Adding “Color” to MRI

MRI may be widely used, but the technology is still essentially where black and white film was in the early 20th century. Researchers have now figured out a way to add the equivalent of color to MRI. The advance could help doctors tell different structures and types of cells apart in images of your insides.

MRI, which is short for magnetic resonance imaging, uses a large magnet and radio waves to look at organs and structures inside your body. Doctors use MRI scans to examine the brain and spinal cord and to diagnose a variety of conditions, from torn ligaments to tumors.

MRI scanners create a magnetic field that locks hydrogen atoms in the body's water-filled tissues into an aligned magnetic state. When a radio wave is applied, it knocks the hydrogen atoms out of alignment. The atoms in different tissues realign at different rates, producing radiofrequency (RF) signals that a computer converts into an image.

Researchers have developed compounds called contrast agents, typically metal complexes, that alter the magnetic fields around them, affecting RF signals and creating contrast in MRI images. Unlike the colored compounds that allow researchers to identify different probes in optical imaging techniques, however, the MRI contrast agents developed to date don’t provide a range of distinct signals. Researchers can’t easily tell contrast agents apart in MRI images and so can’t, for example, distinguish between multiple cell types.

A team led by Dr. Gary Zabow, a joint fellow at NIH’s National Institute of Neurological Disorders and Stroke (NINDS) and the National Institute of Standards and Technology (NIST), set out to add the equivalent of color to MRI. In the June 19, 2008, issue of Nature, the researchers explained how they used geometrical features to create particles that could be distinguished in MRI images.

The team designed and microfabricated particles with 2 microscopic nickel disks separated by non-magnetic spacers. Water molecules can freely diffuse in and out of the space between these disks. When they’re placed in an MRI scanner's magnetic field, the particles align with the field. The disks create a different magnetic field strength in the space between them than in the field outside. Water molecules diffusing in and out of the space between the disks respond to different RFs than those outside, creating contrast.

The frequencies the water molecules respond to are determined by the particle’s structure, allowing the researchers to create distinct RF signals that a computer can convert into a rainbow of colors. The researchers say that they should also be able to design particles that can be turned on or off by making coatings or spacers of material that dissolves or breaks down under certain conditions.

This study demonstrates the potential of microfabrication techniques, so critical to the electronics industry, for making multiple probes that can be distinguished by MRI. Much work still needs to be done before these structures can be used in the body, including demonstrating their safety and developing methods of delivering them to their targets.

—by Harrison Wein, Ph.D.