Researchers from the University of California Irvine (UCI; Irvine, CA) and the University of Southern California (USC; Los Angeles, CA) collaborated on a new method for identifying atherosclerotic plaques, the encased deposits in arteries that restrict blood flow and can lead to heart attack or stroke. The technique combines two different types of imaging—intravascular ultrasound (IVUS) and optical coherence tomography (OCT)—to allow for an unprecedented combination of depth and detail.
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The hybrid approach, performed in rabbits and on human tissue, provides both the imaging depth and resolution necessary to identify thin-cap fibroatheromas (TCFAs). These plaques are susceptible to rupture because they contain a large necrotic core, filled with lipids and dead cells, and the region surrounding the core (the fibrous cap) has thinned.
Until now, both technical issues and safety concerns prevented combining the two techniques. Each technology has a different optimal imaging speed; IVUS takes about 30 frames/s, while OCT uses a speed of more than 100 frames/s. Increasing IVUS's speed sacrifices image quality, while slowing OCT extends procedure time, increasing the risk of both side effects from the imaging agents used and false diagnosis from catheter spasm. The research team's previous attempt at integrating the two used 20 frames/s, but was abandoned because of the long procedure time. Alternatively, doing one procedure after the other would require two separate catheters and cost a lot more.
This time, the breakthrough came as a result of many technical advances, including a customized catheter, improved ultrasound transducer, and faster graphical processing unit. The result was a hybrid IVUS-OCT imaging system that could take pictures at a rate of 72 frames/s. Although there was initially a slight decrease in IVUS image quality, it was offset by increased sensitivity of the transducer. The hybrid system can visualize 7 cm of artery in 4 s.
The new system was first tested in healthy rabbits and rabbits with a build-up of plaque (atherosclerosis), and proved the mechanics of the ultrafast system worked well and that plaques could be clearly characterized from the resulting images. Next the team used the hybrid approach to visualize plaques in human tissue obtained from cadavers. After IVUS-OCT imaging, the tissue was sliced and stained to confirm locations of TCFA plaques. Two doctors independently evaluated images from IVUS alone, OCT alone, or the IVUS-OCT images together. While they were unable to identify TCFAs using either IVUS alone or OCT alone, the doctors made accurate classification of these plaques when IVUS and OCT images were evaluated together.
The technique could also be used to image other structures and lesions, such as calcification, notes Kirk Shung, Ph.D., a professor of biomedical engineering at USC and one of the authors of the paper describing the work. Most people who are older than 60 have deposits of calcium mineral in their major arteries, which cause the vessels to be less elastic over time and are a factor in heart disease.
The OCT-IVUS technology has been licensed by OCT Medical Imaging Inc. (Irvine, CA), a company that Zhongping Chen, Ph.D., a UCI professor of biomedical engineering and senior author of the paper, co-founded. The company recently received a Small Business Innovation Research (SBIR) program grant from NHLBI to fast-track the translation of this technology for clinical applications.
Full details of the work appear in the journal Scientific Reports; for more information, please visit http://dx.doi.org/10.1038/srep18406.