Multilayer thin-film phantoms for axial contrast transfer function measurement in optical coherence tomography

Abstract

In optical coherence tomography (OCT), axial resolution is one of the most critical parameters impacting image quality. It is commonly measured by determining the point spread function (PSF) based on a specular surface reflection. The contrast transfer function (CTF) provides more insights into an imaging system’s resolving characteristics and can be readily generated in a system-independent manner, without consideration for image pixel size. In this study, we developed a test method for determination of CTF based on multi-layer, thin-film phantoms, evaluated using spectral- and time-domain OCT platforms with different axial resolution values. Phantoms representing six spatial frequencies were fabricated and imaged. The fabrication process involved spin coating silicone films with precise thicknesses in the 8-40 μm range. Alternating layers were doped with a specified concentration of scattering particles. Validation of layer optical properties and thicknesses were achieved with spectrophotometry and stylus profilometry, respectively. OCT B-scans were used to calculate CTFs and results were compared with convetional PSF measurements based on specular reflections. Testing of these phantoms indicated that our approach can provide direct access to axial resolution characteristics highly relevant to image quality. Furthermore, tissue phantoms based on our thin-film fabrication approach may have a wide range of additional applications in optical imaging and spectroscopy.

Model eyes with curved multilayer structure for the axial resolution evaluation of an ophthalmic optical coherence tomography device

“…4) with progressive increase of the thicknesses for each layer pair (except for top and bottom layers), switching between brightness (with signals in OCT devices) and darkness (without signals in OCT devices). 7 Among them, the bright layers have homogeneous particles, while no particles are in dark layers. The cross-section of the retina phantom with eight layers (A, 1, 2, 3, 4, 5, 6, B) is illustrated in Fig.…”

“…Currently, there are two common methods, i.e., Point Spread Function [4][5][6] (PSF) and the use of a phantom, for the evaluation of the axial resolution of an OCT device. Since post-processing of PSF is very complicated and not suitable for clinical applications 7 , the use of a phantom plays a more important role in clinical standardization and validation due to the advantages of its intuitiveness and e±ciency.…”

“…Agrawal et al fabricated a practicable phantom, however, it was composed of the layers with same thickness leading to low e±-ciency, and the dissimilarity of physical characteristics compared with natural eyes may not guarantee the measurement accuracy. 7 Hu et al proposed a method of manufacturing a 3D test target with 3D printing technology for measuring the axial resolution of an ophthalmic OCT device, nevertheless, the highest axial precision of the 3D printers is 16 m, which is still too low to detect that of OCT devices. 13 In this study, novel standard model eyes were designed and fabricated, and the evaluation criterion for the axial resolution of an ophthalmic OCT device was proposed.…”

Variations in optical coherence tomography resolution and uniformity: a multi-system performance comparison

“…The FWHM correction that was applied to SDOCT axial resolution (fitting the Gaussian only between the minima immediately surrounding the peak) was intended only to remove the influence of the side lobes on the Gaussian fit, to more accurately represent the theoretical FWHM. The true effect of side lobes on the axial FWHM may require an MTF approach to quantify [7]. Axial side lobes arise from the non-Gaussian SLD spectrum, and are common in ultrahigh resolution OCT [30] and other optical imaging modalities [31].…”

“…In recent years, there has been a movement towards development of phantom-based techniques for OCT system resolution and other image quality parameters [1][2][3][4][5][6][7][8]. These approaches parallel those found in international consensus performance standards for established imaging modalities such as X-ray computed tomography, magnetic resonance imaging and ultrasound [9].…”