Endomicroscopy is a technique which allows us to obtain cellular level images of human tissue without the need for biopsy. As it’s a particular research interest of mine I will be adding a number of articles and blog posts related to the technology behind it:
- Introduction to Endomicroscopy : A description of the technology and its applications, suitable for the layman.
- Key Publications in Endomicroscopy : Links to a selection of important journal articles related to the development of endomicroscopy.
- Fibre Bundle Endomicroscopy : A discussion of how fibre bundle are used in endomicroscopy.
- Endomicroscopy Research Groups and Companies
Posts related to endomicroscopy are listed below.
In the last few years, several papers have looked at how it might be possible to use a multimode fibre as an ultra-narrow endoscope (see this post and this post for a bit of background). The most common approach is to use a spatial light modulator to shape the wavefront entering the fibre. If this is done in precisely the right way, interference between light coupled into the different modes of the fibre will result in a focused spot at the far end. By adjusting the input wavefront it’s then possible to scan the spot in two dimensions, allowing point-by-point imaging. Of course, we need to know what wavefronts to use, making it necessary to perform a calibration which requires access to the far end of the fibre. Unfortunately, this calibration is highly dependent on the configuration of the fibre – if the fibre is bent then the calibration changes. This means the technique is only applicable to rigid probes, greatly limiting the scope of potential applications. Now, in a paper published in Nature Photonics, Tomáš Čižmár and colleagues from the University of Dundee have suggested a possible solution to this problem.
A team from Massachusetts General Hospital (MGH) has developed a tethered endomicroscopy capsule, offering a potential alternative to endoscopic tissue biopsy. It uses optical coherence tomography (OCT) to generate high-resolution cross-sections through the walls of the oesophagus. Capsules are already in use for video endoscopy, and OCT has been shown to have value in the diagnosis of Barrett’s oesophagus, but this is the first time the two technologies have been combined.
When biopsies are reviewed by pathologists in histology, they are typically viewed in ‘cross-section’ – a slice is cut perpendicular to the surface of the tissue. In contrast, microscopy performed directly on the patient (in vivo or endomicroscopy) usually generates en face images – slices in the plane of the tissue surface. Some endomicroscopes designs allow vertical cross-sections to be generated from stacks of en face images at varying depths, but this is nowhere near to being a real time approach. A group from the University of Michigan has now developed a handheld endomicroscope which can generate vertical cross-sections with a frame rate of 10 Hz.
I recently discussed a paper by a group from Korea University,Seoul, who had developed a reflection-mode widefield endomicroscope using a multi-mode fibre. Other people have been thinking along the same lines, including a Swiss group who have recently published details of another multi-mode fibre based device. Unlike the widefield implementation, this method uses digital phase conjugation to scan a point of light at the far-end of the fibre, making fluorescence imaging possible. The basic idea isn’t new, but the group was able to demonstrate better resolution than previous reports through the use of a very high NA, double clad fibre.
Fibre bundle endomicroscopes usually operate in fluorescence mode: the tissue is stained with a fluorescent dye which, when illuminated at a certain wavelength, emits light at a longer wavelength. Collecting this fluorescent emission tends to produce clear, high contrast images, and also allows back-reflections from the fibre bundle to be removed using wavelength selective filters. Reflectance mode endomicroscopes, which create an image from the light back-scattered from the tissue, have been demonstrated several times, but have found little practical application. Now, Cha et al. from John Hopkins University have developed a dual-mode device that simultaneously collects both fluorescence and reflectance images. They have used this device to measure the efficacy of gene transfection – the deliberate insertion of genes into cancerous cells.
Most modern endoscopes use miniaturised cameras to capture macroscopic images, but this technology is not suitable for very high resolution endomicroscopy systems. The current generation of endomicroscopes either have a scanning head at the distal tip of the probe, or use a fibre bundle to relay the image out of the patient. The fibre bundle approach allows for the smallest diameter probes, but also has disadvantages, including a severe resolution/field of view trade-off. A recent paper in Physics Review Letters has suggested there might be another possibility. The authors managed to transmit an image through a single multimode optical fibre with a diameter of only 200 microns. They achieved a resolution of around 2 microns and a field of view equal to the fibre core diameter, opening up the prospect of an ultra-thin endomicroscope reaching parts of the body which are currently inaccessible.
The Bioengineering Department at Rice University in Texas has been developing fibre-bundle based widefield endomicroscopes for several years. While these devices lack the depth sectioning capabilities of confocal endomicroscopes, they can still produce useful images from certain tissues if a suitable topical fluorophore is applied. A recent paper from Richards-Kortum’s Group at Rice has demonstrated ‘real time’ mosaicing using their endomicroscope, allowing characterisation of much larger areas of tissue than would otherwise be possible.
Many diseases, including cancer, can be diagnosed by extracting tissue samples from the patient’s body and studying them under a microscope. This is called histopathology. An alternative technique called endomicroscopy has recently been developed. It allows tissue to be imaged at the microscopic level without removing it from the patient. This post introduces the topic of endomicroscopy – sometimes known as ‘confocal laser endomicroscopey’ or CLE – to the non-specialist.
I’ve recently co-authored a perspective called Robotics and Smart Instruments for Translating Endomicroscopy to In situ, In vivo Applications in the journal Computerized Medical Imaging and Graphics. The editorial discusses some of the challenges of in vivo confocal laser endomicroscopy (CLE), and how robotics technology may help to overcome them. This forms the basis for quite a lot of my current research so I would be interested to hear any comments or ideas people may have.
Microscopes are great tools – they make modern biology possible, and are essential for the diagnosis of many diseases. But they have one big disadvantage when it comes to medicine: they can’t be used directly on patients. Or at least they couldn’t, because now a new technology has emerged that allows us to perform ‘endomicroscopy’ – microscopy inside of the patient. This in vivo microscopy has lots of potential applications and is a pretty exciting area to be working in. The technology still has a few problems, but it’s shaping up to be one of the key new imaging techniques of the twenty-first century. And it has become practical because of the use of optical fibre bundles.