| A quick peek: A Fourier domain scan of a stented left anterior coronary artery. The stent struts can be seen as bright, short dashes in the upper image. On the lower longitudinal scan, the stent, embedded in the irregular walls of the artery, can be seen stretching from 20 millimeters to 32 millimeters. |
Credit: LightLab Imaging/Helios Heart Center
Getting a high-resolution picture of the interior of a coronary artery is difficult: to take a scan using existing technology, the heart has to be kept free of blood for 30 seconds. A new approach could dramatically reduce the time required for imaging, making it safer and easier for doctors to check stents for stability and keep track of new scar tissue.
The new method builds on an old technique called optical coherence tomography (OCT). This high-resolution medical imaging system has been in use in ophthalmology for more than a decade and is occasionally used to scan coronary arteries. But OCT is problematic because it cannot see through blood, so any area being scanned has to be flushed with saline. During the procedure, a special balloon is used to block incoming blood, which can cause damage to the tissue. Two US companies are working individually on a scanning method that would take a fraction of the time, greatly reducing the risk of damage to the heart.
OCT works by projecting a beam of light onto a surface, which then reflects a small amount of light back to the device. Due to the high speed at which light travels, reflection time is too brief to be measured directly. Instead, OCT relies on an interferometer, which measures the interference of noncoherent light. Because these light waves have a short wavelength, high-resolution images can be generated. (Intravascular ultrasound (IVUS) could also be used, but it typically has a resolution of 80 microns to 130 microns. OCT devices already on the market are able to measure down to the 15-micron level, providing far more detail.)
A number of companies are working on improving OCT using what is known as the Fourier domain. This mathematical formula is used to process a complex signal so that it can be differentiated into its component parts and analyzed. For OCT, this means that multiple wavelengths of data can be gathered simultaneously rather than sequentially, an improvement on previous generations of the technology. Sometimes called optical Fourier-domain imaging, this method substantially reduces the time required to perform a scan. Traditional OCT scanning requires multiple exposures of light aimed at specific points to make a full image. Fourier domain OCT exposes the entire area at once, reducing the time required to obtain a section from 30 seconds to two seconds. This reduction dramatically reduces the associated risks of the procedure. In two seconds, an area of artery 40 millimeters to 50 millimeters in size can be scanned, with an accompanying improvement in scanning resolution down to 10 microns.
Optical Fourier domain imaging is "a fairly well-established approach in principle," says Thomas Milner, associate professor in the Department of Biomedical Engineering at the University of Texas, "and it's just starting to work its way into instruments that will be on the market soon."
Even though the basis of the Fourier domain OCT was conceived more than four years ago, the technological advances that make it practical are comparatively recent. Fourier domain OCT requires three important pieces of equipment: the scanning laser, the processing electronics, and the light detectors. "It's really in the last 24 months that all of this [technology] started to become available so you could look at it as a system," says Chris Peterson, vice president of research and development at LightLab.
An initial application for this technology will be to image stents after insertion to ensure they haven't shifted. A stent is an artificial buttress placed in an artery to keep it open, allowing the blood to flow freely. Research from Harvard University Medical School has shown that stent prolapsing can occur with shifts of less than 100 microns, a level that would go undetected by IVUS. The increased accuracy of OCT technology allows doctors to observe how well the stent is adhering to the arterial walls and to track small amounts of endothelial regrowth that would go unnoticed by IVUS. It could also be used postoperatively to check healing. The resolution of this scan is fine enough to allow doctors to identify small but significant plaque deposits that existing technology might overlook. The technology could also be used to carefully target biopsies, as cancerous cells could be identified in much smaller quantities than currently possible.
LightLab is not the only company trying to improve heart-imaging technology. CardioSpectra of Austin, TX, is working in a similar vein with Fourier domain OCT. The company was recently purchased by Volcano, one of the leading manufacturers of IVUS products. This $25 million acquisition would seem to suggest a strong future for the technology.
Chris Milner attests that work continues to expand the range of the device into other regions of medicine. "In the world of research, it's being explored in a number of areas; the [gastrointestinal] track, the bladder, and refinements to OCT technology are also being investigated."
This technology will be coming into use "in the very near future...by the end of 2009 at the latest," says Craig Kelley, director of marketing for LightLabs.