Optical micro-endoscopes for 3D in-vivo imaging

Abstract

Optical coherence tomography (OCT) is a medical optical imaging technique that can achieve high-resolution 3D images (1 15 m)1 by detecting infrared light scattered by body tissue. It has an imaging depth range of about 3mm in highly-scattered tissue and has already been widely used for external imaging, e.g., for eye diseases and skin cancer diagnosis. It would be useful to apply OCT to internal imaging, since about 85% of all cancers originate from the surface layers of internal organs. As these layers are within its imaging depth range, OCT can detect cancer at an early stage as well as precisely determine tumor margins. However, it is very challenging to apply this technique to internal organs because of stringent size and imaging time constraints. One promising approach is to use microelectromechanical systems (MEMS) scanning micromirrors. Several research groups have demonstrated actuation mechanisms to generate rotational scanning for micromirrors for endoscopic OCT. Among them, electrostatic micromirrors can be very fast and consume low power, but the angular scan range is limited and the required driving voltage is too high ( 100V) to be safe for patients. Electromagnetic micromirrors have also been used for OCT probes, but the assembly process is complicated and the potential electromagnetic interference caused by the use of permanent magnets is another concern. Another problem these mirrors share is that they all have a small fill factor (the ratio of the mirror area to the device area). For a given resolution, the required mirror area is fixed. Thus, a small fill factor leads to a large device, which in turn increases the imaging probe size. Electrothermal mirrors, on the other hand, can achieve a large scan range at low drive voltage, but most of them have large initial tilt angles and significant shift of rotation centers, causing serious optical alignment and coupling problems. Figure 1. Scanning electron micrograph of an electrothermal MEMS mirror. Chip size: 2mm 2mm.