MEMS based Dual-Axes Confocal Clinical Endoscope for Real Time in vivo Imaging W. Piyawattanametha 1,2,3 , M. J. Mandella 1,3 , H. Ra 1 , J. T. C. Liu 1,3 , E. Garai 1,3 , G. S. Kino 1 , O. Solgaard 1 , and C. H. Contag 3 1 Edward L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA 2 NECTEC/NANOTEC, Pathumthani, Thailand 12120, 3 James H. Clark Center for Biomedical Engineering & Sciences Stanford University, Stanford, CA 94305, USA. E-mail:wibool@gmail.com ABSTRACT We demonstrate a dual-axes confocal endoscope in a 5.5 mm diameter package for clinical use. Miniaturization is achieved by using a barbell-shaped, gimbaled, two-dimensional MEMS scanner that is actuated by self-aligned, vertical-comb actuators. The maximum DC optical scan angles are ±2.6º on the inner axis and ±0.8º on the outer axis, and the corresponding resonance frequencies are 3.1 kHz and 1.1 KHz. The maximum imaging rate is 10 frames/second, and the microscope achieves full- width-half-maximum transverse and axial resolutions of 5μm and 7 μm, respectively when operated in the near infrared wavelength (785 nm). INTRODUCTION Confocal microscopy offers opportunities for three- dimensional (3-D) imaging due to its optical sectioning capabilities, and has had a tremendous impact on the study of cells in culture and small transparent organisms. However, transition to a miniature in vivo confocal microscope has been limited by high numerical aperture (NA) optics and the need for a scalable beam-scanning mechanism. Advances will be required to further develop the field of intravital microscopy and clinical imaging. A conventional single-axis confocal (SAC) microscope uses a large NA lens while a dual-axes confocal (DAC) microscope utilizes two overlapping low NA beams from two smaller lenses. The transverse and axial resolutions of the DAC are both proportional to 1/NA, while in the SAC they are proportional to 1/NA and 1/NA 2 , respectively. For 3-D imaging, the axial resolution is most important for achieving optical sectioning. Performance tradeoffs among axial resolution, field of view (FOV), and objective lens size of both systems can be further discussed as follows: By assuming the SAC and DAC to have the same FOV (implying the same focal length, f) as shown in Fig. 1a, lens diameter, D, is defined from the geometry as D DAC ~f[(tan α+θ) - (tan θ-α)] and D SAC ~2f×tan ρ. The normalized axial resolution [1], given by δ, is defined as δ SAC = λ/Δz SAC =1.1n(1-cos ρ) and δ DAC = λ/Δz DAC =2.7nαsin θ. Figure 1b shows a plot of D versus δ for both SAC and DAC systems. It reveals that for a SAC system to achieve the same axial resolution and FOV, it requires ~4x the lens diameter of a DAC system, thus requiring ~8x the scan-mirror area. DACs can therefore be implemented in much smaller packages than SACs. Our current DAC design parameters, θ=23ͦ and α= 6ͦ (δ DAC =0.15) have an optimum ratio α/θ=0.26 and enables miniaturization down to a 5.5. mm diameter package. Another advantage of the DAC design is improved image contrast of optical sections in a scattering medium (tissue) because the light scattered along the input illumination path before the focal volume (scattered photon noise) has very low probability of coupling back to the output collection path [1]. Previously, MEMS scanner based DAC microscopes have been demonstrated in 10 mm diameter packages [2-4]. In this abstract, we present an endoscopic MEMS DAC with a 5.5 mm diameter package capable of in vivo real time imaging. A photograph of the DAC endoscope with its top open showing internal parts and b a 0.00 0.05 0.10 0.15 0.20 0.25 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Normalized Obj. lens dia. (D/f) Normalized axial resolution (δ) SAC DAC α θ ρ f DSAC DDAC Fig. 1: a) Basic architecture of SAC versus DAC. b) Objective lens diameter vs. axial resolution for SACs and DACs. parabolic mirror cap risley prisms MEMS scanner a b Clinical upper GI scope hemispherical lens Figure 2: 5-mm diameter DAC endoscope: a) Photograph with its top open. b) Photograph while in a biopsy channel of a therapeutic upper GI endoscope. The scale bars are 2 mm. 42 Tu1.2 09:00 – 09:15 978-1-4244-1918-0/08/$25.00©2008 IEEE