Magnetic resonance imaging (MRI) has been used to investigate soft bodily tissues for diagnostic purposes for over 40 years. Current machines can resolve structures to about 0.1 millimeters—a figure that has remained relatively stable in recent years. A team of physicists have designed a small MRI machine that was able to detect a structure over one million times smaller: a single hydrogen atom. It is hoped that this technology will eventually be used to determine the structure of specific molecules. The research was led by Christian Degen of ETH Zurich and the paper was published in Science.
As the name implies, MRI machines use electromagnetic coils to generate a magnetic field and send short bursts of radio waves around a tissue of interest. This field excites protons in the nucleus of hydrogen atoms which causes them to align. Radio waves from the machine disrupt this alignment, but in the absence of the signal, the atoms realign and emit their own radio signal. This is detected by the machine and interpreted in cross-sectional images.
"MRI is nowadays a mature technology and its spatial resolution has remained largely the same over the last ten years. Physical constraints preclude increasing the resolution much further," Degen said in a press release.
In order to drastically increase the resolution, Degen’s team replaced the electromagnetic coil with a diamond sensor chip in a fluorescent microscope. The machine was used to detect what is called a nitrogen-vacancy center. Essentially, the perfect carbon lattice structure of a diamond is slightly flawed. Two carbon atoms are replaced by one nitrogen atom, creating a very small spot that is both magnetic and fluorescent.
The diamonds used had nitrogen-vacancy centers located just a few nanometers below the surface. The MRI machine was able to get an optical readout from those spots, which the team used to determine the location of individual hydrogen atoms. The machine was able to resolve the atoms to about one angstrom. As a comparison, a strand of DNA is about 20 Å wide.
"Quantum mechanics then provides an elegant proof of whether one has detected an individual nucleus, or rather a cluster of several hydrogen atoms," explained Degen.
A machine working on such a fine scale would never be a practical solution for patient diagnostics, but it does have incredibly important implications. Degen’s team hopes to develop this technology to the point where it can resolve small molecules and eventually use it to study the structure of proteins which has countless applications, including drug discovery.
Protein molecular structure is typically studied through x-ray crystallography, but many have properties that make them difficult to crystallize. Since the technique relies on billions of the proteins all being uniformly crystallized, this presents a challenge. However, this new small-yet-mighty MRI could be able to offer an eventual solution.
"This is an important intermediate step toward the mapping of entire molecules," Degen concluded.