"If you hit it off-centre, it just rolls out of the way." This week, Zeman will demonstrate the device at the Frontiers in Optics conference in Rochester, New York. The prototype of the system, which he calls a vein contrast enhancer (VCE), uses a near-infrared camera to capture a real-time video image of the patient's veins, a PC to enhance the contrast of the image and a desktop video projector to display it on the skin in real time . An array of near-infrared LEDs surrounding the camera's lens illuminates the skin at a wavelength of 740 nanometres. This wavelength is strongly absorbed by blood, but is scattered by the surrounding tissue.
"Fat and tissue look light, veins and blood look dark," says Zeman. The image from the camera is fed to a PC running imaging software that maps the image onto a bright green background in real time and boosts the contrast between the veins and surrounding tissue. The PC then feeds this image to a projector that beams it onto the skin. The tricky part is making sure the image of the veins is projected in exactly the right place. Get this wrong, and the system becomes worse than useless. The key is device called a "hot mirror", which is transparent to visible light but reflects infrared (hot) wavelengths.
The video projector and camera are set at 90 degrees to each other facing the mirror, which is set at 45 degrees to both of them. After calibration, this ensures that a vein always appears within 0.06 millimetres of its correct position, Zeman says. The vein contrast enhancer can detect veins up to 8 millimetres below the surface of the skin, Zeman says. Green light is used as the backdrop because it is not sensed by the infrared camera. Zeman has now miniaturised the VCE system to fit it in a package the size of a shoebox, making it portable enough to be mounted on an intravenous drip stand. Three prototypes will begin clinical trials at a hospital in Tennessee later this year.
"From the patient's standpoint, anything that makes the placement of the intravenous drip easier is great," says Dennis Ernst, director of the Center for Phlebotomy Education in Ramsey, Indiana. He predicts that patients, especially children, will enjoy the spooky green image of the vasculature on their arms. That's a boon, he says, because normally the small veins and puppy fat of young children means that more than half the attempts to find a vein are unsuccessful.
Written by CELESTE BIEVER, BOSTON
This article appears in New Scientist issue: 9 October 2004
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