Extending MRI to nanoscale resolution

October 2, 2013

Illustration of the experimental setup shows the two unique components of the team’s novel MRI technique that was successful in producing a 2D MRI image with spatial resolution on the nanoscale (credit: Budakian et al./University of Illinois at Urbana-Champaign)

University of Illinois at Urbana-Champaign and Northwestern University researchers have devised a novel nuclear magnetic resonance imaging (MRI) technique that delivers about 10­ nanometers spatial resolution.

This represents a significant advance in MRI sensitivity. Current MRI techniques commonly used in medical imaging yield spatial resolutions on the millimeter length scale, with the highest-resolution experimental instruments giving spatial resolution of a few micrometers.

“Our approach brings MRI one step closer in its eventual progress toward atomic-scale imaging,” said U. of I. physicist Raffi Budakian, who led the research effort.

The new breakthrough technique introduces two unique components to overcome obstacles to applying classic pulsed magnetic resonance techniques in nanoscale systems:

  • A novel protocol for spin manipulation applies periodic radio-frequency magnetic field pulses to encode temporal correlations in the statistical polarization of nuclear spins in the sample.
  • A nanoscale metal constriction focuses current, generating intense magnetic-field pulses.

In their proof-of-principle demonstration, the team used an ultrasensitive magnetic resonance sensor based on a silicon nanowire oscillator to reconstruct a two-dimensional projection image of the proton density in a polystyrene sample at nanoscale spatial resolution.

“We expect this new technique to become a paradigm for nanoscale magnetic-resonance imaging and spectroscopy into the future,” added Budakian. “It is compatible with and can be incorporated into existing conventional MRI technologies.”

Exclusive KurzweilAI interview with Prof. Budakian

What are the major applications of your research and when can we expect to see operational devices?

Specifically, we are focused on imaging biological systems between 1–100 nm. These include proteins and viruses. MRI is a powerful tool for studying biological systems because it offers a host of unique modalities for imaging. It is nondestructive, fully three-dimensional, and chemically specific. Extending these capabilities to the nanometer scale would, among other things, transform our understanding of protein structure, which would enable more effective drug development. Our approach would permit the application of established techniques in clinical MRI to the nanometer scale.

We are in the beginning stages of this new technology. We need several more years of technique development before we can apply this technique to answer biologically relevant questions. Of course, the speed of progress depends a great deal on the funding situation. The application of this technique will not be in a clinical setting. I have not approached any commercial companies with this idea. It’s still very new.

What is the highest resolution available in current MRI devices and how do they compare with your work?

The highest resolution inductively-coupled MRI measurements that I am aware of is 3.7 x 3.3 x 3.3 micrometers [2]. There  is a number of people in the force-detected MRI community working on developing nanoscale MRI.. There is also a growing community of people trying to apply nitrogen vacancy centers to nanoscale MRI [3,4].

The first work that demonstrated nanoscale MRI imaging was by Degan et al.[5]. Like the previous work, our approach uses force-detected magnetic resonance imaging. Our approach differs in several important aspects to that work. One of the most significant differences is the use of time-dependent magnetic field gradients for spin detection and imaging. The ability to control the time dependence of the magnetic fields permits the use of all other pulsed magnetic resonance techniques for nanoscale imaging and spectroscopy.

What are your plans for future development?

The goal of our work is to extend the capabilities of MRI to the nanometer scale. In this initial proof-of-concept work, we demonstrated 10-nm spatial resolution imaging of proton spins in polystyrene. In the next 2–3 years, our goal is to demonstrate proton spin imaging in biological systems with 1–3 nm spatial resolution.

Abstract of Physical Review X paper [1]

We report a method for nanometer-scale pulsed nuclear magnetic resonance imaging and spectroscopy. Periodic radio-frequency pulses are used to create temporal correlations in the statistical polarization of a solid organic sample. The spin density is spatially encoded by applying a series of intense magnetic field gradient pulses generated by focusing electric current through a nanometer-scale metal constriction. We demonstrate this technique using a silicon nanowire mechanical oscillator as a magnetic resonance sensor to image 1H spins in a polystyrene sample. We obtain a two-dimensional projection of the sample proton density with approximately 10-nm resolution.