Ultrafast, light-sensitive video cameras are needed for observing high-speed
events such as shockwaves, communication between living cells, neural activity,
laser surgery and elements of blood analysis. To catch such elusive moments,
a camera must be able to capture millions or billions of images continuously
with a very high frame rate. Conventional cameras are simply not up to the task.
Now, researchers at the UCLA
Henry Samueli School of Engineering and Applied Science have developed a novel,
continuously running camera that captures images roughly a thousand times faster
than any existing conventional camera.
In a paper in the April 30 issue of Nature (currently available online), UCLA
Engineering researchers Keisuke Goda, Kevin Tsia and team leader Bahram Jalali
describe an entirely new approach to imaging that does not require a traditional
CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor)
video camera. Building on more than a decade of research on photonic time stretch,
a technique for capturing elusive events, the team has demonstrated a camera
that captures images at some 6 million frames per second.
"The most demanding application for high-speed imaging involves fast events
that are very rare, rogue events or the proverbial needle in the haystack —
in other words, unusual events that carry important information," said
Jalali, a professor of electrical engineering and principal investigator of
the project.
One of the applications he envisions for the camera is flow cytometry, a technique
used for blood analysis. Traditional blood analyzers can count cells and extract
information about their size, but they cannot take pictures of every cell because
no camera is fast and sensitive enough for the job. At the same time, images
of cells are needed to distinguish diseased cells from healthy ones. Today,
pictures are taken manually under a microscope from a very small sample of blood.
But what if you needed to detect the presence of very rare cells that, although
few in number, signify the early stages of a disease? Circulating tumor cells
are a perfect example. Typically, there are only a handful of them among a billion
healthy cells; yet these cells are precursors to metastasis, the spread of cancer
that causes about 90 percent of cancer mortalities.
"The chance that one of these cells will happen to be on the small sample
of blood viewed under a microscope is negligible," Jalali said. "To
find these rogue cells — needles in the haystack — you need to analyze
billions of cells, the entire haystack. Ultra-high-speed imaging of cells in
flow is a potential solution for detection of rare abnormal cells."
The new imager operates by capturing each picture with an ultrashort laser
pulse — a flash of light only a billionth of a second long. It then converts
each pulse to a serial data stream that resembles the data in a fiber optic
network rather than the signal coming out of a camera. Using a technique known
as amplified dispersive Fourier transform, these laser pulses, each containing
an entire picture, are amplified and simultaneously stretched in time to the
point that they are slow enough to be captured with an electronic digitizer.
The fundamental problem in performing high-speed imaging, Jalali says, is that
the camera becomes less and less sensitive at higher and higher speeds. It is
simple to see why: At high frame rates, there is less time to collect photons
in each frame before the signal becomes weaker and more prone to noise. The
new imager overcomes this because it is the first to feature optical image amplification.
"Our serial time-encoded amplified microscopy (STEAM) technology enables
continuous real-time imaging at a frame rate of more than 6 MHz, a shutter speed
of less than 450 ps and an optical image gain of more than 300 — the world's
fastest continuously running camera, useful for studying rapid phenomena in
physics, chemistry and biology," said research co-author Goda, a postdoctoral
researcher in the group.
One such phenomenon the group has studied with the new camera is laser ablation,
an important technology that is the basis of laser medicine. The camera can
capture laser ablation happening in real time, providing important clues for
understanding the process and optimizing its effectiveness.
"Unlike other high-speed imaging methods, our approach does not require
cooling of the camera or high-intensity illumination — problems that plague
conventional CCD and CMOS cameras," said Kevin Tsia, a graduate student
in the group and a co-author of the research.
Posted April 30th, 2009
