Fig 1: Epoxy resin based PSF phantom
developed at NPL

The key figures of merit for Optical Coherence Tomography (OCT) systems are sensitivity and resolution. These parameters can be used to determine the suitability of instruments for purpose and to gauge the practical improvements to instruments afforded by modifications to the design. Therefore, NPL is developing reproducible and stable phantoms for depth resolved sensitivity and three-dimensional resolution or point-spread-function (PSF).

In August 2009, we completed preliminary development of PSF phantoms that can be used to evaluate OCT system resolution.

Our phantoms consist of a low density of sub-resolution sized particles. Images of these particles can be used to measure both the statistical properties of the PSF (Fig 2) and its spatial distribution throughout the image plane (Fig 4).

Fig 2: Statistical distribution of axial (left) and lateral (right) resolution in a commercial OCT instrument

Fig 3: Spatial map of lateral (top) and axial (bottom) resolution,
measured with a commercial OCT microscope using NPL PSF phantoms

The final report for this study can be downloaded from the NPL publications database (P Tomlins, NPL Report OP2, 'Point Spread Function Phantoms for Optical Coherence Tomography').

Furthermore, in collaboration with Aston University, our group is developing relative sensitivity phantoms for OCT. The former consists of localised differences in refractive index that form points of decreasing brightness. The sensitivity limit of an OCT instrument is reached when no fainter point can be identified (Fig 4).

Fig 4: OCT image of a relative sensitivity phantom.
Points of decreasing intensity are used to quickly identify the sensitivity limit of an OCT instrument

Funded by the UK Technology Strategy Board through the OMICRON project, we have been able to initiate development of contrast phantoms for OCT. These phantoms use two layers of different scattering coefficient material to determine the ability of OCT to distinguish between layers of material having different scattering properties. These bi-layer phantoms are produced with a range of scattering substrates (Fig 5) and imaged using OCT (Fig 6).

Fig 5: Titanium dioxide doped epoxy resin contrast phantoms with different scattering intensities

Fig 6: OCT image of a contrast
phantom. The two layers of different
scattering coefficient are resolved
by the OCT instrument

Related Publications

• P H Tomlins, 'Point-Spread Function Phantoms for Optical Coherence Tomography', NPL Report OP 2 (2009)

For further information, please contact Peter Woolliams