Posts with the tag: OCT
Optical microscopy can only penetrate a few hundred microns into thick tissue, a limit imposed by scattering. High resolution imaging requires single-scattering events, so when we have multiple-scattering from particles above and below the focal plane, the resolution and signal to noise ratio quickly degrade. The thicker the tissue (i.e. the deeper the plane of interest) the more the multiple-scattering events dominate over single scattering. Techniques such as optical coherence tomography (OCT) enhance the penetration depth by rejecting multiple-scattered light using what is effectively a time-of-flight measurement. This works because light that has been scattered multiple times will tend to have travelled further than light that has been scattered only once. However, even with this technique, the penetration depth seldom exceeds 1-2 mm, as some multiple-scattered photons will (by chance) have a time of flight close to that of the single scattered photons. As we try to go deeper, these events will begin to dominate again. Now, a group mainly from Korea University in Seoul have suggested an additional method of discriminating between single and multiple-scattered photons, using a technique they call “collective accumulation of single-scattered waves”.
Recently, there has been a lot of interest in the application of optical coherence tomography (OCT) to retinal surgery. While OCT is already established as a tool for diagnosis and pre-surgical planning, the idea of imaging during the surgery itself hasn’t found much traction. Initially, this was partly due to the lack of commercial OCT systems that were well-integrated with ophthalmic microscopes. This meant that the surgery had to be halted, the ophthalmic microscope removed, and the OCT slid into place every time an OCT image was wanted. More recently, Carl Zeiss and Haag-Steit have begun marketing devices where the OCT is integrated into the surgical microscope, so that both can be used simultaneously. The OCT images can then be displayed to the surgeon in the microscope view. However, the authors of a recent paper in Biomedical Optics Express claim that these integrated OCT systems are still not ideal. Instead, they propose an OCT scanner which is built into the surgical instrument itself.
Researchers at the University of Maryland have developed a low-cost fibre sensor for detecting blood flow. It has a diameter of just 125 microns, meaning it could be incorporated into a surgical instrument to alert the operator that they are approaching a blood vessel. It combines features of laser Doppler flowmetry and low coherence interferometry to provide a measurement with high spatial localisation, but without many of the complications of a full Doppler optical coherence tomography (OCT) system.
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.
I recently discussed several ways of generating a simple optical coherence tomography (OCT) cross-section by manually scanning a fibre optic probe. The idea is that we simplify the engineering of a miniaturised OCT system by dispensing with the need to incorporate scanning mirrors. It looks like this is a popular topic at the moment because another paper on the theme of handheld probes has just appeared in Biomedical Optics Express. This article takes things a little further, and tries to use motion compensation to improve the stability of the image. This unfortunately re-introduces some complexity into the system, but the authors think it could have still have applications for image guided surgical interventions.
Doppler Optical Coherence Tomography (Doppler OCT) allows us to measure blood flow on a microscopic scale. Sometimes we only want to distinguish between what is moving and what isn’t, perhaps so that we can distinguish blood vessels from surrounding tissue. On other occasions we might want to actually estimate the direction and velocity of the blood flow. Either way, Doppler OCT has rarely been used in vivo with endomicroscopes, partly because of the difficulties in using phase-based techniques outside the comfort of the optics lab. A recent paper in Biomedical Optics Express (an open access journal) has reported the use of a commercial OCT endomicroscope to obtain Doppler OCT images of intra-vascular blood flow in a pig. It shows that the phase stability problem can be overcome using simple subtraction techniques, paving the way for rapid clinical translation.
Optical Coherence Tomography (OCT) images are usually built up scanning a laser spot rapidly over the sample. For the common applications of eye and skin imaging there aren’t any particularly onerous size constraints on OCT systems, so bulky galvanometer mirrors can be used to generate the scan. But for endoscopic imaging we need to minimise the size of the mechanism. This has resulted in the development of a number of miniaturised scanning systems, mostly involving MEMS mirrors and fibre scanning cantilevers. One of the more esoteric solutions is the idea of free-hand scanning, where the operator builds up a scan simply by moving the probe manually. This reduces the hardware requirements to a minimum, but leads to the tricky problem of how to correctly assemble the image when we don’t know how the probe has been moved.
Optical Coherence Tomography (OCT) has a lot of advantages over confocal microscopy, especially for applications where it’s useful to have a large working distance between the probe and the tissue. But a big limitation is that it can only detect reflected light, and so can’t be used with fluorescent stains. Fluorescence is often preferred in conventional microscopy because it allows us to visualise structures that we can’t easily identify in reflectance images. So the race is on to find a way to make OCT work with fluorescent emission as well as reflected light.