Endoscopy describes techniques allowing the visualisation of internal structures of a “sample” from the outside via an imaging channel that can reach those internal structures in a minimally invasive manner. In medical imaging, an endoscope can be a then flexible or rigid instrument that is introduced into the body via a natural orifice or through a surgically prepared opening. Imaging can be provided by a miniature camera at the “distal” end of the endoscope (i.e. inside the body) or by an image guide that conveys radiation from inside the body to an external camera at the “proximal” end of the instrument.
Optical endoscopes broadly fall into three categories: flexible video endoscopes, rigid optical endoscopes and flexible optical endoscopes. Video endoscopes are those that utilise a miniature CCD camera and associated image-forming optics at the distal end and the flexible section of the endoscope is essentially a cable conduit for the electronic signals and power etc. Video endoscopes are wide-field imaging instruments that are analogous to wide-field microscopes. Rigid optical endoscopes are typically constructed from a series of lenses enclosed in a rigid metal cylinder and these relay an optical image from the distal to proximal end. They are usually employed as wide-field microscopes with a CCD camera at the proximal end but they can be used in scanning microscope configurations and this approach has recently been adapted to multiphoton microscopy for intravital imaging where a “stick lens” made from GRIN lenses is employed.
Rigid endoscopes are typically used during surgical procedures. For internal diagnostic imaging it is usual to employ flexible endoscopes that can pass through internal pathways, cavities or vessels in live subjects. Video endoscopes are most commonly used for intensity imaging but for more sophisticated imaging modalities, such as spectrally resolved imaging or fluorescence lifetime imaging, confocal or multiphoton microscopy (to provide higher resolution and optical sectioning), it is necessary to use a flexible optical endoscope. Flexible optical endoscopes can be divided into wide-field optical endoscopes, microconfocal endoscopes and multiphoton endoscopes.
Wide-field flexible optical endoscopes typically utilise an optical fibre bundle to relay the optical image from the sample (distal) end to the detector (proximal) end, as illustrated in figure (a). These optical fibre bundles typically comprise ~30,000 optical fibres that each correspond to an image pixel. This is a small number of pixels compared to a typical CCD camera and so such optical endoscopes offer a smaller number of image resolution elements than video endoscopes or optical microscopes and consequently lower quality images. The spacing between individual fibre cores (and consequent fill-factor) also impacts the efficiency of light collection and the image quality.
Endoscopes can also be used to provide optically sectioned images analogous to laser scanning confocal microscopes. Confocal endomicroscopes either utilise a proximal scanner with an imaging fibre bundle (b) or a distal miniature scanner with a single optical fibre (c) to convey the light from the sample to the (proximal) detector. In the former case, the fibre bundle can be an array of single mode fibre “cores” that are fabricated together to form a “coherent” bundle.
We are developing new technology for endoscopy, particularly focussing on the implementation of advanced fluorescence imaging techniques translated from microscopy, including wide-field FLIM endoscopy, laser scanning confocal endomicroscopy and a novel concept for ultrathin multiphoton endoscopy utilising adaptive optics.