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. 2008 Nov 24;16(24):19712-23.
doi: 10.1364/oe.16.019712.

Long-wavelength optical coherence tomography at 1.7 microm for enhanced imaging depth

Utkarsh Sharma  1 , Ernest W Chang, Seok H Yun
Affiliations

Long-wavelength optical coherence tomography at 1.7 microm for enhanced imaging depth

Utkarsh Sharma et al. Opt Express. .

Abstract

Multiple scattering in a sample presents a significant limitation to achieve meaningful structural information at deeper penetration depths in optical coherence tomography (OCT). Previous studies suggest that the spectral region around 1.7 microm may exhibit reduced scattering coefficients in biological tissues compared to the widely used wavelengths around 1.3 mum. To investigate this long-wavelength region, we developed a wavelength-swept laser at 1.7 microm wavelength and conducted OCT or optical frequency domain imaging (OFDI) for the first time in this spectral range. The constructed laser is capable of providing a wide tuning range from 1.59 to 1.75 microm over 160 nm. When the laser was operated with a reduced tuning range over 95 nm at a repetition rate of 10.9 kHz and an average output power of 12.3 mW, the OFDI imaging system exhibited a sensitivity of about 100 dB and axial and lateral resolution of 24 mum and 14 mum, respectively. We imaged several phantom and biological samples using 1.3 mum and 1.7 microm OFDI systems and found that the depth-dependent signal decay rate is substantially lower at 1.7 microm wavelength in most, if not all samples. Our results suggest that this imaging window may offer an advantage over shorter wavelengths by increasing the penetration depths as well as enhancing image contrast at deeper penetration depths where otherwise multiple scattered photons dominate over ballistic photons.

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Figures

Fig. 1
Fig. 1
Experimental setup of the wavelength swept laser in the 1.7-µm spectral range.
Fig. 2
Fig. 2
(a) Output spectrum of the laser in Design 1. (b) Optical absorption curve of water (blue) reported in Ref. [22], superimposed with the laser spectrum (black).
Fig. 3
Fig. 3
Measured laser output of Design 2. (a) Peak hold output spectrum. (b) Time domain trace.
Fig. 4
Fig. 4
Schematic of the OFDI system.
Fig. 5
Fig. 5
Signal measured with a −55 dB partial reflector. The detection sensitivity is 103 dB at a depth of 0.5 mm (red) and decreases with the increasing depth (dotted line: a Gaussian fit). A typical single A-line is shown in dark cyan; 500 A-lines at each depth were averaged to reduce the fluctuations in noise floor (red, dark red, navy, and dark yellow). Asterisks (*) denotes the ghost peaks due to the etalon effect of the SOA chip.
Fig. 6
Fig. 6
10% Intralipid solution. (a,b) OCT images obtained with the 1.3 µm and 1.7 µm systems, respectively. The dynamic range of grayscale is 45 dB, same in both images. (c,d) Depth profiles, averaged over 150 A-lines in (a) and (b), respectively. Dashed lines are linear regressions. The slope was used to determine the total attenuation and scattering coefficients.
Fig. 7
Fig. 7
Silicone red rubber. (a,b) OCT images obtained with the 1.3 µm and 1.7 µm system, respectively. The dynamic range is 47 dB same in both images. (c,d) Depth profiles, averaged over 150 A-lines in (a) and (b), respectively. Dashed line represents linear fit over the region beyond which multiple scattering starts to cause nonlinear dependence on depth.
Fig. 8
Fig. 8
Human tooth. (a,b) OFDI intensity images obtained (a) with the 1.3 µm system and (b) 1.7 µm system. The grayscale used in both images has the same dynamic range of 40 dB. (c) A-line profile averaged over the area enclosed between dashed vertical bars shown in (a). (d) A-line profile obtained from (b). The slope of the signal decay is distinctly different between the pre- and post-natal regions in the enamel. Inset shows a photograph of the sample where the green line represents the imaged site.
Fig. 9
Fig. 9
Human fingertip imaged with (a) 1.3 µm system and (b) 1.7 µm system. The images have the same dynamic range of 40 dB. (c,d) Space-averaged A-line profiles obtained at λ = 1.3 µm, (c), and 1.7 µm, (d).
Fig. 10
Fig. 10
Enhanced penetration depths at 1.7 µm versus 1.3 µm wavelength. (a) Depth profiles from 10% Intralipid solution (Fig. 6). The ballistic penetration depth is 2.5 mm at λ =1.7 µm versus 2.0 mm at λ =1.3 µm. (b) Depth profiles from silicone rubber (Fig. 7). The ballistic penetration depth is 1.0 mm at λ =1.7 µm versus 0.7 mm at λ =1.3 µm. The ballistic penetration is limited by the onset of multiple scattering marked by the deviation from the linear slope (Lines). (c) The magnitude of multiple-scattering signal estimated by taking the difference of the depth profile from the linear regression in (b).
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