Synthetic Aperture Phase Microscopy Enables Subcellular Imaging

Researchers at The Chinese University of Hong Kong (CUHK) have developed a phase-reliant method of synthetic aperture microscopy (SAM). The method, introduced as “high spatial and temporal resolution synthetic aperture phase microscopy,” or “HISTR-SAPM,” features a setup of digital micromirror devices (DMDs) — electronic components commonly used in digital projectors and that contain a matrix of micromirrors — and overcomes previously existing limitations to SAM tied to spatial resolution and frame rate.

Scientists can individually and electronically control the orientation of micromirror matrices in DMDs at high speeds, which allowed the researchers to develop and implement a system in which they were able to alter the angle of a laser beam arriving at an imaged sample thousands of times per second. The system featured two DMDs and optimized lenses. As soon as light passed through the sample, the team reported, it combined with a portion of the original laser light to yield an interferogram. This resulting pattern carried phase information: Information derived from the relative delay between two electromagnetic waves.

Lightwaves passing through a sample causes their relative phases to change, depending on the optical properties at each point in the sample and the light’s angle of incidence. SAM enables users to quickly capture multiple images, including those with different incident angles. Once processed, those captured images combine to produce an image crisper than that of any of the original images.

Observation of subcellular structures in unlabeled living cells. Courtesy of Advanced Photonics via SPIE.

Existing implementations of SAM, though, do not produce the spatial resolution or frame rate necessary to image in many emerging applications.

In the new method, multiple produced interferograms combine via specially designed algorithms to create the final phase of the image.

“Using our DMD-based approach, we could accurately image material structures with features as small as 132 nm, quantify millisecond fluctuations in the membranes of red blood cells, and observe dynamic changes in cellular structure in response to exposure to chemicals,” said Renjie Zhou, who led the research team from CUHK. The team also tested its label-free technique with nanometric gratings and cancer cells. The method additionally avoided interference caused by laser speckle: The incorporation of multiple interferograms to compute a common image eliminates the unpredictable influence and presence of speckle in individual interferograms.

The spatial spectrum synthesis process in HISTR-SAPM. Courtesy of SPIE.

The researchers also demonstrated the ability to increase the imaging frame rate by decreasing the number of interferograms they used.

“We envision that our high-speed imaging technique will find applications in biology and materials research, such as studying the motions and interactions of live cells and monitoring material manufacturing processes in real time for quality control purposes,” Zhou said. Pairing the existing approach with different algorithms could additionally allow the researchers to use their method to build a 3D imaging system, Zhou said.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.2.6.065002).