The start of something30 October 2018
Developments in areas such as 3D printing and sensor technology are ushering in a new era of MRI design, with two current projects highlighting the growing potential for wearable MRI devices. Paul Miller hears from research teams at NYU School of Medicine, the University of Nottingham and University College London who are leading the charge.
Densely packed resonant structures used for magnetic resonance imaging (MRI), such as nuclear magnetic resonance phased array detectors, suffer from resonant inductive coupling, which restricts the coil design to fixed geometries, imposes performance limitations and narrows the scope of MRI experiments to motionless subjects.
Now, a new kind of MRI component in the shape of a glove promises to deliver the first clear images of bones and ligaments moving together. Led by NYU School of Medicine and recently published in Nature Biomedical Engineering, the study shows how a new MRI element design woven into garment-like detectors can capture high-quality images of moving joints.
The study authors say their MRI glove prototype promises to become useful in the future diagnosis of repetitive strain injuries like carpal tunnel syndrome in office workers, athletes, and musicians. Because the invention shows how different tissue types impinge on each other as they move, the team believes it could enable the construction of a more versatile atlas of hand anatomy, guide surgery with hand images in more realistic positions, or aid in the design of better prosthetics.
“Our results represent the first demonstration of an MRI technology that is flexible and sensitive enough to capture the complexity of soft-tissue mechanics in the hand,” says lead author Bei Zhang, research scientist at the Center for Advanced Imaging Innovation and Research (CAI2R), within the Department of Radiology at NYU Langone Health.
Past and current trends
Since its emergence in the 1970s, MRI has given physicians a better look inside tissues, helping to diagnose millions of maladies per year, from brain tumours to internal bleeding to torn ligaments.
The technology works by immersing tissues in a magnetic field such that any hydrogen atoms present align to create an average magnetic force in one direction in each tissue slice. These little magnets can then be tipped out of equilibrium by waves of electromagnetic force, or ‘radio waves’. Once tipped, they spin like tops and emit radio signals, which reveal their positions and can be rebuilt into images.
Another factor that is fundamental to MRI is the ability of radiofrequency coils to convert radio waves into a detectable electric current. Despite this impact, the technology has long struggled with a basic limitation.
the captured ‘spinning top radio waves’ produce little currents inside receiver coils, which in turn create their own magnetic fields and prevent nearby coils from capturing clean signals.
Over the past 30 years, attempts to manage interactions between neighbouring coils have resulted in state-of-the-art MRI scanners in which receiver coils are painstakingly arranged to cancel out magnetic fields in neighbouring coils. Once the best arrangement is set, coils can no longer move relative to one another, constraining the ability of MRI to complex, moving joints.
As all current MRI scanners measure signals that create currents in receiver coils (detectors), such coils have always been designed as low impedance structures that let the current flow easily. The leap made by the study authors was to design a high impedance structure that blocks current, and then measures how hard the force in magnetic waves pushes (the voltage) as it attempts to establish a current in the coil.
With no electric current created by the MR signal, the new receiver coils no longer create magnetic fields that interfere with neighbouring receivers, removing the need for rigid structures. The researchers found that their system, with the new coils stitched into a cotton glove, generated exquisite images of freely moving muscles, tendons and ligaments in a hand as it played piano and grabbed objects.
The MRI signal is produced by hydrogen atoms (protons), and so this technology excels at imaging soft tissue structures rich in water, each molecule of which includes two atoms of hydrogen. For this reason, MRI is great at capturing muscles, nerves, and even cartilage, which are difficult to study using other non-invasive methods. Tendons and ligaments, however, which are made of dense proteins, remain difficult to see independently, because they appear as black bands alongside bone.
The next verb generation
The study found that, in visualising fingers as they flexed, the new coils revealed how the black bands moved in concert with the bones, which could help to catalogue differences that come with injury.
“We wanted to try our new elements in an application that could never be done with traditional coils, and settled on an attempt to capture images with a glove,” says senior author Martijn Cloos, assistant professor from the CAI2R institute in the Department of Radiology at NYU Langone Health. “We hope that this result ushers in a new era of MRI design, perhaps including flexible sleeve arrays around injured knees, or comfy beanies to study the developing brains of newborns.”
This movement towards wearable or mobile MRI devices is gathering pace elsewhere. In the UK, scientists have carried out the first study of human cognition using a new generation of brain scanner that can be worn like a helmet, marking an important step forward in the translation of their new technique from the laboratory bench to a genuinely useful tool for cognitive neuroscience and clinical application. The study was undertaken in collaboration by researchers at the Sir Peter Mansfield Imaging Centre, University of Nottingham and the Wellcome Centre for Human Neuroimaging, University College London. It is part of a five-year Wellcome-funded project that has the potential to revolutionise the world of human neuroimaging.
Brain cells operate and communicate by producing electrical currents. These currents generate tiny magnetic fields that are detected outside the head. Researchers use magnetoencephalography (MEG) to map brain function by measuring these magnetic fields. This allows a millisecond-by-millisecond picture of which parts of the brain are engaged when we undertake different tasks, such as speaking or moving.
Here, subjects wearing the MEG scanner were shown nouns on a screen and told to think of related words without speaking. They were instructed to continue doing this until the word disappeared from the screen after a three-second period.
Each verb generation period was followed by a period of two seconds where the subject was asked to do nothing. Images captured exactly how the language network was engaged when subjects undertake the task.
“This is the first study of human cognition using this new scanner and it highlights this technology’s potential as a tool for cognitive neuroscience,” says Dr Matt Brookes, who leads MEG work in the School of Physics and Astronomy at the University of Nottingham. The study shows the potential of our system to improve the accuracy of surgical planning, via mapping eloquent cortex.
“If we can map, for example, the language network, then that will provide useful information for surgeons who may be planning resections in, for example, epilepsy. We hope the methods will be particularly beneficial for young children, who are often difficult to scan accurately using the fixed scanners which rely on the patient staying very still for long periods of time. This therefore represents an exciting step forward as it demonstrates the utility of a new generation of wearable MEG sensors for cognitive and clinical neuroscience.”
Potential in development
Conventional MEG scanners are large and weigh around 0.5t. This is because the sensors used to measure the brain’s magnetic field need to be kept very cold (–269°C), which requires bulky cooling technology. With current scanners, the patient must remain very still while being scanned, as even a 5mm movement can make the images unusable. This means it is often difficult to scan people who find it hard to remain still, such as young children or patients with movement disorders.
The new OPM-MEG system uses quantum sensors, mounted in a 3D-printed prototype helmet. As the new sensors are very light in weight and can work at room temperature, they can be placed directly onto the scalp surface. Positioning the sensors much closer to the brain increases the amount of signal that they can pick up.
“From a neuroscience perspective, this work is very exciting as it allows us to study tasks that we could never have contemplated before with conventional scanners where the head has to remain fixed,” says Professor Gareth Barnes of the Wellcome Centre for Human Neuroimaging.
“For example, people interacting naturally or people navigating through virtual worlds and laying down memories. Importantly, we can do this throughout the lifespan–allowing us to understand how key functions like memory or language develop and how they degrade in dementia. We soon expect delivery of even smaller sensors, which we should be able to put within a bicycle helmet and we are building a new room where subjects are free to move around naturally. We will be able to allow people to interact with one another or within virtual worlds where we can study how they make decisions and lay down memories. This will mean we will be able to study natural human movement and how it is compromised in diseases, such as Parkinson’s.”
These projects remain at the developmental stage, but they clearly highlight the potential for smaller, wearable forms of imaging technology to transform what is possible in the imaging sphere. As more of these similiar developments come to the fore, it is clear the trend is unlikely to go out of fashion any time soon.