High resolution single-mode-fiber-based sensor for intravascular detection of fluorescent molecular probes

Early detection of coronary atherosclerosis is an unmet clinical challenge. The detection system has to be highly sensitive and possess high spacial resolution, in order to provide precise information of the vulnerable plaque location and size. Recently molecular fluorescence probes have been identified as efficient inflammation biomarkers for the inflammation process within vulnerable plaques¹ and being used in the proposed application to detect inflamed lesions in the blood vessel wall. The general principle of the proposed solution is based on a sensor whose head is guided by an intravascular catheter to the region of interest (coronary artery). When the sensor illuminates an activated fluorescent probe, located in inflamed areas of vulnerable plaques, the fluorescence is excited and light is emitted with a slightly shifted spectrum. The emitted light is being collected by the same sensor head, guided through the optical fiber and finally detected by photo-detectors. In this way, by detecting emitted fluorescence one can obtain information about the location of vulnerable plaques. The localization resolution is critically depending on the spot size of the illuminating light beam. Moreover, for a high signal to noise ratio in the detection electronics, as much fluorescent light as possible has to be collected from the plaque location. It has been already demonstrated that using single-mode fibers in combination with graded index fibers, a Gaussian beam, with adjustable waist position and diameter can be formed, representing the fundamental limit of achievable spot size2. However, when using single mode fibers in this application, the collection efficiency would be very low due to the small core diameter of this fiber and thus signal to noise ratio would be strongly reduced. In this work, we present a solution to this challenge, combining both principles. A single mode fiber in combination with a graded index fiber is used for illumination purposes, while the fluorescence light is collected by the same fiber, but employing the cladding/coating total reflection to form a multimode fiber for the backwards propagating light. Thus, a narrow spot size can be obtained allowing high resolution images, with high signal to noise ratio due to the multimodal collection scheme. We show preliminary results of spot size and beam diameter measurements from the sensor head and discuss the implication for the improvement of the current catheter-based detection systems.