Pollution from microplastics – small plastic particles less than 5 mm in size – poses an ongoing threat to human health. Independent studies have found microplastics in human tissues and within the bloodstream. And as blood circulates throughout the body and through vital organs, these microplastics reach can critical regions and lead to tissue dysfunction and disease. Microplastics can also cause functional irregularities in the brain, but exactly how they exert neurotoxic effects remains unclear.

A research collaboration headed up at the Chinese Research Academy of Environmental Sciences and Peking University has shed light on this conundrum. In a series of cerebral imaging studies reported in Science Advances, the researchers tracked the progression of fluorescent microplastics through the brains of mice. They found that microplastics entering the bloodstream become engulfed by immune cells, which then obstruct blood vessels in the brain and cause neurobehavioral abnormalities.

“Understanding the presence and the state of microplastics in the blood is crucial. Therefore, it is essential to develop methods for detecting microplastics within the bloodstream,” explains principal investigator Haipeng Huang from Peking University. “We focused on the brain due to its critical importance: if microplastics induce lesions in this region, it could have a profound impact on the entire body. Our experimental technology enables us to observe the blood vessels within the brain and detect microplastics present in these vessels.”

In vivo imaging

Huang and colleagues developed a microplastics imaging system by integrating a two-photon microscopy system with fluorescent plastic particles and demonstrated that it could image brain blood vessels in awake mice. They then fed five mice with water containing 5-µm diameter fluorescent microplastics. After a couple of hours, fluorescence images revealed microplastics within the animals’ cerebral vessels.

As they move through rapidly flowing blood, the microplastics generate a fluorescence signal resembling a lightning bolt, which the researchers call a “microplastic flash” (MP-flash). This MP-flash was observed in four of the mice, with the entire MP-flash trajectory captured in a single imaging frame of less than 208 ms.

Three hours after administering the microplastics, the researchers observed fluorescent cells in the bloodstream. The signals from these cells were of comparable intensity to the MP-flash signal, suggesting that the cells had engulfed microplastics in the blood to create microplastic-labelled cells (MPL-cells). The team note that the microplastics did not directly attach to the vessel wall or cross into brain tissue.

To test this idea further, the researchers injected microplastics directly into the bloodstream of the mice. Within minutes, they saw the MP-Flash signal in the brain’s blood vessels, and roughly 6 min later MPL-cells appeared. No fluorescent cells were seen in non-treated mice. Flow cytometry of mouse blood after microplastics injection revealed that the MPL-cells, which were around 21 µm in dimeter, were immune cells, mostly neutrophils and macrophages.

Tracking these MPL-cells revealed that they sometimes became trapped within a blood vessel. Some cells exited the imaging field following a period of obstruction while others remained in cerebral vessels for extended durations, in some instances for nearly 2.5 h of imaging. The team also found that one week after injection, the MPL-cells had still not cleared, although the density of blockages was much reduced.

“[While] most MPL-cells flow rapidly with the bloodstream, a small fraction become trapped within the blood vessels,” Huang tells Physics World. “We provide an example where an MPL-cell is trapped at a microvascular turn and, after some time, is fortunate enough to escape. Many obstructed cells are less fortunate, as the blockage may persist for several weeks. Obstructed cells can also trigger a crash-like chain reaction, resulting in several MPL-cells colliding in a single location and posing significant risks.”

The MPL-cell blockages also impeded blood flow in the mouse brain. Using laser speckle contrast imaging to monitor blood flow, the researchers saw reduced perfusion in the cerebral cortical vessels, notably at 30 min after microplastics injection and particularly affecting smaller vessels.

Changing behaviour

Lastly, Huang and colleagues investigated whether the reduced blood supply to the brain caused by cell blockages caused behavioural changes in the mice. In an open-field experiment (used to assess rodents’ exploratory behaviour) mice injected with microplastics travelled shorter distances at lower speeds than mice in the control group.

The Y-maze test for assessing memory also showed that microplastics-treated mice travelled smaller total distances than control animals, with a significant reduction in spatial memory. Tests to evaluate motor coordination and endurance revealed that microplastics additionally inhibited motor abilities. By day 28 after injection, these behavioural impairments were restored, corresponding with the observed recovery of MPL-cell obstruction in the cerebral vasculature at 28 days.

The researchers conclude that their study demonstrates that microplastics harm the brain indirectly – via cell obstruction and disruption of blood circulation – rather than directly penetrating tissue. They emphasize, however, that this mechanism may not necessarily apply to humans, who have roughly 1200 times greater volume of circulating blood volume than mice and significantly different vascular diameters.

“In the future, we plan to collaborate with clinicians,” says Huang. “We will enhance our imaging techniques for the detection of microplastics in human blood vessels, and investigate whether ‘MPL-cell-car-crash’ happens in human. We anticipate that this research will lead to exciting new discoveries.”

Huang emphasizes how the use of fluorescent microplastic imaging technology has fundamentally transformed research in this field over the past five years. “In the future, advancements in real-time imaging of depth and the enhanced tracking ability of microplastic particles in vivo may further drive innovation in this area of study,” he says.

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Braille is a tactile writing system that helps people who are blind or partially sighted acquire information by touching patterns of tiny raised dots. Braille uses combinations of six dots (two columns of three) to represent letters, numbers and punctuation. But learning to read braille can be challenging, particularly for those who lose their sight later in life, prompting researchers to create automated braille recognition technologies.

One approach involves simply imaging the dots and using algorithms to extract the required information. This visual method, however, struggles with the small size of braille characters and can be impacted by differing light levels. Another option is tactile sensing; but existing tactile sensors aren’t particularly sensitive, with small pressure variations leading to incorrect readings.

To tackle these limitations, researchers from Beijing Normal University and Shenyang Aerospace University in China have employed an optical fibre ring resonator (FRR) to create a tactile braille recognition system that accurately reads braille in real time.

“Current braille readers often struggle with accuracy and speed, especially when it comes to dynamic reading, where you move your finger across braille dots in real time,” says team leader Zhuo Wang. “I wanted to create something that could read braille more reliably, handle slight variations in pressure and do it quickly. Plus, I saw an opportunity to apply cutting-edge technology – like flexible optical fibres and machine learning – to solve this challenge in a novel way.”

Flexible fibre sensor

At the core of the braille sensor is the optical FRR – a resonant cavity made from a loop of fibre containing circulating laser light. Wang and colleagues created the sensing region by embedding an optical fibre in flexible polymer and connecting it into the FRR ring. Three small polymer protrusions on top of the sensor act as probes to transfer the applied pressure to the optical fibre. Spaced 2.5 mm apart to align with the dot spacing, each protrusion responds to the pressure from one of the three braille dots (or absence of a dot) in a vertical column.

As the sensor is scanned over the braille surface, the pressure exerted by the raised dots slightly changes the length and refractive index of the fibre, causing tiny shifts in the frequency of the light travelling through the FRR. The device employs a technique called Pound-Drever-Hall (PDH) demodulation to “lock” onto these shifts, amplify them and convert them into readable data.

“The PDH demodulation curve has an extremely steep linear slope, which means that even a very tiny frequency shift translates into a significant, measurable voltage change,” Wang explains. “As a result, the system can detect even the smallest variations in pressure with remarkable precision. The steep slope significantly enhances the system’s sensitivity and resolution, allowing it to pick up subtle differences in braille dots that might be too small for other sensors to detect.”

The eight possible configurations of three dots generate eight distinct pressure signals, with each braille character defined by two pressure outputs (one per column). Each protrusion has a slightly different hardness level, enabling the sensor to differentiate pressures from each dot. Rather than measuring each dot individually, the sensor reads the overall pressure signal and instantly determines the combination of dots and the character they correspond to.

The researchers note that, in practice, the contact force may vary slightly during the scanning process, resulting in the same dot patterns exhibiting slightly different pressure signals. To combat this, they used neural networks trained on large amounts of experimental data to correctly classify braille patterns, even with small pressure variations.

“This design makes the sensor incredibly efficient,” Wang explains. “It doesn’t just feel the braille, it understands it in real time. As the sensor slides over a braille board, it quickly decodes the patterns and translates them into readable information. This allows the system to identify letters, numbers, punctuation, and even words or poems with remarkable accuracy.”

Stable and accurate

Measurements on the braille sensor revealed that it responds to pressures of up to 3 N, as typically exerted by a finger when touching braille, with an average response time of below 0.1 s, suitable for fast dynamic braille reading. The sensor also exhibited excellent stability under temperature or power fluctuations.

To assess its ability to read braille dots, the team used the sensor to read eight different arrangements of three dots. Using a multilayer perceptron (MLP) neural network, the system effectively distinguished the eight different tactile pressures with a classification accuracy of 98.57%.

Next, the researchers trained a long short-term memory (LSTM) neural network to classify signals generated by five English words. Here, the system demonstrated a classification accuracy of 100%, implying that slight errors in classifying signals in each column will not affect the overall understanding of the braille.

Finally, they used the MLP-LSTM model to read short sentences, either sliding the sensor manually or scanning it electronically to maintain a consistent contact force. In both cases, the sensor accurately recognised the phrases.

The team concludes that the sensor can advance intelligent braille recognition, with further potential in smart medical care and intelligent robotics. The next phase of development will focus on making the sensor more durable, improving the machine learning models and making it scalable.

“Right now, the sensor works well in controlled environments; the next step is to test its use by different people with varying reading styles, or under complex application conditions,” Wang tells Physics World. “We’re also working on making the sensor more affordable so it can be integrated into devices like mobile braille readers or wearables.”

The sensor is described in Optics Express.

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Brain tumours are notoriously difficult to treat, resisting conventional treatments such as radiation therapy, where the deliverable dose is limited by normal tissue tolerance. To better protect healthy tissues, researchers are turning to microbeam radiation therapy (MRT), which uses spatially fractionated beams to spare normal tissue while effectively killing cancer cells.

MRT is delivered using arrays of ultrahigh-dose rate synchrotron X-ray beams tens of microns wide (high-dose peaks) and spaced hundreds of microns apart (low-dose valleys). A research team from the Centre for Medical Radiation Physics at the University of Wollongong in Australia has now demonstrated that combining MRT with targeted radiosensitizers – such as nanoparticles or anti-cancer drugs – can further boost treatment efficacy, reporting their findings in Cancers.

“MRT is famous for its healthy tissue-sparing capabilities with good tumour control, whilst radiosensitizers are known for their ability to deliver targeted dose enhancement to cancer,” explains first author Michael Valceski. “Combining these modalities just made sense, with their synergy providing the potential for the best of both worlds.”

Enhancement effects

Valceski and colleagues combined MRT with thulium oxide nanoparticles, the chemotherapy drug methotrexate and the radiosensitizer iododeoxyuridine (IUdR). They examined the response of monolayers of rodent brain cancer cells to various therapy combinations. They also compared conventional broadbeam orthovoltage X-ray irradiation with synchrotron broadbeam X-rays and synchrotron MRT.

Synchrotron irradiations were performed on the Imaging and Medical Beamline at the ANSTO Australian Synchrotron, using ultrahigh dose rates of 74.1 Gy/s for broadbeam irradiation and 50.3 Gy/s for MRT. The peak-to-valley dose ratio (PVDR, used to characterize an MRT field) of this set-up was measured as 8.9.

Using a clonogenic assay to measure cell survival, the team observed that synchrotron-based irradiation enhanced cell killing compared with conventional irradiation at the same 5 Gy dose (for MRT this is the valley dose, the peaks experience 8.9 times higher dose), demonstrating the increased cell-killing effect of these ultrahigh-dose rate X-rays.

Adding radiosensitizers further increased the impact of synchrotron broadbeam irradiation, with DNA-localized IUdR killing more cells than cytoplasm-localized nanoparticles. Methotrexate, meanwhile, halved cell survival compared with conventional irradiation.

The team observed that at 5 Gy, MRT showed equivalent cell killing to synchrotron broadbeam irradiation. Valceski explains that this demonstrates MRT’s tissue-sparing potential, by showing how MRT can maintain treatment efficacy while simultaneously protecting healthy cells.

MRT also showed enhanced cell killing when combined with radiosensitizers, with the greatest effect seen for IUdR and IUdR plus methotrexate. This local dose enhancement, attributed to the DNA localization of IUdR, could further improve the tissue-sparing capabilities of MRT by enabling a lower per-fraction dose to reduce patient exposure whilst maintaining tumour control.

Imaging valleys and peaks

To link the biological effects with the physical collimation of MRT, the researchers performed confocal microscopy (at the Fluorescence Analysis Facility in Molecular Horizons, University of Wollongong) to investigate DNA damage following treatment at 0.5 and 5 Gy. Twenty minutes after irradiation, they imaged fixed cells to visualize double-strand DNA breaks (DSBs), as shown by γH2AX foci (representing a nuclear DSB site).

The images verified that the cells’ biological responses corresponded with the MRT beam patterns, with the 400 µm microbeam spacing clearly seen in all treated cells, both with and without radiosensitizers.

In the 0.5 Gy images, the microbeam tracks were consistent in width, while the 5 Gy MRT tracks were wider as DNA damage spread from peaks into the valleys. This radiation roll-off was also seen with IUdR and IUdR plus methotrexate, with numerous bright foci visible in the valleys, demonstrating dose enhancement and improved cancer-killing with these radiosensitizers.

The researchers also analysed the MRT beam profiles using the γH2AX foci intensity across the images. Cells treated with radiosensitizers had broadened peaks, with the largest effect seen with the nanoparticles. As nanoparticles can be designed to target tumours, this broadening (roughly 30%) can be used to increase the radiation dose to cancer cells in nearby valleys.

“Peak broadening adds a novel benefit to radiosensitizer-enhanced MRT. The widening of the peaks in the presence of nanoparticles could potentially ‘engulf’ the entire cancer, and only the cancer, whilst normal tissues without nanoparticles retain the protection of MRT tissue sparing,” Valceski explains. “This opens up the potential for MRT radiosurgery, something our research team has previously investigated.”

Finally, the researchers used γH2AX foci data for each peak and valley to determine a biological PVDR. The biological PDVR values matched the physical PVDR of 8.9, confirming for the first time a direct relationship between physical dose delivered and DSBs induced in the cancer cells. They note that adding radiosensitizers generally lowered the biological PVDRs from the physical value, likely due to additional DSBs induced in the valleys.

The next step will be to perform preclinical studies of MRT. “Trials to assess the efficacy of this multimodal therapy in treating aggressive cancers in vivo are key, especially given the theragnostic potential of nanoparticles for image-guided treatment and precision planning, as well as cancer-specific dose enhancement,” senior author Moeava Tehei tells Physics World. “Considering the radiosurgical potential of stereotactic, radiosensitizer-enhanced MRT fractions, we can foresee a revolutionary multimodal technique with curative potential in the near future.”

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Assessing lung function is crucial for diagnosing and monitoring respiratory diseases. The most common way to do this is using spirometry, which measures the amount and speed of air that a person can inhale and exhale. Spirometry, however, is insensitive to early disease and cannot detect heterogeneity in lung function. Techniques such as chest radiography or CT provide more detailed spatial information, but are not ideal for long-term monitoring as they expose patients to ionizing radiation.

Now, a team headed up at Newcastle University in the UK has demonstrated a new lung MR imaging technique that provides quantitative and spatially localized assessment of pulmonary ventilation. The researchers also show that the MRI scans – recorded after the patient inhales a safe gas mixture – can track improvements in lung function following medication.

Although conventional MRI of the lungs is challenging, lung function can be assessed by imaging the distribution of an inhaled gas, most commonly hyperpolarized ³He or ¹²⁹Xe. These gases can be expensive, however, and the magnetic preparation step requires extra equipment and manpower. Instead, project leader Pete Thelwall and colleagues are investigating ¹⁹F-MRI of inhaled perfluoropropane – an inert gas that does not require hyperpolarization to be visible in an MRI scan.

“Conventional MRI detects magnetic signals from hydrogen nuclei in water to generate images of water distribution,” Thelwall explains. “Perfluoropropane is interesting to us as we can also get an MRI signal from fluorine nuclei and visualize the distribution of inhaled perfluoropropane. We assess lung ventilation by seeing how well this MRI-visible gas moves into different parts of the lungs when it is inhaled.”

Testing the new technique

The researchers analysed ¹⁹F-MRI data from 38 healthy participants, 35 with asthma and 21 with chronic obstructive pulmonary disease (COPD), reporting their findings in Radiology. For the ¹⁹F-MRI scans, participants were asked to inhale a 79%/21% mixture of perfluoropropane and oxygen and then hold their breath. All subjects also underwent spirometry and an anatomical ¹H-MRI scan, and those with respiratory disease withheld their regular bronchodilator medication before the MRI exams.

After co-registering each subject’s anatomical (¹H) and ventilation (¹⁹F) images, the researchers used the perfluoropropane distribution in the images to differentiate ventilated and non-ventilated lung regions. They then calculated the ratio of non-ventilated lung to total lung volume, a measure of ventilation dysfunction known as the ventilation defect percentage (VDP).

Healthy subjects had a mean VDP of 1.8%, reflecting an even distribution of inhaled gas throughout their lungs and well-preserved lung function. In comparison, the patient groups showed elevated mean VDP values – 8.3% and 27.2% for those with asthma and COPD, respectively – reflecting substantial ventilation heterogeneity.

In participants with respiratory disease, the team also performed ¹⁹F-MRI after treatment with salbutamol, a common inhaler. They found that the MR images revealed changes in regional ventilation in response to this bronchodilator therapy.

Post-treatment images of patients with asthma showed an increase in lung regions containing perfluoropropane, reflecting the reversible nature of this disease. Participants with COPD generally showed less obvious changes following treatment, as expected for this less reversible disease. Bronchodilator therapy reduced the mean VDP by 33% in participants with asthma (from 8.3% to 5.6%) and by 14% in those with COPD (from 27.2% to 23.3%).

The calculated VDP values were negatively associated with standard spirometry metrics. However, the team note that some participants with asthma exhibited normal spirometry but an elevated mean VDP (6.7%) compared with healthy subjects. This finding suggests that the VDP acquired by ¹⁹F-MRI of inhaled perfluoropropane is more sensitive to subclinical disease than conventional spirometry.

Supporting lung transplants

In a separate study reported in JHLT Open, Thelwall and colleagues used dynamic ¹⁹F-MRI of inhaled perfluoropropane to visualize the function of transplanted lungs. Approximately half of lung transplant recipients experience organ rejection, known as chronic lung allograft dysfunction (CLAD), within five years of transplantation.

Transplant recipients are monitored frequently using pulmonary function tests and chest X-rays. But by the time CLAD is diagnosed, irreversible lung damage may already have occurred. The team propose that ¹⁹F-MRI may find subtle early changes in lung function that could help detect rejection earlier.

The researchers studied 10 lung transplant recipients, six of whom were experiencing chronic rejection. They used a wash-in and washout technique, acquiring breath-hold ¹⁹F-MR images while the patient inhaled a perfluoropropane/oxygen mixture (wash-in acquisitions), followed by scans during breathing of room air (washout acquisitions).

The MR images revealed quantifiable differences in regional ventilation in participants with and without CLAD. In those with chronic rejection, scans showed poorer air movement to the edges of the lungs, likely due to damage to the small airways, a typical feature of CLAD. By detecting such changes in lung function, before signs of damage are seen in other tests, it’s possible that this imaging method might help inform patient treatment decisions to better protect the transplanted lungs from further damage.

The studies fall squarely within the field of clinical research, requiring non-standard MRI hardware to detect fluorine nuclei. But Thelwall sees a pathway towards introducing ¹⁹F-MRI in hospitals, noting that scanner manufacturers have brought systems to market that can detect nuclei other than ¹H in routine diagnostic scans. Removing the requirement for hyperpolarization, combined with the lower relative cost of perfluoropropane inhalation (approximately £50 per study participant), could also help scale this method for use in the clinic.

The team is currently working on a study looking at how MRI assessment of lung function could help reduce the side effects associated with radiotherapy for lung cancer. The idea is to design a radiotherapy plan that minimizes dose to lung regions with good function, whilst maintaining effective cancer treatment.

“We are also looking at how better lung function measurements might help the development of new treatments for lung disease, by being able to see the effects of new treatments earlier and more accurately than current lung function measurements used in clinical trials,” Thelwall tells Physics World.

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Magnetic particle imaging (MPI) is an emerging medical imaging modality with the potential for high sensitivity and spatial resolution. Since its introduction back in 2005, researchers have built numerous preclinical MPI systems for small-animal studies. But human-scale MPI remains an unmet challenge. Now, a team headed up at the Athinoula A Martinos Center for Biomedical Imaging has built a proof-of-concept human brain-scale MPI system and demonstrated its potential for functional neuroimaging.

MPI works by visualizing injected superparamagnetic iron oxide nanoparticles (SPIONs). SPIONs exhibit a nonlinear response to an applied magnetic field: at low fields they respond roughly linearly, but at larger field strengths, particle response saturates. MPI exploits this behaviour by creating a magnetic field gradient across the imaging space with a field-free line (FFL) in the centre. Signals are only generated by the unsaturated SPIONs inside the FFL, which can be scanned through the imaging space to map SPION distribution.

First author Eli Mattingly and colleagues propose that MPI could be of particular interest for imaging the dynamics of blood volume in the brain, as it can measure the local distribution of nanoparticles in blood without an interfering background signal.

“In the brain, the tracer stays in the blood so we get an image of blood volume distribution,” Mattingly explains. “This is an important physiological parameter to map since blood is so vital for supporting metabolism. In fact, when a brain area is used by a mental task, the local blood volume swells about 20% in response, allowing us to map functional brain activity by dynamically imaging cerebral blood volume.”

Rescaling the scanner

The researchers began by defining the parameters required to build a human brain-scale MPI system. Such a device should be able to image the head with 6 mm spatial resolution (as used in many MRI-based functional neuroimaging studies) and 5 s temporal resolution for at least 30 min. To achieve this, they rescaled their existing rodent-sized imager.

The resulting scanner uses two opposed permanent magnets to generate the FFL and high-power electromagnet shift coils, comprising inner and outer coils on each side of the head, to sweep the FFL across the head. The magnets create a gradient of 1.13 T/m, sufficient to achieve 5–6 mm resolution with high-performance SPIONs. To create 2D images, a mechanical gantry rotates the magnets and shift coils at 6 RPM, enabling imaging every 5 s.

The MPI system also incorporates a water-cooled 26.3 kHz drive coil, which produces the oscillating magnetic field (of up to 7 mT_peak) needed to drive the SPIONs in and out of saturation. A gradiometer-based receive coil fits over the head to record the SPION response.

Mattingly notes that this rescaling was far from straightforward as many parameters scale with the volume of the imaging bore. “With a bore about five times larger, the volume is about 125 times larger,” he says. “This means the power electronics require one to two orders of magnitude more power than rat-sized MPI systems, and the receive coils are simultaneously less sensitive as they become larger.”

Performance assessment

The researchers tested the scanner performance using a series of phantoms. They first evaluated spatial resolution by imaging 2.5 mm-diameter capillary tubes filled with Synomag SPIONs and spaced by between 5 and 9 mm. They reconstructed images using an inverse Radon reconstruction algorithm and a forward-model iterative reconstruction.

The system demonstrated a spatial resolution of about 7 mm with inverse Radon reconstruction, increasing to 5 mm with iterative reconstruction. The team notes that this resolution should be sufficient to observe changes in cerebral blood volume associated with brain function and following brain injuries.

To determine the practical detection limit, the researchers imaged Synomag samples with concentrations from 6 mg_Fe/ml to 15.6 µg_Fe/ml, observing a limit of about 1 µg_Fe. Based on this result, they predict that MPI should show grey matter with a signal-to-noise ratio (SNR) of roughly five and large blood vessels with an SNR of about 100 in a 5 s image. They also expect to detect changes during brain activation with a contrast-to-noise ratio of above one.

Next, they quantified the scanner’s imaging field-of-view using a G-shaped phantom filled with Synomag at roughly the concentration of blood. The field-of-view was 181 mm in diameter – sufficient to encompass most human brains. Finally, the team monitored the drive current stability over 35 min of continuous imaging. At a drive field of 4.6 mT_peak, the current deviated less than 2%. As this drift was smooth and slow, it should be straightforward to separate it from the larger signal changes expected from brain activation.

The researchers conclude that their scanner – the first human head-sized, mechanically rotating, FFL-based MPI – delivers a suitable spatial resolution, temporal resolution and sensitivity for functional human neuroimaging. And they continue to improve the device. “Currently, the group is developing hardware to enable studies such as application-specific receive coils to prepare for in vivo experiments,” says Mattingly.

At present, the scanner’s sensitivity is limited by background noise from the amplifiers. Mitigating such noise could increase sensitivity 20-fold, the team predicts, potentially providing an order of magnitude improvement over other human neuroimaging methods and enabling visualization of haemodynamic changes following brain activity.

The MPI system is described in Physics in Medicine & Biology.

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Popularized in the late 1950s as a child’s toy, the hula hoop is undergoing renewed interest as a fitness activity and performance art. But have you ever wondered how a hula hoop stays aloft against the pull of gravity?

Wonder no more. A team of researchers at New York University have investigated the forces involved as a hoop rotates around a gyrating body, aiming to explain the physics and mathematics of hula hooping.

To determine the conditions required for successful hula hoop levitation, Leif Ristroph and colleagues conducted robotic experiments with hoops twirling around various shapes – including cones, cylinders and hourglass shapes. The 3D-printed shapes had rubberized surfaces to achieve high friction with a thin, rigid plastic hoop, and were driven to gyrate by a motor. The researchers launched the hoops onto the gyrating bodies by hand and recorded the resulting motion using high-speed videography and motion tracking algorithms.

They found that successful hula hooping is dependent on meeting two conditions. Firstly, the hoop orbit must be synchronized with the body gyration. This requires the hoop to be launched at sufficient speed and in the same direction as the gyration, following which, the outward pull by centrifugal action and damping due to rolling frication result in stable twirling.

This process, however, does not necessarily keep the hoop elevated at a stable height – any perturbations could cause it to climb or fall away. The team found that maintaining hoop levitation requires the gyrating body to have a particular “body type”, including an appropriately angled or sloped surface – the “hips” – plus an hourglass-shaped profile with a sufficiently curved “waist”.

Indeed, in the robotic experiments, an hourglass-shaped body enabled steady-state hula hooping, while the cylinders and cones failed to successfully hula hoop.

The researchers also derived dynamical models that relate the motion and shape of the hoop and body to the contact forces generated. They note that their findings can be generalized to a wide range of different shapes and types of motion, and could be used in “robotic applications for transforming motions, extracting energy from vibrations, and controlling and manipulating objects without gripping”.

“We were surprised that an activity as popular, fun and healthy as hula hooping wasn’t understood even at a basic physics level,” says Ristroph in a press statement. “As we made progress on the research, we realized that the maths and physics involved are very subtle, and the knowledge gained could be useful in inspiring engineering innovations, harvesting energy from vibrations, and improving in robotic positioners and movers used in industrial processing and manufacturing.”

The researchers present their findings in the Proceedings of the National Academy of Sciences.

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From tumour-killing quantum dots to proton therapy firsts, this year has seen the traditional plethora of exciting advances in physics-based therapeutic and diagnostic imaging techniques, plus all manner of innovative bio-devices and biotechnologies for improving healthcare. Indeed, the Physics World Top 10 Breakthroughs for 2024 included a computational model designed to improve radiotherapy outcomes for patients with lung cancer by modelling the interaction of radiation with lung cells, as well as a method to make the skin of live mice temporarily transparent to enable optical imaging studies. Here are just a few more of the research highlights that caught our eye.

Marvellous MRI machines

This year we reported on some important developments in the field of magnetic resonance imaging (MRI) technology, not least of which was the introduction of a 0.05 T whole-body MRI scanner that can produce diagnostic quality images. The ultralow-field scanner, invented at the University of Hong Kong’s BISP Lab, operates from a standard wall power outlet and does not require shielding cages. The simplified design makes it easier to operate and significantly lower in cost than current clinical MRI systems. As such, the BISP Lab researchers hope that their scanner could help close the global gap in MRI availability.

Moving from ultralow- to ultrahigh-field instrumentation, a team headed up by David Feinberg at UC Berkeley created an ultrahigh-resolution 7 T MRI scanner for imaging the human brain. The system can generate functional brain images with 10 times better spatial resolution than current 7 T scanners, revealing features as small as 0.35 mm, as well as offering higher spatial resolution in diffusion, physiological and structural MR imaging. The researchers plan to use their new NexGen 7 T scanner to study underlying changes in brain circuitry in degenerative diseases, schizophrenia and disorders such as autism.

Meanwhile, researchers at Massachusetts Institute of Technology and Harvard University developed a portable magnetic resonance-based sensor for imaging at the bedside. The low-field single-sided MR sensor is designed for point-of-care evaluation of skeletal muscle tissue, removing the need to transport patients to a centralized MRI facility. The portable sensor, which weighs just 11 kg, uses a permanent magnet array and surface RF coil to provide low operational power and minimal shielding requirements.

Proton therapy progress

Alongside advances in diagnostic imaging, 2024 also saw a couple of firsts in the field of proton therapy. At the start of the year, OncoRay – the National Center for Radiation Research in Oncology in Dresden – launched the world’s first whole-body MRI-guided proton therapy system. The prototype device combines a horizontal proton beamline with a whole-body MRI scanner that rotates around the patient, a geometry that enables treatments both with patients lying down or in an upright position. Ultimately, the system could enable real-time MRI monitoring of patients during cancer treatments and significantly improve the targeting accuracy of proton therapy.

Also aiming to enhance proton therapy outcomes, a team at the PSI Center for Proton Therapy performed the first clinical implementation of an online daily adaptive proton therapy (DAPT) workflow. Online plan adaptation, where the patient remains on the couch throughout the replanning process, could help address uncertainties arising from anatomical changes during treatments. In five adults with tumours in rigid body regions treated using DAPT, the daily adapted plans provided target coverage to within 1.1% of the planned dose and, in over 90% of treatments, improved dose metrics to the targets and/or organs-at-risk. Importantly, the adaptive approach took just a few minutes longer than a non-adaptive treatment, remaining within the 30-min time slot allocated for a proton therapy session.

Bots and dots

Last but certainly not least, this year saw several research teams demonstrate the use of tiny devices for cancer treatment. In a study conducted at the Institute for Bioengineering of Catalonia, for instance, researchers used self-propelling nanoparticles containing radioactive iodine to shrink bladder tumours.

Upon injection into the body, these “nanobots” search for and accumulate inside cancerous tissue, delivering radionuclide therapy directly to the target. Mice receiving a single dose of the nanobots experienced a 90% reduction in the size of bladder tumours compared with untreated animals.

At the Chinese Academy of Sciences’ Hefei Institutes of Physical Science, a team pioneered the use of metal-free graphene quantum dots for chemodynamic therapy. Studies in cancer cells and tumour-bearing mice showed that the quantum dots caused cell death and inhibition of tumour growth, respectively, with no off-target toxicity in the animals.

Finally, scientists at Huazhong University of Science and Technology developed novel magnetic coiling “microfibrebots” and used them to stem arterial bleeding in a rabbit – paving the way for a range of controllable and less invasive treatments for aneurysms and brain tumours.

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