Collimated ultrasound from an optical fibre

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Ultrasound is a safe and low-cost medical imaging tool which allows us to probe inside the human body without the risks associated with ionising radiation. The concept is simple: a transducer sends a pulse of ultrasound into the tissue, where it reflects and scatters off various structures and back to the transducer. The time taken for these echoes to return depends on the depth of the structure, and so timing the returning ultrasound allows the position of the scattering structures to be inferred. This provides what is known as an ‘A-scan’ – information along a single line going into the tissue. 2D or 3D images are then formed from multiple A-scans acquired as the ultrasonic beam is scanned in one or two directions.
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Why do animals have differently shaped pupils?

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Only the least observant could fail to notice that pupils come in striking different shapes. Not in humans, of course, where they are invariable circular, but in many other terrestrial animals where they are sometimes round, but also often elongated and slit-like. The orientation of these slits also varies; most predators, including your neighbourhood tabby, have vertical slits, while in prey animals they are more likely to be horizontal. This intrigued a team of optometrists and physicists from the University of California, Berkeley, and Durham University (UK), and so they set out to discover what drives these variations. And in a paper published in Science Advances1, they think they have the answer.
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  1. Banks, Martin S., et al. “Why do animal eyes have pupils of different shapes?.” Science advances 1.7 (2015): e1500391

Absorption spectroscopy in newborn baby lungs

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Any kind of non-invasive optical measurement of structures deep inside the human body is challenging, and absorption spectroscopy is no exception. But despite the apparent difficulties, a research team from Lund University want to use spectroscopy to measure the concentration of gases in the lungs of newborn babes. They particularly want to do this for premature babies, because they are often born with respiratory problems. At the present, their lungs are monitored with blood tests and x-rays, and there are obvious limits to how regularly these can be performed. If non-invasive optical sensing could give some indication of how well the newborns’ lungs are functioning, it could be an attractive way of providing continual monitoring during treatment.
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No benefit to structured illumination microscopy for scattering imaging

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Super-resolution microscopy has received a lot of interest in the past few years, culminating in the 2014 Nobel Prize in Chemistry for the development of the STED and STORM/PALM family of techniques. Around the same time, an interesting (and mischievously titled) commentary appeared in Nature Photonics, claiming to resolve (ho ho) a misconception about a third approach to super-resolution – SIM or ‘structured illumination microscopy’. This is a technique which can be used to improve the resolution by a factor of two. The paper argues that structured illumination microscopy only provides true resolution enhancement for fluorescence imaging and none at all for scattering imaging. This is despite recent papers making claims – and apparently providing experimental evidence – to the contrary.
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Imaging through multi-mode fibres using model-based calibration

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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.

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Deep Tissue Imaging by Collective Accumulation of Single-Scatterers

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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”.

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OCT Integrated into Robotic Opthalmic Forceps

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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.
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Optical Phase Conjugation is Linked to Speckle Decorrelation

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Optical imaging into thick tissue is generally limited to a penetration depth of only a few millimetres. Beyond this depth, almost every photon is scattered multiple times before reaching the detector, meaning that we no longer have much of an idea where it originated from. It’s possible to sidestep this limit if we can somehow differentiate photons that have come from a single point in the sample (for example by using a guide star or ultrasound to modulate the optical signal), and if we measure the wavefront that emerges from the sample. Then, using a technique called optical phase conjugation, a reversed wavefront can be sent back into the sample. Since this wavefront will undergo the opposite series of scattering events to the outcoming beam, it will be focused back to the original point. This ‘time-reversal’ technique can then allow us to image (by scanning the beam and collecting all the returning light or fluorescence) or to deliver various kinds of laser-therapy. However, the difficulty is that the scattering depends on the positions of all the scattering particles in the tissue; if there is movement then wavefront measurement becomes invalid. An international group of researchers have recently shown that the time-constant of the optical phase conjugation is linked to another property of scattering media – speckle correlation. The paper reports measurements of these time constants, giving an indication of how often the wavefront measurement would be needed.
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Fourier Ptychographic Microscopy

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Two of the key parameters that describe the performance of the optical microscope are its resolution and its field-of-view. In fact, these two parameters are coupled: switching to a higher magnification objective will improve the resolution, but also tend to reduce the field-of-view. This trade-off is encapsulated in the idea of the space-bandwidth product, which is (conceptually at least) a measure of how many useful pixels of information an imaging system can transmit. Typical microscopes and microscope objectives are limited to around 10 Megapixels; if we make these pixels smaller by increasing the resolution then the area covered must be reduced as well. So if we want to image large areas at high resolution, as we might want to do in histology for example, then we have to mechanically scan the slide underneath the microscope and stitch multiple images together.

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Imaging Through Scattering Media Using the Speckle Memory Effect

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A research team from the University of Twente has found a way to obtain high resolution images of a fluorescent object through a strongly scattering medium. This has been a goal of bioimaging scientists for some time, as it would allow us to use visible light to image much deeper into tissue. Various methods have been suggested, but they generally need some kind of ‘guide-star’ behind the scattering layer. The new approach uses the ‘memory effect’ of speckle to avoid the need for any calibration or guide-star, potentially making it much more applicable to real situations.

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