Toward better treatment of clogged arteries
Intravascular optical coherence tomography (IVOCT) is a catheter-based technique that provides detailed cross-sectional imaging of the artery wall with a resolution of the order of ten micrometers. The clinical relevance of IVOCT has been demonstrated in many studies for the diagnosis and treatment monitoring of atherosclerosis1 (hardening and/or thickening of arteries by deposited plaques of cholesterol and other lipids, inflammatory cells, and calcium deposits). The technology is mature enough to be distributed commercially.2 Images may be acquired during a ‘pullback’ (backward translation of the imaging probe usually at a predetermined speed), so that pictures are obtained at different positions along the inside of the artery. In OCT, an intravascular pullback also usually includes a rotation of the probe. This minimally invasive technology has evolved tremendously over the last few years to allow fast pullbacks (rotation and translation) over tens of millimeters during a few seconds. IVOCT not only provides a detailed imaging of the atherosclerotic structures over a large segment of an artery, but also allows realistic 3D reconstructions.
Our research group investigates further applications of the IVOCT technology. One field of interest is the development of percutaneous (through-the-skin) coronary intervention (PCI) devices. For example, IVOCT can be used to monitor the inflation of an angioplasty balloon in a deployment tester.3 In balloon angioplasty, a balloon-tipped catheter is inserted into an artery and inflated. The balloon must be well designed to ensure success of the procedure. A deployment tester is used to test the quality of balloons through inflation under predetermined conditions. Traditionally, it can be equipped with an external laser scanner that provides measurements of the balloon's outer diameter at a given location. However, these measurements are only obtained from a side view and from a particular orientation. In contrast, IVOCT allows a full characterization of balloon diameter and wall thickness during the inflation process. With a pullback, the whole balloon can be imaged at a given pressure to provide complete 3D monitoring. This setup provides detailed information not currently available to balloon manufacturers, providing them with a tool to develop better angioplasty balloons.
In our technique, we insert our custom-built catheter-based OCT probe in an inflated semi-compliant balloon, that is, one that expands easily to a certain diameter and will expand more if required, although less easily (see Figure 1). The IVOCT probe is composed of single-mode fiber, a gradient index lens, and a small prism to redirect light in a direction perpendicular to the probe axis. The whole assembly rotates and translates in a liquid within a polymer sheath. We connect the semi-compliant balloon at both ends to a tube and introduce the OCT probe into the balloon through a T-connector (not shown) that also connects the balloon to the high pressure line of the deployment tester. Figure 2(a) shows a typical IVOCT cross-section obtained at a given pressure. Since IVOCT measures distances in optical length (refractive index times geometrical length), a conversion to geometrical length is provided in Figure 2(b), where the color indicates the thickness. We obtain the compliance and the elastic modulus by measuring the average diameter of the balloon as a function of the pressure. We can also compare the complete data to a 3D simulation of the balloon deformation to infer mechanical properties at all locations.
To monitor inflation under more realistic conditions, we use the same setup and insert the balloon inside an excised artery or an artery phantom (see Figure 3). The artery phantom is a silicone-based structure composed of three layers that provide the same optical signatures as the three artery layers: the intima, the media, and the adventitia.4IVOCT measurement immediately after balloon insertion reveals flaps in the balloon surface because the semi-compliant balloon was manually folded to ease insertion: see Figure 3(a). The flaps distort the IVOCT image due to refraction of light at the various interfaces. Later in the same inflation process, the flaps have disappeared: see Figure 3(b). The complete real-time IVOCT monitoring of the balloon inflation is provided in a short video.5
IVOCT technology can provide powerful development information for angioplasty balloon manufacturers. The applicability of the technology is even larger. A similar setup can be used to ease the development of numerous devices for percutaneous coronary intervention procedures. Examples include the assessment of new stent designs or of the efficiency of new atherectomy devices that remove accumulated plaque on the wall of the artery. We are now developing the same technology to monitor the angioplasty or stent deployment procedures performed on animals. Not only can we monitor balloon inflation, but we can also monitor tissue deformation and possible tissue damage. This should lead to new tools to improve the outcome of the percutaenous coronary intervention procedures.
The authors would like to acknowledge the financial support of the Genomics and Health Initiative of the National Research Council and of the Natural Sciences and Engineering Research Council of Canada.