Researchers from the Michigan Technological University have made high-frequency MRIs more precise by creating a better, more uniform magnetic field.
The team found that radio frequency probes with structures inspired by microstrip patch antennas (MPA) would increase the MRI resolution in high-frequency MRI machines, when compared to the conventional surface coils that are commonly used now.
“When frequencies become higher, wavelengths become shorter, and your magnetic field loses uniformity,” Elena Semouchkina, an associate professor of electrical and computer engineering at Michigan Technological University, said in a statement. “Uniformity is important for high-resolution images, so we proposed a new approach to developing these probes.”
MPAs, which are often used in telecommunication applications, are made of a flat piece of metal grounded by a larger piece of metal. These antennas are inexpensive and simple to produce.
MRIs work by issuing radio frequency pulses in a magnetic field through probes with coils that are used to create an image. However, these conventional coils have frequency limits where too high of a frequency prevents them from creating uniformed magnetic fields at the volume needed.
MPAs are an alternative where waves oscillate in the cavity formed between the patch and ground plane electrodes, which are accompanied by currents in the patch electrode and, respectively, oscillating magnetic fields around the patch, providing a magnetic field that is both even and strong.
“While the complexity of birdcage coils increases with the increase in operation frequency, patch-based probes can provide quality performance in the higher microwave range while still having a relatively simple structure,” Semouchkina said.
The researchers also showed smaller radiation losses, which makes them competitive with or even better than conventional coils.
“The addition of high permittivity inserts to the patch substrate was beneficial for increasing B1 field uniformity,” the authors write in the study. “It was also shown by simulations that two vis-à-vis placed identical patches fed with 180° phase difference could produce uniform B1 field in the space between patches and could be used as volume RF probes.”
High-frequency radio waves can often cause damage to humans, limiting the researchers to examine high frequency machines and not the metal tube that is seen in hospitals and other medical centers. Human can only sustain frequencies up to seven Teslas, but ultrahigh fields up to 21.1 Teslas can be used in testing on animal models and tissue samples.
The study was published in IEEE Transactions on Microwave Theory and Techniques.