NEWS FEATURE CENTRE FOR ADVANCED IMAGING, UNIV. QUEENSLAND UNIV. IMAGING, ADVANCED CENTRE FOR Cerebral blood vessels glow orange in this picture, generated by a 7-tesla magnetic resonance imaging scanner at The University of Queensland in Australia. THE STRONGEST SCANNERS Researchers are pushing non-invasive brain imaging to new limits. n a cold morning in Minneapolis BY ANNA NOWOGRODZKI faced long delays because the liquid helium last December, a man walked into needed to fill it was in short supply. After the Oa research centre to venture where check-up that included a baseline test of his machine was finally delivered, on a below- only pigs had gone before: into the strongest sense of balance to make sure that any dizzi- freezing day in 2013, it took four years of ani- magnetic resonance imaging (MRI) machine ness from exposure to the magnets could be mal testing and ramping up the field strength built to scan the human body. assessed properly. In the MRI room at the before Ugurbil and his colleagues were com- First, he changed into a hospital gown, and University of Minnesota’s Center for Mag- fortable sending in the first human. Even then, researchers made sure he had no metal on his netic Resonance Research, he lay down inside a they didn’t quite know what they’d see. But it body: no piercings, rings, metal implants or 4-metre-long tube, surrounded by 110 tonnes was worth the wait: when the scan materialized pacemakers. Any metal could be ripped out of magnet and 600 tonnes of iron shielding, on screen, the fine resolution revealed intricate by the immensely powerful, 10.5-tesla mag- for an hour’s worth of imaging of his hips, details of the wafer-thin cartilage that protects net — weighing almost 3 times more than a whose thin cartilage would test the limits of the hip socket. “It was extremely exciting and Boeing 737 aeroplane and a full 50% more the machine’s resolution. very rewarding,” Ugurbil says. powerful than the strongest magnets approved The centre’s director, Kamil Ugurbil, had The US$14-million scanner is one of a for clinical use. Days earlier, he had passed a been waiting for years for this day. The magnet handful around the world that are pushing 24 | NATURE | VOL 563 | 1 NOVEMBER 2018 ©2018 Spri nger Nature Li mited. All ri ghts reserved. ©2018 Spri nger Nature Li mited. All ri ghts reserved. FEATURE NEWS MRI to new limits of magnetic strength. Today, brain and other tissue. lower-resolution scanners — is involuntary hospitals routinely use machines with field The stronger the magnetic field, the greater movements of the brain inside the skull. “If I strengths of 1.5 T or 3 T. But ultra-high-field the fraction of protons that become aligned, stretch my toes while I’m in the scanner, my scanners are on the rise. There are already and the bigger the energy difference between brain will move because my toes are connected dozens of 7-T machines in research labs around them and those that remain unaligned. This through the spinal cord to the brain,” Menon the world, and last year, the first 7-T model was produces a signal that can be better detected says. And thanks to the heartbeats, he adds, the cleared for clinical use in both the United States over background noise. But every jump in brain pulsates “on the scale of half a millimetre and Europe. At the extreme end are three scan- field strength comes with some uncertainty. to a millimetre”. Tackling these artefacts is an ners designed for humans that reach beyond “At the beginning of the MRI era, many sci- ongoing area of research, he says. 10 T. In addition to the University of Minne- entists were thinking that 0.5 T would be the Even so, scientists say, 7 T has already sota’s machine, researchers are readying two maximum magnet strength for MRI” because opened a new window onto the living brain, by 11.7-T devices for their first tests on people: they thought the ion conductivity of live tis- revealing structures smaller than 1 millimetre. a gargantuan one for whole-body scanning at sue would stop radio waves from penetrat- This regime, dubbed the mesoscopic scale by the NeuroSpin Centre at CEA Saclay outside ing far enough inside the body, says Victor neuroscientists, is something that previously Paris, and a smaller one for head scans at the US Schepkin of the US National High Magnetic was accessible only by surgeons, says Klaus National Institutes of Health (NIH) in Bethesda, Field Laboratory in Tallahassee, Florida. Then, Scheffler, head of the magnetic-resonance Maryland. Germany, China and South Korea the 1980s saw the emergence of 1.5-T scanners centre at the Max Planck Institute for Biologi- are considering building 14-T human scanners. for clinical use. And in 2002, 3-T scanners won cal Cybernetics in Tübingen, Germany. With The appeal of ultra-high-field scanners approval. Even before then, researchers were 7 T, Scheffler says, “you see all the details with- is clear. The stronger the magnetic field, the pushing for higher field strength; the first 7-T out opening the brain”. greater the signal-to-noise ratio, which means research scanners began to emerge in 1999. Among the structures that have been the body can be imaged either at greater reso- revealed are the six layers of the cerebral lution, or at the same resolution, but faster. cortex, the 3-millimetre-thick outer region of At 3 T, MRI machines can resolve details of the brain that is responsible for humans’ high the brain as small as 1 millimetre. That reso- level of cognition. Each layer has a speciali- lution can be as fine as 0.5 millimetres in a “EVEN THE zation: one handles inputs from other brain 7-T machine — enough to discern the func- areas, some process information and still tional units inside the human cortex and per- others convey the outputs of that processing haps see for the first time how information STARTING IMAGES to other parts of the brain. The jump to 7-T flows between collections of neurons in a live machines has enabled researchers to measure human brain. Scanners with even higher field LOOK PRETTY the relative activity in different layers, which strengths are expected to have resolving power can reveal how that information is travelling. that is at least double that of the 7-T devices. SPECTACULAR.” “That’s the huge advance over imaging at 3 T The push to achieve higher field strengths or 1.5 T,” says Menon. “Normally, we just say A presents a range of challenges. The scanners is connected to B, and we can’t tell much about are bigger, more expensive and more tech- which way the information is flowing.” nically demanding. They also require more The move from 3T to 7 T presented some Some teams have used this capability to attention to safety. But work at 7 T has already challenges. Biological side effects, although measure activity as people undergo verbal and resulted in gains, researchers say, for both neu- temporary, are more pronounced: people can behavioural tests, and the results are illuminat- roscience and clinical applications: clinicians experience dizziness and vertigo when they ing how activity in different layers alters how can guide electrodes for deep-brain-stimula- move in and out of the scanner, researchers say. various areas of the cortex process experiences tion treatments more accurately, and might When people move inside the machine, they (S. J. D. Lawrence et al. NeuroImage http://doi. also be able to detect osteoarthritis at an earlier can sometimes taste metal, see white flashes or org/cwbr; 2017). “It’s not just that area A is in stage than was possible before. experience involuntary eye movements called charge of vision, but that it is modulated by The scanners offer detail that was once nystagmus. attention, mood, memory,” says Menon. “And seen only in thinly sliced postmortem sam- Tissue can also overheat. Because hydrogen those kinds of questions are extremely diffi- ples imaged by powerful microscopes. “This nuclei resonate at higher frequencies as the field cult to answer in animal models. They obvi- is a window we’ve just never had in the intact strength increases, ultra-high-field MRIs must ously don’t think or verbalize the way we can.” human brain,” says Ravi Menon, a neuro- use shorter-wavelength, and thus higher-energy, Now, with 7-T scans of humans, “a picture of imaging scientist at Robarts Research Institute radio pulses to make the protons wobble. human memory is emerging that was really at Western University in London, Canada. Human tissue absorbs more energy from these unavailable before”, he says. waves. So to avoid creating hotspots — and Researchers also hope to learn more about IF YOU BUILD IT to make usable images — this energy must be the columnar organization of the brain. Cortical The nuts and bolts of MRI technology have not smoothed out as much as possible inside the columns are thought to carry out computations changed much since the first human scanner tube. Researchers have devised various ways and respond preferentially to particular stim- was developed in the mid-1970s. The heart of of accomplishing this. One tactic, says Gregory uli, such as the orientation of objects, although the MRI is still a tube-like superconducting Chang, a musculoskeletal radiologist at the there’s fierce debate over their exact role in this magnet, which generates a static electromag- New York University School of Medicine, is to context.