physicsworld--2020-12-10--China launches GECAM all-sky scanner to study extreme cosmic events

China has successfully launched a space telescope to study some of the most energetic events in the universe .
Lifting off today from the Xichang Satellite Launch Center at 4.14 a.m. local time , the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor ( GECAM ) represents one of the first new “ all-sky ” devices that will monitor fast-radio bursts , high-energy neutrinos and magnetars .
The craft is set to begin operations as soon as it enters orbit , doing so for three years .
To study these events , GECAM consists of two satellites – each weighing 160 @ @ @ @ @ @ @ sides of the Earth at an altitude of about 600 km .
Each GECAM satellite features a dome-shaped array of 25 gamma-ray detectors and eight charged particle detectors .
They will search for cosmic events happening in the energy range of 6 keV – 5 MeV and , within a couple of minutes of detection , GECAM will send out alerts to telescopes around the world for follow-up observations .
The idea of GECAM emerged following the announcement in February 2016 of the first detection of gravitational waves by the US-based Laser Interferometer @ @ @ @ @ @ @ year later and LIGO , working together with the VIRGO gravitational-wave detector in Italy , spotted the first gravitational wave produced by the merger of two neutron stars – an observation that was followed up by the Fermi Gamma-ray Space Telescope and other observatories around the world , kick-starting the era of multimessenger astronomy .
Likewise , GECAM spot the gamma-rays bursts that are related to the production of gravitational waves by such comic mergers .
“ We ’ re really excited to see GECAM fly on time , as we overcame numerous @ @ @ @ @ @ @ pandemic , ” says GECAM ’ s principal investigator Shaolin Xiong from the Institute of High Energy Physics , Chinese Academy of Sciences ( CAS ) in Beijing .
GECAM is the first of a line-up of space science missions to be launched by China in the coming five years .
Other missions include the Advanced Space-based Solar Observatory , the Einstein Probe as well as the Solar wind Magnetosphere Ionosphere Link Explorer – a joint mission between CAS and the European Space Agency .

Eye in the sky: the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor will study fast-radio bursts as well as gamma-ray bursts related to gravitational wave events (Courtesy: GECAM team/Institute of High Energy Physics, Chinese Academy of Sciences)
China has successfully launched a space telescope to study some of the most energetic events in the universe. Lifting off today from the Xichang Satellite Launch Center at 4.14 a.m. local time, the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) represents one of the first new “all-sky” devices that will monitor fast-radio bursts, high-energy neutrinos and magnetars. The craft is set to begin operations as soon as it enters orbit, doing so for three years.
We’re really excited to see GECAM fly on time, as we overcame numerous technical difficulties and made it through the pandemic Shaolin Xiong
To study these events, GECAM consists of two satellites – each weighing 160 kg – that will orbit on opposite sides of the Earth at an altitude of about 600 km. Each GECAM satellite features a dome-shaped array of 25 gamma-ray detectors and eight charged particle detectors. They will search for cosmic events happening in the energy range of 6 keV – 5 MeV and, within a couple of minutes of detection, GECAM will send out alerts to telescopes around the world for follow-up observations.
The idea of GECAM emerged following the announcement in February 2016 of the first detection of gravitational waves by the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO). A year later and LIGO, working together with the VIRGO gravitational-wave detector in Italy, spotted the first gravitational wave produced by the merger of two neutron stars – an observation that was followed up by the Fermi Gamma-ray Space Telescope and other observatories around the world, kick-starting the era of multimessenger astronomy. Likewise, GECAM spot the gamma-rays bursts that are related to the production of gravitational waves by such comic mergers.
“We’re really excited to see GECAM fly on time, as we overcame numerous technical difficulties and made it through the pandemic,” says GECAM’s principal investigator Shaolin Xiong from the Institute of High Energy Physics, Chinese Academy of Sciences (CAS) in Beijing.
GECAM is the first of a line-up of space science missions to be launched by China in the coming five years. Other missions include the Advanced Space-based Solar Observatory, the Einstein Probe as well as the Solar wind Magnetosphere Ionosphere Link Explorer – a joint mission between CAS and the European Space Agency.


physicsworld--2020-12-10--MRI during treatment could improve paediatric proton therapy

Identifying anatomic changes shown on MRI scans during a course of pencil-beam proton therapy , and adapting treatment plans accordingly , could improve the treatment quality , effectiveness and safety .
In a retrospective study , researchers at St. Jude Children ’ s Research Hospital determined that 27 % of paediatric patients , the majority with brain tumours , would have benefited from mid-treatment plan changes .
Pencil-beam proton therapy delivers radiation doses to a target with high accuracy , but if the target shifts , the dosimetric advantage afforded by the Bragg peak may be compromised and the risk @ @ @ @ @ @ @ Prior clinical trials comparing outcomes of paediatric patients treated with either proton or photon therapy have produced mixed results with regard to toxicity .
The researchers hypothesized that adherence to the treatment plan without adjusting for anatomic changes may have compromised the ability of proton therapy to maximally spare normal tissue while adequately covering the target volume .
As such , they investigated whether altering treatment plans to compensate for changes in tumours or adjacent organs-at-risk ( OARs ) could improve target dose delivery and reduce normal tissue dose .
They published their @ @ @ @ @ @ @ Oncology , Biology , Physics .
The study included 73 children with a variety of cancers who received proton therapy at St. Jude during a 30-month period starting January 2017 .
The patients had between one and seven MRI exams , most of which were performed between weeks two and four of their treatments .
Children with low-grade gliomas or rhabdomyosarcomas underwent weekly MRI exams .
The researchers evaluated 230 MRI scans acquired in the treatment position during the patients ’ proton therapy courses , recording patterns of temporal and spatial anatomic changes @ @ @ @ @ @ @ significantly from re-optimization , they compared the calculated radiation dose for the anatomy-of-the-day without treatment adaptation with the dose that would have been delivered with an adapted plan .
The majority of patients had brain tumours ( 79 % ) or tumours located in the head-and-neck ( 12 % ) .
They were prescribed a median dose of 54 Gy ( RBE ) in 1.8 Gy ( RBE ) per fraction .
MRI analysis revealed that 20 patients had anatomic changes in their gross tumour volume ( GTV ) and/or changes in the @ @ @ @ @ @ @ This included all eight patients with rhabdomyosarcomas and 24 % of patients with low-grade gliomas .
Led by radiation oncologist Sahaja Acharya , the researchers determined that 11 of the 20 re-optimized treatment plans had significantly changed plan quality , defined as a 5 % or greater decrease in CTV V95 ( clinical target volume receiving at least 95 % of the prescription dose ) or a 5 % or greater increase in the dose–volume parameter used as an OAR constraint .
Because of anatomic changes , seven patients experienced a significant reduction @ @ @ @ @ @ @ , ranging from 5–16 % decrease in CTV V95 .
Four patients experienced a significant increase in dose to the brain stem , hippocampus and/or optic apparatus , which could have been prevented if the delivered plan had been adapted .
The team note that none of the patients who had re-optimized plans experienced local or distant failures .
Additionally , none experienced grade 3 or higher toxicities or developed radiation necrosis .
“ Of the 27 % of patients who demonstrated anatomic change during proton therapy , more than 50 % had @ @ @ @ @ @ @ authors .
“ Re-optimization of these plans resulted in an improvement of CTV coverage up to 16 % and a decrease in OAR dose up to 46 % .
” Because most changes were identified during the second or third week of a six-week course of treatment , the re-optimization effect would have been significant .
“ Certain tumour histologies and locations might derive more benefit than others from this type of approach .
Specific patterns of anatomic change , unrelated to tumour or patient characteristics , might help in selecting patients for @ @ @ @ @ @ @ write .
“ We are planning to conduct a prospective pilot study that will specify timing and frequency of on-treatment MRI for different disease categories , ” Acharya tells Physics World .
“ Through this study , we seek to understand the number of patients we need to image in order to detect one plan deviation.
” Acharya is also the principal investigator of an accruing phase II study ( NCT04065776 ) investigating the feasibility of reducing radiation doses to the hippocampi by using proton therapy for midline or suprasellar low-grade gliomas .
@ @ @ @ @ @ @ a weekly MRI scan , and treatment plans are adapted to shrinking tumour volumes if there is more than a 20 % change in GTV .
All children are also followed with longitudinal neurocognitive testing .
“ This study might provide some insight on whether adapting to a smaller target volume can spare critical regions of the brain , such as the hippocampus , for children with midline or suprasellar low-grade gliomas , and whether sparing such regions will preserve neurocognition , ” explains Acharya .

Sahaja Acharya and colleagues are investigating the potential role of MRI-guided proton therapy in the treatment of paediatric patients. (Courtesy: Seth Dixon/St. Jude Children’s Research Hospital)
Identifying anatomic changes shown on MRI scans during a course of pencil-beam proton therapy, and adapting treatment plans accordingly, could improve the treatment quality, effectiveness and safety. In a retrospective study, researchers at St. Jude Children’s Research Hospital determined that 27% of paediatric patients, the majority with brain tumours, would have benefited from mid-treatment plan changes.
Pencil-beam proton therapy delivers radiation doses to a target with high accuracy, but if the target shifts, the dosimetric advantage afforded by the Bragg peak may be compromised and the risk of toxicities to adjacent tissue increased. Prior clinical trials comparing outcomes of paediatric patients treated with either proton or photon therapy have produced mixed results with regard to toxicity.
The researchers hypothesized that adherence to the treatment plan without adjusting for anatomic changes may have compromised the ability of proton therapy to maximally spare normal tissue while adequately covering the target volume. As such, they investigated whether altering treatment plans to compensate for changes in tumours or adjacent organs-at-risk (OARs) could improve target dose delivery and reduce normal tissue dose. They published their findings in the International Journal of Radiation Oncology, Biology, Physics.
The study included 73 children with a variety of cancers who received proton therapy at St. Jude during a 30-month period starting January 2017. The patients had between one and seven MRI exams, most of which were performed between weeks two and four of their treatments. Children with low-grade gliomas or rhabdomyosarcomas underwent weekly MRI exams.
The researchers evaluated 230 MRI scans acquired in the treatment position during the patients’ proton therapy courses, recording patterns of temporal and spatial anatomic changes. To identify scenarios that would benefit significantly from re-optimization, they compared the calculated radiation dose for the anatomy-of-the-day without treatment adaptation with the dose that would have been delivered with an adapted plan.
The majority of patients had brain tumours (79%) or tumours located in the head-and-neck (12%). They were prescribed a median dose of 54 Gy(RBE) in 1.8 Gy(RBE) per fraction. MRI analysis revealed that 20 patients had anatomic changes in their gross tumour volume (GTV) and/or changes in the tissue density within the beam path. This included all eight patients with rhabdomyosarcomas and 24% of patients with low-grade gliomas.
Led by radiation oncologist Sahaja Acharya, the researchers determined that 11 of the 20 re-optimized treatment plans had significantly changed plan quality, defined as a 5% or greater decrease in CTV V95 (clinical target volume receiving at least 95% of the prescription dose) or a 5% or greater increase in the dose–volume parameter used as an OAR constraint.
Because of anatomic changes, seven patients experienced a significant reduction in CTV coverage with the delivered plan, ranging from 5–16% decrease in CTV V95. Four patients experienced a significant increase in dose to the brain stem, hippocampus and/or optic apparatus, which could have been prevented if the delivered plan had been adapted. The team note that none of the patients who had re-optimized plans experienced local or distant failures. Additionally, none experienced grade 3 or higher toxicities or developed radiation necrosis.
“Of the 27% of patients who demonstrated anatomic change during proton therapy, more than 50% had suboptimal delivered plans,” write the authors. “Re-optimization of these plans resulted in an improvement of CTV coverage up to 16% and a decrease in OAR dose up to 46%.” Because most changes were identified during the second or third week of a six-week course of treatment, the re-optimization effect would have been significant.
“Certain tumour histologies and locations might derive more benefit than others from this type of approach. Specific patterns of anatomic change, unrelated to tumour or patient characteristics, might help in selecting patients for MRI-based adaptive therapy,” the researchers write.
“We are planning to conduct a prospective pilot study that will specify timing and frequency of on-treatment MRI for different disease categories,” Acharya tells Physics World. “Through this study, we seek to understand the number of patients we need to image in order to detect one plan deviation.”
Acharya is also the principal investigator of an accruing phase II study (NCT04065776) investigating the feasibility of reducing radiation doses to the hippocampi by using proton therapy for midline or suprasellar low-grade gliomas. Children enrolled in this study also undergo a weekly MRI scan, and treatment plans are adapted to shrinking tumour volumes if there is more than a 20% change in GTV. All children are also followed with longitudinal neurocognitive testing.
“This study might provide some insight on whether adapting to a smaller target volume can spare critical regions of the brain, such as the hippocampus, for children with midline or suprasellar low-grade gliomas, and whether sparing such regions will preserve neurocognition,” explains Acharya.


physicsworld--2020-12-10--Madness in the method: why your notions of how science works are probably wrong

You know what the scientific method is until you try to define it : it ’ s a set of rules that scientists adopt to obtain a special kind of knowledge .
The list is orderly , teachable and straightforward , at least in principle .
But once you start spelling out the rules , you realize that they really don ’ t capture how scientists work , which is a lot messier .
In fact , the rules exclude much of what you ’ d call science , and includes even more of what you don ’ t .
@ @ @ @ @ @ @ thought it necessary to specify a “ scientific method ” at all .
In his new book The Scientific Method : an Evolution of Thinking from Darwin to Dewey , the University of Michigan historian Henry Cowles explains why some people thought it necessary to define “ scientific method ” in the first place .
Once upon a time , he writes , science meant something like knowledge itself – the facts we discover about the world rather than the sometimes unruly way we got them .
Over time , however , science @ @ @ @ @ @ @ that we obtain those facts independent of the humans who follow the method , and independent of the facts themselves .
Cowles ’ s story of this transformation begins with the naturalist Charles Darwin , who worried that professional scientists would regard his mixture of observing , recording and analysing things like barnacles , birds and worms as not rigorous enough to count as legitimate science .
Darwin ’ s account of scientific method , in short , was largely motivated by his own professional anxiety .
That anxiety motivated him to reflect @ @ @ @ @ @ @ how you thought what you thought ” .
Darwin found the answer in what he was studying – nature itself .
Just as nature takes alternative forms of life and selects among them , Darwin argued , so scientists take hypotheses and choose the most robust .
Nature has its own “ method ” , and humans acquire knowledge in an analogous way .
Darwin ’ s scientific work on living creatures is indeed rigorous , as I think contemporary readers will agree , but in the lens of our notions of scientific @ @ @ @ @ @ @ and disorganized .
He was , after all , less focused on justifying his beliefs than on understanding nature .
Following Darwin , the American “ pragmatists ” – 19th-century philosophers such as Charles Peirce and William James – developed more refined accounts of the scientific method that meshed with their philosophical concerns .
For Peirce and James , beliefs were not mental judgements or acts of faith , but habits that individuals develop through long experience .
Beliefs are principles of action that are constantly tested against the world , reshaped and @ @ @ @ @ @ @ .
The scientific method is simply a careful characterization of this process .
For Cowles , though , the turning point into the modern conception of scientific method occurred in 1910 when fellow pragmatist John Dewey published a book called How We Think .
In it , Dewey specified five steps that epitomize what we now think of as the scientific method , which was no longer intimately connected with the experience of the individual scientist but instead became a set of rules that turns those who adopt them into scientists .
These @ @ @ @ @ @ @ a felt difficulty ; ( ii ) its location and definition ; ( iii ) suggestion of possible solution ; ( iv ) development by reasoning of the bearings of the suggestion ; ( v ) further observation and experiment leading to its acceptance or rejection ; that is , the conclusion of belief or disbelief ” .
While Dewey intended these steps to describe organic human thinking and how they can function in education , it was all too easy to interpret them as a special way to think , and to @ @ @ @ @ @ @ a manual for technical practice ” .
Authors of textbooks seized on the idea as a way of explaining what science was and why it worked .
Dewey ’ s steps , Cowles says , were quickly turned into “ a symbolism of the separation of science from everyday thinking , a talisman of scientific exceptionalism ” .
The result , he argues , was “ a shared method that seemed less and less natural as the 20th century wore on ” though seemingly confirmed by episodes carefully selected from the history of @ @ @ @ @ @ @ and the early pragmatists on method had assumed that it was a way of thinking , in the end professional anxieties and the desire to elevate and insulate scientific knowledge above other kinds of knowledge transformed it into something apart from thinking , something potentially programmable into a computer .
In this way “ the scientific method ” – an abstract set of rules purporting to be about scientific practice – had been turned into a logo ; an identifier and attestation .
Reviewing Cowles ’ s book , the Stanford University historian @ @ @ @ @ @ @ scientific method ” originated not within science itself , but “ in the popular , professional , industrial , and commercial exploitation of its authority ” ( New York Review of Books 2 July 2020 ) .
Integral to this idea , she writes , was the claim that “ science held an exclusive monopoly on truth , knowledge , and authority , a monopoly for which ‘ the scientific method ’ was a guarantee ” .
Yet it is possible , Cowles argues , to reject such a view of scientific method @ @ @ @ @ @ @ the flawed , fallible activity of some imperfect , evolving creatures and as a worthy , even noble pursuit ” .
To me , the scientific method is a good example of how philosophers and others create a problem by overstressing the orderly and formal character of scientific practice , and then set themselves the task of solving the problem they have created .
As I argued in my own book The Workshop and the World , scientists working in a laboratory should be compared to a jury who place themselves for a @ @ @ @ @ @ @ .
The jury members bring their experience to the deliberations , which is essential to being able to judge fairly .
Those who think that hard-and-fast rules can be drawn up to guarantee justice have never been on a jury .

Physics World, where it appeared under the headline "Madness in the method".. Members of the Institute of Physics can enjoy the full issue Taken from the December 2020 issue of, where it appeared under the headline "Madness in the method".. Members of the Institute of Physics can enjoy the full issue via the Physics World app
A new book suggests that traditional notions about “the scientific method” are flawed and misleading, as Robert P Crease discovers
Messy business Science is less of a rigid process and more like a jury weighing up evidence in a trial. (Courtesy: iStock/Chris Ryan)
You know what the scientific method is until you try to define it: it’s a set of rules that scientists adopt to obtain a special kind of knowledge. The list is orderly, teachable and straightforward, at least in principle. But once you start spelling out the rules, you realize that they really don’t capture how scientists work, which is a lot messier. In fact, the rules exclude much of what you’d call science, and includes even more of what you don’t. You even begin to wonder why anyone thought it necessary to specify a “scientific method” at all.
In his new book The Scientific Method: an Evolution of Thinking from Darwin to Dewey, the University of Michigan historian Henry Cowles explains why some people thought it necessary to define “scientific method” in the first place. Once upon a time, he writes, science meant something like knowledge itself – the facts we discover about the world rather than the sometimes unruly way we got them. Over time, however, science came to mean a particular stepwise way that we obtain those facts independent of the humans who follow the method, and independent of the facts themselves.
Cowles’s story of this transformation begins with the naturalist Charles Darwin, who worried that professional scientists would regard his mixture of observing, recording and analysing things like barnacles, birds and worms as not rigorous enough to count as legitimate science. Darwin’s account of scientific method, in short, was largely motivated by his own professional anxiety. That anxiety motivated him to reflect on, as Cowles says, “how you thought what you thought”. Darwin found the answer in what he was studying – nature itself.
A natural view
Just as nature takes alternative forms of life and selects among them, Darwin argued, so scientists take hypotheses and choose the most robust. Nature has its own “method”, and humans acquire knowledge in an analogous way. Darwin’s scientific work on living creatures is indeed rigorous, as I think contemporary readers will agree, but in the lens of our notions of scientific method it was hopelessly anecdotal, psychological and disorganized. He was, after all, less focused on justifying his beliefs than on understanding nature.
In the lens of our notions of scientific method, Charles Darwin’s scientific work was hopelessly anecdotal, psychological and disorganized Robert P Crease
Following Darwin, the American “pragmatists” – 19th-century philosophers such as Charles Peirce and William James – developed more refined accounts of the scientific method that meshed with their philosophical concerns. For Peirce and James, beliefs were not mental judgements or acts of faith, but habits that individuals develop through long experience. Beliefs are principles of action that are constantly tested against the world, reshaped and tested again, in an endless process. The scientific method is simply a careful characterization of this process.
For Cowles, though, the turning point into the modern conception of scientific method occurred in 1910 when fellow pragmatist John Dewey published a book called How We Think. In it, Dewey specified five steps that epitomize what we now think of as the scientific method, which was no longer intimately connected with the experience of the individual scientist but instead became a set of rules that turns those who adopt them into scientists. These rules were: “(i) a felt difficulty; (ii) its location and definition; (iii) suggestion of possible solution; (iv) development by reasoning of the bearings of the suggestion; (v) further observation and experiment leading to its acceptance or rejection; that is, the conclusion of belief or disbelief”.
While Dewey intended these steps to describe organic human thinking and how they can function in education, it was all too easy to interpret them as a special way to think, and to calcify them into what Cowles calls “a manual for technical practice”. Authors of textbooks seized on the idea as a way of explaining what science was and why it worked. Dewey’s steps, Cowles says, were quickly turned into “a symbolism of the separation of science from everyday thinking, a talisman of scientific exceptionalism”. The result, he argues, was “a shared method that seemed less and less natural as the 20th century wore on” though seemingly confirmed by episodes carefully selected from the history of science.
While the ruminations of Darwin and the early pragmatists on method had assumed that it was a way of thinking, in the end professional anxieties and the desire to elevate and insulate scientific knowledge above other kinds of knowledge transformed it into something apart from thinking, something potentially programmable into a computer. In this way “the scientific method” – an abstract set of rules purporting to be about scientific practice – had been turned into a logo; an identifier and attestation.
The critical point
Reviewing Cowles’s book, the Stanford University historian Jessica Riskin recently argued that the “scientific method” originated not within science itself, but “in the popular, professional, industrial, and commercial exploitation of its authority” (New York Review of Books 2 July 2020). Integral to this idea, she writes, was the claim that “science held an exclusive monopoly on truth, knowledge, and authority, a monopoly for which ‘the scientific method’ was a guarantee”. Yet it is possible, Cowles argues, to reject such a view of scientific method and to think of science “as the flawed, fallible activity of some imperfect, evolving creatures and as a worthy, even noble pursuit”.
To me, the scientific method is a good example of how philosophers and others create a problem by overstressing the orderly and formal character of scientific practice, and then set themselves the task of solving the problem they have created. As I argued in my own book The Workshop and the World, scientists working in a laboratory should be compared to a jury who place themselves for a short time in isolation to evaluate evidence. The jury members bring their experience to the deliberations, which is essential to being able to judge fairly. Those who think that hard-and-fast rules can be drawn up to guarantee justice have never been on a jury.


physicsworld--2020-12-10--Laser ‘speed limit’ leaves defects in 3D-printed parts

3D printing techniques are transforming many areas of manufacturing .
One such technique , laser powder bed fusion ( LPBF ) , is particularly attractive because it can be used to make complex metal parts that would be difficult or impossible to manufacture conventionally .
However , it suffers from a major drawback in the form of tiny voids that weaken and degrade the metal .
Researchers in the US and China have now identified how these voids are generated , and how they become trapped as the metal solidifies – findings that could help manufacturers find ways to control @ @ @ @ @ @ @ processes .
In LPBF , a high-power laser , guided by a digital computer-aided design and drafting model , is scanned across a thin layer of metal powder .
The heat from the laser melts the metal powder in localized regions and fuses it to the layer of metal directly underneath .
While this process is highly versatile and fairly speedy , the objects it creates are often riddled with microscopic , vapour-filled pores that drastically limit the material ’ s toughness and fatigue resistance .
When a high-power laser boils molten metal @ @ @ @ @ @ @ gaps .
These gaps are known as keyholes , and it had long been suspected that they were related to defects in the finished 3D-printed part .
The exact relationship between keyholes and porosity was not , however , fully understood .
Researchers led by Anthony Rollett of Carnegie Mellon University , Tao Sun of the University of Virginia and Cang Zhao of Tsinghua University have now used high-energy X-rays derived from the Argonne National Laboratory ’ s Advanced Photon Source to image how keyholes and pores form in a titanium alloy ( @ @ @ @ @ @ @ observations was that keyholes can change shape during the process .
In particular , the tip of the structure can morph into a “ J ” shape and pinch itself off , leaving a large bubble behind .
If this bubble never reconnects to the main keyhole , Rollett explains that it can instead collapse , generating an acoustic shock wave as it does so .
This shock wave pushes the remaining pores away from the keyhole , thereby ensuring that they survive long enough to become trapped in the re-solidifying metal .
@ @ @ @ @ @ @ creating voids , Rollett and colleagues emphasize that keyholes themselves are not flaws .
In fact , they have a positive role to play in LPBF , as they increase the efficiency of the laser : once inside a keyhole , the beam undergoes multiple reflections that enhance laser light absorption .
It is only under certain conditions that the keyhole changes shape and becomes unstable , generating unwanted pores in the process .
Stay out of the “ danger zone ” To find out what these conditions are , the researchers studied @ @ @ @ @ @ @ is scanned over the metal power relates to keyhole instability .
They found that keyholes become more unstable if the laser is scanned too slowly across the metal powder .
Such slow scanning speeds increase the laser ’ s power in a localized region , allowing the metal there to overheat .
The team found that there is a well-defined boundary between stable and unstable keyholes .
“ You can think of the boundary as a speed limit , except it is the opposite of driving a car , ” Rollett says .
@ @ @ @ @ @ @ more dangerous as you go slower .
If you ’ re below the speed limit , then you are almost certainly generating a defect.
” However , he continues , “ As long as you stay out the ‘ danger zone ’ ( that is , too hot , too slow ) , the risk of leaving defects behind is quite small.
” The researchers , who report their work in Science , say they now plan to investigate ultrafast keyhole dynamics as well as other modes of instability in the laser melting @ @ @ @ @ @ @ study the development of microstructure from the rapid solidification and cooling inherent in LPBF additive manufacturing , ” Rollett tells Physics World .

Artwork showing the boundary and origin of keyhole porosity. Credit: Ye Feng and Cang Zhao, Tsinghua University
3D printing techniques are transforming many areas of manufacturing. One such technique, laser powder bed fusion (LPBF), is particularly attractive because it can be used to make complex metal parts that would be difficult or impossible to manufacture conventionally. However, it suffers from a major drawback in the form of tiny voids that weaken and degrade the metal. Researchers in the US and China have now identified how these voids are generated, and how they become trapped as the metal solidifies – findings that could help manufacturers find ways to control them, and thereby improve 3D metal-printing processes.
In LPBF, a high-power laser, guided by a digital computer-aided design and drafting model, is scanned across a thin layer of metal powder. The heat from the laser melts the metal powder in localized regions and fuses it to the layer of metal directly underneath. While this process is highly versatile and fairly speedy, the objects it creates are often riddled with microscopic, vapour-filled pores that drastically limit the material’s toughness and fatigue resistance.
“Keyhole” structures
When a high-power laser boils molten metal, it often generates deep and narrow gaps. These gaps are known as keyholes, and it had long been suspected that they were related to defects in the finished 3D-printed part. The exact relationship between keyholes and porosity was not, however, fully understood.
Researchers led by Anthony Rollett of Carnegie Mellon University, Tao Sun of the University of Virginia and Cang Zhao of Tsinghua University have now used high-energy X-rays derived from the Argonne National Laboratory’s Advanced Photon Source to image how keyholes and pores form in a titanium alloy (Ti-6l-4V) during LPBF. Among their observations was that keyholes can change shape during the process. In particular, the tip of the structure can morph into a “J” shape and pinch itself off, leaving a large bubble behind.
If this bubble never reconnects to the main keyhole, Rollett explains that it can instead collapse, generating an acoustic shock wave as it does so. This shock wave pushes the remaining pores away from the keyhole, thereby ensuring that they survive long enough to become trapped in the re-solidifying metal.
Despite the role that keyholes play in creating voids, Rollett and colleagues emphasize that keyholes themselves are not flaws. In fact, they have a positive role to play in LPBF, as they increase the efficiency of the laser: once inside a keyhole, the beam undergoes multiple reflections that enhance laser light absorption. It is only under certain conditions that the keyhole changes shape and becomes unstable, generating unwanted pores in the process.
Stay out of the “danger zone”
To find out what these conditions are, the researchers studied how the speed at which the laser is scanned over the metal power relates to keyhole instability. They found that keyholes become more unstable if the laser is scanned too slowly across the metal powder. Such slow scanning speeds increase the laser’s power in a localized region, allowing the metal there to overheat.
The team found that there is a well-defined boundary between stable and unstable keyholes. “You can think of the boundary as a speed limit, except it is the opposite of driving a car,” Rollett says. “In this case, it gets more dangerous as you go slower. If you’re below the speed limit, then you are almost certainly generating a defect.” However, he continues, “As long as you stay out the ‘danger zone’ (that is, too hot, too slow), the risk of leaving defects behind is quite small.”
The researchers, who report their work in Science, say they now plan to investigate ultrafast keyhole dynamics as well as other modes of instability in the laser melting process. “We also plan to study the development of microstructure from the rapid solidification and cooling inherent in LPBF additive manufacturing,” Rollett tells Physics World.


physicsworld--2020-12-09--Fast quantum random number generator could advance cryptography on the cheap

While world events are often difficult to predict , true randomness is surprisingly hard to find .
In recent years , physicists have turned to quantum mechanics for a solution , using the inherently unpredictable behavior of photons to generate the truly random numbers that underpin many modern cryptographic protocols .
Now , a new study promises to make this process of quantum random number generation more accessible , by showing that it is possible to produce certifiably random numbers quickly using a system built with off-the-shelf components .
When numbers are used to securely encode information , the randomness @ @ @ @ @ @ @ string of truly random numbers is one that a hacker can never guess .
In classical physics , however , all processes – even chaotic ones – are deterministic , making true randomness impossible .
To illustrate this , study lead author David Drahi , a physicist at the University of Oxford , UK , notes that classically , a simple coin flip is about as random as it gets .
However , he continues , “ if you know the mass of the coin , if you can see the coin , @ @ @ @ @ @ @ , you can predict where it is going to land ” .
Classical randomness is therefore limited by the existence of information about the physical process meant to produce it .
In the quantum world , in contrast , “ there are these fundamentally non-deterministic processes , ” says Nathan Walk , a physicist at Freie Universitat Berlin , Germany and a co-author on the study .
The results of quantum measurements , he adds , are inherently unpredictable , because their outcome does not exist in any meaningful way until the measurement @ @ @ @ @ @ @ the system has , famously , collapsed .
In developing their random number generator ( RNG ) , the study ’ s authors focused not only on producing randomness , but also on confirming that this randomness originates from a non-deterministic quantum process rather than some incidental classical noise in the experiment .
“ There is not a test you can do on a string of numbers to tell if it ’ s random , ” Walk notes .
“ You can ’ t certify strings .
But you can certify processes.
” @ @ @ @ @ @ @ a quantum certification process – essentially an additional measurement – into their protocol for generating random numbers .
They also developed rigorous theoretical proofs of its effectiveness and demonstrated that it can be implemented in practice by performing experiments using quantum light .
According to Renato Renner , a physicist at ETH Zurich in Switzerland who was not affiliated with the study , such steps are important for creating a practical system .
“ You really want to have a device that produces some certificate , otherwise you don ’ t really profit @ @ @ @ @ @ @ says .
In their experiments , the researchers send laser light ( a photonic state ) into one input of a beam splitter while the other input is kept void , resulting in a zero signal ( a vacuum state ) .
The consequent pair of output beams is then measured using two separate detectors .
Because each photon that arrives at the beam splitter has an equal ( 50 % ) chance of being reflected or transmitted , the difference between the numbers of photons recorded by each detector is unpredictable .
@ @ @ @ @ @ @ number , Drahi explains .
To confirm that randomness generated in this way is reliable and useful , the researchers performed another measurement on the photonic state before it reaches the beam splitter .
In this certification measurement , the light signal is discarded if it would not generate the desired amount of randomness at the end of the experiment .
This can happen if the laser signal contains either too few photons or too many .
Too few , and the number of possible unpredictable events will be too low for the @ @ @ @ @ @ @ many , and the detectors will hit their maximum value , making the measurement fully predictable .
The inclusion of this certification measurement means that the researchers have theorized and built a device that not only produces randomness at a fast rate of 8.05 gigabits per second , but also ensures the quality of that randomness in real time .
According to Feihu Xu , a physicist at the University of Science and Technology of China who was not involved in the work , this “ development of a formal framework to monitor @ @ @ @ @ @ @ even though some other ideas in the study have been explored before .
The researchers ’ inclusion of real-time randomness certification in their experiment also has consequences for its possible future applications , because the study ’ s theoretical framework proves that the RNG protocol is partly independent of the devices used to implement it .
For instance , the light source used can be “ untrusted ” – the randomness analysis is independent of any information about it .
As long as the light signal passes the certification measurement , the properties @ @ @ @ @ @ @ not affect the quality of final randomness .
This flexibility means that the group ’ s quantum RNG could in principle be used by “ a person or a computer that knows nothing about quantum physics ” , Walk notes .
The numbers obtained in this fashion would be reliably random regardless of operator ’ s expertise , he adds , since the protocol automatically ensures its own performance .
As a bonus , the research team managed to construct their experiment using affordable off-the-shelf components – a feature that allowed Drahi to @ @ @ @ @ @ @ some of his co-authors in Moscow .
“ It got there and it worked , ” he says , noting that it would have been virtually impossible to ship all of the components for a more exotic quantum device across a continent , have a different researcher assemble them upon arrival , and then successfully generate random numbers at the same high speed .
This level of practicality , combined with the rigorous approach to confirming the randomness of their random numbers as well as generating them , sets up this study as @ @ @ @ @ @ @ of real-world quantum devices providing reliably true randomness .
“ It could have very broad applications , ” Renner concludes .
The new quantum RNG is described in Physical Review X .

Thumbs-up for randomness David Drahi in the quantum optics lab. (Courtesy: David Drahi)
While world events are often difficult to predict, true randomness is surprisingly hard to find. In recent years, physicists have turned to quantum mechanics for a solution, using the inherently unpredictable behavior of photons to generate the truly random numbers that underpin many modern cryptographic protocols. Now, a new study promises to make this process of quantum random number generation more accessible, by showing that it is possible to produce certifiably random numbers quickly using a system built with off-the-shelf components.
When numbers are used to securely encode information, the randomness of those numbers is crucial: a string of truly random numbers is one that a hacker can never guess. In classical physics, however, all processes – even chaotic ones – are deterministic, making true randomness impossible. To illustrate this, study lead author David Drahi, a physicist at the University of Oxford, UK, notes that classically, a simple coin flip is about as random as it gets. However, he continues, “if you know the mass of the coin, if you can see the coin, if you can look at the wind, you can predict where it is going to land”. Classical randomness is therefore limited by the existence of information about the physical process meant to produce it.
In the quantum world, in contrast, “there are these fundamentally non-deterministic processes,” says Nathan Walk, a physicist at Freie Universitat Berlin, Germany and a co-author on the study. The results of quantum measurements, he adds, are inherently unpredictable, because their outcome does not exist in any meaningful way until the measurement has been made and the wavefunction of the system has, famously, collapsed.
Certifiably random
In developing their random number generator (RNG), the study’s authors focused not only on producing randomness, but also on confirming that this randomness originates from a non-deterministic quantum process rather than some incidental classical noise in the experiment. “There is not a test you can do on a string of numbers to tell if it’s random,” Walk notes. “You can’t certify strings. But you can certify processes.”
Drahi, Walk and collaborators built such a quantum certification process – essentially an additional measurement – into their protocol for generating random numbers. They also developed rigorous theoretical proofs of its effectiveness and demonstrated that it can be implemented in practice by performing experiments using quantum light. According to Renato Renner, a physicist at ETH Zurich in Switzerland who was not affiliated with the study, such steps are important for creating a practical system. “You really want to have a device that produces some certificate, otherwise you don’t really profit from any quantum advantages,” he says.
In their experiments, the researchers send laser light (a photonic state) into one input of a beam splitter while the other input is kept void, resulting in a zero signal (a vacuum state). The consequent pair of output beams is then measured using two separate detectors. Because each photon that arrives at the beam splitter has an equal (50%) chance of being reflected or transmitted, the difference between the numbers of photons recorded by each detector is unpredictable. It is bound to be a random number, Drahi explains.
To confirm that randomness generated in this way is reliable and useful, the researchers performed another measurement on the photonic state before it reaches the beam splitter. In this certification measurement, the light signal is discarded if it would not generate the desired amount of randomness at the end of the experiment. This can happen if the laser signal contains either too few photons or too many. Too few, and the number of possible unpredictable events will be too low for the measurement to be sufficiently random. Too many, and the detectors will hit their maximum value, making the measurement fully predictable.
The inclusion of this certification measurement means that the researchers have theorized and built a device that not only produces randomness at a fast rate of 8.05 gigabits per second, but also ensures the quality of that randomness in real time. According to Feihu Xu, a physicist at the University of Science and Technology of China who was not involved in the work, this “development of a formal framework to monitor and certify the randomness” stands out even though some other ideas in the study have been explored before.
“Untrusted” light source
The researchers’ inclusion of real-time randomness certification in their experiment also has consequences for its possible future applications, because the study’s theoretical framework proves that the RNG protocol is partly independent of the devices used to implement it. For instance, the light source used can be “untrusted” – the randomness analysis is independent of any information about it. As long as the light signal passes the certification measurement, the properties of the device that produced it do not affect the quality of final randomness. This flexibility means that the group’s quantum RNG could in principle be used by “a person or a computer that knows nothing about quantum physics”, Walk notes. The numbers obtained in this fashion would be reliably random regardless of operator’s expertise, he adds, since the protocol automatically ensures its own performance.
As a bonus, the research team managed to construct their experiment using affordable off-the-shelf components – a feature that allowed Drahi to ship the experimental setup from Oxford to some of his co-authors in Moscow. “It got there and it worked,” he says, noting that it would have been virtually impossible to ship all of the components for a more exotic quantum device across a continent, have a different researcher assemble them upon arrival, and then successfully generate random numbers at the same high speed. This level of practicality, combined with the rigorous approach to confirming the randomness of their random numbers as well as generating them, sets up this study as a promising starting point for the development of real-world quantum devices providing reliably true randomness. “It could have very broad applications,” Renner concludes.
The new quantum RNG is described in Physical Review X.


physicsworld--2020-12-09--Acoustic metamaterial helps moths avoid being eaten by bats

Natural acoustic metamaterials found on the wings of some moths could help the insects avoid being eaten by bats – according to Marc Holderied and colleagues at the UK ’ s University of Bristol .
By doing a combination of simulations and experiments , the team found that coupled vibrations of wing scales enable the moths to absorb ultrasound over a broad range of frequencies .
The discovery could lead to the development of bio-inspired sound proofing materials with the potential to perform far better than current designs .
As a key source of prey for echolocating bats , moths @ @ @ @ @ @ @ .
While many moth species are very good at hearing the ultrasound used by bats , many other species lack this ability .
Instead , these insects use acoustic camouflage to avoid detection .
This involves sound waves being scattered or absorbed by sub-wavelength size structures on the moth ’ s wings .
Scientists have studied similar metamaterial structures on butterfly wings , which can have special optical properties that produce vibrant iridescent colours .
However , these scales are not suitable for dampening sound , leading Holderied ’ s team to ask @ @ @ @ @ @ @ ?
In their study , the researchers investigated sound dampening mechanisms in two earless species of moth – Antheraea pernyi and Dactyloceras lucina .
They first used a combination of scanning electron microscopy and micro-computed tomography to study the shapes and arrangements of the wing scales of both species .
The images revealed overlapping tile arrangements of paddle-shaped scales , with stalks attached to stiff , lightweight membranes .
These layers were typically less than 0.3 mm thick , which is far shorter than the ultrasound wavelengths used by bats .
In subsequent @ @ @ @ @ @ @ these layers could significantly dampen sound waves within a broad range of ultrasound frequencies .
Sound at frequencies as low as 20 kHz is absorbed and the layers display a maximum absorption of 72 % at 78 kHz .
Using computer models , the researchers worked out the mechanisms responsible for these advanced absorption properties .
The scales vary significantly in terms of their sizes and shapes – depending on their location on the wing .
The team found that this leads to variations in the natural resonant frequencies of the scales .
@ @ @ @ @ @ @ coupling between the vibrations of neighbouring scales on shared membranes .
This means that collectively , the scales act to dampen acoustic waves over a broad range of frequencies .
This is wholly unlike the acoustic behaviour of butterfly wings , whose uniform scales can only resonate in small , localized clusters .
The wings of the two moth species are the first confirmed examples of natural acoustic metamaterials .
Their design could be mimicked to create lightweight , ultrathin soundproofing panels , with the potential to out-perform the thick , porous absorbers @ @ @ @ @ @ @ research is described in Proceedings of the National Academy of Science .

Ultrasound absorbers: false-colour 3D representation of a 0.21×0.28 mm wing section of the moth Lasiocampa quercus showing the structure, diversity, and arrangement of base scales (orange) and cover scales (blue and yellow). (Courtesy: Simon Reichel/Thomas Neil/Zhiyuan Shen/Marc Holderied)
Natural acoustic metamaterials found on the wings of some moths could help the insects avoid being eaten by bats – according to Marc Holderied and colleagues at the UK’s University of Bristol. By doing a combination of simulations and experiments, the team found that coupled vibrations of wing scales enable the moths to absorb ultrasound over a broad range of frequencies. The discovery could lead to the development of bio-inspired sound proofing materials with the potential to perform far better than current designs.
As a key source of prey for echolocating bats, moths are under evolutionary pressure to evade capture. While many moth species are very good at hearing the ultrasound used by bats, many other species lack this ability. Instead, these insects use acoustic camouflage to avoid detection. This involves sound waves being scattered or absorbed by sub-wavelength size structures on the moth’s wings.
Scientists have studied similar metamaterial structures on butterfly wings, which can have special optical properties that produce vibrant iridescent colours. However, these scales are not suitable for dampening sound, leading Holderied’s team to ask what is different about moths’ wings? In their study, the researchers investigated sound dampening mechanisms in two earless species of moth – Antheraea pernyi and Dactyloceras lucina.
Microscope and tomography
They first used a combination of scanning electron microscopy and micro-computed tomography to study the shapes and arrangements of the wing scales of both species. The images revealed overlapping tile arrangements of paddle-shaped scales, with stalks attached to stiff, lightweight membranes. These layers were typically less than 0.3 mm thick, which is far shorter than the ultrasound wavelengths used by bats.
In subsequent experiments, Holderied and colleagues showed that these layers could significantly dampen sound waves within a broad range of ultrasound frequencies. Sound at frequencies as low as 20 kHz is absorbed and the layers display a maximum absorption of 72% at 78 kHz.
Using computer models, the researchers worked out the mechanisms responsible for these advanced absorption properties. The scales vary significantly in terms of their sizes and shapes – depending on their location on the wing. The team found that this leads to variations in the natural resonant frequencies of the scales. Furthermore, their models revealed a strong coupling between the vibrations of neighbouring scales on shared membranes. This means that collectively, the scales act to dampen acoustic waves over a broad range of frequencies. This is wholly unlike the acoustic behaviour of butterfly wings, whose uniform scales can only resonate in small, localized clusters.
The wings of the two moth species are the first confirmed examples of natural acoustic metamaterials. Their design could be mimicked to create lightweight, ultrathin soundproofing panels, with the potential to out-perform the thick, porous absorbers currently used for sound insulation.
The research is described in Proceedings of the National Academy of Science.


physicsworld--2020-12-09--Online MR imaging verifies proton beam range

Proton therapy holds the promise of delivering highly conformal dose distributions to a target tumour volume , due to the finite range of the proton beam .
However , the accuracy of this delivery can be affected by factors such as target motion , which may lead to overdosing regions of healthy tissue and underdosage of the tumour volume .
“ Proton therapy is very sensitive to changes in the position and shape of the patient as a whole and the patient ’ s organs , both between fractions and within one fraction , ” explained Sonja Schellhammer , recipient @ @ @ @ @ @ @ best abstract presented at the recent ESTRO 2020 congress .
“ It would be ideal if we could monitor the proton range simultaneously with the patient anatomy in real time.
” Currently , verification of proton range is mostly based on detection of secondary radiation such as prompt gamma rays or acoustic waves .
Another indirect approach involves using MRI to visualize long-term radiation-induced biological effects .
MRI benefits from excellent soft-tissue contrast , offers real-time imaging , delivers no radiation dose and could prove ideal for integration with proton therapy .
With @ @ @ @ @ @ @ Dresden-Rossendorf ( HZDR ) are working to integrate proton therapy with MRI .
Previous studies by others have shown that MRI can determine proton beam range using an off-line approach , with contrast-enhanced images recorded weeks or months after irradiation .
“ The question we asked in this study , is whether it would be possible to visualize the proton beam with online MRI during irradiation , ” Schellhammer explained in her ESTRO presentation .
To test this idea , the researchers designed a phantom experiment using the Dresden proton therapy facility , @ @ @ @ @ @ @ and ASG Superconductors – Paramed MRI Unit , had integrated an open 0.22T MRI scanner with a fixed proton research beamline .
The proton beam was incident upon a plastic bottle filled with deionized water and placed in the scanner ’ s magnetic isocentre .
The team used a PMMA range shifter to tailor the proton energy such that the beam stopped within the bottle , and recorded MR image slices through the centre of the beam .
The researchers performed one scan during proton irradiation , and three immediately after irradiation , @ @ @ @ @ @ @ For two of the six MR sequences – the proton density-weighted gradient echo ( GE ) sequence and the inversion recovery gradient echo ( IRGE ) sequence – they saw signatures in the MR image that were likely induced by the proton beam .
They also observed that these signals didn ’ t disappear immediately after irradiation , but faded over tens of seconds .
“ With this experiment , we confirmed that there is a measurable proton beam-induced signal on the MR images , ” said Schellhammer .
“ We then asked @ @ @ @ @ @ @ verification.
” Next , the team irradiated the water phantom using proton beams with energies of between 190 and 225 MeV ( corresponding to different beam ranges ) .
The observed MR signal changed in depth with increasing range , and the measured residual ranges agreed with the calculated values to within 2 mm .
Repeating the experiment with four different beam currents ( 1 , 3 , 9 and 27 nA , corresponding to dose rates of 1.7 , 5 , 15 and 45 Gy/s ) demonstrated that the signal intensity increased @ @ @ @ @ @ @ ( recorded for 20 s ) were only visible using beam currents of 3 nA or more .
To investigate whether this approach may work in a patient , the team used the same experimental set-up to irradiate other liquid phantoms ( ethanol and petroleum ) , highly viscose materials ( sugar syrup , mayonnaise and gelatine ) and a tissue-mimicking material ( a pork chop ) .
While images of ethanol and petroleum showed similar beam-induced signals as seen with water , for the more viscose materials and the pork chop , @ @ @ @ @ @ @ appears that the signal is only present in liquids , so likely may not be transferable to patients , ” explained Schellhammer .
“ But this approach may well prove useful for quality assurance of proton range in future hybrid MR-proton therapy systems.
” The mechanisms underlying the observed effects are still to be unravelled .
The most probable hypothesis , Schellhammer suggested , is that irradiating a liquid increases its temperature locally , creating a density difference in which the lighter heated water rises out of the imaging volume and creates a @ @ @ @ @ @ @ to test this hypothesis , she noted .
“ I have demonstrated that proton beam range can be accurately verified with online MRI , ” Schellhammer concluded .
“ However , today only at high doses and in fluid-filled phantoms .
The method holds potential for dosimeter-free online quality assurance of MR-integrated proton therapy .
Further research towards MR-based range proton beam verification is clearly justified .
”

MR images acquired using GE (top row) and IRGE (bottom row) sequences during proton irradiation, with expected proton ranges of (left to right) 4.2, 6.4, 8.6 and 12.2 cm at 9 nA. (Courtesy: S Schellhammer, HZDR/OncoRay)
Proton therapy holds the promise of delivering highly conformal dose distributions to a target tumour volume, due to the finite range of the proton beam. However, the accuracy of this delivery can be affected by factors such as target motion, which may lead to overdosing regions of healthy tissue and underdosage of the tumour volume.
“Proton therapy is very sensitive to changes in the position and shape of the patient as a whole and the patient’s organs, both between fractions and within one fraction,” explained Sonja Schellhammer, recipient of the Donal Hollywood Award for the best abstract presented at the recent ESTRO 2020 congress. “It would be ideal if we could monitor the proton range simultaneously with the patient anatomy in real time.”
Currently, verification of proton range is mostly based on detection of secondary radiation such as prompt gamma rays or acoustic waves. Another indirect approach involves using MRI to visualize long-term radiation-induced biological effects. MRI benefits from excellent soft-tissue contrast, offers real-time imaging, delivers no radiation dose and could prove ideal for integration with proton therapy.
With this in mind, scientists at OncoRay/Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are working to integrate proton therapy with MRI. Previous studies by others have shown that MRI can determine proton beam range using an off-line approach, with contrast-enhanced images recorded weeks or months after irradiation. “The question we asked in this study, is whether it would be possible to visualize the proton beam with online MRI during irradiation,” Schellhammer explained in her ESTRO presentation.
To test this idea, the researchers designed a phantom experiment using the Dresden proton therapy facility, where OncoRay, in collaboration with IBA and ASG Superconductors – Paramed MRI Unit, had integrated an open 0.22T MRI scanner with a fixed proton research beamline. The proton beam was incident upon a plastic bottle filled with deionized water and placed in the scanner’s magnetic isocentre. The team used a PMMA range shifter to tailor the proton energy such that the beam stopped within the bottle, and recorded MR image slices through the centre of the beam.
The researchers performed one scan during proton irradiation, and three immediately after irradiation, using a variety of MR sequences. For two of the six MR sequences – the proton density-weighted gradient echo (GE) sequence and the inversion recovery gradient echo (IRGE) sequence – they saw signatures in the MR image that were likely induced by the proton beam. They also observed that these signals didn’t disappear immediately after irradiation, but faded over tens of seconds.
“With this experiment, we confirmed that there is a measurable proton beam-induced signal on the MR images,” said Schellhammer. “We then asked whether this could be useful for range verification.”
Next, the team irradiated the water phantom using proton beams with energies of between 190 and 225 MeV (corresponding to different beam ranges). The observed MR signal changed in depth with increasing range, and the measured residual ranges agreed with the calculated values to within 2 mm.
Repeating the experiment with four different beam currents (1, 3, 9 and 27 nA, corresponding to dose rates of 1.7, 5, 15 and 45 Gy/s) demonstrated that the signal intensity increased with increasing beam current. MR images (recorded for 20 s) were only visible using beam currents of 3 nA or more.
To investigate whether this approach may work in a patient, the team used the same experimental set-up to irradiate other liquid phantoms (ethanol and petroleum), highly viscose materials (sugar syrup, mayonnaise and gelatine) and a tissue-mimicking material (a pork chop). While images of ethanol and petroleum showed similar beam-induced signals as seen with water, for the more viscose materials and the pork chop, no signal was seen.
“It appears that the signal is only present in liquids, so likely may not be transferable to patients,” explained Schellhammer. “But this approach may well prove useful for quality assurance of proton range in future hybrid MR-proton therapy systems.”
The mechanisms underlying the observed effects are still to be unravelled. The most probable hypothesis, Schellhammer suggested, is that irradiating a liquid increases its temperature locally, creating a density difference in which the lighter heated water rises out of the imaging volume and creates a signal void. Further investigations are needed to test this hypothesis, she noted.
“I have demonstrated that proton beam range can be accurately verified with online MRI,” Schellhammer concluded. “However, today only at high doses and in fluid-filled phantoms. The method holds potential for dosimeter-free online quality assurance of MR-integrated proton therapy. Further research towards MR-based range proton beam verification is clearly justified.”