Beating the dark current for safer X-ray imaging

A detection technology that can create sharp images from ultra-low X-ray doses could improve the safety of X-ray medical imaging. The invention achieves high sensitivity using a novel arrangement of perovskite single crystals as X-ray detecting materials^[1].

Although X-ray machines remain a key form of medical imaging, X-rays are a high energy form of ionizing radiation and high doses are associated with an increased risk of cancer.  Keeping X-ray exposure to within safe limits curtails medical use.

An intense search is underway to identify materials that could increase the sensitivity of X-ray detectors, enabling high-quality medical images using very low X-ray doses.

“In recent years, many perovskite single crystal materials have demonstrated excellent X-ray detection performance,” says KAUST researcher Xin Song, a member of Omar Mohammed’s research group, who led the research.

When an X-ray photon strikes a perovskite semiconductor crystal, it generates a pair of electric charges, one positive and one negative. When these charges reach electrodes at the perovskite edges, they create a photocurrent from which X-ray images can be generated.

To push the performance of perovskite X-ray detectors further, the team has targeted the materials’ ‘dark current’. “The dark current of an X-ray detector semiconductor refers to the electrical current that flows through the device when it is not exposed to X-rays,” says Song. Dark current is primarily caused by heat-generated charge carriers and leakage currents within the device, she says.

The dark current in an X-ray detector can obscure low-dose X-ray signals and introduce additional noise. “This reduces the signal-to-noise ratio and negatively impacts the overall performance of the device,” Song says.

The research team has now shown that an approach called cascade engineering can effectively suppress dark current. “Cascade engineering connects a type of single crystals in series,” Mohammed explains. Connecting crystals in series increases the electrical resistance through the devices. “This can effectively reduce the dark current and noise of the device without impacting the X-ray generated charge carriers, improving detector performance and reducing its detection limit.”

The team developed a device based on a perovskite called methylammonium lead bromide (MAPbBr₃) to test the cascade engineering concept. “This material exhibits relatively high stability, and the synthesis process enables MAPbBr₃ single crystal preparation with excellent reproducibility,” Song says. These attributes make the material an ideal candidate for X-ray detector fabrication with significant commercial potential, she says.

The researchers tested connections of 1 to 4 single crystals in series, and showed that increasing the number of series connections effectively reduced the dark current. Higher numbers of crystals also weakened device detection sensitivity, however. “We found that the connection of two crystals in series achieved the lowest detection limit while maintaining higher sensitivity,” Song says. Pairing two crystals using cascade engineering lowered the detection limit from 590 nGy·s−1 to just 100 nGy·s−1.

“We are now investigating the cascade structure for other perovskite single crystals, with the goal of reducing their detection limits still further,” Mohammed says. The team is also working on the packaging of MAPbBr₃ cascade single crystals for real-world medical imaging.

Reference

1. Song, X., Zhang, X., He, T., Wang, J., Zhu, H., Zhou, R., Ahmad, T., Bakr, O.M.& Mohammed, O.F. Revolutionizing X‐ray imaging: A leap toward ultra-low-dose detection with a cascade-engineered approach. ACS Central Science10, 2082–2089 (2024).| article