Monday, October 24, 2011

FiO 2011 - part 5

Hey everybody.. FiO is officially over. But I am still processing the information from the sessions. In this post I'm going to summarize some of the talks I thought were interesting work. More will come later..

David Brady at the Hot Topics. Pic courtesy: OSA
In the Hot Topics session for information sensing and processing David Brady emphasized that optics is increasingly penetrating digital systems. Compressive measurement has started being used in optics and is increasingly more practical. Compressive measurement has now been demonstrated for projection tomography, snapshot hyperspectral imaging, OCT, holography, digital superresolution, etc. This year FiO had a tutorial talk by Rebecca Willett and a whole session dedicated to the topic. 

Serhan Isikman and M. J. Lee from the Ozcan group talked about...
... Holographic Optofluidic Tomographylensfree on-chip imaging (in-line holography). A partially coherent light source like an LED illuminates weakly scattering transmissive objects in a microfluidic channel. Some of the light gets through directly, some carries object information. Together they interfere and an in-line hologram is imaged on a CMOS sensor array. The low pixel resolution is compensated by taking multiple images using the motion of the object or light source for shifted images. Using a light source at different angles like in an X-ray CT allows them to obtain a tomographic effect, and using some sort of filtered back-projection they can reconstruct 3D data. They now have a portable lensfree tomographic microscope. This technique is good mostly for transmissive, weakly scattering objects. 

They also spoke about compact, field portable, cost-effective, reflective and transmissive microscopy. The reflection mode does lensfree digital off-axis holography and digital reconstruction to obtain the object image. This mode is useful for dense media. For transmission mode they use in-line holography. They used this technique for malaria detection, sperm analysis, etc. They showed a dual arm microscope which contains 2 CMOS sensors and combines both reflection and transmission modes using a beam splitter. This can image transmission amplitude and phase and reflection. 

Changhuei Yang from Caltech spoke about their ePetri Dish Project. Their group has a similar method for imaging without lenses. They place a sensor at the bottom of a petri dish and cells in a culture medium inside the dish. They shine a beam of light from above and capture the “shadows” of the cells that fall on the sensor. This lets them image large fields of view and a lot of cells all together. They improve their resolution beyond the size of the sensor pixels by taking multiple images with different angles of illumination shifting the “shadows”. The multiple frames with these shifted images are then used with digital superresolution algorithms to obtain high resolution images. Very nice use of cheap sensors.  

The above three talks all have lower sampling during image capture and then use multiple images and superresolution techniques to overcome the low sampling resolution. 

Thomas Kohlgraf-Owens, from CREOL, had a very interesting talk on imaging spectrometry using a random scattering medium. He uses the idea that the captured data can be thought of as Out(r) = M(r,w) x In(r,w) where M is the transfer matrix which maps the spread of spectra over detector pixels. Therefore, In = Minv x Out. You can think of a random material/scattering medium as M. One colour goes through, scatters randomly, and falls over different pixels at the detector with different ratios. A second colour scatters in a different manner and again is collected with different ratios at the same detector. Thus every pixel at the detector contains a sum of different spectra weighted with different proportions. The scattering medium acts as a spatially varying color filter. This kind of a device can be very small, as small as the scattering material. He used an imaging fiber bundle as scattering material. 

Spectrally Encoded Confocal Microscopy: DongKyun Kang talked about endoscopy for esophageal disease diagnosis. Traditionally this requires a biopsy, histology and microscopy. The limited tissue samples in biopsy allow examination of less than 0.1% of the region under risk. Complete scanning of the esophagus (without scrapings) at micron resolution would be ideal. They have used optical frequency domain imaging (OFDI) which is like a swept source OCT or a Michelson interferometer. They use a balloon centering catheter for esophageal imaging, with a helical scanning pattern to cover the entire esophagus.  This technique allows visualizing the architectural features (shape presumably) but misses cellular features.

Confocal microscopy/optical biopsy can see cellular features. There exist some commercial products but apparently they have low fields of view. His team uses spectrally encoded confocal microscopy (SECM). In this technique a fiber emits broadband light, which falls on a grating. This diffracts different wavelengths, which travel through the objective lens and different wavelength bands fall at different locations on the sample. Thus, spatial information gets encoded into different wavelengths. Basically this is a spectrometer on the sample side. On the detector side the light goes back through the same fiber and comes out of the body. Now this can be put through a spectrometer to analyze the spectrum and generate a line image. This technique increases scanning speed and enables large area imaging. He showed images of diseased esophagus and compared them to histology images. They also use an angled objective SECM probe where the focal plane is at an angle to the sample. This allows some depth sampling and adaptive focusing using the 1st reflection from the top of the sample. Was a great informative talk with good technology as well as interesting results from the application! 

Umm.. the amount of detail this post has gone into does not bode well for how much time I could spend summarizing.. oh well.. we'll see how it goes.  

Cheers and thanks for stopping by!

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