Kye Sung Lee

Task-based optimization and performance assessment in optical coherence imaging

Optimization of an optical coherence imaging (OCI) system on the basis of task performance is a challenging undertaking. We present a mathematical framework based on task performance that uses statistical decision theory for the optimization and assessment of such a system. Specifically, we apply the framework to a relatively simple OCI system combined with a specimen model for a detection task and a resolution task. We consider three theoretical Gaussian sources of coherence lengths of 2, 20, and 40 microm. For each of these coherence lengths we establish a benchmark performance that specifies the smallest change in index of refraction that can be detected by the system. We also quantify the dependence of the resolution performance on the specimen model being imaged.

Dispersion control with a Fourier-domain optical delay line in a fiber-optic imaging interferometer

Recently, Fourier-domain (FD) optical delay lines (ODLs) were introduced for high-speed scanning and dispersion compensation in imaging interferometry. We investigate the effect of first- and second-order dispersion on the photocurrent signal associated with an optical coherence imaging system implemented with a single-mode fiber, a superluminescent diode centered at 950 nm +/- 35 nm, a FD ODL, a mirror, and a layered LiTAO3 that has suitable dispersion characteristics to model a skin specimen. We present a practical and useful method to minimize the effect of dispersion through the interferometer and the specimen combined, as well as to quantify the results using two general metrics for resolution. Theoretical and associated experimental results show that, under the optimum solution, the maximum broadening of the point-spread function through a 1-mm-deep specimen is limited to 57% of its original rms width value (i.e., 8.1 microm optimal, 12.7 microm at maximum broadening) compared with approximately 110% when compensation is performed without the specimen taken into account.

Phantom study of tear film dynamics with optical coherence tomography and maximum-likelihood estimation

In this Letter, we implement a maximum-likelihood estimator to interpret optical coherence tomography (OCT) data for the first time, based on Fourier-domain OCT and a two-interface tear film model. We use the root mean square error as a figure of merit to quantify the system performance of estimating the tear film thickness. With the methodology of task-based assessment, we study the trade-off between system imaging speed (temporal resolution of the dynamics) and the precision of the estimation. Finally, the estimator is validated with a digital tear-film dynamics phantom.

Maximum-likelihood estimation in Optical Coherence Tomography in the context of the tear film dynamics

Understanding tear film dynamics is a prerequisite for advancing the management of Dry Eye Disease (DED). In this paper, we discuss the use of optical coherence tomography (OCT) and statistical decision theory to analyze the tear film dynamics of a digital phantom. We implement a maximum-likelihood (ML) estimator to interpret OCT data based on mathematical models of Fourier-Domain OCT and the tear film. With the methodology of task-based assessment, we quantify the tradeoffs among key imaging system parameters. We find, on the assumption that the broadband light source is characterized by circular Gaussian statistics, ML estimates of 40 nm +/- 4 nm for an axial resolution of 1 μm and an integration time of 5 μs. Finally, the estimator is validated with a digital phantom of tear film dynamics, which reveals estimates of nanometer precision.

A 5mm catheter for constant resolution probing in Fourier domain optical coherence endoscopy

A 5mm biophotonic catheter was conceived for optical coherence tomography (OCT) with collimation optics, an axicon lens, and custom design imaging optics, yielding a 360 degree scan aimed at imaging within concave structures such as lung lobes. In OCT a large depth of focus is necessary to image a thick sample with a constant high transverse resolution. There are two approaches to achieving constant lateral resolution in OCT: Dynamic focusing or Bessel beam forming. This paper focuses on imaging with Bessel beams. A Bessel beam can be generated in the sample arm of the OCT interferometer when axicon optics is employed instead of a conventional focusing lens. We present a design for a 5mm catheter that combines an axicon lens with imaging optics and the coupling of a MEMS mirror attached to a micromotor that allow 360 degree scanning with a resolution of about 5 microns across a depth of focus of about 1.2mm.

Dispersion manipulation in optical coherence tomography with Fourier-domain optical delay line

In the last decade, Fourier-domain optical delay lines (FD-ODL) based on pulse shaping technology have emerged as a practical device for high-speed scanning and dispersion compensation in imaging interferometry such as optical coherence tomography(OCT). In this study, we investigate the effect of first- and second-order dispersion on the photocurrent signal associated with a fiber-optic OCT system implemented using a superluminescent diode centered at 950nm±35nm, an FD-ODL, and a mirror and a layered LiTaO3 which owns suitable dispersion characteristics to model a skin specimen. We present a practically useful method associated with FD-ODL to minimize the effect of dispersion through the OCT system and the specimen combined, and quantify the results using two general metrics for axial resolution.

2mm catheter design for endoscopic optical coherence tomography

A biophotonics catheter was conceived with collimation optics, an axicon lens, and custom design imaging optics yielding a 360 degree scan aimed at imaging within concave structures such as arteries and lung lobes. The large depth of focus is necessary to image a long-depth-range sample with constant transverse resolution in optical coherence tomography (OCT). There are two approaches to achieving constant invariant resolution in OCT: Dynamic focusing or Bessel beam formation. This paper focuses on imaging with Bessel beams. The Bessel beams may be created with axicon optics which can be used instead of a conventional focusing lens in the sample arm of the OCT interferometer. In this paper we present the design of a 2mm catheter for optical coherence endoscopy with resolution of about 5 micron across a depth of focus of about 1.6mm. Importantly, we investigated the fabrication of a 800μm diameter axicon lens and the associated lateral resolution obtained over a long depth range in our OCT system, compared to the same OCT system using a conventional lens.

Fourier domain optical coherence tomography with an 800-μm diameter axicon lens for long-depth-range probing

Recently, Fourier domain optical coherence tomography (FDOCT) has attracted much attention due to the significantly improved sensitivity and imaging speed compared to time domain OCT. The large depth of focus is necessary to image a long-depth-range sample with constant transverse resolution in FDOCT where dynamic focusing is not considered. Under such imaging scheme, an axicon lens can be used instead of a conventional focusing lens in the sample arm of OCT to achieve both high lateral resolution and a long depth of focus simultaneously. In this study, a 800μm diameter axicon lens was fabricated on a silica wafer. We incorporated the fabricated axicon lens into the sample arm of our FD OCT system and investigated the lateral resolution over a long depth range, compared to the same FD OCT system using a conventional lens.

2mm Catheter Design for Optical Coherence Microscopy

A 2 mm biophotonics catheter was conceived with collimation optics, an axicon lens, and custom design imaging optics yielding a 360 degree scan within concave structures such as arteries and lung lobes.

AUC-based resolution in optical coherence tomography

Optical coherence tomography (OCT) is an interferometric technique using the low coherence property of light to axially image at high resolution in biological tissue samples. Transverse imaging is obtained with two-dimensional scanning and transverse resolution is limited by the size of the scanning beam at the imaging point. The most common metrics used for determining the axial resolution of an OCT system are the full-width-at-half-maximum (FWHM), the absolute square integral (ASI), and the root-mean-square (RMS) width of the axial PSF of the system, where the PSF of an OCT system is defined as the envelope of the interference fringes when the sample has been replaced by a simple mirror. Such metrics do not take into account the types of biological tissue samples being imaged. In this paper we define resolution in terms of the instrument and the biological sample combined by defining a resolution task and computing the associated detectability index and area under the receiver operating characteristic curve (AUC). The detectability index was computed using the Hotelling observer or best linear observer. Results of simulations demonstrate that resolution is best quantified as a probability of resolving two layers, and the impact on resolution of variations in the index of refraction between the layers is clearly demonstrated.