Feature Of The Week 6/13/10: Gabor Domain Optical Coherence Microscopy: Assessment of a Liquid Lens Enabled In Vivo Optical Coherence Microscope
Feature Of The Week 6/13/10: Optical coherence tomography has reached unprecedented axial resolution with the development of broader and broader band sources. The promise to also reach micron level lateral resolution in OCT emerged in the mid 90s with the development of related technology Optical Coherence Microscopy (OCM). The basic concept to increasing lateral resolution is the increase in the numerical aperture (NA) of the imaging optics as resolution varies linearly with NA. The drawback to simply imaging with higher numerical aperture optics however is that the depth of focus over which the targeted lateral resolution will be maintained varies inversely as the NA square. The depth of focus is often written as ±2l/(NA)^2. Thus increasing lateral resolution has typically gone hand-in-hand with seriously impacting depth of imaging or imaging speed, and unfortunately most often both.
The researchers Professor Jannick Rolland and Dr. Kye-Sung Lee who a year ago relocated from CREOL, the College of Optics and Photonics at the University of Central Florida to the University of Rochester, Dr. Supraja Murali (who graduated from CREOL and is now with General Optics Asia Limited who manufactured the optical device), Panomsak Meemon from CREOL, Dr. William Kuhn of Opt-E, and Dr. Kevin Thompson of Optical Research Associates developed a methodology combined with assessments that delivers high lateral resolution in an optical coherence microscope without compromising axial resolution. This has been a longstanding impediment to subcellular imaging with OCM technology. A first step in accomplishing this longstanding goal was the design and implementation of a modest numerical aperture (NA) microscope that results in an optimal optical package of an OCM with liquid lens focusing. This method of focusing can occur at video rates with no moving parts. The second successful and enabling step was the introduction of a Gabor-type image fusion algorithm to automatically window the imagery in-depth and to merge only the in-focus data into a single image. The desire to simultaneously obtain subcellular resolution (< 3 µm) and significant depth of focus (1-2 mm) are conflicting requirements at a fundamental level. Resolution increases as the numerical aperture (NA) increases, where for this application centered on 800 nm, a NA of 0.2 corresponds to a limiting resolution of 2 µm. Assessments of the technology demonstrate that not all metrics are equally useful in quantifying resolution, and the Modulation Transfer Function was adopted to measure resolution, assess the impact of gravity on the liquid lens as well as the effect of optical aberrations on resolution and contrast, and estimate the effective depth of focus to guide the associated algorithms for image fusion.