Ultrathin fluorescence microscope in a needle – kick off

This month, I started work on my recently awarded grant from the Engineering and Physical Sciences Research Council. The project is called “Ultrathin fluorescence microscope in a needle”, which is hopefully fairly self-explanatory; we want to make a needle which can penetrate into tissue and provide us with high resolution microscopy images. This would allow real time microscopic imaging to support surgery and other interventions, something that simply isn’t possible at the moment. There will be a postdoc joining me for a year, and the project meshes well with Andy Thrapp‘s PhD project, so there will be a strong project team focusing on this idea.

I should be clear that the idea isn’t to put an entire microscope in a needle. The needle only needs to contain an image relay, something to transfer an image from the end of the needle which will go into the tissue (the distal end), to the end which is outside of the tissue (the proximal end). We can then attach whatever bulky components we like to this outside end, such as cameras, lasers, and spectrometers. So, the challenge is to make a conduit which can fit inside a needle and transfer the image without loss of resolution.

One way of doing this is with a special type of long-thin lens called a GRIN lens, which can be made less than half a millimetre in diameter and a few centimetres long. The downside is that to maintain high resolution and efficient light capture, the maximum image diameter ends up being several times smaller than the diameter. In a common configuration, the image size is 1/5th of the diameter, meaning that for a half millimetre lens, the image is only one tenth of millimetre, or 100 microns. At that point the information we acquired from the tissue starts to become limited. And at a half millimetre diameter, the outer diameter of the needle (which also needs to include the metal wall) is still fairly large.

An alternative is to use a fibre bundle image guide. An image guide contains thousands of individual fibre optic elements, each of which can transmit a ‘pixel’ of information. Image guides were traditionally used to make endoscopes, although this approach is now being superseded by miniature ‘chip on tip’ cameras. For a microscope in a needle, a fibre guide with no lens on the tip would provide an image equal to its diameter (less a little bit for the outer coating on the fibre), but the resolution would be fairly poor.

The aim of this project is to find a way of improving the resolution of the fibre bundle. We have a good idea how to do it, by making use of the multiple ‘modes’ each fibre in the image guide can support (you can think of these as different paths the light takes through the fibre). Each mode can carry some information, meaning that each fibre in the image guide can carry more than one pixel of image data. Encoding and extracting this information is the tricky part; many different ways have been suggested, all with some drawbacks. Our approach is related to the idea of single pixel cameras. As we get things working, future posts will explain how it works in more detail, so stay tuned.

See also the project page on the Applied Optics Group website.