Combined fluorescence & OCT

OCT creates images based on the refractive index distribution within biological tissue. For certain applications such as cancer diagnosis, it is desirable to combine these “structural” images with “molecular” contrast that shows the distribution of certain biochemical compounds. One of the most widespread techniques for detecting the presence of biochemical compounds using optics is fluorescence imaging, typically via the use of fluorescent antibodies. The OBEL research group was the first to combine OCT and fluorescence imaging capabilities within a needle probe [1]. Our OCT + fluorescence needle probe uses an elegant trick to combine the two imaging techniques. It uses a special type of optical fibre (double-clad fibre, or DCF) that has a “small” single-mode core with a diameter of 9 micrometres surrounded by a “big” multi-mode core with a diameter of 105 micrometres (the so-called inner cladding), thereby creating an optical fibre link with two separate channels, as illustrated in the figure below.

Illustration of our combined OCT + fluorescence imaging needle probe using double-clad fibre (DCF). (a) shows the OCT light path. The 1300-nm light from the OCT system is guided in the core of the DCF and expanded/focused by sections of no-core fibre (NCF) and graded-index fibre (GRIN). (b) shows the fluorescence light path. The 488-nm light for fluorescence excitation is guided in the core of the DCF and is focused into the sample via the same path as the OCT light. The green fluorescence, however, is efficiently collected and guided back to the detector by the large-diameter inner cladding.

Illustration of our combined OCT + fluorescence imaging needle probe using double-clad fibre (DCF). (a) shows the OCT light path. The 1300-nm light from the OCT system is guided in the core of the DCF and expanded/focused by sections of no-core fibre (NCF) and graded-index fibre (GRIN). (b) shows the fluorescence light path. The 488-nm light for fluorescence excitation is guided in the core of the DCF and is focused into the sample via the same path as the OCT light. The green fluorescence, however, is efficiently collected and guided back to the detector by the large-diameter inner cladding.

We have built an all-fibre based imaging system for combined OCT + fluorescence imaging using these needle probes (see figure below). A key component in this system is a novel type of fibre coupler [2] which was fabricated by our collaborators in Canada (Castor Optics) and which allows us to separate the OCT and the fluorescence signals traveling in the DCF into two separate paths for detection. We are using this system to explore various potential applications of this new technique in collaboration with researchers in medicine and biology.

Schematic of our all-fibre system for combined OCT and fluorescence imaging using needle probes

Schematic of our all-fibre system for combined OCT and fluorescence imaging using needle probes

Our OCT + fluorescence needle probes can capture weak fluorescence signals so efficiently that they can detect fluorescently labelled cells with a sensitivity comparable to state-of-the-art commercial fluorescence microscopes. As an example, the figure below shows OCT and fluorescence images obtained using one of our needle probes, from a section of human liver that was excised from a patient undergoing tumour resection and labelled with a fluorescent antibody. A small region of dense fluorescence has been magnified (d), showing liver progenitor cells (arrowhead) as well as ductal epithelial cells (arrow) which also get labelled by the fluorescent antibody. These results are validated against images obtained using a 4x wide-field fluorescence microscope ((c) and (e)). These results indicate that it will be feasible to detect fluorescently labelled cells deep inside living tissue using needle probes, rather than having to cut out tissue samples for inspection under a conventional microscope. This has enormous potential for future applications in cancer diagnosis as well as for the development of new targeted pharmaceuticals and their delivery protocols.

OCT and fluorescence images of a section of human liver. The images on the left (a,b,c) were obtained with our OCT + fluorescence needle. For comparison, the images on the right (c,e) were obtained using a widefield fluorescence microscope.

OCT and fluorescence images of a section of human liver. The images on the left (a,b,c) were obtained with our OCT + fluorescence needle. For comparison, the images on the right (c,e) were obtained using a widefield fluorescence microscope.

1) References:

[1] D. Lorenser, B.C. Quirk, M. Auger, W.-J. Madore, R.W. Kirk, N. Godbout, D.D. Sampson, C. Boudoux, and R.A. McLaughlin. “Dual-modality needle probe for combined fluorescence imaging and three-dimensional optical coherence tomography.” Optics Letters 38, 266-268 (2013).

[2] S. Lemire-Renaud, M. Rivard, M. Strupler, D. Morneau, F. Verpillat, X. Daxhelet, N. Godbout, and C. Boudoux, “Double-clad fiber coupler for endoscopy,” Optics Express 18, 9755-9764 (2010).