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Tuesday, August 14, 2007

3D images of living cell

A new imaging technique developed at MIT has allowed scientists to create the first 3D images of a living cell, using a method similar to the X-ray CT scans doctors use to see inside the body.

The technique, described in a paper published in the Aug. 12 online edition of Nature Methods, could be used to produce the most detailed images yet of what goes on inside a living cell without the help of fluorescent markers or other externally added contrast agents, said Michael Feld, director of MIT's George R. Harrison Spectroscopy Laboratory and a professor of physics.

"Accomplishing this has been my dream, and a goal of our laboratory, for several years," said Feld, senior author of the paper. "For the first time the functional activities of living cells can be studied in their native state."

Using the new technique, his team has created three-dimensional images of cervical cancer cells, showing internal cell structures. They've also imaged C. elegans, a small worm, as well as several other cell types.

The researchers based their technique on the same concept used to create three-dimensional CT (computed tomography) images of the human body, which allow doctors to diagnose and treat medical conditions. CT images are generated by combining a series of two-dimensional X-ray images taken as the X-ray source rotates around the object.

"You can reconstruct a 3D representation of an object from multiple images taken from multiple directions," said Wonshik Choi, lead author of the paper and a Spectroscopy Laboratory postdoctoral associate.

Cells don't absorb much visible light, so the researchers instead created their images by taking advantage of a property known as refractive index. Every material has a well-defined refractive index, which is a measure of how much the speed of light is reduced as it passes through the material. The higher the index, the slower the light travels.

The researchers made their measurements using a technique known as interferometry, in which a light wave passing through a cell is compared with a reference wave that doesn't pass through it. A 2D image containing information about refractive index is thus obtained.

To create a 3D image, the researchers combined 100 two-dimensional images taken from different angles. The resulting images are essentially 3D maps of the refractive index of the cell's organelles. The entire process took about 10 seconds, but the researchers recently reduced this time to 0.1 seconds.

The team's image of a cervical cancer cell reveals the cell nucleus, the nucleolus and a number of smaller organelles in the cytoplasm. The researchers are currently in the process of better characterizing these organelles by combining the technique with fluorescence microscopy and other techniques.

"One key advantage of the new technique is that it can be used to study live cells without any preparation," said Kamran Badizadegan, principal research scientist in the Spectroscopy Laboratory and assistant professor of pathology at Harvard Medical School, and one of the authors of the paper. With essentially all other 3D imaging techniques, the samples must be fixed with chemicals, frozen, stained with dyes, metallized or otherwise processed to provide detailed structural information.

"When you fix the cells, you can't look at their movements, and when you add external contrast agents you can never be sure that you haven't somehow interfered with normal cellular function," said Badizadegan.

The current resolution of the new technique is about 500 nanometers, or billionths of a meter, but the team is working on improving the resolution. "We are confident that we can attain 150 nanometers, and perhaps higher resolution is possible," Feld said. "We expect this new technique to serve as a complement to electron microscopy, which has a resolution of approximately 10 nanometers."

Other authors on the paper are Christopher Fang-Yen, a former postdoctoral associate; graduate students Seungeun Oh and Niyom Lue; and Ramachandra Dasari, principal research scientist at the Spectroscopy Laboratory.

The research was conducted at MIT's Laser Biomedical Research Center and funded by the National Institutes of Health and Hamamatsu Corporation.

About Researcher George Russell Harrison

George Russell Harrison was born on July 14, 1898 in San Diego, California. His early years and schooling were spent in California. Young George's interest in physics may have been fostered by friends of his father the Varian brothers, who were later to invent the Klystron and to head the electronics firm in Palo Alto bearing their name. Harrison entered Stanford University in 1915, and chose physics as his major field of study. Despite a brief interruption in his studies, associated with World War I, he received the bachelor's degree on schedule in June 1919.

That autumn he enrolled in the Stanford graduate school as a candidate for the master's degree in physics. During this period he was chosen to tutor Herbert Hoover, Jr., and for several years he lived on the Stanford campus with the Hoover family, thereby beginning a long and close friendship. He received the master's degree in 1920 and continued on for the doctorate. A new Physics Department chairman, David L. Webster, had recently arrived from Harvard University and influenced his decision to enter the field of spectroscopy. His doctoral thesis, supervised by Webster, was completed in the spring of 1922, and was the basis of a paper, "The Absorption of Light by Sodium and Potassium Vapors," published later that year in the Proceedings of the National Academy of Sciences.

His promise as a research physicist led to the award of a National Research Council Fellowship for work with the renowned spectroscopist of the vacuum ultraviolet, Professor Theodore Lyman of Harvard. The two years he spent in Lyman's laboratory (1923-25) deepened and broadened his knowledge of the world of physics. He then returned to Stanford where, as an assistant professor, he began building up a laboratory, which soon included a 21-foot vacuum spectrograph, the largest of its day. He was promoted to associate professor in 1927.

In 1930 Dr. Harrison accepted the offer of a professorship of experimental physics at the Massachusetts Institute of Technology by its new president, Karl T. Compton. Compton was a specialist in vacuum spectroscopy and perhaps had been attracted by Dr. Harrison's early work. In any case, the two men had this common interest, which resulted in the founding of the MIT Spectroscopy Laboratory, the first building designed and constructed for the particular needs of spectroscopy.

Dr. Harrison's activities in the Spectroscopy Laboratory are described in his own words in the accompanying history. The most noteworthy of his many achievements there were the development of a high-speed automatic comparator for the recording of intensities and wavelengths of spectral lines (1938), the compilation of the MIT Wavelength Tables (1939), and the invention of the echelle spectrograph (1949). He was the first to devise a practical ruling engine, servo-mechanically controlled by means of optical interferometric techniques, which he used to produce diffraction gratings of unprecedented optical quality and size (1948-72). He was the author or coauthor of more than 100 scientific papers. In 1948 he published the well-known text, Practical Spectroscopy, with Richard C. Lord and John R. Loofbourow. He also wrote books for the layman on scientific and engineering subjects, the best known of which, Atoms in Action (1939), was translated from English into more than a dozen other languages.

During World War II Dr. Harrison was chief of the Optics Division of the National Defense Research Committee (1942-43), and later head of the Office of Scientific Research and Development's Office of Field Service in the Pacific Theater (1944-45). He was awarded the U.S. Medal of Freedom and the Presidential Medal of Merit for his contributions.

Dr. Harrison became Dean of Science at MIT in 1942 and oversaw the postwar development of the School of Science until his retirement in 1964. He had a clear sense of the structure and purpose of MIT, and he provided the conceptual leadership during this 22-year period that brought the School of Science to its eminent position among the world's foremost academic institutions. His encouragement and support of science, not only for its own sake, but also as the indispensable partner of engineering, were basic to fundamental changes in the character of the Institute.

Dean Harrison received many medals, awards, and honorary degrees for his scientific accomplishments, including the Rumford Medal of the American Academy of Arts and Sciences, the Cresson Medal of the Franklin Institute, the Ives and Mees Medals and the Meggers Award of the Optical Society of America, and the Pittsburgh Spectroscopy Award. He was one of six honorary members of the Optical Society, and was a Fellow of the American Philosophical Society, the American Physical Society, the American Academy of Arts and Sciences, and the Australian Academy of Science, and he held many high offices in these and other scholarly organizations.

Dean Harrison's devotion to his research until his death in 1979 was characteristic of his disciplined mind and strong work ethic. In the years of his retirement, it was a common sight for workers in the Spectroscopy Laboratory to see him bounding down the basement corridor with the vigor and sense of purpose, which were his trademark. Throughout his career he was admired and respected for his candor and fairness. His dominant thought seemed always to be, "If it is worth doing, how can it best be done?" Ingrained in his nature was the desire to experiment, to work with his hands, to invent new devices for the solution of seemingly insoluble problems. For George Harrison, inventing, as he used that term, was a challenge and a source of pleasure. Naming the Spectroscopy Laboratory in his honor is a fitting tribute, and will serve as an example of excellence to which students of science can aspire.

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1 comment:

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