Doctors and physicists at Yale developed the new type of MRI to read radio waves absorbed in and emitted from atoms of phosphorus found in hard and soft solids and create images based on the signals. The resulting high-resolution images and a description of the experiment were published in the March 19 Proceedings of the National Academy of Sciences (1). The researchers say the technique could lead to improved images showing the internal structure of bone and brain tissue and also of archeological artifacts and even coal and sand.
As a result, standard MRI would be an "excellent candidate" to probe deeper into bone and other solids because it doesn’t require smashing open or slicing a sample to look inside of it, he said. Instead, an MRI uses powerful magnets and bursts of radio waves to ping hydrogen atoms in the water content of a sample. The hydrogen atoms absorb and emit the radio waves, and each atom gives off a GPS-like signal that a computer then maps into an image of the material.
But bone fragments are dry. They don't have much water or the required hydrogen atoms to reproduce the radio signals to create a clear image. A standard MRI also falls short for imaging soft solids, such as brain and heart tissue, because the tool detects the signal coming from hydrogen in the free water inside and outside of the cells, rather than from the actual cells’ membranes, said Yale physics doctoral student Merideth Frey, who also worked on the new phosphorous-based MRI tool.
Barrett and Frey knew from another line of research related to quantum computing that specific changes to the pulse of the radio waves used in MRI could manipulate atoms of phosphorous and cause them to absorb and emit radio signals. They and their collaborators reconfigured a standard MRI machine to send and receive a signal they call a quadratic echo radio pulse pattern.
"Our pulses are short, sharp, and rectangular to hit the whole sample at once," Barrett said, explaining that the pulse pattern makes the signal from a solid look as though it is coming from a liquid. The pulse pattern is the key development that makes phosphorous-based MRI possible, he said.
To test the new technique, the team put cow bone and mouse liver, heart, and brain tissue into the reconfigured MRI and made both two- and three-dimensional images of the samples, as well as a video that shows the different structures of the mouse brain based on the density of phosphorous in each region.
Frey said that images from the new type of MRI could be combined with X-rays and other diagnostic imagers as a way to better measure bone and tissue quality. But it’s not likely that people with fractures and breaks will be getting phosphorous-based MRIs any time soon. The new method uses high-frequency radio pulses and creates too much heat, both of which would be dangerous for living samples.
1. Frey, M., et. al. Phosphorus-31 MRI of hard and soft solids using quadratic echo line-narrowing. PNAS: Early Edition. March 19, 2012. doi: 10.1073/pnas.1117293109.