CU research finding opens applications in medical imaging

Posing in the Developmental Resource for Biophysical Imaging and Optoelectronics (DRBIO) in Cornell's Clark Hall are, from left, Rebecca M. Williams, Warren R. Zipfel and Watt W. Webb, researcher, associate director and director, respectively, of DRBIO. Barry De Libero/University Photography

By Roger Segelken

Twelve years after its patent was granted, the technique called multiphoton microscopy appears ready to move from the realm of biological research to medical imaging.

In a series of three papers published this spring in the scientific literature, Watt W. Webb, Cornell's Eckert Professor of Engineering and co-inventor, with Winifried Denk, of multiphoton microscopy, displayed images of living tissue with details that rivaled -- and in some cases surpassed -- those from more traditional imaging processes.

Multiphoton microscopy produces high-resolution, three-dimensional pictures of tissues with minimal damage to living cells, Webb explained, adding: "We use a laser that produces a stream of extremely short, intense pulses, so the probability that two or three photons interact with an individual biological molecule at the same time is greatly increased. When this occurs, their individual energies can combine, and the cumulative effect is the equivalent of delivering one photon with twice the energy (in the case of two-photon excitation) or three times the energy (in three-photon excitation)."

When a scanning laser microscope moves a focused beam of pulsed photons across a sample at a precise depth (plane of focus), living cells above or below the plane are not affected. Repeated scans at different focal planes are "stacked" to produce three-dimensional pictures of biological features, such as capillaries feeding adipose tissue around a mouse ovary or Alzheimer's disease-damaged nerve cells in a human brain.

Commercially produced multiphoton microscopes, which are now in place in biological laboratories around the world, are still at the heart of Webb's experimental set-ups. But custom-built add-ons and some well-informed tweaking enable researchers in the Developmental Resource for Biophysical Imaging and Optoelectronics (DRBIO), of which Webb is the director, to extend the frontiers of medical imaging.

Reporting in the May 30 issue of the journal Science, researchers in the Webb laboratory said that molecule-sized nanocrystals, called quantum dots, can function as fluorescence imaging labels to circulate in the bloodstream and illuminate capillaries in unprecedented detail. Viewed beneath the skin of a living mouse, the quantum dot images are so vivid that researchers can see blood vessel walls ripple with each heartbeat -- 640 times a minute.

The Cornell team comments on the Science report, titled "Water-soluble Quantum Dots for Multiphoton Fluorescence Imaging in vivo," in an article at http://www.news.cornell.edu/releases/May03/quantum_dots.hrs.html.

Two articles in the June 10 Proceedings of the National Academy of Sciences reported further progress in developing new imaging technologies. The article, titled "Uniform polarity microtubule assemblies imaged in native brain tissue by second harmonic generation microscopy," proposed a biomedical imaging technique to highlight the cytoskeletal infrastructure (microtubules) of nerve cells and map the nervous system as it develops and struggles to repair itself.

"Never before has there been a satisfactory way of detecting polarity in microtubule assembles in living brain tissue," said Webb, who credited Cornell physics graduate student Daniel A. Dombeck with critical work in the project. More details about microtubule imaging are in an article at: http://www.news.cornell.edu/releases/June03/microtubule.hrs.html.

A new way to perform noninvasive "optical biopsies" was described in the second PNAS report, "Live tissue intrinsic emission microscopy using multiphoton-excited native fluorescence and second harmonic generation." Cornell biophysicists, who conducted the study with colleagues at Harvard Medical School and Cornell's College of Veterinary Medicine, predicted that it should be possible to obtain endoscopic and laparoscopic images of tissues at the cellular level from deep within living animals, including humans. More information on this finding is at: http://www.news.cornell.edu/releases/June03/Intrinsic.Fluor.hrs.html.

Commenting on the optical biopsy technique, Warren R. Zipfel, associate director of DRBIO, said: "Multiphoton microscopy is extremely well-suited to take advantage of the natural fluorescence -- the ability to give off light under bombardment by radiant energy -- of certain constituents in living tissue."

DRBIO is funded by the National Institutes of Health. Denk is now director of the Max-Planck-Institut für Medizinische Forschung Biomedizinische Optik, Germany.