension methods. Dynamic focusing synchronizes the focal plane position with coherence gate scanning, which is more suitable for time-domain OCT. The multi-focus approach requires parallel acquisition at different focal depths. In comparison, depth-of-focus extension has been more successful. Till now, the methods proposed for focus-depth extension can be categorized into three groups: (1) Bessel beam illumination with an axicon lens, (2) phase apodization and (3) digital refocusing. The axicon and phase apodization approaches are inefficient in illumination and collection from the extended focus, leading to signal losses of tens of decibels. Interferometric synthetic aperture methods (ISAM) have been developed for digital refocusing by solving the inverse scattering problem. It is computationally expensive and requires phase stable acquisition of consecutive depth profiles. Other digital refocusing methods based on deconvolution or scalar diffraction models also require phase-stable OCT measurements.
In this study, we demonstrated a novel method to extend the focus-depth, which is analogous to synthetic aperture radar. This method does not require phase-stable OCT A-scan over the whole volume and is compatible with endoscopy and computationally inexpensive. The idea is to use an annular phase plate to separate the light into different optical apertures that are encoded by a depth offset in an OCT A-scan. When the scattering object is out of focus, an additional phase difference takes place among the scattered light fields through different optical apertures as compared to the situation of the scattering object in focus. The depth-offset mentioned above leads to the acquisition of multiple OCT images by different optical apertures in parallel in a single B-scan. This allows for manipulation of the phase of those images to correct the defocus-induced phase difference among different optical apertures. As a consequence, a coherent summation of those images can be achieved for reconstructing a new image with a significantly extended depth-of-focus.
For more information see recent Article. Courtesy of Jianhua Mo from VU University Amsterdam.