Our device setup is based on a parallel Fourier-domain OCT (pfdOCT) system, which uses an imaging spectrograph that allows simultaneous detection of multiple spectrograms in parallel [6
]. In this particular system, a super-continuum laser source (Fianium, SC450) is used, where light from the laser source is filtered to produce a center wavelength of 575 nm and a bandwidth of 240 nm. The filtered light is input to the pfdOCT system, which is based on a Michelson interferometer with the addition of a 4-f imaging system (
]. Here, light from the source is collimated by lens, L1, and then focused on one axis by a cylindrical lens, L2. L3 and L4 are used to form a line of illumination on the sample and reference arm, respectively. The scattered light returned from the sample is combined with the reflected light from the reference arm at the beam-splitter and imaged onto the entrance slit of the spectrograph. With this setup, up to 400 interferograms, limited by the CCD and beam size, are sampled in parallel. An axial resolution of 1.2 µm and a transverse resolution of 6.9 µm were determined experimentally.
Parallel frequency domain OCT system and sample. L = 120 µm is the thickness of the sample used in the concentration measurement. Red dashed lines and black lines show the propagation of light in two orthogonal dimensions.
Data collected by the CCD are processed with the DW method, which is a bilinear processing approach that produces spatially resolved spectroscopic information with high resolution in both the spatial and spectral domains [8
]. In this method, two short-time Fourier transforms (STFTs) are computed, one using a wide spectral window (Δkw
= 0.907 µm) and another using a narrow spectral window (Δkw
= 0.016 µm). The two resulting time-frequency distributions (TFDs) are then multiplied on a point-by-point basis, forming a TFD with high resolutions in both domains. Thus, the DW method avoids the trade-off between spatial and spectral resolutions that is associated with the use of a single STFT and approaches the high resolution seen for Cohen’s bilinear distributions (e.g., the Wigner distribution) as representations of time frequency distributions. We have shown that this method is equivalent to probing the Wigner distribution of the scattered sample field with two orthogonal windows that independently adjust the spatial and spectral resolutions [8
]. METRiCs OCT, which utilizes the DW method, has been applied to produce true-color, quantitative, tomographic images of an in vivo
rat dorsal skin fold window chamber model [5
]. Quantification of hemoglobin oxygen saturation levels of the vasculature was also demonstrated [4
The nanoparticles used in this study are gold nanospheres (GNS, BBInternational, 60 ± 3 nm) and gold nanorods (GNR). These nanoparticles have been extensively studied in drug delivery [9
] or gene delivery [10
], and do not exhibit cytotoxicity with proper polymer coating [11
]. The GNS has an extinction peak at 535 nm in deionized water. The synthesis of nanorods follows the standard seed-mediated method [12
]. Two batches of GNR were used. The aspect ratio of the first batch of rods is manually measured from TEM images of 110 nanorods to be 1.55 ± 0.26, with the longitudinal axes having the size of 55.5 ± 10.2 nm, and the transverse axes 36.2 ± 7.2 nm; the aspect ratio of the second batch is 1.48 ± 0.27, with the longitudinal axes being 57.6 ± 11.4 nm and the transverse axes being 39.6 ± 8.2 nm. Both batches of GNR have an extinction peak at 603 nm in deionized water.