A number of vascular diseases, such as coronary artery disease and peripheral vascular disease, are caused by the build-up of atherosclerotic deposits (plaque) in the arteries, which limit blood flow to the tissues that are supplied by that particular artery. Disorders caused by occluded body vessels, including coronary artery disease (CAD) and peripheral artery disease (PAD), may be debilitating and life-threatening. Chronic Total Occlusion (CTO) can result in limb gangrene, requiring amputation, and may lead to other complications and eventually death. Increasingly, treatment of such blockages may include interventional procedures in which a guidewire is inserted through a catheter into the diseased artery and threaded to the blocked region. There the blockage may be either expanded into a more open position, for example, by pressure from an inflated catheter balloon (e.g., balloon angioplasty) and/or the blocked region may be held open by a stent. Treatment of such blockages can also include using a catheter to surgically remove the plaque from the inside of the artery (e.g., an atherectomy).
There is medical interest in equipping catheters with sensors that can help direct the catheter for atherectomy, occlusion-crossing, and/or other surgical procedures. For example, it would be useful to have sensors that can give the surgeon immediate visual feedback as to whether a particular tissue is diseased and/or how far away the cutting portion of a catheter is from the boundary of a particular blood vessel layer to minimize the risk of accidental damage. Conventional radiological imaging methods and ultrasound imaging systems have been attempted for such surgical procedures. However, neither ultrasound nor radiological imaging methods have enough resolution to help guide the operation of the catheter through small dimensions. Moreover, standard radiological techniques cannot easily discriminate between healthy tissue and diseased tissue unless the tissue has become heavily calcified. Further, the components of an ultrasound system are generally too large to implement on a small scale, such as with a system configured to be used within blood vessels.
Optical Coherence Tomography (OCT) has been proposed as one technique that may be particularly helpful for imaging regions of tissue, including within a body lumen such as a blood vessel. At a basic level, OCT relies on the fact that light traveling from a source and scattering from more distant objects takes longer to travel back than light scattering from nearby objects. Due to the wave nature of light, very small timing differences caused by light signals traveling different distances on the micron scale can cause constructive or destructive interference with reference light signals. OCT systems measure the resulting interference to obtain an image of the target. A typical OCT system requires one or more interferometers to distinguish the signal from the applied light. In addition, most known OCT systems, when applied to catheters, include a fiber that is rotated (often at high rates) within the catheter in order to scan the lumen and a second, large reference arm.
A typical OCT device includes a target arm and a reference arm to generate a reference signal. In order to provide the interference reference signal, the OCT device will split an illuminating light signal from the source in two equal or unequal parts, send part of the illuminating light to the target of interest through one target optical “target arm” and send the other part of the illuminating light down a separate reference arm. Light from the separate reference arm reflects off of a mirror, and then returns and interferes with the scattered light that is returning from the target optical arm after bouncing off of the target. In a traditional OCT device, the reference arm length is engineered to be exactly the same length as the target arm so that the interference effect is maximized. The resulting interference between the two beams creates interference that can be measured to extract depth information related to the target. Using this depth information, an image of the object can be generated. A typical OCT device can further include a focusing lens in the target arm, such as a graded index (GRIN) lens, configured to focus the light coming out of the optical fiber into the tissue.
These traditional OCT systems, however, are large and cumbersome due to the required reference arm and are therefore generally ineffective for use in a medical catheter, particularly for use with a low cost and disposable catheter. Using a common path OCT system, i.e., a system without a separate reference arm, is one way to eliminate the cost and size of such an imaging catheter. There are several challenges, however, associated with developing a catheter having common path OCT. For example, a common path OCT system requires that the reference reflection be formed within the same optical conduit as the target reflection. This reference reflection must be finely tuned to avoid noise in the system, requiring that the path from the light source to the reflection interface be free of unnecessary components, such as focusing elements that could interfere with the reference reflection. Further, the common path system must have components that are small enough to fit inside of a single small catheter, making it difficult to include additional components. Finally, for common path OCT, it is desirable to have the reference reflection as close to the tissue as possible to maintain the imaging range within the coherence length of the source and avoid data processing burden, as data processed for the distance between the reference and the start of the imaging is not useful. Accordingly, a common path OCT system that solves some of these problems is desired.
The distal imaging tip of a common-path OCT catheter should perform two main functions: (1) direct the beam towards the imaging object and (2) focus the beam on the imaging object for improved image quality. In addition, for common path OCT, the geometry and properties of the distal imaging assembly should be such that it introduces only one primary source of back-reflection (reference reflection) and avoid any other parasitic reflection which could causes artifacts in the images.
One way to address these needs that has been proposed (see, e.g., US-2015-0099984) which uses a common-path OCT system with a graded index (GRIN) fiber attached to the distal tip of a single mode optical fiber in the catheter so as to act as a lens for focusing light. Unfortunately, this solution has proven problematic. For example, the addition of any lens (e.g., a graded index (GRIN) lens) is difficult and results in potential failure modes. FIG. 1 illustrates a prior-art device 100, such as a catheter, having housing 109 holding a graded index (GRIN) fiber 103 at the end of an optical fiber 110, forming a grins lens assembly. In this example, deflection and focusing of the beam is accomplished using two separate components. First, a mirror 101 is mounted at 45 degrees to the axis of the optical fiber 110 to deflect the beam perpendicular to axis of the catheter, as shown by beam 113. Second, the graded index fiber 103 is spliced to the optical fiber 110, which is a single mode fiber (SMF) assembly, in order to focus the beam. The GRIN fiber 103 is spliced in front of the SMF fiber 110 and then cleaved to precise length and angle to meet the focusing and reference reflection requirements in conjunction with the epoxy 102 used to secure the GRIN fiber 103 at the distal tip. The precision splicing and cleaving requirements for this SMF-GRIN assembly makes it an expensive component for the device 100.
Further, the manufacturing processes required to make these SMF-GRIN assemblies, such as that in device 100, is often difficult and requires precision alignment. Moreover, in order to splice the GRIN fiber 103 in front of the SMF fiber 110, the SMF fiber must be stripped to certain length (see stripped section 105 in FIG. 1). Stripping of the fiber SMF fiber, especially polyimide fibers, typically renders a long fragile portion 132 of fiber. The total length of the fragile portion 132 of the SMF-GRIN assembly may be greater than 2.5 mm. This fragile portion 132 often must be encapsulated inside a solid hypo tube, to prevent the distal portion from breaking while subjected to stress/strain while imaging in tortuous anatomy. The presence of the long (e.g., >2.5 mm) hypo tube in the device 100 reduces the flexibility of the distal tip. Thus, when the device 100 is part of a catheter, such as an atherectomy catheter, this reduced flexibility can make it difficult to access the tortuous section of the anatomy.
Moreover, focusing length may be sacrificed by using a GRIN fiber 103 and separate mirror 101. The focusing capability of the GRIN fiber 103 is a function of the diameter of the fiber. Using 125 micron GRIN fiber 103, to match the diameter of the SMF 110, the focus point is limited to 1-1.5 mm from the tip of the fiber. In imaging catheters where the fiber is deployed in the middle of the catheter, significant portion of the Rayleigh range of the beam could be lying inside the catheter. This reduces the imaging depth within which the imaging catheter is able to maintain high resolution.
Described herein are apparatuses, including tip assemblies for OCT (common path) imaging systems, and methods of making an using them, that may address the issues raised above.