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Medical physicists pioneer virtual meeting

In any normal year, the American Association for Physicists in Medicine (AAPM) would be preparing to welcome around 4000 attendees to its annual meeting and exhibit, which in 2020 was due to take place in Vancouver, Canada, as a joint meeting with the Canadian Organization for Medical Physicists (COMP). But this year will be different: the bustle of the traditional conference venue will be replaced by a virtual meeting platform that will allow delegates to attend scientific presentations, take part in live Q&A sessions, and interact with leading equipment vendors in an online exhibit hall.

The scientific programme for the Joint AAPM | COMP Virtual Meeting will feature six tracks of specially curated presentations, along with a series of interactive online ePosters. Delegates can opt to attend the live presentations, which will each be followed by an interactive Q&A, or catch up a via an on-demand service that will be available for six weeks after the event.

The virtual meeting platform will also host networking and social events, such as an online fitness challenge that will enable delegates to earn points for completing a daily 30-minute activity. Meanwhile, the online exhibit hall will be open for delegates to browse company and product information, view video presentations, and schedule meetings with almost 60 exhibitors. A vendor-focused seventh track of presentations offers delegates a deeper dive into specific products and services, with experts available to answer questions immediately after their talks. As a taster, some of the latest innovations from the vendor community are highlighted below.

Robotic radiosurgery system delivers precision, speed and motion synchronization

The CyberKnife S7 System from Accuray is the first robotic and fully automatic system for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT), enabling clinicians to deliver personalized treatments to more patients. The system delivers non-surgical stereotactic treatments in any part of the body with sub-millimetre precision, with powerful motion tracking capabilities and adaptive treatment planning to enable high doses of radiation to be targeted directly at the tumour.

The CyberKnife system offers real-time motion synchronization to automatically adapt the treatment in response to any movement of the target or patient. It exploits Accuray’s proven Synchrony technology, which uses artificial intelligence to continuously synchronize the radiation beam to the target location.

Such real-time adaptation allows clinicians to escalate the radiation dose delivered to the tumour while minimizing the exposure of healthy tissue, and also enables more precise and effective hypofractionation treatments. The fully automated system ensures there is no impact on standard workflows or delivery speed, while also avoiding unnecessary manual tasks and reducing the potential for error.

The CyberKnife S7 is the first SRS/SBRT system to have a linear accelerator mounted on a robotic arm, which provides flexibility and freedom of motion for delivering complex beam profiles to any part of the body. An integrated stereoscopic X-ray system provides continual image guidance and real-time tracking information to maintain accuracy and precision during treatment.

The fully automated system is designed for operational efficiency, providing personalized treatment planning while also maintaining patient throughput. Treatment plans can be optimized in as little as 60 seconds, in some cases enabling same-day planning and treatment delivery, while hypofractionated treatments can be delivered in just 15 minutes.

More information about the CyberKnife S7 System can be found on the Accuray website.

Visit Accuray in the virtual exhibit hall.

Seeing is believing: a new way to visualize radiotherapy

The BeamSite video imaging system from start-up company DoseOptics enables clinicians to visualize in real time how the radiation beam is being delivered to the patient. The system captures video-rate images of both the entry and exit beams during treatment, and also provides record and playback capabilities for sharing observations and investigating any anomalies with the clinical team.

The BeamSite system offers direct video imaging for most common radiotherapy modalities, as well as total skin electron therapy. Such real-time visualization is particularly useful for monitoring any stray radiation during treatment, checking that the patient remains in the correct position while the beam is being delivered, and identifying and understanding any errors or near misses.

The system works by detecting the faint Cherenkov radiation that is emitted by human tissue when it is irradiated with high-energy electrons or photons. Unique time-gating technology ensures that each pulse of radiation delivered by the linac contributes to the image recovered, and time-integrating software accumulates the Cherenkov emission to create an image that overlaid in real time on the area being irradiated. The camera and software operate remotely to provide an independent check and measurement tool for beam shape and delivery.

The team from DoseOptics will be presenting several talks and posters during the virtual AAPM meeting to explain the BeamSite system in more detail. They will be:

Talk: Treatment Verification from Cherenkov Imaging During Radiation Therapy Sunday 12 July, 14.00 ET

Talk: First Imaging of Intrinsic Light Emission from Biological Tissue Visualized Proton Pencil Beam Scanning Monday 13 July, 15.30 ET

ePoster: Evaluating the Clinical Utility of Cherenkov Imaging in Radiotherapy

Visit DoseOptics in the virtual exhibit hall.

The BeamSite system enables treatment verification during radiation therapy. (Courtesy: DoseOptics)

Optimized DCAT planning delivers faster treatments

A recent study by medical physicists at the UT Health San Antonio Cancer Center reveals that patients with simple lung or liver lesions could be treated more efficiently – and just as effectively – with optimized dynamic conformal arc therapy (DCAT). While traditional DCAT generally does not achieve the same plan quality as volumetric modulated arc therapy (VMAT), optimizing the DCAT delivery by varying the dose rate and gantry speed delivers highly conformal dose distributions.

“At our institution, this would mean that 40–60% of lung and liver patients who would normally be treated using VMAT could be treated more efficiently using optimized DCAT, while maintaining the same plan quality,” conclude Sotiri Stathakis and Niko Papanikolaou. “These patients could benefit from significantly shorter treatment times, which are easier to tolerate and reduce the risk of intrafraction movement.”

Stathakis and Papanikolaou exploited the Monaco treatment planning system from Elekta, which offers a variable dose-rate feature for DCAT delivery. It also includes segment shape optimization for DCAT, which optimizes the beam weights and shapes to improve conformality and prevent damage to healthy organs.

Comparing the plans produced for 19 patients using both VMAT and optimized DCAT revealed that both techniques achieved the same plan quality when the targets were located away from other critical organs. But DCAT could be delivered 2.5 times faster than VMAT – which could be particularly beneficial for treatments exploiting the deep-inspiration breath-hold technique.

“Based on the results of this study, lung and liver patients with simple, spherical lesions that are not close to organs-at-risk are ideal candidates for optimized DCAT,” conclude Stathakis and Papanikolaou. “The potential for application of optimized DCAT to other treatment sites, such as pancreas, brain and prostate, is of great interest and the subject for future investigation.”

Read more about the study in a white paper available from Elekta.

Visit Elekta in the virtual exhibit hall.

Optimized DCAT (top) can deliver a similar treatment plans to VMAT for lesions on the liver, as shown in this example, and the lung. (Courtesy: Elekta)

Adaptive radiotherapy delivers intelligent treatments

The new Ethos™ therapy system from Varian enables clinicians to adapt radiotherapy treatments to daily changes in the patient’s anatomy. It has been designed to help deliver treatments that better target the tumour, reduce the dose to healthy tissue, and achieve the clinical objectives for each treatment plan.

Ethos incorporates artificial intelligence (AI) to increase the capability, flexibility and efficiency of radiotherapy. It has been designed to allow physicians to assess and adapt treatment plans daily, enabling them to deliver more personalized cancer care and offering the potential for improved patient outcomes.

Ethos therapy integrates Iterative cone-beam CT and multimodality images on the treatment console, providing clinicians with an up-to-date view of the patient’s anatomy to help make better informed treatment decisions. The streamlined workflow of Ethos therapy is enabled by its AI-driven planning and contouring capabilities, making it possible to better visualize daily changes and enabling physicians to make any adjustments to the treatment within minutes.

Find out more by visiting varian.com/ethos and downloading the Ethos therapy brochure.

Visit Varian in the virtual exhibit hall.

The Ethos radiotherapy system from Varian offers the flexibility to adapt treatment plans on a daily basis. (Courtesy: Varian)

Optimization technique enables more effective treatment planning

RaySearch Laboratories explains in a new white paper how multi-criteria optimization (MCO) helps clinicians select the optimal treatment plan for their patients through a streamlined and intuitive workflow. The company’s RayStation treatment planning system supports MCO for intensity-modulated radiotherapy, as well as volumetric modulated arc therapy, tomotherapy and proton pencil-beam scanning.

Such MCO techniques start with a series of ideal clinical objectives, such as delivering a uniform dose to the target volume and zero dose to critical organs, from which a series of possible plans are generated. Clinicians can explore these different plans through RayStation’s interactive interface, which exploits slider controls to alter the dose distribution and examine in real time the impact on the defined clinical goals.

As well as such manual navigation, the module also offers an automatic option that optimizes the treatment plan based on a prioritized list of clinical objectives. The dose distribution selected through the chosen navigation process can be converted to machine parameters through a dose mimicking optimization that minimizes discrepancies between the navigated dose and the deliverable plan.

A number of research studies have shown how MCO can speed up the treatment planning process, while also delivering higher quality plans than standard inverse planning. MCO has also been shown to enable novice dosimetrists to create treatment plans of comparable quality to more experienced planners using traditional techniques, while a proof-of-concept suggests that MCO could enable treatment plans to be created in a single meeting between a physician and a planner to save time and improve clinical decision making.

Read the full white paper on the RaySearch website.

Visit RaySearch in the virtual exhibit hall.

In this example, the MCO module in the RayStation treatment planning system was used to reduce the dose delivered to the right parotid for a head-and-neck patient. (Courtesy: RaySearch)

Autocontouring delivers equivalent results in half the time

DLC Expert is an automatic contouring system from Mirada Medical that exploits artificial intelligence and deep learning to delineate organs-at-risk and other anatomical structures, which is an important but time-consuming element of radiotherapy treatment planning. An expert assessment by oncologists at MAASTRO Clinic in the Netherlands suggests that the time needed for thoracic contouring can be reduced from 20 minutes when done manually using existing clinical routine to just 10 minutes with the deep-learning approach, while validation results available from Mirada show that DLCExpert produces OAR contours of a similar quality to those drawn by professional clinicians.

DLCExpert exploits Mirada’s Zero-Click Contouring platform, which uses background processing to deliver contours before planning gets underway. Contours can be validated using any treatment planning software, or the RTx imaging workstation available from Mirada.

DLCExpert supports all major anatomical sites including breast, lung, head and neck, and prostate. To test out its capabilities, visit www.autocontouring.com to take Mirada’s modified version of the Turing test to try to identify which contours are drawn by radiation oncology professionals and which have been automatically drawn by Mirada software.

More information about the deep-learning techniques used in DLCExpert can be found in a technical white paper from Mirada Medical.

Visit Mirada Medical in the virtual exhibit hall.

Deep learning contouring — The contours produced by deep learning (yellow) compared with those manually drawn (red) and an atlas-based contour (blue) for the lung (left), oesophagus (middle) and heart. (Courtesy: T Lustberg *et al Radiotherapy and Oncology* 10.1016/j.radonc.2017.11.012 ©2018, with permission from Elsevier)

Imaging guides proton therapy

Tony Lomax, Chief Medical Physicist at the Paul Scherrer Institute in Villingen, Switzerland, offers an insight into the different imaging modalities that are needed for proton therapy in a new white paper for Siemens Healthineers. Most important for making initial clinical decisions are the offline imaging techniques used to diagnose the disease, which determines whether the patient will be treated with protons or conventional radiotherapy, and to map out the extent of the tumour and any critical structures and organs nearby.

Such anatomical imaging is generally achieved with computed tomography and magnetic resonance imaging, which offer excellent spatial resolution and good anatomical contrast. MR imaging is also used to image the activity and function of the tumour, often in combination with positron-electron tomography (PET). Lomax points out that it can be difficult to match these two datasets, since PET has a lower spatial resolution and does not provide anatomical information, which means that imaging systems such as PET-CT and PET-MRI are now emerging to improve the precision with which the tumour can be defined.

For proton therapy it is particularly important to understand how the tumour or surrounding organs might move during treatment, which requires time-resolved images to be captured before treatment gets underway. By far the most popular is 4D CT, although major artifacts in the reconstructed data can be caused by any variability in the patient’s breathing. Interest is therefore growing in other techniques, such as 4D MRI, that can reduce these motion artifacts and also capture data over longer periods of time.

Lomax also reviews the options for online imaging during proton therapy. Commercial proton therapy machines are now equipped with cone-beam CT (CBCT) systems for in-room 3D imaging, which is widely used to ensure that the patient is positioned in the same way for each treatment. But some proton centres have opted instead for in-room CT systems that provide the same diagnostic quality as pre-treatment CT, and which also offer useful information for adapting the treatment in response to any anatomical changes, such as the size and shape of the tumour.

While MR-guided systems are now emerging for real-time imaging in radiotherapy, Lomax points out that integrating MR with a proton machine presents several practical challenges that have yet to be overcome. Online 4D imaging is also not yet available for proton therapy, which means that real-time motion tracking must instead rely on time-resolved 1D or 2D imaging. Lomax highlights two recent studies in which such 2D surrogates based on X-ray fluoroscopy or ultrasound monitoring have been used to reconstruct 3D movements using motion models derived from 4D MRI imaging.

Read the full white paper by Tony Lomax on the Siemens Healthineers website.

Visit Siemens Healthineers in the virtual exhibit hall.

The Paul Scherrer Institute in Villingen, Switzerland, is a leading centre for proton therapy (Courtesy: Paul Scherrer Institute)

Beetle-inspired film reflects 95% of solar radiation

A new flexible material for passive cooling that was inspired by a volcano dwelling beetle has been developed by scientists in China, the US and Sweden. The film reflects around 95% of solar irradiance, and can reduce the surface temperature of objects by around 5 °C. It could be used to cool everything from buildings to electronics, the researchers say.

There are some 30,000 species of longhorn beetle, often characterized by having antennae that are much longer than the insect’s body. In southeast Asia one bright golden species, Neocerambyx gigas, is often found living on the slopes of active volcanoes, particularly on the Indonesian islands of Java and Sumatra. In these extreme environments summer temperatures often top 40 °C, while ground temperatures can exceed 70 °C. When it gets really hot, the beetles stop moving and foraging to help shed excess heat and reduce heat absorption.

Han Zhou, a materials scientist at the Shanghai Jiao Tong University in China, told Physics World that this ability to survive in extreme climates and the beetles “very brilliant appearance” attracted her team’s interest. Zhou and colleagues in Shanghai, the University of Texas at Austin and Sweden’s KTH Royal Institute of Technology wondered if the beetle had microstructures that increased its light reflectance to help it regulate its body temperature.

Keeping cool

“We measured their temperature under the light irradiation, and we found that this beetle can lower their surface temperature by 1.5 °C in air and by about 3 °C in vacuum, Zhou explains. “We also measured their optical properties. We found that these beetles have very high optical reflectivity in the visible and the near-infrared light region.”

Using scanning electron microscopy, they discovered that the hardened forewing – called the elytra – of these longhorn beetles is covered in tiny fluffs. These hair like-structures are triangular in cross section and reach densities of around 25,000 per square centimetre.

The researchers found that these forewings reflect 65% of solar irradiation in the visible to near-infrared range, but only 30% if the triangular fluffs are removed, they report in the Proceedings of the National Academy of Sciences.

Pleated fluff

In cross section, the fluffs have two smooth sides and one corrugated side with frills, the scanning electron microscopy revealed. The pleats created by the corrugation and frills are around 1 micron in width and 0.18 micron in height. The team modelled how light interacts with the fluffs and this has shed light on the origins of the forewings’ optical properties.

The study reveals that the pleats create a strong scattering effect that increases reflectivity, no matter the incident angle of the light. The corrugated edge also adjusts the angle of transmitted light, thereby increasing the chance that it experiences total internal reflection inside the fluffs. And light that passes through the smooth sides at most angles also experiences total internal reflection when it hits the inside of the corrugated edge.

“We used this beetle as a model to synthesize some bio-inspired materials,” Zhou explains. They developed a polymer film with triangular surface undulations to mimic the reflective properties of the fluff-covered forewings. To recreate the performance of the fluffs’ pleats they embedded spherical aluminium oxide ceramic particles into the polymer. These are ideal, the team says, because they exhibit a strong light scattering affect and have negligible absorption of visible to near-infrared light, so generate very little heat under direct sunlight.

Pyramid power

Experiments showed that pyramid structures on the surface of the film produced the highest reflectance, compared with cones and prisms. The final film was 500 micron thick, embedded with randomly distributed 2 micron aluminium oxide particles, and covered in an array of 8 micron wide and 5.7 micron high pyramids.

The film has a reflectance of around 95% and real-world tests highlighted its cooling ability. A mobile phone case made from the material reduce the temperature of the device by as much as 4.5 °C, compared to a case without the film. And on a sunny day, sheets of the material on the bonnet of a car reduced the surface temperature by an average of almost 4 °C and a maximum of more than 7 °C, compared with patches of white paper of a similar size and thickness. The researchers say that this demonstrates that the flexible film is a promising passive cooling material for electronic devices and vehicles.

Zhou believes that the material could have many uses, from cooling fabrics to helping cool buildings. Next, she says, they would like to work out how to manufacture the film on much larger scales, and they are looking to see it can doped with other materials to give it new properties, such as high conductivity.

Near infrared fluorescence imaging provides early diagnosis of cracked teeth

Tooth cracks that are not detected in X-ray CT scans (left) can be seen using indocyanine green near‐infrared fluorescence (ICG-NIRF) imaging. (Courtesy: Jian Xu and Zhongqiang Li)

Cracked teeth can be identified in their early stages using near infrared fluorescence (NIRF) imaging, researchers in the US have demonstrated. The approach – which can distinguish between different types of crack and reveal their depth – is more reliable than existing modalities and may help better diagnose the source of otherwise inexplicable toothache.

Cracked teeth are a common condition and, thanks to their potential to allow bacteria across the enamel–dentin junction, also the third highest cause of tooth loss. Despite this, the condition is often overlooked in its early stages. “Cracked teeth can be difficult to diagnose clinically as patients’ symptoms often aren’t reproducible and cracks can be barely visible to the naked eye,” explains oral surgeon James Allison of Newcastle University, who was not involved in the present study.

At present, there is no dependable clinical method for detecting the presence of cracks in tooth enamel. Visual and surgical-microscope-aided inspection is an unreliable approach, and dye-staining is time consuming and cannot reveal cracks beneath the surface of teeth. Common imaging modalities like X-ray and cone-beam CT, meanwhile, do not offer a high enough resolution. MicroCT scanning, with its higher resolution, can reveal larger cracks – but is only viable on extracted teeth, rendering it useless in a clinical setting.

Conventional near-infrared imaging – in which light is passed through dental structures and scattered to produce image contrast – has, like X-ray imaging, been shown capable of detecting some cracks in teeth. But it cannot distinguish between crack types or provide further information on the extent of the damage.

In their study, engineer Jian Xu of the Louisiana State University and colleagues have instead turned to NIRF – in which image contrast is generated by the differential accumulation within teeth of a fluorescent dye (here indocyanine green) which is excited by infrared light. To demonstrate the potential of the technique, the researchers compared the images of 16 extracted cracked teeth produced by NIRF with both those from near-infrared transillumination and X-ray imaging.

They found that the fluorescence approach was consistently able to reveal cracks in enamel that were not visible in the X-ray images – and was able to highlight more cracks than conventional near infrared imaging. They also report that an angled exposure gave better image contrast, as it created shadows under each crack. From these, one can determine crack depth and whether the crack is in enamel alone or has also reached the dentin.

Furthermore, the team noted that cracks could be revealed by immersing teeth in the fluorescence agent for only one minute – although longer periods produced clearer images. In practice, the dye could be applied to patient teeth via a mouthwash. In fact, indocyanine green has the benefit of being entirely safe to swallow – although has been known to cause allergic reactions.

“We use indocyanine-green-assisted near-infrared imaging to address the major drawback of the current state-of-the-art dental imaging: failure to detect some critical dental diseases, for example, early stage cracks and caries,” Xu tells Physics World. Alongside this, he adds, the new technique does not rely on the use of bulky imaging sensors and avoids the ionizing radiation-based health risks associated with X-ray techniques.

“The idea of checking all teeth for cracks would be unlikely to be cost effective as a health intervention,” notes dental radiologist Keith Horner of University Dental Hospital Manchester. Enamel cracks are not normally treated, he explains, but the approach could be useful for diagnosing patients with toothache where decay or restoration is not an obvious cause of the pain.

The research is described in the Annals of the New York Academy of Sciences.

Probing the foundations of quantum physics

Manipulating atoms into quantum entangled states is hard enough, but then proving that you have achieved that entanglement is harder still. But that is the goal of an experiment at the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna, as explained in this interview with physicist Michael Keller – recorded before the COVID-19 pandemic.

Keller’s team is seeking to create momentum-entangled particles by cooling helium atoms into a Bose-Einstein condensate and “kicking” them with lasers. As observing entangled states directly tends to destroy the entanglement, the researchers have devised indirect methods for study the helium atoms’ properties. The group’s long-term goal is to carry out tests on these momentum-entangled states to probe the foundations of quantum physics.

David Leckrone on his 33 years with the Hubble Space Telescope mission, Twitter poster conference takes off

Launched three decades ago, the Hubble Space Telescope is a triumph of the modern scientific era. In this episode of the Physics World Weekly podcast we hear from David Leckrone, who began working on Hubble before it launched and was the mission’s senior project scientist in 1992–2009.

Leckrone, whose new book Life with Hubble is an insider’s view of the Hubble mission, talks about the shock of discovering that the telescope’s optics were blighted by spherical aberration and how scientists and engineers banded together to work-out a solution. He describes how the Space Shuttle programme played a crucial role in maintaining Hubble and he also pays tribute to the people who made the mission such a success.

The COVID-19 pandemic has changed the very nature of the scientific conference and physicists have responded by using various online platforms to keep the lines of communication open. IOP Publishing’s Tim Smith talks about a Twitter poster conference that he is helping to organize for 15–16 July and how you can get involved – and even win a prize for best poster.

In a separate audio clip on the Physics World blog, David Leckrone talks about some of the outstanding science that has been done by the Hubble Space telescope over the past 30 years. His highlights include the 1994 impact of fragments of the comet Shoemaker-Levy 9 with Jupiter, and the landmark 2001 measurement of the Hubble constant by Wendy Freedman and colleagues.

David Leckrone’s scientific highlights from three decades of the Hubble Space Telescope

David Leckrone's Hubble science highlights

Recently, I had a fantastic conversation with David Leckrone, an astrophysicist who spent 33 years working on NASA’s Hubble Space Telescope. Much of that interview is featured in today’s episode of the Physics World Weekly podcast, where Leckrone explains what it was like when he and his colleagues realized that the telescope suffered from spherical aberration –and how they saved the mission from disaster.

Unfortunately, the interview is too long for all of it to be included in the podcast, so I have decided to present Leckrone’s scientific highlights in a standalone clip. Enjoy this insider’s view on the comet Shoemaker-Levy 9 impact of Jupiter and other Hubble triumphs of discovery.

Leckrone is author of Life with Hubble.

Why insulated metals cool down faster than their bare counterparts

A phenomenological model devised by scientists at the University of Twente in the Netherlands sheds new light on the behaviour of metals cooled by liquid nitrogen – and specifically the somewhat counter-intuitive classical observation that insulated metals cool down faster than their bare counterparts (Cryogenics).

“Our model and the related understanding of the low-temperature physics are novel,” says Srinivas Vanapalli, head of Twente’s Applied Thermal Sciences Laboratory. In addition to the fundamental insights, Vanapalli and his PhD student Sahil Jagga reckon that their findings will make it possible to design faster and more efficient cryogenic cooling systems.

A significant barrier to rapid cool-down in cryogenic systems is the evolution of a vapour film between the liquid-nitrogen coolant and the stainless-steel tubing that connects the cryogen bath to the cryogen storage tank. This phenomenon – known as the Leidenfrost effect – results in a low heat transfer rate and inefficient usage of precious coolant.

Now, Vanapalli and Jagga say they have come up with a workaround that addresses this problem while simultaneously optimizing the associated workflow. Their data-driven model is the result of a systematic set of quenching experiments on copper cylinders coated with different thicknesses of low-conductive epoxy. During these experiments, the researchers evaluated the heat flux (W/m²) and cool-down time of cylinders in both saturated – that is, just about to boil – and subcooled liquid nitrogen.

The researchers found that an early transition between two liquid-nitrogen boiling regimes – from film boiling (as seen with the Leidenfrost effect) to the so-called nucleate boiling regime – is a major driver of enhanced cooling, as it promotes the formation of cold spots between the liquid and insulating layer. “In terms of the underlying physics,” explains Vanapalli, “coating the tube with an insulating material increases the liquid–solid contacts at the surface to yield a more efficient cooling process.”

The utility of the University of Twente model is that it predicts the right coating thickness of the cylinders, since coatings that are too thin or too thick will slow the cooling rate. According to Vanapalli, theirs are the first results to show that the optimum coating thickness depends not just on the thermal properties of the coating material, but also on the thermodynamic state of the liquid nitrogen – in other words, whether it’s saturated or sub-cooled.

Previous studies elsewhere have sought to evaluate the effect of insulating coating thickness at the minimum film-boiling temperature – the point where the vapour film between liquid and solid surface collapses – but the microscale mechanisms in play have proved frustratingly elusive. The Twente researchers also note that, until now, scientists seeking to optimize the thickness of the insulation coating in a way that minimizes cool-down time relied exclusively on empirical investigation – essentially trial and error.

Cold science, hot stuff

More widely, Vanapalli notes that there are commercial drivers in play, since liquid-nitrogen cooling finds diverse applications within research and industry. These applications include cryopreservation of biological samples in “biobank” repositories; studies of high-temperature superconductivity; whole-body cryotherapy in professional sports; and quenching of tool-steels for improved hardness and durability, to name just a few.

Study of liquid-nitrogen droplets gives new insight into cryogenic spray-cooling

With this in mind, Vanapalli and Jagga plan to scale their new model to cover a range of coating materials, as well as other cryogenic liquids such as argon, hydrogen and helium. Two main inputs will be needed: the thermal properties of the coating material and critical heat flux data for specific pairings of metal surfaces and cryogenic liquids. “As a next step, we will acquire experimental data with argon and hope that the cryogenics community will contribute for other fluid–surface pairings,” Vanapalli says. “Ultimately, the aim is to translate this effort into an open-access web tool to benefit cryogenic system designers.”

The research is sponsored by Dutch funding agency NWO-TTW within the CryoOn project.

Matrix approach removes distortions in ultrasound images

Ultrasound imaging uses the reflection of waves to image soft tissues within the body. However, soft tissues have inherent inhomogeneities that can distort the incident and reflected propagating waves. Consequently, wave speed fluctuations and multiple scattering events lead to inevitable blur in the resulting images. To resolve this problem of image quality degradation, researchers have employed the adaptive focusing approach, which can compensate for image distortions caused by uneven interfaces or tissue inhomogeneities.

Conventional adaptive focusing methods are based on a dominant scatterer – an artificial reference point from which the signal to be optimized is reflected. However, it is not always straightforward to generate a reference point that accounts for all the imperfections of a heterogeneous medium. In addition, conventional adaptive focusing methods that correct for aberrations to optimize the image quality are often based on the assumption that these aberrations do not change over the entire field-of-view. In reality, the extent of image distortions can vary as a function of imaging depth inside a biological tissue.

To overcome these limits for ultrasound imaging, several acoustic imaging groups have developed methods to estimate and compensate for the time delays of emitted and reflected signals (as sound waves travel through different tissue depths) and mapped the speed-of-sound distribution to enable reconstruction of an image. But a more general approach that could be extended to any type of wave was not previously available. Now, researchers at the Institut Langevin (ESPCI Paris, PSL University, CNRS), led by Alexandre Aubry, have developed a universal matrix approach for wave imaging, using a multi-sensor network.

In collaboration with SuperSonic Imagine, the researchers published their imaging method – a reflection matrix approach for quantitative imaging of scattering media – in Physical Review X in early June. A week later, they published the experimental proof-of-principle of their theoretical results in PNAS. In this latter study, the researchers showed that their mathematical formalism can correct for aberrations that are inevitably present in in vivo images of the human body.

CNRS and SuperSonic Imagine researchers (left to right): William Lambert, Laura Cobus, Thomas Frappart, Mathias Fink and Alexandre Aubry. (Courtesy: Alexandre Aubry)

A novel approach

The novelty of the team’s work is that, unlike adaptive focusing methods, the reflection matrix approach is not limited to a single isoplanatic patch (area over which aberrations are spatially invariant). Instead, their approach retrieves the transmission matrix which connects any point inside the tissue with an array of external ultrasound sensors, to tackle more rigorously the challenges induced by random scattering media in medical ultrasound applications.

Enhanced image resolution — Application of reflection matrix approach in imaging of the liver enhances the image resolution. (Courtesy: Alexandre Aubry)

This new technique records the amplitude and phase of the tissue response, i.e., the ultrasound signal reflected from the tissue. Then, by using a position-dependent matrix, elements corresponding to the angle of rotation of the imaging plane and signals emitted/reflected by each transducer in the array can be accessed to link each sensor and image voxel.

As a random distribution of unresolved scatterers makes adaptive focusing more challenging, the reflection matrix approach considers the reflectivity of the medium to be continuous and random. Thus, the reflection matrix can be projected to each isoplanatic patch separately and therefore distinguish and resolve aberrations in multiple fields-of-view.

Finally, the team applied their aberration correction technique to ultrasonic data recorded from the calf of a healthy human volunteer. This in vivo imaging experiment confirmed that the distortion matrix approach can be extended to cases in which the speed-of-sound distribution in the medium is unknown, as the matrix can correct for complex position-dependent distortions.

The researchers tested and confirmed their theoretical developments using one particular type of wave. However, they believe that the potential of this work goes beyond ultrasound imaging as it can be applied to all fields of wave physics. Aubry notes that his group’s work on broadening the application of the distortion matrix approach to optical microscopy will shortly be published in Science Advances.

White papers: WITec, Ocean Insight and Photonic Science & Engineering

Innovation in action: (left to right) WITec’s UHTS300 Raman spectrometer system (Courtesy: WITec); Ocean Insight’s spectral tools are used in a wide range of areas including food processing (Courtesy: Ocean Insight); Photonic Science and Engineering’s Laue tool used to study sapphire (Courtesy: Photonic Science and Engineering).

This time we are featuring white papers from WITec, Ocean Insight, and Photonic Science and Engineering.

Studying semiconductors

Raman spectroscopy is a long-established, non-destructive technique for studying solids, liquids and gases that involves measuring how light scatters inelastically off the component molecules. In Correlative Raman Imaging of Semiconducting Materials – the latest application note from WITec – you can find out how confocal Raman microscopy, which combines Raman spectroscopy with confocal microscopy, is a great way for characterizing semiconducting materials. The document shows how it can reveal the chemical composition of a sample, identify possible contaminants and even visualize strain fields in 3D volumes. Confocal Raman microscopy can even be combined with atomic-force microscopy or scanning-electron microscopy to allow semiconductors to be comprehensively investigated.

Innovative insights

Navigating today’s industrial landscape is an ongoing challenge for manufacturers, with dynamic regulatory issues, emerging technologies and global competition placing a premium on innovation and creativity. Manufacturers are therefore constantly looking for better ways to manage their supply chains, reduce waste and off-specification product, and increase production yields, which is where spectrometers, cameras and multispectral sensors come in. In its latest white paper Unlocking the Unknowns of Industrial Sensing, Ocean Insight shows how it offers robust, scalable sensing tools to help users unlock the unknowns of industrial applications especially in industrial sensing – from measuring colour and spotting defects to testing samples and sorting and grading products. Machine learning and custom algorithm development capabilities add another layer of insight, the paper adds.

Cleverly orientated

Laue diffraction is a powerful method for investigating crystal orientation. UK firm Photonic Science and Engineering has now developed a special Laue tool that fires an intense, polychromatic “white” beam onto a single crystal material positioned behind a very-low-noise CMOS detector. A back-reflection geometry records as many Bragg reflections as possible within a single exposure, delivering a “misorientation” value against the primary reference axis. In Laue Crystal Orientation System used in Micro-Diffraction Experiment you can find out how this technique has been used at the European Synchrotron Radiation Facility in Grenoble, France, and elsewhere, to study semiconductor detectors, sapphire substrates, laser or gamma scintillation materials, ceramics and metal alloys.

Nanolubricant makes household fridges more efficient

Adding multi-walled carbon nanotubes (MWCNTs) to a common refrigerant can reduce a domestic refrigerator’s power consumption by nearly 30%, according to researchers at the University of Johannesburg in South Africa. Such a huge energy saving would be particularly welcome in low-income countries where electricity blackouts are frequent and refrigerators often have to run off expensive generators.

A conventional refrigerator works by pumping a refrigerant around a closed system. The refrigerant expands from a liquid to a gas, and then contracts to become a liquid again – a process that requires electrical energy, which is acquired by cooling the surroundings on the cold side of the system. This heat is then released on the hot side when the gas condenses back into a liquid.

Many domestic, and indeed industrial, refrigerators use a chemical called R134a as their refrigerant. Also known as tetrafluoroethene, R134a has the chemical formula CF₃CH₂F and belongs to the hydrofluorocarbon family of refrigerants. Although it is non-flammable and boasts a low ozone depleting potential (ODP) compared to chlorofluorocarbons and hydrochlorofluorocarbons (both of which were phased out in the 1990s), it does have a high global warming potential (GWP). For this reason, other refrigerants are being sought to replace it.

Replacing R134a with a R600a/MWCNT mix

In this work, researchers led by Daniel Madyira studied an alternative refrigerant called R600a, or isobutane. This chemical has a low GWP as well as a low ODP and is starting to replace R134a in new refrigeration units, especially domestic and light commercial systems such as fridges, freezers and drink dispensers.

To improve the cooling efficiency of their R600a, the researchers dosed it with 0.4 g/L to 0.6 g/L MWCNTs in a mineral oil lubricant. They then removed R134a and its compressor oil from a household refrigerator (which was manufactured to work with 100 g of R134a) and fed their new R600a/MWCNT mix into the inlet of the fridge’s compressor.

Performance tests by the University of Johannesburg team revealed that the fridge cooled faster when running on R600a/MWCNTs and had a reduced evaporation temperature of –11 °C after 150 minutes. “This is lower than the –8 °C for R134a,” Madyira explains. “It also exceeds the ISO 8187 standard, which calls for –3 °C at 180 minutes.”

29% less energy consumption

The Johannesburg team found that their modified appliance consumed 29% less energy than one using R134a – a figure that Madyira calls “a significant energy gain for users, especially low-income households”. He adds that both the reduction in energy consumption and the resulting improvement in cooling performance are due to the nanoparticles, which “not only improve the refrigerant’s thermal conductivity but also reduce friction and wear on the fridge’s vapour compressor,” he tells Physics World. The reduction in friction, in turn, enhances the heat transfer rate and reduces the pumping power required by the compressor for effective operation. Previous work by other groups also showed reduced energy use when refrigerants made with water or ethylene glycol were dosed with MWCNT particles in a similar fashion.

Don’t try this at home

Despite the potential boost to cooling efficiency, Madyira advises against simply replacing the refrigerant in your fridge with R600a. Instead, a trained refrigeration technician should do the job – not least because R600a, unlike R134a, is flammable. Also, while some refrigerator manufacturers have begun to commercialize appliances that use R600a, they use no more than 150 g of the material in a domestic fridge. In their work, Madyira and colleagues used between 50–70 g to stay within safety parameters.

Another barrier to do-it-yourself replacement is that the mineral oil used in the compressor needs to be mixed in the right concentrations, with a magnetic stirrer or sonic bath used to agitate and homogenize the ingredients before adding them to the compressor. The main challenge in this step, Madyira explains, is to overcome coagulation of the particles before adding the oil to the compressor – a task that requires careful preparation.