A micromachined endoscopic scanner that combines a 2-D scanning mirror with optical-coherence tomography (OCT) can scan living tissue and provide real-time 3-D images. The microelectromechanical-systems (MEMS) device has the potential to perform OCT imaging inside the human body to facilitate the diagnosis of cancer and other diseases, according to its inventors.
Built by a team of researchers from the University of California, Los Angeles (UCLA) and the Massachusetts Institute of Technology (MIT; Cambridge, MA), the scanner measures 5.5 mm diagonally across and incorporates a 1-mm-diameter scanning mirror (see figure). The team reported on the design of the MEMS endoscopic scanner and the OCT-imaging results at the 2004 Conference on Lasers and Electro-Optics (CLEO; May 16–21 in San Francisco, CA). The project is supported by National Science Foundation's biophotonics program.
The MIT researchers scanned human skin in vivo at a rate of up to 20 frames per second, with 5-µm axial image resolution in air and 4-µm resolution in tissue. They could visualize the stratum corneum, epidermis layer, and a spiraling sweat duct. When imaging the human nailfold, they obtained clear delineation of the nailfold structure—including the stratum corneum, epidermis, and nail—at acquisition rates from 2 to 20 Hz (160 transverse pixels per image). Because of these fast acquisition rates, the researchers could perform real-time video capture and frame averaging to improve image contrast.
A minimally invasive optical-imaging technology, OCT offers high-speed, high-resolution endoscopic imaging of biological tissues in situ and in real time, explains Wibool Piyawattanametha, one of the UCLA researchers. Endoscopic OCT can distinguish architectural layers in vivo and can differentiate normal from tumor lesions within the human gastrointestinal tract at micron-scale resolution. Researchers perform OCT by measuring the intensity of reflected or backscattered light, rather than acoustical waves. While the penetration of OCT imaging in nontransparent tissue is only a few millimeters, the typical image resolution of OCT is greater than ultrasound, magnetic-resonance imaging, or computed tomography.
A need for compact, robust scanning devices for endoscopic applications has led to the development of MEMS scanning mirrors for OCT, according to Piyawattanametha. However, demonstrations of MEMS-scanning OCT endoscopes to date have been limited to single-axis scanning. The UCLA/MIT team used a 2-D MEMS-based endoscopic scanner for OCT imaging. The device—which incorporates a modelocked chromium-doped forsterite laser centered at 1250 nm with an approximate 180-nm bandwidth—is small enough for minimally invasive endoscopic procedures.
Side-scanning configuration
The compact aluminum housing of the fiber-coupled MEMS scanning endoscope can be machined at low cost. Optics consist of a graded-index fiber collimator followed by an antireflection-coated achromatic focusing lens producing a beam diameter of about 13 µm. Mounted at 45°, the 2-D MEMS scanner directs the beam orthogonal to the endoscope axis in a side-scanning configuration. The 1-mm-diameter mirror facilitates focusing for precise adjustment of optical alignment, while the actuator provides high-angle scanning at low applied voltage. A waveform of 30 to 70 V is used to scan the mirror at frequencies up to 20 Hz for imaging.
"The main challenge was integrating the scanning mirror and the detection optics in a compact package," says Piyawattanametha. "These results are preliminary and were obtained with uncoated mirror optics. Further improvement in sensitivity should be possible with the next generation of devices in process. We believe that the 2-D MEMS scanner can also be used for real-time endoscopic microscopy applications by using fast-axis resonant scanning."
Ilene Schneider | Freelance writer
Ilene Schneider is a freelance writer living in Irvine, CA.