Point spread function (PSF) phantoms based on unstructured distributions of sub-resolution particles in a transparent matrix have proven effective for evaluating resolution and its spatial variation in optical coherence tomography (OCT) systems. Measurements based on PSF phantoms have the potential to become a standard test method for consistent, objective and quantitative inter-comparison of OCT system performance. Towards this end, we have evaluated three PSF phantoms and investigated their ability to compare the performance of four OCT systems. The phantoms are based on 260-nm-diameter gold nanoshells, submicron-diameter iron oxide particles and 1.5-micron-diameter silica particles. The OCT systems included spectral-domain and swept source systems in free-beam geometries as well as a time-domain system in both free-beam and fiberoptic probe geometries. Results indicated that iron oxide particles and gold nanoshells were most effective for measuring spatial variations in the magnitude and shape of PSFs across the image volume. The intensity of individual particles was also used to evaluate spatial variations in signal intensity uniformity. Significant system-to-system differences in resolution and signal intensity and their spatial variation were readily quantified. The phantoms proved useful for identification and characterization of irregularities such as astigmatism. Particle concentrations of 5000 per cubic millimeter or greater provided accurate determination of performance metrics. Our multi-system inter-comparison provides evidence of the effectiveness of PSF-phantom-based test methods for comparison of OCT system resolution and signal uniformity.
Optical coherence tomography (OCT) is a non-invasive three-dimensional imaging system that is capable of producing
high resolution in-vivo images. OCT is approved for use in clinical trials in Japan, USA and Europe. For OCT to be used
effectively in a clinical diagnosis, a method of standardisation is required to assess the performance across different
systems. This standardisation can be implemented using highly accurate and reproducible artefacts for calibration at
both installation and throughout the lifetime of a system. Femtosecond lasers can write highly reproducible and highly
localised micro-structured calibration artefacts within a transparent media. We report on the fabrication of high quality
OCT calibration artefacts in fused silica using a femtosecond laser. The calibration artefacts were written in fused silica
due to its high purity and ability to withstand high energy femtosecond pulses. An Amplitude Systemes s-Pulse Yb:YAG
femtosecond laser with an operating wavelength of 1026 nm was used to inscribe three dimensional patterns within the
highly optically transmissive substrate. Four unique artefacts have been designed to measure a wide variety of
parameters, including the points spread function (PSF), modulation transfer function (MTF), sensitivity, distortion and
resolution - key parameters which define the performance of the OCT. The calibration artefacts have been characterised
using an optical microscope and tested on a swept source OCT. The results demonstrate that the femtosecond laser
inscribed artefacts have the potential of quantitatively and qualitatively validating the performance of any OCT system.
The recent expansion of clinical applications for optical coherence tomography (OCT) is driving the development of
approaches for consistent image acquisition. There is a simultaneous need for time-stable, easy-to-use imaging targets
for calibration and standardization of OCT devices. We present calibration targets consisting of three-dimensional
structures etched into nanoparticle-embedded resin. Spherical iron oxide nanoparticles with a predominant particle
diameter of 400 nm were homogeneously dispersed in a two part polyurethane resin and allowed to harden overnight.
These samples were then etched using a precision micromachining femtosecond laser with a center wavelength of 1026
nm, 100kHz repetition rate and 450 fs pulse duration. A series of lines in depth were etched, varying the percentage of
inscription energy and speed of the translation stage moving the target with respect to the laser. Samples were imaged
with a dual wavelength spectral-domain OCT system (λ=800nm, ▵λ≈180nm, and λ=1325nm, ▵λ≈100nm) and point-spread function of nanoparticles within the target was measured.
As optical coherence tomography (OCT) becomes widespread, validation and characterization of
systems becomes important. Reference standards are required to qualitatively and quantitatively
measure the performance between difference systems. This would allow the performance degradation
of the system over time to be monitored. In this report, the properties of the femtosecond inscribed
structures from three different systems for making suitable OCT characterization artefacts (phantoms)
are analyzed. The parameter test samples are directly inscribed inside transparent materials. The
structures are characterized using an optical microscope and a swept-source OCT. The high
reproducibility of the inscribed structures shows high potential for producing multi-modality OCT
calibration and characterization phantoms. Such that a single artefact can be used to characterize
multiple performance parameters such the resolution, linearity, distortion, and imaging depths.
In this work we present contrast measurement phantoms for optical coherence tomography. In this initial study, contrast
has been evaluated from OCT images of a USAF-1951 bar target, buried beneath scattering layers of different density.
Preliminary results indicate that scattering does not degrade imaging contrast significantly, however further work is
required to verify these findings. This work has important implications from a quality control perspective as well for
OCT inter-comparisons at, for example, different wavebands.
In this paper a novel method for determining refractive indices of a multi-layered samples using low coherence
interferometry (LCI), developed at the National Physical Laboratory, UK, is introduced. Conventional Optical
Coherence Tomography (OCT) utilises a lateral scanning optical probe beam to construct a depth resolved image of the
sample under investigation. All interfaces are detected in optical path length, resulting in an image depending on the
refractive index of all prior layers. This inherent ambiguity in optical and geometric path length reduces OCT images to
purely qualitative ones. We have demonstrated that by optically probing the sample at multiple angles we can determine
bulk refractive index of layers throughout plane parallel samples. This method improves upon current approaches of
extracting refractive index parameters from multi-layered samples as no prior geometrical information is required of the
sample and the phase index for each layer is obtained as opposed to the group index. Consequently the refractive index
result for each layer is independent of the refractive index of surrounding layers. This technique also improves on
conventional measurements, as it is less susceptible to error due to surface defects. This technique is easily implemented,
and can easily be modified to obtain in situ measurements. Investigating a silica test piece and comparing the refractive
index obtained by that of standard critical angle refractometry has validated the robustness of the technique.
Two significant figures of merit for optical coherence tomography (OCT) systems are the axial and transverse resolutions.
Transverse resolution has been defined using the Rayleigh Criterion or from Gaussian beam optics. The axial resolution
is generally defined in terms of the coherence length of a Gaussian shaped source. Whilst these definitions provide a
useful mathematical reference they are somewhat abstracted from the three dimensional resolution that is encountered
under practical imaging conditions. Therefore, we have developed a three-dimensional resolution target and measurement
methodology that can be used to calibrate the three-dimensional resolution of OCT systems.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.