Dramatic reductions in the emissions of greenhouse gases are essential to help mitigate the effects of climate change, according to two reports issued last week by the National Research Council of the US National Academy of Sciences. Written by a 22-strong committee, the reports argue that removing carbon dioxide from the atmosphere is the most promising approach to tackle climate change, but conclude that the use of geoengineering to reduce the amount of solar energy that reaches the atmosphere is too risky. That type of geoengineering, the reports state, should instead be restricted to small-scale experiments rather than full deployment.
Dual approach
The reports identify two specific geoengineering approaches that could have a significant impact on the climate: injecting aerosols into the stratosphere – to mimic the effect of large volcanic eruptions – and brightening clouds to make them more reflective. Although these methods are cheaper than removing carbon dioxide from the air and would not require “major technological innovation”, the reports note that any future decisions on managing solar radiation – known as albedo modification – will be judged mainly on questions of risk. Indeed, many of the processes most relevant to albedo modification – such as those controlling the formation of clouds and aerosols – are among the most difficult to measure and monitor. Nor can current observational techniques monitor the environmental effects of the approach.
Marcia McNutt, a former director of the US Geological Survey who chaired the committee, says the fact that scientists are even thinking about using geoengineering to mitigate climate change should be a “wake-up call”. “The longer we wait, the more likely it will become that we will need to deploy some forms of carbon-dioxide removal to avoid the worst impacts of climate change,” she insists.
Conflicting views
However, some climate scientists urge the need for caution. “No reputable scientist I know thinks placing tiny reflecting particles in the stratosphere is a good idea, although some support studying it,” says Philip Duffy, president and executive director of the Woods Hole Research Center, an institution that focuses on climate change. Pennsylvania State University climatologist Michael Mann takes an equally sceptical view. “I believe that we should continue to fund studies of geoengineering approaches,” he says, “if only for one purpose: to expose just how dangerous many of these schemes might be.”
Marine geochemist Scott Doney of the Woods Hole Oceanographic Institution, who helped write the reports, thinks that deploying geoengineering – or even carrying out field research into it – is not on the immediate agenda and would require further discussion. “This is not just an academic science conversation, it needs to involve society: NGOs, governments and industry,” he says. “We’re saying that we should not move forward with deployment now – and not with field research until we have more detailed conversations on governance issues.”
A new type of nanoparticle could allow patients to be imaged in six different ways with an injection of just one contrast agent, reports an international research team. If put into clinical use, the technology could give doctors the ability to combine the strong points of a number of imaging techniques, providing a clearer overall picture of patient organs and tissue.
Many ways and means
The nanoparticles – averaging 74 nm in diameter – comprise a core surrounded by a porphyrin-phospholipid (PoP) wrapper. Each part has properties that facilitate different imaging modes. The core component fluoresces blue when struck with near-infrared light, allowing imaging with the potential for deep-tissue penetration. In addition, the core’s ytterbium component, which is dense in electrons, enables detection by CT scanning. The outer PoP shell has biophotonic properties that make it suitable for use with both fluorescence and photoacoustic imaging. At the same time, the copper affinity of the porphyrin wrapper allows the nanoparticles to be easily coated in radioactive copper-64 for the purposes of PET and Cerenkov-luminescence imaging. The team initially tested each imaging method in vitro and subsequently in a turkey breast to determine the level of signal attenuation with tissue depth. Turkey breast was chosen as a medium comparable to that of human breast tissue, as lymph-node mapping is a current challenge in the assessment of breast cancer. The nanoparticles were then used to image the lymph nodes of live mice, demonstrating the potential of this approach.
Multiscale imaging
“Photoacoustic imaging provides information about endogenous blood vessels, while CT provides information about local bone structure,” says team member Jonathan Lovell, a biomedical engineer at the University of Buffalo in the US. The CT and PET scans were seen to afford the deepest tissue penetration, while fluorescence imaging provided additional information on the uptake of the nanoparticles into cells.
Along with the nanoparticles’ convenient compatibility with the different imaging methods, the researchers also point out that their nanoparticles are both inexpensive and simple to manufacture in comparison with other contrast agents. “Another advantage of this core/shell imaging contrast agent is that it could enable biomedical imaging at multiple scales, from single-molecule to cell imaging, as well as from vascular and organ imaging to whole-body bioimaging,” adds Guanying Chen of the University of Buffalo’s Institute for Lasers Photonics and Biophotonics and the Harbin Institute of Technology in China.
All in one
An imaging machine capable of performing all six scans at once does not currently exist, but the researchers are optimistic that devices capable of exploiting some or all of the nanoparticles’ potential will soon be developed. “Creating a higher-order integrated scanner is not a simple task, but luckily there has been a lot of research and development into just that topic recently,” says Lovell, noting that integrating just some of the six methods – such as the optical techniques, for example – might provide a simpler device that would still be suitable for clinical applications.
Tumour mapping
David Cormode, a radiologist from the University of Pennsylvania who was not involved in the work, is impressed with the nanoparticles’ unique potential as a flexible contrast agent. “I look forward to future work where the agent is used for targeted imaging,” he says, while noting that “safety and excretion will have to be studied prior to translation to patients”.
In addition to confirming the suitability of the nanoparticles for clinical use, the team is also seeking additional applications for the technology. One avenue of investigation would involve the addition of a targeting molecule to the surface of the nanoparticles, allowing them to be taken in by cancer cells to allow better mapping of tumours.
The 71-year-old theoretical physicist Paul Frampton, who was arrested in Argentina in 2012 with 2 kg of cocaine in his luggage, has released his own version of events.
The British-born physicist was in Argentina after thinking he had struck up a correspondence on the Internet with Czech-born lingerie model Denise Milani.
However, when he arrived, Milani was nowhere to be seen and Frampton was apparently asked by someone else to carry a suitcase for her, which turned out to contain the drugs.
Despite protesting his innocence, Frampton was sentenced in November 2012 to 56 months in jail in Buenos Aires, some of which he spent under house arrest.
Now, in a 45-page e-book – Tricked!: the Story of an Internet Scam – Frampton outlines “the true story of an adventure that I would rather not have had”. According to the book’s blurb, it provides an “important lesson” that is “essential reading for everybody who uses the Internet”.
Like the legend of Isaac Newton seeing an apple fall, the story of young James Watt carefully observing steam coming from a tea kettle has become part of British folklore. A popular painting shows him being watched by an aunt, whose puzzled and disapproving expression holds dramatic irony: unlike us, she doesn’t know that her nephew would go on to invent the modern steam engine and so change the world.
The title notwithstanding, Ben Russell’s main theme in James Watt: Making the World Anew is not Watt the man. Rather, he uses Watt as a “lens” through which to examine how the process of “making things” evolved during the industrial revolution. He does, indeed, guide us through the varied phases of Watt’s long and productive life, but also invites us to study a somewhat neglected aspect of the industrial revolution: the part played by craftsmen such as clockmakers, instrument makers, potters, blacksmiths and millwrights. As Russell points out, without the people who actually made things, the ideas that inspired progress would have remained just ideas.
Watt himself was both a thinker and a doer. His most famous invention – the steam engine with a separate condenser – drew on both attributes and demonstrates his ability to go on thinking (and doing) long after most people would have given up. He realized that contemporary engines wasted a great deal of heat every time water was injected into the cylinder to condense the steam, and began to experiment with ways to reduce the waste, using his skill as an instrument maker in improvising apparatus for the purpose. He found that the problem was not caused by heat escaping through the cylinder walls, as he had first thought, but by the cast-iron cylinder cooling and having to be reheated at every stroke. The solution was to send the steam to a separate vessel to be condensed; then the cylinder would remain hot and the condenser could be kept cool, thus wasting far less heat. So stated, it sounds simple, but the idea came to Watt only after he had gained understanding of the properties of steam – specific heat, latent heat and elasticity – through his experiments and his discussions with the natural philosopher, Joseph Black.
Although trained as a maker of scientific instruments, Watt turned his hand to all manner of crafts wherever he saw potential markets. Despite being tone-deaf, he made and sold flutes and guitars. No-one matched Watt’s almost superhuman versatility but, as Russell points out, ambitious craftsmen everywhere were developing new techniques and identifying new materials – anything that could give them an edge over their competitors. In doing so, they improved existing products and found ways to make new ones. Some, notably Josiah Wedgwood, were able to anticipate the needs of society in a spectacularly successful way. Others fell prey to the whims of fashion. When, in the late 1700s, people abandoned fancy shoe buckles in favour of cheaper and more practical shoelaces, 20,000 buckle-makers petitioned the Prince of Wales, and the Birmingham Gazette ran a despairing article in May 1790 contrasting the “manly buckle” with “that most ridiculous of all ridiculous fashions, the effeminate shoestring”.
An important part of the ambitious craftsman’s business was to deal with people – to negotiate contracts, chase slow payers and discipline recalcitrant workers. Though far from shy when in congenial company, Watt hated this rough stuff. He was, however, keen to strike good bargains and to secure his legal ownership of inventions, and was lucky to find a partner, the manufacturer Matthew Boulton, who was a master of such arts. It was Boulton who persuaded Watt to patent the principles of his separate condenser rather than the means of applying them. This patent eventually made Watt a very rich man, but only after he and Boulton had pursued many people who had infringed the patent or withheld royalty payments, and, in the end, got a significant proportion of them to pay.
Anyone who opens this book expecting to read a story is likely to be disappointed. Russell gives us instead a course of lectures, meaty ones, that draw on a great many sources. (He lists 787 references.) The text is copiously illustrated but is not all easy going, and at times one may wish for some smooth paraphrasing rather than clunky quotes from original letters and papers. It’s good, authentic stuff though, and we learn some surprising facts (surprising to me, at least). For example, waterwheels persisted far longer during the industrial revolution than I had thought. Moreover, they were sometimes used together with steam engines in a hybrid arrangement, rather like that in today’s petrol/electric cars. In some installations the steam engine would be brought in only when the water supply failed; in others the steam engine actually fed water to the water wheel.
Perhaps from a wish to emphasize the importance of craft work in the industrial revolution (as a companion to the importance of science which has already been much written about), Russell has made a strange omission. He tells us nothing about Birmingham’s Lunar Society, of which Watt and Boulton were leading members along with Joseph Priestley, Erasmus Darwin and Josiah Wedgwood. (The society was an informal group that met monthly around the time of the full moon for dinner, gossip, and scientific talk. Its members strongly supported each other and, together, wielded great influence.) The omission is especially odd as Russell has included an illustration of society members sitting round a table drinking coffee.
James Watt epitomises the fusion of craft with science that brought about the industrial revolution and created the profession of engineering. Anyone with more than a passing interest in the comparatively neglected role of craft in this process will find this book good value.
The European Physical Society (EPS) has hit out at plans to remove €2.7bn from the €80bn budget of Horizon 2020 – the European Union’s main research funding programme – and use the money instead to help finance a new European Union economic-stimulus initiative. In a letter to European Commission president Jean-Claude Juncker – signed by EPS president John Dudley and EPS president-elect Christophe Rossel – it warns that ignoring the “importance of research and development as key drivers of prosperity is sending the wrong message to the scientific communities who are essential for Europe’s future”.
The economic-stimulus initiative – which was announced in December last year by Juncker and is officially called the European Fund for Strategic Investment (EFSI) – aims to bolster weaker European economic regions and boost employment. To fund the EFSI, the Horizon 2020 budget would be cut by €70m this year, €860m in 2016, €871m in 2017, €479m in 2018, €150m in 2019 and €270m in 2020. Among the biggest losers would be the European Institute of Innovation and Technology (EIT) in Budapest with total cuts of €350m, while the European Research Council (ERC), which gives grants to individual researchers, would have its funds slashed by €221m.
Official approval?
Juncker has sought to ease concerns among researchers by saying that the EFSI investments will also benefit research. While the European Commission has stated it would like the EFSI to become operational in the coming months, the plan still needs official approval from the European Parliament and the European Council. The EPS letter to Juncker, dated 16 February, does not directly ask Junker to reconsider his plan to divert Horizon 2020 money to the EFSI but hints at the political implications, noting that the EPS comprises 42 member societies spanning Europe and represents the interests of 130,000 physicists. “We urge you to send a clear signal to the scientific community of your continued commitment to supporting scientific research and co-operation in Europe,” the letter says.
The letter also highlights the benefits physics plays to the European economy by employing 15.4 million people across the continent in 2010 and generating €3800bn in turnover. The EPS is particularly concerned about cuts to the ERC budget, saying it would lead to the axing of as many as 150 ERC grants, which would remove funds for 150 European scientists. “This loss of support will lead to a decline in Europe’s capacity to attract top-rank researchers and compete on a global scale,” the letter states.
Protest proclamations
Several other scientific and academic organizations have also either written letters of protest to Juncker or issued public statements, including the European Association for Chemical and Molecular Sciences, the League of European Research Universities and the European University Association.
Rüdiger Voss, head of international relations at CERN, told physicsworld.com that the European Parliament “can only accept or reject the EFSI as a whole”, noting that it would be difficult to reduce or remove the specific cuts to Horizon 2020. “Science is the ultimate driver of innovation and economic development, with a much larger multiplicative effect than direct investments to stimulate the economy,” he says. “It may appear short-sighted to divert significant funds from Horizon 2020 to the EFSI.”
Ove Poulsen, a retired optical physicist at Aarhus University in Denmark who was president of the EPS from 2005 to 2007, says it is hard to gauge the potential negative effects of the cuts on physics research. “Noting the central role of the physical sciences in emerging technology development, in problem solving of a societal nature and a growing impact in future energy scenarios, the proposed cuts surely will be felt by the European physics community,” he says.
The first stars known to pulsate in a fractal manner have been discovered by physicists in the US and Germany. According to the researchers, the variable stars may be the first “strange non-chaotic attractors” seen outside the laboratory. The objects were found in data from the Kepler space telescope by looking for stars with two characteristic pulsation frequencies that have a “golden ratio” of approximately 1.62. The discovery could shed light on the physics that drives variable stars and also help astronomers come up with better classification systems for these objects.
A variable star dims and brightens as its size and sometimes its shape oscillates at one or more frequencies. Since it was launched in 2009, NASA’s Kepler mission has been a boon to astronomers studying variable stars because the telescope has been monitoring the brightness of more than 100,000 stars in its search for distant planets. However, John Learned of the University of Hawaii and Michael Hippke of the Institute for Data Analysis in Neukirchen-Vluyn in Germany noticed the first strange non-chaotic “golden star” when searching the Kepler data for evidence that advanced extraterrestrial civilizations modulate variable stars to communicate between galaxies.
Geometry of life and art
This star – called KIC 5520878 – is a type of periodic variable star known as an “RRc Lyrae” variable. It pulsates at a large number of frequencies that are all related to two frequencies – f1 and f2 – that have a golden ratio. The golden ratio or “golden mean” is an irrational number that has significance in geometry, biology and art. Its presence in a dynamical system can mean that the system behaves as a “strange non-chaotic attractor”. In this case, “strange” means that the system can be characterized as fractal, and “non-chaotic” means that the dynamics falls in the middle ground between order and chaos.
To study the dynamics of the star, Learned and Hippke joined forces with physicists at the University of Hawaii and the College of Wooster, including John Lindner. To verify that the star is indeed a strange non-chaotic attractor, the team did a “spectral scaling” analysis. First, the researchers did a Fourier transform of a time sequence of the brightness of the star that was acquired by Kepler over a four-year period, creating a power spectrum with peaks at a large number of frequencies. Then, they counted the number of peaks above a threshold value, repeating the process over a wide range of threshold values. Finally, they plotted the number of peaks above the threshold as a function of the threshold.
Norwegian link
They found that the number of peaks was pretty well constant until the threshold reached an inflection point (a point on a curve at which the sign of the curvature changes). When this occurred, the number dropped rapidly and obeyed a power law. According to the team, this behaviour is indicative of strange non-chaotic dynamics. Interestingly, Lindner points out that a similar analysis of the variability of the coastline of Norway yields the same power-law exponent of –1.5. The team also did an “attractor reconstruction”, whereby the time evolution of the brightness of the star is plotted. The researchers found the attractor to be a “warped torus” (see figure), which is indicative of a non-chaotic system defined by two frequencies and involving non-linear dynamics.
While several strange non-chaotic attractors have been created in the lab based on magnetic, electrochemical, electronic and atomic systems, Lindner and colleagues believe that KIC 5520878 is the first to be found in nature. The physicists then applied their analysis to three other RRc Lyrae variable stars that also have two frequencies with ratios close to the golden mean. They found exactly the same power-law relationship. Finally, they looked at two variable stars that have frequency ratios that are not the golden ratio – but rather are simple fractions – and found that these did not obey power laws.
Quasiperiodic instabilities
As well as offering astronomers a new way to classify variable stars, the strange non-chaotic nature of some stars could help scientists gain a better understanding of the physics underlying stellar pulsations. In a stable star the inward gravitational pressure is balanced by the outward pressure of light. In some regions of a star, however, an increase in temperature can make the star more opaque, thereby reducing the amount of light that can escape. This can cause a build-up of pressure that is relieved by the star expanding. It is this sort of instability that could be responsible for driving the pulsations in a non-linear manner involving two frequencies.
The team is now studying other variable stars that could be strange non-chaotic attractors, and Lindner points out that many RR Lyrae and Cepheids appear to have frequency ratios near the golden mean. “Currently, we suspect that the RRc Lyrae stars, a previously recognized subclass of about 9% of RR Lyraes, may all be golden and strange non-chaotic,” he says. He also says that an important open question is whether all golden-ratio stars exhibit strange non-chaotic behaviour.
Medical imaging is a multidisciplinary science encompassing a wide range of powerful techniques with applications in both patient care and fundamental biological studies. In this latest Physics World focus issue, we examine how imaging technologies such as X-ray computed tomography (CT), magnetic resonance imaging and other nuclear, ultrasound and optical imaging techniques have evolved in recent years. We also take a look at what improvements can be expected in the future.
Created in collaboration with our sister website medicalphysicsweb, the new focus issue on medical imaging can be accessed free of charge in digital-magazine format.
Hot topic: A hydrogen plasma is held in a compact tokamak. (Courtesy: Tokamak Energy)
Researchers from the UK firm Tokamak Energy say that future fusion reactors could be made much smaller than previously envisaged – yet still deliver the same energy output. That claim is based on calculations showing that the fusion power gain – a measure of the ratio of the power from a fusion reactor to the power required to maintain the plasma in steady state – does not depend strongly on the size of the reactor. The company’s finding goes against conventional thinking, which says that a large power output is only possible by building bigger fusion reactors.
The largest fusion reactor currently under construction is the €16bn ITER facility in Cadarache, France. This will weigh about 23,000 tonnes when completed in the coming decade and consist of a deuterium–tritium plasma held in a 60 m-tall, doughnut-shaped “tokamak”. ITER aims to produce a fusion power gain (Q) of 10, meaning that, in theory, the reactor will emit 10 times the power it expends by producing 500 MW from 50 MW of input power. While ITER has a “major” plasma radius of 6.21 m, it is thought that an actual future fusion power plant delivering power to the grid would need a 9 m radius to generate 1 GW.
Low power brings high performance
The new study, led by Alan Costley from Tokamak Energy, which builds compact tokamaks, shows that smaller, lower-power, and therefore lower-cost reactors could still deliver a value of Q similar to ITER. The work focused on a key parameter in determining plasma performance called the plasma “beta”, which is the ratio of the plasma pressure to the magnetic pressure. By using scaling expressions consistent with existing experiments, the researchers show that the power needed for high fusion performance can be three or four times lower than previously thought.
Combined with the finding on the size-dependence of Q, these results imply the possibility of building lower-power, smaller and cheaper pilot plants and reactors. “The consequence of beta-independent scaling is that tokamaks could be much smaller, but still have a high power gain,” David Kingham, Tokamak Energy chief executive, told Physics World.
The researchers propose that a reactor with a radius of just 1.35 m would be able to generate 180 MW, with a Q of 5. This would result in a reactor just 1/20th of the size of ITER. “Although there are still engineering challenges to overcome, this result is underpinned by good science,” says Kingham. “We hope that this work will attract further investment in fusion energy.”
Many challenges remain
Howard Wilson, director of the York Plasma Institute at the University of York in the UK, points out, however, that the result relies on being able to achieve a very high magnetic field. “We have long been aware that a high magnetic field enables compact fusion devices – the breakthrough would be in discovering how to create such high magnetic fields in the tokamak,” he says. “A compact fusion device may indeed be possible, provided one can achieve high confinement of the fuel, demonstrate efficient current drive in the plasma, exhaust the heat and particles effectively without damaging material surfaces, and create the necessary high magnetic fields.”
The work by Tokamak Energy follows an announcement late last year that the US firm Lockheed Martin plans to build a “truck-sized” compact fusion reactor by 2019 that would be capable of delivering 100 MW. However, the latest results from Tokamak Energy might not be such bad news for ITER. Kingham adds that his firm’s work means that, in principle, ITER is actually being built much larger than necessary – and so should outperform its Q target of 10.
Image of Smith’s Cloud taken by the Green Bank Telescope. (Courtesy: Bill Saxton, NRAO/AUI/NSF)
By Margaret Harris at the AAAS meeting in San Jose
A giant cloud of hydrogen gas is barrelling towards the Milky Way faster than the speed of sound, and dark matter may hold it together long enough to produce a spectacular outburst of new stars in the night sky – but not for another 30 million years.
The cloud – which is known as Smith’s Cloud after Gail Bieger-Smith, who discovered it as an astronomy student in 1963 – is one of several starless blobs of hydrogen known to exist in the space between galaxies. According to Felix “Jay” Lockman, principal scientist at the US National Radio Astronomy Observatory’s Green Bank Telescope, such gas clouds are, in effect, “construction debris” left over from an earlier age of galaxy formation. “These are parts for remodelling your house that didn’t arrive by the time the contractor left,” Lockman told an audience at the 2015 AAAS meeting in San Jose, California.
“I was completely blown away,” says Alessandro Olivo of University College London (UCL) in the UK, recalling the fine detail of a wasp revealed in his first X-ray phase-contrast image. Then working at the University of Trieste in Italy, Olivo and colleagues took the image in 1997 at Elettra, a synchrotron on the outskirts of the city. The research was part of a global resurgence of interest in X-ray phase-contrast imaging (XPCi) in the 1990s led by Japanese pioneer Atsushi Momose.
The first XPCi image was acquired by Ulrich Bonse and Michael Hart at Cornell University in New York in 1965. The renewal in interest was motivated by the technique’s potential to improve on conventional X-ray images, in medicine as well as in other applications.
Introduced to hospitals shortly after their discovery by Wilhelm Röntgen in 1895, X-rays’ ability to non-invasively peer inside a patient rapidly made them indispensable. Today, 3D computed tomography (CT) and real-time fluoroscopy (which uses X-rays to obtain real-time moving images of a patient’s internal anatomy) have also become invaluable. The underlying physical principle, which is to create images based on the fact that different materials in the body attenuate X-rays to different extents, has remained unchanged in 120 years. However, this approach hides subtle composition fluctuations in organs and other soft tissues. Refraction at an interface – a manifestation of phase change – is a significantly more sensitive indicator of composition. In fact, the resulting variations in speed are up to 1000 times greater than changes in attenuation.
Better images and lower doses
The potential implications for medicine of the XPCi technique are profound. It could improve the detection and characterization of abnormalities. Alternatively, increased image sensitivity could be traded for shorter exposures and lower radiation doses, reducing a small but finite risk of radiation-induced cancer.
Increased image sensitivity could be traded for shorter exposures and lower radiation doses, reducing a small but finite risk of radiation-induced cancer
In imaging the breast – a radiosensitive soft-tissue structure – mammography arguably has the most to gain from XPCi and is attracting a lot of attention from researchers. Lower doses in cancer screening and improvements in image quality could have large health and economic benefits, particularly for the majority of people who do not have cancer. Currently, absorption-based mammography returns around nine false-positive results for every tumour correctly identified, resulting in unnecessary and invasive follow-up investigations.
The simplest XPCi technique is free-space propagation. It involves firing a highly coherent beam through an object, which changes its phase and refracts it by angles of the order of 1 μrad – equivalent to a 1 mm deflection 1 km from the object. X-rays adjacent to the object provide an unshifted reference that combines with the refracted X-rays to generate an interference pattern. Unlike absorption-based imaging, where the detector is placed directly behind the object, a detector recording the pattern is placed between tens of centimetres and several metres downstream where the interference pattern can be resolved.
Highly coherent, intense and energy-tunable synchrotrons provide perfect beams for free-space propagation and other XPCi techniques. Starting in 2006, researchers co-led by Renata Longo of the University of Trieste undertook the first clinical XPCi synchrotron study. Using free-space propagation with monochromatic 17–22 keV X-rays at Elettra, they imaged 71 women for whom conventional mammography and ultrasound returned inconclusive findings.
Despite the tough challenge the cohort posed, the images were better than conventional mammograms and doses were comparable or lower. Excitingly, XPCi also cleared 17 individuals subsequently confirmed as tumour-free where absorption imaging had failed. “We have a technique that has the potential to decrease the number of healthy people undergoing further examinations,” says Longo.
From synchrotron to lab
However, before all patients can benefit from XPCi, compact, robust and affordable techniques are needed. The only commercial system, which was launched by Japanese company Konica Minolta in 2005, has so far had limited impact, says Olivo. A partially coherent X-ray source combined with a free-space propagation approach restricted improvements in image contrast over conventional X-ray images.
An alternative approach that sidesteps the requirement for beam coherence is edge illumination, which measures how much X-rays get deflected when refracted by an object. Developed by Olivo, the method uses two “masks”: one in front of the X-ray source and another in front of the detector. Each comprises a set of apertures with a period – or slit spacing – of around 70–80 μm, the first creating a set of beamlets.
During a reference scan with no object present, the masks are aligned so that only half of each beamlet passes through an aperture onto a detector pixel. When an object is introduced, more or less of the beamlet hits the pixel, depending on its deflection. The measured intensity change can be used to derive the phase change.
The latest mammography prototype at UCL uses a powerful polychromatic X-ray source that enables quick exposures and results in breast doses comparable to those in clinical practice. The masks are easy to make in larger sizes, enabling extended fields of view. Imaging excised breast tissue, the system has detected features invisible in absorption images and increased the contrast of “microcalcifications” – tiny deposits that can indicate cancer – by up to a factor of nine.
For manufacturers, a potential limitation is that the devices have to be quite tall because the further the X-rays travel from the patient, the more they get deflected and the more sensitive the system is. In fact, standing 1.5 m high, the UCL system may be too large to be incorporated into existing commercial systems.
Another technique – grating-based interferometry – involves placing a diffraction grating in front of a regular polychromatic X-ray tube to increase coherence. Originally developed in synchrotrons, it was first achieved by Franz Pfeiffer and colleagues at the Paul Scherrer Institute (PSI) in Zurich, Switzerland. Pfeiffer now leads a group at the Technical University of Munich (TUM) in Germany.
X-rays transmitted through an object pass directly through a second grating that creates a diffraction pattern downstream at a specific distance. A third “analyser” grating in front of a detector interrogates the pattern. The phase shifts can be deduced by comparing the intensities with a reference scan with the object absent.
Imaging challenges
Researchers are tackling several issues that dictate the performance of grating-based XPCi. In general, the X-rays needed to image structures deep in the body are several times more energetic than those that have been used to date in XPCi studies of the breast, tissue samples and laboratory animals. Higher energies require gratings thick enough to absorb X-rays incident between slits that would otherwise degrade the diffraction pattern. However, high-sensitivity imaging requires a small period and fabricating a stable grating with both properties is difficult.
Several groups are beginning to overcome this challenge. Using gratings made by researchers at the Karlsruhe Institute of Technology in Germany, a collaboration led by Pfeiffer has acquired 82 keV and 133 keV images – the highest to date – at the European Synchrotron Radiation Facility in Grenoble, France. “Grating fabrication has improved such that gratings with high aspect ratios – structures up to 150 μm high with very small periods down to 2.4 μm – are now possible,” says Julia Herzen, who is leading research at TUM that aims to translate advances in the lab into clinical applications.
By absorbing X-rays, gratings also reduce the number of photons reaching the detector and are a complicating factor in the trade-off between image sensitivity and dose. The challenge is to identify the sweet spot that improves contrast beyond that of absorption images, yet keeps patient doses within prescribed limits.
Enhanced visibility Conventional absorption (left) and phase-contrast (right) mammography images show how phase contrast increases the sharpness of soft-tissue details. (Courtesy: M Stampanoni, ETHZ/PSI)
With simultaneous absorption, phase-contrast and dark-field images of excised breast tissue, a Swiss Federal Institute of Technology Zurich and PSI group led by Marco Stampanoni is using grating-based imaging to provide further evidence of the clinical potential of phase measurements. In a first for the technique, the researchers demonstrated significant improvements in image quality in the lab.
Dark-field imaging is a variation of the phase-contrast principle and is a hot topic in the XPCi community. Instead of detecting phase shifts that highlight boundaries between macroscopic objects, it detects ultra-small-angle scattering by microscopic structures. Pioneered in the early 2000s, dark-field imaging can be achieved with the same pixelated detectors used for XPCi. “This is a very important new signal because you can get information from structures that are much smaller than pixels in a coarse detector,” says Stampanoni.
In a separate study, the group has taken the first steps towards a quantitative measure that could help assess the risk of cancerous or precancerous cells in clusters of microcalcifications in the breast. Analysing microcalcifications in mastectomy samples, the researchers found that some scattered a lot yet absorbed few X-rays, while others exhibited the opposite characteristics. Stampanoni’s team hypothesizes that the ratio of the two signals could discriminate between two types of microcalcification: one rarely associated with cancer and another commonly found in so-called proliferative lesions that include the most aggressive breast cancers.
However, more work is needed. As all of the tissue came from patients with breast cancer sufficiently advanced to require mastectomies, the sample was biased. “We have an indication where this threshold could be, but we need to have much stronger statistical evidence,” says Stampanoni.
From the lab to the hospital
The only company with an imager in a hospital is Konica Minolta, which is working with researchers at Japanese universities, including Momose. In a simple application, its second, grating-based prototype system has imaged the finger joints of volunteers in 2D, using a small field of view and mean beam energy of 28 keV, where each image takes tens of seconds to acquire. There are plans to adapt the system for mammography.
One long-term goal for the XPCi community is 3D imaging in patients. A large Italian consortium co-led by Longo is planning the first patient scans in a second breast-imaging study at Elettra, which is due to start by the end of 2016. With the ideal conditions that the synchrotron provides, the study will give a performance benchmark for phase-contrast breast CT. Like conventional CT, it requires multiple projections taken over 360° around the patient. Achieving this without a synchrotron, within non-negotiable dose limits and in short exposures in particular, is an even tougher challenge than for 2D imaging.
While several groups have links with companies, aside from the newer Konica Minolta prototype there is no 2D or 3D commercial imager on the horizon, making it hard to predict when patients might start benefiting from XPCi. Funding is a big issue because XPCi has reached a stage where it is no longer eligible for blue-sky funding from research councils, yet is not advanced enough to gain serious financial support from business, which means that – according to Olivo – groups might struggle to find the funding needed to push XPCi into the clinic. “You can make anything work if you throw enough money at it, once you have the proper principle,” he says. “I cannot build a perpetual motion machine, but I think I can build a phase-contrast imager!”
Beyond mammography
Lung studies Propagation-based phase-contrast image showing a neonate lung (in a rabbit kitten) that inflated at birth and one that did not – the speckle indicates healthy, aerated lung. (Courtesy: Marcus Kitchen and Stuart B Hooper, Monash University)
Researchers around the world are using phase-contrast X-ray imaging to study many different diseases and areas of the body, including cartilage, bone, lungs, blood vessels and kidneys, typically using animals and tissue samples because human scanners are still under development.
In the lungs, the interface between air and soft tissue provides the strongest phase gradient in the body. Potential applications include diagnosing and monitoring conditions that impair lung function, like emphysema and cystic fibrosis. With multiple overlapping airways, the lungs appear in phase-contrast images as speckled masses that are hard to read by eye, and research has focused on quantitative analyses.
Examining absorption and dark-field signals in the lungs of live mice using a miniature CT scanner developed in-house, researchers at the Technical University of Munich measured dramatically lower scattering signals in emphysematous lungs, while conventional imaging showed little change.
Marcus Kitchen’s group at Monash University in Australia, meanwhile, has developed techniques that quantify lung function by analysing the speckle texture. “Our technique uses a statistical approach to say what is the average size and number of the airways that are contributing to the speckle pattern,” he explains.
Using animal models, the researchers have employed XPCi to identify optimal approaches for the resuscitation of newborn babies with great success. Their findings have resulted in changes to clinical practice around the world.
Also at Monash, Kaye Morgan and colleagues were the first to measure the depth of fluid lining the airways in live mice, using a high-speed XPCi technique they developed. Thinner fluid layers in people with cystic fibrosis make them more prone to infection, and the researchers are using the technique to investigate new therapies. On the physics side, the group is working on approaches that could enable monitoring of individual patients. “It’s definitely a long-term goal,” says Morgan.
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