Smaller and more informative medical endoscopy

Recent advances in medical endoscopes allow clinicians to perform surgical and diagnostic procedures less invasively, reducing patient discomfort and recovery times. While current state-of-the-art video endoscopes transmit high-quality images, their overall size is dictated by the dimensions of the sensors and electronics that deliver clinically useful images. Smaller and more flexible endoscopes that use optical fiber bundles have recently become available where each individual fiber is used to transmit one pixel in the image. However, maintaining acceptable image quality is challenging when only a limited number of fibers can be packed into an endoscope of a given diameter. Several research groups have attempted to rapidly scan light from an optical fiber to obtain an image.^1–3 While high-quality images have been obtained using this technique, the size of the scanning mechanism makes this method difficult to implement in the smallest medical endoscopes.

We have recently introduced a new approach for single-fiber endoscopy—spectrally encoded endoscopy (SEE)—which could overcome the limitations of today's fiber bundles. In the SEE system, broadband light emanating from a fiber is separated into different colors (or wavelengths) using a lens-grating pair at the distal end (tip) of the endoscopic probe (see Figure 1). This optical configuration focuses each color onto a different location on the tissue. The light reflected back from the tissue through the SEE probe is then decoded outside of the body using a fast spectrometer to form a single transverse line of the image. Two-dimensional coverage is obtained by rotating the fiber using motors or galvanometric scanners at the proximal end of the probe, outside the patient.

Since a high-speed scanning mechanism is not required at the distal end of the endoscope, the diameter of the SEE probe can be as narrow as the optical fiber itself, typically in the range of 80–250μm. Furthermore, the number of pixels in an SEE image can be larger than that of fiber-bundle endoscopes with comparable diameters.

Figure 1. Photograph of the SEE probe (top) next to a human hair (bottom) for size comparison. Visible light is coupled into the probe, illuminating a piece of paper below.

Three-dimensional SEE imaging can be accomplished when the endoscopic probe is placed in one arm of an optical interferometer. We have reported video-rate three-dimensional SEE imaging using our first 350μm diameter probe⁴ with a spectral-domain heterodyne interferometry detection scheme.^4,5 Volumetric images were obtained from a variety of specimens,⁶ including murine intraperitoneal metastatic ovarian tumors in vivo,⁴ and mice embryos (see Figure 2). In a recent publication we demonstrated that SEE can also measure the function and dynamics of tissue.⁷ Using the optical Doppler effect, the SEE system can image rapid tissue motion such as blood flow and auditory vibrations (see Figure 3).

Figure 2. Three-dimensional SEE imaging: (a) Surface rendering of metastatic tumor nodules within a mouse peritoneum. (b) Hind paws and tail of a stage E18.5 mouse embryo. (c) A photograph of a stage E14 mouse embryo. (d) SEE image of the area marked by the rectangle in (c). (e) Two optical sections of image (d) located 0.3 and 1.3mm below the front tissue surface. P: Hind paw. T: Tail. U: Umbilical cord. Scale bars represent 0.5mm.

Figure 3. (a) Photograph of an excised human stapes showing the footplate and the two crura. (b) Height map obtained by SEE where gray levels represent distance from the metal base. (c) The raw fringe data corresponding to the region marked by a white rectangle in (b). Oscillation frequency was measured to be approximately 1kHz. (d) Combined intralipid flow and average reflectance images of two 0.5mm-diameter tubes with similar magnitude but opposite flow velocities. (e) Cross-sectional imaging of intralipid flow within a 1mm-diameter tube. Scale bars represent 0.2mm.

Our current research is dedicated toward adding functional capabilities such as color and fluorescence SEE imaging, and improving probe design and usability for specific clinical applications that call for minimally invasive examination. Within its diminutive dimensions, SEE offers functional, high-quality three-dimensional video-rate imaging that can allow clinicians to better view and diagnose a wide variety of medical conditions, opening up new diagnostic possibilities in previously inaccessible areas of the body.