A relatively portable device that can image inside arteries and gastrointestinal tracts of living beings enables doctors to see features as small as 10 µm, including early signs of cancer and heart disease. The device is based on optical coherence tomography (OCT) and could be used in place of surgical biopsies and in some situations in which regular biopsies are not possible.
The system is a culmination of work done by James Fujimoto’s group at the Massachusetts Institute of Technology (Cambridge, MA) and Mark Brezinski at Massachusetts General Hospital (Boston, MA). The group recently reported in vivo imaging in a rabbit esophagus.1
Tomography by interference
The OCT system is based on a fiber interferometer. A Kerr-lens-modelocked, chromium-doped forsterite laser emits 30-fs pulses with 30 mW of average power at 1280 nm. This wavelength falls between the water-absorption peak at about 1500 nm and the spectral region below 1000 nm, at which tissue scatters most light. The group chose the wavelength because it can penetrate through about 3 mm of tissue.
One arm of the interferometer goes through a catheter or endoscope. The distal end of the single-mode fiber is cemented to a GRIN lens, which is cemented to a prism that acts as a turning mirror, directing the light perpendicular to the fiber out through the side of the transparent sheath over the end of the device. The fiber, lens, and prism rotate at the end of the sampling arm, which allows the device to direct the light in a plane around the end of the fiber. Backscattered light from the tissue re-enters the prism, is collected in the core of the GRIN lens, and returns through the fiber. The reference arm of the interferometer is scanned through 3 mm, and a detector records the interference pattern. The system could provide 2000 axial scans/s, allowing a 360° cross section of the area around the fiber tip relatively quickly. An image can be acquired in 250 ms.
Alternative to surgery
The entire endoscopic end of the device is about 1 mm in diameter. It can be used in vessels with a diameter of 5 mm and measures about 3 mm deep into tissue. The 10-µm resolution of the OCT system is about 10 times that of clinical magnetic-resonance imaging or of high-frequency ultrasound imaging. One benefit of the higher resolution is the ability to image damaged tissue, atherosclerotic plaques likely to rupture and cause a heart attack, or precancerous changes. The system uses off-the-shelf components, mostly developed for telecommunications, and is insensitive to temperature changes.
Compared to other clinical imaging systems, OCT is portable and inexpensive. The device could be used to perform optical biopsies in areas where conventional biopsies would be dangerous, such as in the arteries of the heart and brain, as well as detecting cancer in the colon, esophagus, and cervix. Recently, the researchers tested OCT as an imaging aid for surgery and microsurgery. "We are just about ready to test OCT in patients through a high-speed, high-resolution catheter/endoscope-based system," said Fujimoto. Further work includes increasing the resolution and exploring uses of OCT with other techniques, including spectroscopy.