A New Type Of Contrast Medium Can Make MRI More Effective

• Researchers develop a contrast agent that makes MRI more contrast and sensitive. 
• To do this, they created hyperpolarized xenon gas through an optical pumping method.

Magnetic resonance imaging (MRI) is a tomographic technique that uses a strong magnetic field and radio waves to construct detailed images of organs and tissues within the body. It is used to detect anomalies in the brain and spinal cord, breast cancer, certain kinds of heart problems, and many other diseases.

However, the contrast agents (sometimes called dyes) used in MRI face two fundamental issues: they require high-concentrations and are extremely sensitive. Some of them, such as gadolinium-based MRI contrast agents, impose safety issues to some patients.

Recently, a research team at the California Institute of Technology developed a new kind of contrast agent for MRI that not only solves the issues of conventional contrast media but also configures itself to accommodate various amounts of hyperpolarized noble gas (xenon).

Hyperpolarized Xenon

Unlike X-rays and ionizing radiation, MRI generates images of the body by exposing water molecules in the tissue to a strong magnetic field and magnetic field gradients, without harming the organs in the body.

To enhance the MRI sensitivity, contrast agents are either inhaled or injected into the body. These agents contain targeting units that bind them to particular locations of cellular diseases. Agents can be indirectly identified when they bind and exchange hydrogen atoms with water molecules present in the tissue. 

To measure this chemical exchange with higher sensitivity, a technique called hyperpolarized nuclei is applied to different noble gases. In this study, researchers created hyperpolarized xenon gas through an optical pumping method.

Reference: ACS Nano | doi:10.1021/acsnano.8b04222 | Caltech | Physics World 

Researchers pushed the electrons to higher energy levels and aligned (hyperpolarized) their spins with the help of a laser beam. Finally, they used the spin exchange process to transfer this polarization to nearby noble gas (xenon in this case) nuclei.

They used ¹²⁹Xe (an isotope of xenon) to image cavities in porous samples, for example, gas flow within the lungs. The hyperpolarized form of ¹²⁹Xe can last up to several hours in the gaseous state. And since xenon is soluble in hydrophobic solvents and waters, hyperpolarized ¹²⁹Xe could help physicians visualize a wide range of soft tissues.

New MRI contrast agent fills up with the harmless xenon gas | Credit: Barth van Rossum/FMP 

Improving MRI Sensitivity

However, to produce useful signals, MRI needs a high concentration of molecules which cannot be achieved with hyperpolarized xenon. MRI sensitivity still remains low and it cannot detect many cellular biomarkers.

To address this problem, researchers came up with contrast agents that are gas vesicles generated by specific bacteria. They can act like a fish’s swim bladder, enabling bacterial to control their buoyancy in water.

These gas vesicles are made of a porous wall structure through which noble gas could easily flow in and out. Unlike existing contrast agents, they always absorb a specific portion of xenon provided by their surroundings. Thus, the more xenon is there to be absorbed by gas vesicles, the more MRI can benefit from it.

The fraction of xenon dissolved in the blood depends on how much xenon gas a patient inhales. Compared to existing contrast agents, a large amount of xenon passes into the gas vesicles, which enhances both the image contrast and the sensitivity. This makes it possible to detect a disease marker occurring in a low concentration.

Read: New Calcium-Depended MRI Sensor Provides More Detailed Brain Imaging

This is the first time someone has generated an MRI using hyperpolarized xenon and gas vesicles. The team plans to further improve sensitivity and apply gas vesicles to target other disease markers, for instance tracking immune cells or binding to tumor cells.