Nanosecond x-ray video enables high-speed imaging of molecular dynamics

A research group led by Professor Yuji C. Sasaki of the Graduate School of Frontier Sciences at the University of Tokyo and Lecturer Masahiro Kuramochi of the Graduate School of Science and Engineering at Ibaraki University (also serving as a lecturer at the University of Tokyo) has developed a new dynamics measurement technique called "transmitted X-ray blinking (TXB)". It captures micro-scale molecular motion within polymer resins using X-ray video. By analyzing subtle temporal fluctuations in X-ray intensity, the researchers successfully detected clear differences in micro-scale molecular dynamics that could not be distinguished by conventional transmission X-ray imaging.

Sasaki said, "We were able to distinguish whether a polymer material is crystalline or amorphous-something that could not be differentiated with conventional transmission X-ray imaging. Since we can observe differences in material hardness, we would like to develop this into a non-invasive imaging method that can capture states such as the early stages of amyloid β fibril formation." The results were published in Optics Express.

The molecules that make up materials are constantly in motion due to thermal fluctuations. When high-speed continuous imaging is performed using X-rays, these motions appear as blinking. Most researchers have treated this as noise, and X-ray imaging has basically relied on still images.

Sasaki and his team proposed three methods: diffracted X-ray tracking (DXT), diffracted X-ray blinking (DXB), and small-angle X-ray blinking (SAXB), becoming the first in the world to successfully capture the internal motion of atoms and molecules in materials with high precision. This time, they further verified that similar X-ray intensity blinking phenomena can be observed with X-ray intensity levels in the transmission region, comparable to those used in clinical radiography.

The research group built their own palm-sized X-ray source and used it for experiments. By bringing the distance between the X-ray source origin, sample position, and detector down to the millimeter level, they achieved high-intensity X-ray irradiation without introducing X-ray optical systems. As a result, they became able to capture a single image in 900 nanoseconds, thus enabling high-speed continuous imaging.

For TXB samples, two polymer resins with nearly identical X-ray absorption coefficients were used: semi-crystalline polyetheretherketone (PEEK) and amorphous polyetherimide (PEI).

For these two samples, high-speed imaging at 900 nanoseconds per image was repeated 5,000 times. It was confirmed that the two were nearly indistinguishable by comparing the total integrated intensity (still image). Next, when the group analyzed these videos using autocorrelation function (ACF) analysis on 5,000 consecutive images, it was found that PEEK exhibited larger motion amplitude than PEI.

Furthermore, when principal component analysis (PCA) was applied to the autocorrelation analysis data obtained (4,096 pixels), the results could be summarized into 20 types of motion modes (principal components). When linear discriminant analysis (LDA), a type of machine learning, was applied, PEEK and PEI could be distinguished with over 90% accuracy.

This is the world's first demonstration that differences between samples can be identified by analyzing X-ray intensity fluctuations in the transmission region. Kuramochi said, "Training is required for each sample, but by accumulating data, it will become possible to perform analysis with fewer images."

Currently at the proof-of-concept stage, TXB has potential applications not only in materials evaluation but also in basic medical research and may serve as a next-generation radiographic examination method to replace conventional clinical X-ray examinations. It is expected to develop as a new technology unique to Japan.

Journal Information
Publication: Optics Express
Title: Sub-microsecond molecular motion analysis of polymer resins via transmitted X-ray blinking
DOI: 10.1364/OE.573497