McKelvey School of Engineering

New, fundamental limit to ‘seeing and believing’ in imaging

Answers to big questions increasingly require access to the realm of the very small.

As researchers continue to push the limits of imaging, a scientist at Washington University in St. Louis has uncovered a fundamental barrier to accuracy when it comes to measuring the rotational motion of molecules.


Matthew Lew, assistant professor in the Preston M. Green Department of Electrical & Systems Engineering in the McKelvey School of Engineering, likens the consequence of this barrier to something many are familiar with.

“When you look at your sideview mirror in the car, there is a disclaimer: objects are closer than they appear,” said Lew, whose research was published in the Physical Review Letters, the flagship publication of the American Physical Society.

“We have found that objects in the microscope are less confined than they appear. Fluorescent molecules always appear to be more confined in rotational freedom than they actually are,” Lew said.

This discrepancy is a result of measurement noise.

This is important because molecules are not smooth, round balls moving along straight paths, bumping into each other and sticking together — they have a topography of sorts. This is critical to chemical and biological reactions: “There needs to be the right matching of pockets and binding motifs,” Lew said. The puzzle pieces, that is, need to match and connect in order for reactions to occur.

In addition to moving in three dimensions, molecules also rotate, like a ball rolling down an uneven surface they wobble, twist, and spin in all directions. Researchers need to see both the straight, translational movement and the spinning, rotational movement to understand how molecules interact.

In order to see anything, however, an imaging device needs to capture light emitted from the fluorescing object. In the case of these tiny bits of matter, that may mean a relatively small number of photons.

The limit Lew has discovered deals with light: If the object being imaged is too dim, it will appear rotationally constrained and look like it has less rotational movement than it actually does. Like a spinning fan, a rotating molecule should look smooth — like the blurred blades. But if that fan is dimly lit, the blades won’t look perfectly smooth and will instead appear to be “stuttering.” Therefore, they appear to be rotating less than they actually are. (The underlying physics of the fan analogy is different than that of imaging molecules, however).

“If a molecule was completely free to rotate, it would look like a smooth ball,” Lew said. “The ball can never be smooth if there’s noise on top of it. That noise, that roughness makes it look like the ball made up of a molecule that’s not completely free to rotate.”

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