We present a tunable, adaptive optical imaging probe for multimodal imaging such as optical coherence tomography and microscopy. The probe is compatible with forward-looking scanning laser imaging devices such as an endoscope. The lens configuration includes a tunable iris and two varifocal lenses, both driven by microelectrofluidics, as well as several conventional fixed focus lenses. The modulation transfer function and spot size in the focal plane is evaluated, and we show using optical simulations that there are three possible imaging modes with different transverse resolutions and focal depths.
©2013 Optical Society of America
Non-invasive optical diagnosis methodologies such as optical coherence tomography (OCT) [1–4] and endoscopic microscopy [5–7] offer a much higher resolution than conventional medical imaging modalities such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound imaging. OCT applications can now be found in oncology, which typically uses endoscopic methods, but oncology requires a higher resolution than conventional OCT. Optical coherence microscopy (OCM) [8–11] utilizes an optical lens with a higher numerical aperture (NA) than OCT to achieve a higher resolution but the shorter depth of focus (DOF) results in a shorter imaging depth. To compensate for the reduced DOF, dynamic focusing techniques can be introduced using a micro-electromechanical mirror . By controlling the mirror shape and radius of curvature, light is refocused to a different depth in the specimen. A dynamic focusing probe can also be constructed using varifocal lenses, usually liquid lenses with various actuation mechanisms driven, for example, by hydraulic  or pneumatic  pressure. Recently, electrowetting-based varifocal lenses have demonstrated higher resolutions and larger imaging depths . When using dynamic focusing probes, high NA optics are used for higher resolution, and a sequence of images are acquired in the depth direction to reconstruct a complete high-resolution tomographic image from areas of highest contrast in each image.
We propose here a novel adaptive endoscopic optical probe for multimodal imaging such as OCT, OCM, and microscopy. Microelectrofluidic-based tunable optics such as a tunable iris and varifocal lens are used for switching and controlling the operation modes. Section 2 give a brief background of the theory, and section 3 presents the proposed probe and optical design along with simulation results.
One of the key performance indicators of an imaging system is the transverse resolution, and for tomographic imaging such as OCT and OCM, the DOF and depth resolution are essential indices. These parameters are determined by the properties of the optical probe; probes with a higher NA produce a smaller spot size and thus a higher transverse resolution but a reduced DOF, which is detrimental for tomographic imaging. The trade-off between the transverse resolution (∆x) and DOF (∆l) can be expressed as
3. Optical probe design
3.1 Concept design
Diagnostic imaging of a lesion in oncology starts with a large field-of-view or DOF that is reduced to gradually focused in on the lesion to record a high-resolution image. It is desirable for this procedure to be accomplished with single probe for both operational convenience and accurate imaging. A combination of OCT and OCM or endoscopic microscopy offers a promising possible multimodal imaging technique, but conventional OCT and OCM use fixed-focus optics that result in a fixed modality. Employing a varifocal lens  that sequentially increases or decreases focal length allows for depth scanning without any mechanical movement. The series of images is then processed with a Gabor-based algorithm to reconstruct a single highly resolved image by extracting and combining the areas of highest contrast from each image. Changing the focal length of the optical probe changes NA and the focal spot size increases according to the scanning position. This produces a non-uniform, depth-dependent transverse resolution. Thus, to improve the uniformity of the resolution during optical depth scanning (ODS), we propose an optical probe with a variable, controllable NA. To maintain the same NA value during ODS, the beam diameter and focal length are precisely controlled. For example, by controlling the aperture diameter of the iris and the focal length of the varifocal lenses, an NA of 0.3 can be maintained, and a transverse resolution of 1–2 μm is possible depending on the wavelength used. Figure 1 shows a conceptual schematic of ODS using the adaptive probe. The scanning begins by focusing the beam onto the surface of the sample with the desired NA (Fig. 1(a)) and then sequentially scanning in depth into the tissue by increasing the focal length but maintaining the NA (Fig. 1(b) to Fig. 1(d)). Images from each focal step are then processed, and the clearest areas (usually the focal area) is extracted to form a single clear image through the scanned depth.
The tunable iris and focal length of the varifocal lens are controlled using microelectrofluidics (Fig. 2 ) [12,13]. The tunable iris consists of upper and lower channels, with the lower channel containing a varying amount of a light absorbing fluid that defines the aperture. Applying a voltage across the fluid and discrete indium-tin-oxide (ITO) electrodes drives the fluid into the lower channel to change the aperture diameter. The varifocal lens operates on a similar mechanism but only has one channel rather than two. The radius of curvature of the lens is controlled by pulling the liquid through the channel.
3.2 Optical design
The design of the mode-switchable endoscopic optical probe is shown in Fig. 3 . The lens configuration consists of an initial collimation lens set, the tunable iris, a second collimation lens set, a focusing lens set, two varifocal liquid lenses, and an objective lens. All the lenses are spherical except for the objective lens which is conical.
The optical design and simulations were carried out using CodeVTM. We first searched for an appropriate probe configuration and then optimized each lens profile for the OCT, OCM, and ODS modes. An optical fiber with a relatively high NA (0.35) and a core diameter of 3.6 μm is used for OCM or microscopy mode. Figure 4 shows the optimized ray tracing results. The OCT mode (Fig. 4(a)) has a working distance of 28.3 μm, an NA of 0.02, and a scanning range of ±1.4 mm. The diameter of the tunable iris is 1.0 mm, and the radii of curvature of the two varifocal lenses are −3.96 mm and 4.39 mm. In contrast, the OCM and ODS modes (Figs. 4(b) to 4(e)) have a variable working distance of 2.1 mm (Fig. 4(b)) to 4.1 mm (Fig. 4(e)), with a NA of 0.3 and a scanning range of ±0.23 mm. It is shown that the focal plane gradually moves from the surface of the tissue to a depth of 2 mm with a constant NA of 0.3. For the ODS mode, the aperture varies from 3.03 mm (Fig. 4(b)) to 3.72 mm (Fig. 4(e)). The radius of curvature of the first varifocal lens is varied from 5.2 mm to −18.8 mm and that of the second varifocal lens from −5.4 mm to −6.5 mm.
The modulation transfer functions (MTF) of the OCT (Fig. 5 ) and OCM/ODS modes (Fig. 6 ) were analyzed for wavelengths of 970 to 1070 nm (centered at 1020 nm). At a spatial frequency of 20 lp/mm, the MTF of the OCT mode is 0.4, which corresponds to a transverse resolution of 25 μm and is very close to the diffraction limit. In the OCM/ODS mode, the MTF is 0.2 for spatial frequencies up to 250 lp/mm, which corresponds to a transverse resolution of 2 μm. The DOF is approximately 2 mm for the OCT mode and approximately 10 μm for the OCM/ODS mode. For the designed NAs, Eqs. (1) and (2) give the transverse resolution and DOF as 19 μm and 1.6 mm, respectively, for the OCT mode and 1.7 μm and 6.6 μm, respectively, for the OCM/ODS mode.
The transverse resolution is evaluated from the point spread function (PSF) (Figs. 7 and 8 ), and the full width at half maximum (FWHM) of the spot size in each focal plane is 25.7 μm for the OCT mode and 1.7 μm for the OCM mode. The FWHM of the spot size in the OCT mode is larger than the estimated value of 19 μm due to lens aberrations but that in the OCM/ODS agrees well with the calculated value. We can see in Fig. 8 that the transverse resolution is maintained during depth scanning over 2.0 mm using the tunable optics.
We have proposed an adaptive optical probe for OCT and microscopy using tunable optics based on microelectrofluidics. The optical configuration consists of a tunable iris, two varifocal lenses, and eight fixed focus lenses. By controlling the tunable iris and varifocal lenses, the NA can be varied from 0.02 to 0.3, which enables the probe be used in OCT, OCM, and ODS mode. The design is optimized for the highest MTF and smallest spot size; spot sizes of 25.7 μm for the OCT mode (NA = 0.02) and 1.7 μm for the OCM mode (NA = 0.3) were demonstrated. The transverse resolution can be adjusted by controlling the NA. The optical simulations we presented here showed that the proposed adaptive optical probe can be an effective imaging tool on its own or integrated into endoscopy.
References and links
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