Defense Technology Aids Medicine
R. Winn Hardin
WASHINGTON -- A combination of military and commercial technology has created a practical midwave IR spectral imaging microscope.
The system developed by researchers from the National Institute of Standards and Technology and the National Institutes of Health uses a 256 3 256-pixel broadband IR mercury-cadmium-telluride focal plane array developed for the US Ballistic Missile Defense Organization; an IR microscope and Michelson step-scan interferometer from Bio-Rad Laboratories Inc. of Cambridge, Mass.; and a frame grabber from Dipix Technologies Inc. of Ottawa. It produces chemically sensitive spectral images of 0.6-mm2 sample areas with about 12-µm spatial resolution and a spectral resolution of 8 cm21 between 2.5 and 11 µm.
Commercial and research groups have produced similar spectral images in the visible and near-infrared using thermoelectrically cooled charge-coupled devices. Some institutions have even created mid-IR spectral images by using sample raster-scanning and a synchrotron IR source, but the process takes many hours, a lot of space and even more money.
According to NIST's Ted Heilweil, the trade-offs are not too bad: Although both approaches use liquid nitrogen to keep the detectors at 77 K, there is a slight reduction in spatial resolution from perhaps 3 (synchrotron) to 12 µm (spectral imager). However, both systems provide sufficient resolution for biomedical researchers and forensics experts to identify trace molecules in many static systems. Plans call for a comparison of the two approaches to test their performance and for obtaining chemical images of living samples.
In operation, the array collects differences in intensities at each pixel caused by combining the Fourier transform infrared interferometer's beams and passing the combined beam through the sample. The camera signals go through three-stage amplifiers, are digitized and are sent into the frame grabber board for data collection. This is done up to 30 times, staring at the sample for 15 µs. The resulting data are averaged to form a single image at a fixed interferometer position. Then the interferometer takes a "step" creating a new interference image while queuing the frame grabber board. The procedure repeats until 512 interferometric images have been collected. Parallel Fourier transforms and spectral peak intensities then turn the interferograms into spectral or chemically sensitive images.
Scientists have used the FTIR spectral imager to study the relationship between lipid deposits in mice brains and Alzheimer-like symptoms.
"So far the biggest bugaboo is collecting over 150 MB worth of images," Heilweil said. The system's data collection speed and processing time are limited by available memory. The image data is so large it cannot be processed in RAM, but must be temporarily written to hard disk, adding to the processing time.
The National Institutes of Health has licensed the spectral imaging technique to Bio-Rad for production. The commercial system uses smaller HgCdTe arrays (64 3 64 pixels). Larger arrays are difficult to find and more expensive, Heilweil said. G
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