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By Lee J. Nelson
Widefield Heterodyne Laser Doppler imaging could facilitate a better understanding of the middle-ear and its mechanical complexity. The direction, phase, amplitude and frequency response of their vibrations exhibit spatial variation that is difficult to determine using single point measurements. Spectrally-encoded endoscopy—which we presented in 2007—relies on tiny optics and a diffraction grating for three-dimensional imaging. Such spectral-domain interferometry is capable of video-rate imaging with high sensitivity since it records a value at every sampling element in the field-of-view. Pioneering efforts for those applications are being explored in the Department of Biomedical Engineering at the Technion—Israel Institute of Technology (Haifa); Massachusetts General Hospital's Wellman Center for Photomedicine (Boston); and the Eaton-Peabody Laboratory of Auditory Physiology (Boston), a consortium of the Massachusetts Eye and Ear Infirmary, Harvard Medical School, MIT's Research Laboratory of Electronics and Massachusetts General Hospital.
Additionally, since we last reported on the topic, further research in endoscopic ultrasound is under way at the Saint Louis University School of Medicine and the University of Chicago Medical Center. In evaluating pancreatic cancer, endoscopic ultrasonography creates a real-time image, aiding the physician in collecting specimens for biopsy. "An endoscopic ultrasound or endoscopic ultrasound-guided, fine needle aspiration can provide very precise staging and can even detect tumors when other imaging modalities have been unable to provide a diagnosis," notes Dr. Irving Waxman, Director of Endoscopy at the University of Chicago, and a founder of its pancreatic cancer program.
The narrower a scope's tube, the less intrusive is the procedure. Following Micro-Imaging Solutions, Olympus Surgical & Industrial America (Orangeburg, N.Y.) just barely breaks the "5-mm barrier" with the GIF-N180 transnasal upper gastrointestinal endoscope. Despite a 4.9 mm diameter, it clearly portrays the subtle texture of mucosal surfaces and fine capillary networks. Close-focus viewing, to 2 mm, helps ensure detailed observation and inspection of potential lesions.
Olympus maintains GIFN180 makes possible routine patient examinations with minimal or no sedation, absent significant discomfort. The video system, Olympus' EVIS EXERA II Universal Imaging Platform, delivers superb image quality with its visualization enhancement techniques: narrow-band imaging and high-definition video, HDTV1080i.
Narrow-band imaging optically filters the illumination source so it consists of discrete wavelengths (415 and 540 nm). Since dissimilar tissues scatter and absorb light differently (a consequence of variable hemoglobin content), the blue light accentuates superficial capillaries while green light highlights subepithelial vessels. When merged, they emphasize contrast and amplify delicate structures which an examiner otherwise could miss.
To provide the best image achievable, Olympus engineers committed to HDTV1080i compatibility for the EVIS EXERA II series, back in 2005. The 1080 denotes the number of horizontal scan lines (vertical resolution) and the letter "i" signifies interlacing. In an alternate mode, 1080p, "p" indicates progressive scanning. HDTV1080i assumes a widescreen 16:9 aspect ratio, horizontal resolution of 1920 pixels and 1920x1080-pixel frame resolution. The refresh rate is 60 Hz or 30 frames per second.
OmniVision Technologies (Santa Clara, Calif.) adopts a radically different slant. Using backside illumination (BSI), their OmniBSI architecture buttresses image quality while shrinking its pixel roadmap to 0.9 µ, key to continued evolution of digital imaging's miniaturization. Taiwan Semiconductor Manufacturing Corp. (Hsinchu), OmniVision's long-time foundry and process technology partner, was integral to the concept.
BSI inverts the camera chip sensor so it collects light through what was its backside, the silicon substrate. That departs from conventional front-side CMOS imagers where light is attenuated by multiple metal and dielectric layers, the very elements which convert photons into electrons. When light is deflected or blocked from reaching the sensor, it can moderate the fill-factor ratio (photodiode size relative to pixel surface, owing to space occupied by surrounding electronics), compromise detector sensitivity and engender issues like cross-talk, between adjacent pixels. BSI reverses the two layers so they reside below the sensor array and yield a more direct light path. By optimizing light absorption, OmniVision has been able to build a 1.4 µ pixel chip which surpasses the performance metrics of many 1.75 µ, front-side illumination (FSI) devices.
OmniBSI architecture delivers various improvements over FSI including increased sensitivity per unit area as well as heightened quantum efficiency and photo-response uniformity. All lead to superior image fidelity and low-light functionality. The wider chief ray angle—subtended by the central meridian as it passes through the aperture and crosses the optical axis—acts to shorten the lens which begets a thinner camera module. Finally, the inherently large aperture reduces f-stop settings and furthers efficacy.
"Moving FSI pixel architectures down to 1.4 µ and below, under current design rules, poses some real challenges because metal lines and transistors are driving the aperture of the pixel close to the wavelength of light, its physical limit," says Howard Rhodes, Vice President of Process Engineering at OmniVision. "To overcome this with traditional FSI pixel technology would require a migration to 65 nm copper process technologies, which would significantly increase the complexity and cost of manufacturing."
Because BSI achieves significant benefits without necessitating smaller process nodes, routing is simplified and die sizes can be condensed, compared to front-side sensors. Fabrication expenses are contained by obviating retooling and the associated complexities.
As the latest addition to its repertoire, OmniVision introduces OV6930. The low-power 400x400-pixel square graphics array—or SquareGA—is CMOS-based with a 0.1-inch optical configuration in a 1.8 mm2 package. Those dimensions are ideal for endoscopic cameras which typically exact an outside width <2.8 mm. A 3.0 µ pixel (OmniPixel3-HS) imparts exceptional low-light sensitivity.
The OV6930 outputs full-frame or cropped analog images in RAW RGB format, managed by a patent-pending serial interface; a two-wire cable up to 14 feet long. The interface contributes to nominal 80 milliwatt power dissipation. Whether operating at 30 frames per second in full 400x400-pixel mode or 60 frames per second at 400×200 resolution, OV6930 benefits from a simplified, programmable exposure control. Proprietary technology lessens or eliminates common lighting and electrical contamination—as from pattern noise and smearing—and generates a clean, stable color image.
Last year, Visionsense Corp.'s (Orangeburg, N.Y. and Petah-Tikva, Israel) Stereoscopic Vision System was granted pre-marketing approval by the U.S. Food and Drug Administration/Center for Devices and Radiological Health (FDA; Rockville, Md.). Under the terms of the "510(k) clearance," their equipment may be employed during laparoscopic and endoscopic surgery. According to Visionsense, the 3.4mm camera produces real-time, high-resolution, natural stereoscopic images. Its 3D perspective affords the clinician a novel sense of depth-perception, the outcome of a multidisciplinary approach teamed with sophisticated image processing algorithms.
Any image is only as good as the endoscope, and the image degrades as a consequence of a scope's frequent handling, its contact with radio-frequency tools and shaver blades and exposure to corrosive chemicals during sanitation. The solution is a new, sterile instrument for each case. Dr. Marc Philippon (Steadman-Hawkins Clinic, Vail, Colo.) concurs. "The basic premise that drives the need for disposable endoscopes is that if we can't see...we can't operate." A single-use, high-resolution endoscope with a distally-mounted CMOS image sensor extends added benefits: no capital investment, no requirement for resterilization, no downtime for repair and no chance of cross-contamination. Those confer genuine and immediate advantages to patients, endoscopists, hospitals and ambulatory surgical centers, alike.
Also under investigation are higher resolution endoscopes, magnification endoscopy, chromoendoscopy and confocal endomicroscopy...all of which can help identify slight abnormalities not apparent with conventional instrumentation. "The most exciting findings to date are with narrow-band imaging and confocal endomicroscopy," says Dr. Gary Falk of the Department of Gastroenterology and Hepatology, Cleveland Clinic. A confocal microscope "optically sections" living tissue so that individual cell layers become visible. Magnification is possible up to one thousand-fold for observing subcellular structures. By marrying fiber-optics, lasers and high-performance compute technology, Pentax Medical Co. (Hamburg, Germany and Montvale, N.J.) and Optiscan Pty Ltd. (Notting Hill, Victoria, Australia) condensed a confocal microscope into a flexible endoscope. Their product received FDA regulatory clearance in the U.S. and Conformité Européenne for the European Union in 2006. Still, studies are few and require validation in larger numbers of patients. Be sure to stay tuned for the newest developments as they continue to redefine the leading-edge of minimally-invasive medicine and surgery.
Contributing Editor and industry analyst, Lee J. Nelson, is at the forefront of emerging as well as evolving technologies for compute-intensive electronic imaging applications. Contact him at: 1-703-893-0744, email@example.com or http://www.garlic.com/biz.