Magnetic resonance imaging (MRI) is invaluable in the medical world. But despite all the good it does, there is room for improvement. One way to enhance the sensitivity of MRI is called dynamic nuclear polarization (DNP), where target molecules for imaging are modified so they form clearer images when scanned with an MRI machine. But this technique requires some special crystalline materials mixed with polarizing agents which are difficult to create. For the first time, researchers including those from the University of Tokyo demonstrate the use of molecules called fullerenes as polarizing agents. Their new method can make DNP targets sufficient to yield far greater clarity when imaged with an MRI machine, with potential benefits in various medical applications.
If you're lucky enough to have never visited the inside of an MRI machine before, then you are still probably familiar with them: enormous ring-shaped things which engulf a person and noisily scan them to create detailed 3D images for diagnostic purposes. Since their mainstream introduction to the medical world over four decades ago, MRI machines have allowed clinicians and researchers to use 3D data for various diagnostic and research purposes. But as with every machine, there are constant upgrades proposed to improve some aspect or other, whether it's size, cost, noise, functions or abilities.
A typical MRI works by creating a large magnetic field. This forces the protons of water molecules in the body or sample to align. The machine then emits radio waves which knock these protons out of alignment, so that they spring back into alignment again under the force of the magnetic field. As they realign, the protons give off a telltale radio signal which the machine detects and uses to identify the kind of tissue the signal came from. But as you may have already picked up, this means that typical MRI machines are limited to the detection of samples rich in water. So, researchers sought a way to broaden the scope of what the machines can detect, and this is where new research from the Department of Chemistry comes in.
"An established way to improve the detail and information content of MRI images is to use chemical targets in the patient or sample. DNP works this way, but requires agents to polarize the target molecules, and that in turn usually requires extremely cold, or cryogenic, temperatures, and high magnetic field conditions. But we demonstrated an easier way to polarize targets," said Professor Nobuhiro Yanai from the Department of Chemistry. "Our work shows that by using specially designed molecules called fullerenes, we can boost polarization rate to 14.2% in a sample of disordered, glasslike material. This level is high enough for biological applications where a threshold of 10% is the minimum desired; otherwise, polarized molecules decay too quickly for their signals to yield useful images."
Fullerenes, also known as buckyballs, are 3D geometric lattices of carbon atoms, which have attracted researchers' attention as they can be modified in different ways to create functional materials. In this case, Yanai and his team added certain modifications to fullerenes which prevented their rotations such that they would stay polarized. When placed in a sample, electrons from these fullerenes transfer their spin polarization to the nuclei of nearby atoms, and it's this polarization that translates into stronger signals for the imaging sensors to detect. And all the researchers need to do to coax their special fullerenes, called trans-3a isomers, to do this is by shining a certain kind of light on them.
"The polarization of the targets is done outside the body. After polarization, the sample is dissolved, and the fullerene, which could be harmful, is removed before injection into a hypothetical patient," said graduate student Keita Sakamoto. "Because this method, triplet-DNP, avoids the need for a liquid helium coolant, it can run on much simpler, lower-cost equipment. It also makes it possible to bulk-polarize diagnostic chemical probes like pyruvate or anticancer drugs that conventional MRI cannot detect. Our next goal is to develop biocompatible matrices so we can hyperpolarize such medically important molecules. We plan to demonstrate high-sensitivity MRI in animal models first. If those experiments succeed and clinical trials follow, we expect this technology could reach real medical settings in about 10 to 20 years."
