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Video capability revives interest in laser method

05 Mar 2007

Plastic surgery, neuroscience and dermatology could all benefit from the release of the first commercial instrument to image blood flow in real time, at up to 25 frames per second. James Tyrrell speaks to David Briers, one of the pioneers of laser speckle contrast analysis, to discover more about the technique that was first conceived over 25 years ago.

In the 1980s, when Adolf Fercher and David Briers came up with the idea of using laser speckle to monitor blood flow, their dream was to have an instrument that could operate in real time. After 25 years, the technology has caught up in the form of a full-field laser profusion imager (FLPI) from Moor Instruments, UK, and is set to take the medical sector by storm.

“I’m not aware of any other instrument on the market, or even in development, that can give a real-time video image of blood flow over an area,” an excited Briers told OLE upon hearing the news. “If you shine the laser beam on the back of your hand and look at the screen, you can even see the blood pulsing in your veins – it’s an incredible thing to observe.”

Now emeritus professor in applied optics at Kingston University, UK, Briers is pleased to see his original discoveries underpinning the work of Moor Instruments and others. “When our project ran out of funding in 1999, I offered the laser speckle contrast analysis method to anyone who wanted to run with it,” said Briers. “Today, I know of more than 30 groups, in at least 17 countries, which are using or developing the technique.”

For example, neuroscientists at the Harvard Medical School in Boston, US, are using the technique to monitor cerebral blood flow. Other up-and-coming areas include dermatology, diabetes, wound assessment and plastic surgery.

Video-frame-rate (25 images per second) performance opens up many opportunities for research into blood-flow mechanisms. Importantly, it allows scientists to track the changes in microcirculation that occur during a cardiac cycle.

Principle of operation
The technique works by illuminating an area of tissue with laser light to produce a high contrast random interference effect known as a speckle pattern. Blood cells flowing through the region of interest cause the speckle pattern to change and appear blurred, which leads to a reduction in local contrast. High flow rates show up as areas of low contrast and conversely, low flow rates are defined by regions of high contrast.

“A very simple way of capturing this [flow rate] information is to image the speckle pattern,” said Briers. “Velocity distributions are coded as variations in speckle contrast.” For convenience, scientists typ­ically convert the contrast variations into an intensity map, which is much easier for the human eye to perceive.

The early days
When Fercher and Briers began their work in the 1980s at the University of Essen, Germany, suitable digital techniques were unavailable. “It was a two-stage process and nothing like real time, which meant that the medical profession was not particularly interested,” commented Briers. “We’ve had to wait 25 years for [digital] technology to catch up.”

In the early days, Fercher and Briers used a film camera to photograph the flowfield and then applied a form of optical-image processing to capture blood flow in the human retina. In the 1990s, Briers and his team at Kingston University, UK, came up with a digital version of the technique that bypassed the need for a two-stage approach and involved manipulating data directly from a charge-coupled device (CCD).

“About 10 years ago we could do this whole process in about one second,” said Briers. “What Moor Instruments has done is to reduce the processing time down to one-twentyfifth of a second, which means that you can operate at video frame rates and actually see the blood flow changing.”

Moor Instruments’ FLPI system uses a near-infrared laser diode (785 nm) and a 576 x 768 pixel CCD camera to capture blood flow over an area of up to 80 x 120 mm. When operated in zoom mode, the system can deliver a maximum resolution of around 50 µm. The firm is now working on a version that can resolve down to 5 µm. Changes in contrast are colour coded to give a false colour map of velocity distribution.

To make the instrument more access­ible and affordable to its customers, the company is keen to use off-the-shelf PC technology wherever possible. The firm’s Windows-based software is compatible with standard USB and FireWire interfaces, which eliminates the need for dedicated internal frame grabbers, and enables it to be used with laptop computers.

Ease of use and suitability for studies in the clinic and in the ward are key issues for the design team. “The company has packaged the laser delivery system to achieve Class 1 operation,” revealed Briers. “It’s completely eye safe, so you don’t need to wear protective goggles.”

The firm has applied for Food and Drug Administration pre-market approval for its device and is confident that users looking for video-frame-rate performance will appreciate the instrument’s bene­fits. Rival techniques, such as laser Doppler imaging, may offer greater sampling depth, but they are based on a single-point measurement and require scanning. “The main problem is the time taken for the scan to be carried out,” said Briers. “What laser speckle contrast analysis offers is a full-field technique that produces a map of velocity from a single shot.”

Market value
Moor Instruments anticipates that sales of its FLPI over the next five years could be worth about £3 m (€4.5 m) in the research market alone. Over the same period, the clinical sector could deliver around £10 m in sales, but this figure will require a substantial investment in clinical trials.

“It’s really the medical side that has caught the imagination, but there are many other potential applications out there,” commented Briers. “The technique can even be used to watch paint dry by determining when the painted surface has hardened.”

• This article originally appeared in the February 2007 issue of Optics & Laser Europe magazine.

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