Profile: Gabriel Popescu - A passion for light

Editorial

Rebecca Pool

Friday, June 27, 2014 - 07:30

Director of the Quantitative Light Imaging Laboratory, University of Illinois at Urbana Champaign, Professor Gabriel Popescu uses the properties of light to measure cellular structure and dynamics.

Earlier this year, Professor Gabriel Popescu, used white-light diffraction tomography to image unstained live cells at remarkably high resolution in all three dimensions. By fixing a specially-developed 'spatial light interference microscopy module' to a conventional phase contrast microscope he reconstructed 3D structures of E. Coli, blood and cancerous human cells to 350 nm transverse and 900 nm axial resolution.

As Popescu highlighted in Nature Photonics at the time, cells can be imaged over long periods of time, without damage, paving the way to straightforward diagnosis based on confocal microscopy. And not content with imaging blood and cancer cells, the researcher has used the technique to study single cell-growth, and neuron differentiation and growth for more than ten days.

Popescu's breakthrough technique comes under the auspices of quantitative phase imaging, methods that measure how much light is delayed through a specimen at each point in the field of view. This optical path length - or phase information - relates to a sample's refractive index and thickness, enabling detailed studies on cell structure and dynamics.

For Popescu, his tomography breakthrough follows a lifetime fascination with physics and light. "I always knew I was going to study physics, so back in Romania I attended a Maths and Physics high school, in the physics department of the University of Bucharest," he says.

"I had a brilliant physics teacher and she got me excited about this," he adds. "I specialised in optics and following on from some labs in those early college days, I became fascinated with light, especially lasers."

Receiving his B.S and M.S in Physics from Bucharest in 1995 and 1996, the young Popescu had a strong interest in biomedical applications. As he puts it: "I remember reading some papers about brain surgery with carbon dioxide lasers, and maybe because my father had died with brain tumours, this was something that I was very excited about."

Following a year at the National Institute for Lasers, Plasma and Radiation Physics in Romania, Popescu headed out to School of Optics - now the College of Optics and Photonics, CREOL - at the University of Florida, US, to start his Ph.D in 1997. Spurred on by former colleagues moving to CREOL, he explains: "These guys were telling me these great stories about CREOL, and the [School] was pretty famous in Romania. I wanted to do optics so I went."

At CREOL, Popescu spent a lot of time combining light scattering techniques with interferometry and phase measurements, extracting information from inhomogeneous media including microspheres as well as tissue. He also used these light scattering methods to quantify the physical properties of blood as it coagulated, and receiving his Ph.D in 2002, moved over to the G.R. Harrison Spectroscopy Lab at MIT to focus on biomedical applications.

New methods

Thanks to his solid interferometry experience at CREOL, Popescu led the interferometry group at the Lab, supervising a few graduate students, and engineers from Japan-based Hamamatsu Photonics. And this was where his research - soon to be known as quantitative light imaging - began to really take shape.

Neuron with nucleolus, acquired by SLIM

"My goal at CREOL had been to develop new microscopy methods so I kept working on these here," he says, "We were developing microscopes that were very, very sensitive by using interferometry and phase measurements, but also quantitative, so we could get figures for, says cell weight and cell size."

While Popescu and his team developed a host of interferometry-based methods, the years 2004, 2005 and 2006, saw the researchers deliver their big three quantitative light imaging techniques; Fourier phase, Hilbert phase and Diffraction phase microscopy.

According to Popescu, Fourier phase microscopy (FPM) came first, with Hilbert phase microscopy (HPM) following 'in an attempt to make measurements faster'. As the researcher wrote in Optics Letters: "Fourier was developed to extract quantitative phase images with subnanometre path sensitivity over time periods from seconds to a cell life cycle...Hilbert complements FPM and quantifies rapid biological phenomena, such as millisecond-scale red blood cell fluctuations."

But as Popescu explains, the accuracy of the phase measurements using HPM suffered, so his team focused on combining both methods and a year later delivered Diffraction phase microscopy. With a method that could provide fast, accurate and stable phase measurements, the team's biological research proceeded at a rapid pace.

"We published a large number of papers on red blood cells, their biophysics and how they change with disease," says Popescu. "Thanks to our new methods we could quantify nanoscale fluctuations in red blood cells... One study looking at how frequency of vibrations changes in malaria-infected cells, was the clearest experiment we had done."

"Researchers have tried to measure the properties of cells with other methods such as atomic force microscopy but these techniques can be tedious with limited results," he adds.

Come 2007, Popescu was keen to become a member of faculty and moved to the Beckmann Institute for Advanced Science and Technology, at the University of Illinois at Urbana-Champaign, where he remains today. Taking on the role of director of the Quantitative Light Imaging Laboratory, the new Professor, was keen to make his quantitative imaging methods accessible to a broader range of researchers.

"I was always convinced that this type of imaging would have an impact on biology, but knew there was still a lot to be done if biology students were to use this type of technology themselves," he says.

Fibroblasts, acquired by SLIM

As he explains, quantitative imaging methods based on lasers were great for red blood cells, but in less homogeneous samples, such as neurons, the well-known problem of laser speckle interference ultimately limited the technique's resolving power. "Laser speckle washes out the details," he says. "So my number one priority on moving to Urbana was to solve this problem."

Development began apace and several instruments later, the problem was solved by replacing the coherent laser light in quantitative phase imaging methods with white light.

"We always arrive at simple answers after exhausting all other options," laughs Popescu. "But it is much more difficult to build an interferometer with white light, so lasers had been the tradition."

Crucially, Popescu's diffraction tomography breakthrough can be realised on a conventional microscope by adding a so-called spatial light interference microscopy (SLIM) module to the camera port. The module converts interference patterns recorded by a microscope's the CCD device to quantitative phase images, providing subnanometre path-length sensitivity and producing incredibly high resolution images of unstained cells.

With method in hand, the researcher set up 'Phi Optics' in 2009 to commercialise SLIM, unveiled the first prototype earlier this year, the first orders are now coming in. In two to three years, the company aims to supply clinical instruments for blood testing and cancer diagnosis.

Red blood cells, acquired by SLIM

From concept to device, Popescu's progress has been rapid, although he maintains his breakthroughs are rooted firmly in physics, saying: "Unless you understand the basic physics you cannot produce serious advancements in this field."

However, he also attributes a large part of his, and his researchers' successes, to their multi-disciplinary backgrounds; his current research group comprises physicists, bioengineers. electrical engineers and mechanical engineers. "I am convinced that this is how the future should work; attacking a problem from different angles, sharing expertise and adapting information into something new and something better," he says.

So with SLIM on the road to commercialisation, where next for the researcher? "My main mission since I started my post doc has been to advance this field and I truly believe quantitative imaging is going to have a broad impact on biology," he asserts. "We're going to keep focused on this until every laboratory has a quantitative phase microscope."

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