Tricorder Tech: A Miniature Magnetic Resonance Imager Made Of Diamond

Editor’s note: imagine what you could do if your tricorder had this capability in the field – with an embedded AI to interpret what is being imaged inside of a suspected life form.

The development of tumors begins with miniscule changes within the body’s cells; ion diffusion at the smallest scales is decisive in the performance of batteries. Until now the resolution of conventional imaging methods has not been high enough to represent these processes in detail. A research team lead by the Technical University of Munich (TUM) has developed diamond quantum sensors which can be used to improve resolution in magnetic imaging.

Nuclear magnetic resonance (NMR) is an important imaging method in research which can be used to visualize tissue and structures without damaging them. The technique is better known from the medical field as Magnetic Resonance Imaging (MRI), where the patient is moved into a bore of a large magnet on a table. The MRI device creates a very strong magnetic field which interacts with the tiny magnetic fields of the hydrogen nuclei in the body. Since the hydrogen atoms are distributed in a particular way amongst different types of tissues, it becomes possible to differentiate organs, joints, muscles and blood vessels.

NMR methods can also be used to visualize the diffusion of water and other elements. Research for example often involves observing the behavior of carbon or lithium in order to explore the structures of enzymes or processes in batteries. “Existing NMR methods provide good results, for example when it comes to recognizing abnormal processes in cell colonies,” says Dominik Bucher, Professor for Quantum Sensing at TUM. “But we need new approaches if we want to explain what happens in the microstructures within the single cells.”

Sensors made of diamond

The research team produced a quantum sensor made of synthetic diamond for this purpose. “We enrich the diamond layer, which we provide for the new NMR method, with special nitrogen and carbon atoms already during growth,” explains Dr. Peter Knittel of the Fraunhofer Institute for Applied Solid State Physics (IAF). After growth, electron irradiation detaches individual carbon atoms from the diamond’s perfect crystal lattice. The resulting defects arrange themselves next to the nitrogen atoms – a so-called nitrogen-vacancy center has been created. Such vacancies have special quantum mechanical properties needed for sensing. “Our processing of the material optimizes the duration of the quantum states, which allows the sensors to measure for longer,” adds Knittel.

Quantum sensors pass the first test

The quantum state of the nitrogen-vacancy centers interacts with magnetic fields. “The MRI signal from the sample is then converted into an optical signal which we can detect with a high degree of spatial resolution,” Bucher explains.

In order to test the method, the TUM scientists placed a microchip with microscopic water-filled channels on the diamond quantum sensor. “This allows us to simulate microstructures of a cell,” says Bucher. The researchers were able to successfully analyze the diffusion of water molecules within the microstructure.

In the next step the researchers want to develop the method further to enable the investigation of microstructures in single living cells, tissue sections or the ion mobility of thin-film materials for battery applications. “The ability of NMR and MRI techniques to directly detect the mobility of atoms and molecules makes them absolutely unique compared to other imaging methods,” says Prof. Maxim Zaitsev of the University of Freiburg. “We now have found a way, how their spatial resolution, which is currently often deemed insufficient, can be significantly improved in future.”

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Principles of NV-based diffusion imaging within microstructures. (A) Conceptual schematic of diffusion within a microstructure. The apparent diffusion coefficient (ADC) at the two marked locations differs strongly in the x ^ direction, because the free diffusion length is on the same scale as the microstructure itself. The probability to find a diffusing particle at a distance δ δ x ^ from its original position (dashed line) after diffusing for 0.1 s is displayed in the two plots on the top. The microstructure itself is color-coded according to the simulated ADC. (B) Experimental setup. A diamond chip (red) with a highly dense surface-doped NV layer is glued into the microfluidic chip (light gray) and placed between three pairs of magnetic field gradient coils. Each pair produces a B ^ 0 gradient along one of the cardinal directions x ^ , y ^ , and z ^ . The whole experiment is imaged using an optical microscope from above. A (green) laser enters the diamond chip and excites the NV centers in the surface layer, defining the measurement location [see also (C)]. The red NV fluorescence for signal readout is collected and directed to a photodiode using a liquid light guide. The NV electronic spin, used for the quantum sensing protocol, is driven by a microwave (MW) antenna on top of the microfluidic chip. (C) The water sample is confined by a microfluidic channel, whose bottom wall is formed by the NV sensor. Water molecules interacting with the channel walls are hindered in their diffusion and will have a lower ADC. External magnetic field gradients encode the position of the water molecules and allow for the measurement of their ADCs. — Science Advances

Imaging local diffusion in microstructures using NV-based pulsed field gradient NMR, Science Advances (open access)

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