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Seeing large surface areas at the cellular level

Jan 2007
Fiber optic technique based on OCT provides rapid volumetric imaging

Raquel Harper

Researchers continually strive to develop techniques that can detect cancer and other abnormalities as early as possible. Seok H. Yun of the Wellman Center for Photomedicine, Harvard Medical School and Massachusetts General Hospital, all in Boston, and his colleagues at these institutions have developed a microscopy technique based on optical coherence tomography (OCT) that can rapidly image epithelial, mucosal and endothelial tissues at the cellular level over large areas. The method might enable clinicians to find lesions that have been very difficult to detect earlier.

The researchers started creating their fiber optic imaging technique, dubbed optical frequency-domain imaging, when they were studying plaque in coronary arteries and abnormalities in the esophagus. Plaque lesions cause the arteries to narrow or to rupture, reducing blood flow to the heart and possibly leading to a heart attack. And Barrett’s esophagus -- a condition in which the lining of the esophagus has abnormal growths -- can lead to a form of cancer that affects about 200,000 Americans each year.

Such plaque lesions and abnormal growths are too small to detect with 3-D imaging techniques such as MRI and CT. So they investigated the use of OCT -- a technique that provides resolution of 2 to 15 μm and that easily can be incorporated into fiber optic catheters for in vivo imaging.

Researchers have developed a technique based on OCT that allows them to rapidlyimage large surface areas. This 3-D image of a pig’s coronary artery was acquired with an intravascular catheter that contained the fiber optic components within its inner core. Images reprinted with permission of Nature Medicine.

Although OCT allowed them to detect the tiny abnormalities, the technique is too slow for comprehensive microscopic imaging. It can take only point samples, similar to what a conventional biopsy provides. But to make a meaningful decision about a patient’s condition, a clinician must view the entire area of tissue where disease-related changes may develop (usually about 5 cm wide) -- not just one or two small points.

According to Yun, it would take about an hour to screen several centimeters of the esophagus with OCT -- which is impractical in patients. Furthermore, to image coronary arteries with OCT, the blood must be displaced with transparent saline or with an inflated balloon, and that allows only seconds of imaging. With OCT, several seconds barely permits imaging of a few tens of cross-sectional slices, but comprehensive diagnosis requires thousands of cross sections.

A comprehensive microscopy cross section of a pig coronary artery was obtained using the fiber optic technique.

Speed a factor

Clearly, the limitation is speed. The researchers tried to increase that in OCT but discovered that it was impossible to get it to a level that would be practical in patients. The problem is that OCT uses a broadband light source because several hundred to a few thousand wavelengths are needed to measure where the light is scattered within the tissue.

The distribution and location of the detected light scatters show the depth of the tissue structure, including any abnormalities, and this information can be turned into an image. However, by sending all the wavelengths at the same time, some of the information specific to individual wavelengths is lost. As a result, OCT can detect a signal from only one depth point at a time, losing signals from all other points.

Yun said the researchers thought that if they could obtain information from the reflection of each wavelength, they could detect the signals from all depth points simultaneously and, therefore, improve the imaging speed. Unfortunately, a light source that could rapidly send each wavelength one after the other did not exist.

The investigators, therefore, created a unique laser source, called the wavelength swept laser, that uses a semiconductor optical amplifier from Covega Corp. of Jessup, Md., and a custom-made filter based on a polygonal scanner from Lincoln Laser Corp. of Phoenix. The laser rapidly changes from short to long wavelengths, with a sweep range (distance between the wavelengths) of 111 nm at a repetition rate of 64 kHz and an average power of 30 mW. The new laser enabled them to look at approximately 1000 depth points simultaneously.

To support this laser, they had to optimize the interferometry of the traditional OCT technique for the new approach. They modified the interferometry for polarization-diverse detection, and, because the noise characteristics of lasers are different from broadband sources, they implemented dual-balancing detection to suppress the noise unique to lasers.

They added an acousto-optic frequency shifter from Baltimore-based Brimrose Corporation of America to their system, which removed depth degeneracy and doubled the effective ranging depth. The resulting fiber optic system enables them to image depths up to 7.3 mm, accommodating varying distances between the probe and the tissue surface.

After developing fiber optic catheters to support the optical frequency-domain imaging system, the researchers tested the technology in vivo. As reported in the December issue of Nature Medicine, they performed coronary imaging in five pigs and esophageal imaging in two others. For the coronary imaging, they made a small incision in the femoral artery and advanced the catheter (with the fiber optic probe in the inner core) through the chest to the coronary artery. They displaced blood from the field of view by delivering saline through the catheter using an injection pump at a constant rate of 2 to 4 ml/s. They imaged the tissue by rotating the inner core at 108 revolutions per second and pulled the core longitudinally within the sheath with a velocity of 5.4 to 16.2 mm/s. Each image, which included 680 cross-sectional frames, was obtained in about 6.3 s.

For the esophageal imaging, they used an endoscope to confirm the correct position, then inserted the catheter containing the probe into the esophagus. A balloon sheath helped center the system, inflating once the catheter was in the right position. Then they rotated and pulled back longitudinally on the fiber optic probe, just as they did with the coronary artery. The system acquired 3.5 million axial profiles over 5.8 min.

Using Matlab and Osiris data processing programs, the scientists created virtual images from both experiments in which high-resolution, cross-sectional views of different locations in the tissue were obtained.

Overall, they found that the optical frequency-domain imaging technique provided an imaging speed 90 times faster than conventional OCT systems (a three-fold increase in ranging depth along with a thirtyfold increase in acquisition rate). It also enabled them to image tissue areas larger than 25 cm2 and depths up to 2 mm with a 3-D resolution of about 15 × 15 × 10 μm.

Yun and his colleagues will carry out clinical studies with the technique in the near future, using funding from the National Institutes of Health. They believe that their technique will enable diagnostic imaging over large surface areas in other epithelial, mucosal and endothelial tissues as well.

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