A Tiny Mirror for a Small Microscope
When it comes to high-resolution confocal images with miniaturized optics, two fibers may be one too many.
A microscanner used to enable single-fiber confocal microscopy has a 500 × 500-μm mirror and comb teeth to allow scanning in both vertical and horizontal directions. Images courtesy of Optics Express.
A team of scientists from the University of Texas at Austin, from Rice University in Houston and from Stanford University in California has demonstrated cellular imaging using a single-fiber confocal microscope with a microelectromechanical systems (MEMS) scanner. The technique could lead to a cost-effective imaging system suitable for early cancer detection.
The investigators previously had performed imaging with a confocal microscope that features fiber bundles, but that approach suffers from a drawback. “The main limitation of a fiber bundle confocal microscope is the fiber-pixelation in the image, which limits the resolution,” said Kristen C. Maitland of Lawrence Livermore National Laboratory in California, a member of the research group when she was a graduate student at the University of Texas.
One frame of a video acquired at 8 fps depicts a US Air Force resolution test target as viewed with a single-fiber confocal microscope. The leftmost elements are 2.2 μm wide.
The single-fiber approach avoids that problem but faces another. Using a single fiber for illumination and detection requires placing a scanner at the far end — the business end for endoscopy — so the scanner must be small. To do that, they fabricated a 500 × 500-μm mirror out of silicon that could tilt left to right and back and forth.
Rebecca Richards-Kortum, a bioengineering professor at Rice and research team leader, noted that recent advances in MEMS design and fabrication have led to two-axis scanners with large diameters and scanning angles. Using a single two-axis scanner instead of two single-axis mirrors reduces size and simplifies optics.
The scientists tested the scanner using standard optics, employing a 635-nm laser diode from Blue Sky Research of Milpitas, Calif., as light source and an avalanche photodiode from Hamamatsu Corp. in Bridgewater, N.J., as detector. They sent the beam down the fiber, bouncing it off the mirror and scanning it across the sample. They collected the light and sent it back through the fiber, capturing the image with the detector.
They imaged a test target at 8 fps, demonstrating a lateral resolution of 0.82 μm and an axial resolution of 13 μm. The axial results were worse than predicted because of slight misalignments, which could be corrected with an adjustable housing. They also imaged tissue samples, demonstrating expected results.
Plans call for coating the mirror with gold or aluminum to boost performance. The researchers also are “miniaturizing the optical design to enable in vivo imaging,” Richards-Kortum said.
Optics Express, Vol. 14, No. 19, pp. 8604-8612.
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