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Beyond FPM: New imaging technique promises clearer, faster microscopy

A team of engineers at the California Institute of Technology (Caltech) has developed a groundbreaking microscopy method that promises to revolutionise biomedical imaging, digital pathology, and drug screening. The new technique, called Angular Ptychographic Imaging with Closed-form method (APIC), builds upon and surpasses the capabilities of its predecessor, Fourier ptychographic microscopy (FPM).

For centuries, microscope manufacturers grappled with the inherent limitations of optical lenses, struggling to balance high resolution with a wide field of view. The advent of computational microscopy in 2013, marked by the introduction of FPM, offered a solution by combining conventional microscope sensing with advanced computer algorithms to produce sharper, more detailed images over larger areas.

Whilst FPM represented a significant leap forward, it still relied on an iterative process to reconstruct images, potentially introducing inaccuracies. The newly developed APIC method, detailed in a recent Nature Communications [1] paper, addresses this shortcoming whilst retaining all the advantages of FPM. Under the guidance of Changhuei Yang, Thomas G. Myers Professor of Electrical Engineering, Bioengineering, and Medical Engineering at Caltech, the research team devised a novel approach to image reconstruction. APIC employs a linear equation to determine optical aberrations, eliminating the need for the trial-and-error process inherent in FPM.

Ruizhi Cao, co-lead author and former graduate student in Yang’s lab, explained: “We arrive at a solution of the high-resolution complex field in a closed-form fashion, as we now have a deeper understanding of what a microscope captures, what we already know, and what we need to truly figure out, so we don’t need any iteration. In this way, we can basically guarantee that we are seeing the true final details of a sample.”

Yang Changhuei FPM APIC

A stained breast cancer sample was imaged using red, green, and blue LEDs. A new computational microscopy technique developed at Caltech, called APIC, was used to reconstruct the detailed colour image shown on the right. The image shows even higher resolution than the image on the left, which was obtained using FPM, a widely-used microscopy technique.

Phase measurement and aberration correction

Like FPM, APIC measures both light intensity and phase – a property related to light travel distance that is imperceptible to the human eye but crucial for aberration correction. However, APIC’s approach to phase information is markedly different.

Cheng Shen, co-lead author and former member of Yang’s lab, explains: “We have proven that our method gives you an analytical solution and in a much more straightforward way. It is faster, more accurate, and leverages some deep insights about the optical system.”

Practical advantages

APIC offers several practical benefits over its predecessor. Notably, it allows researchers to capture clear images across a large field of view without the need for frequent refocusing. This is particularly advantageous when dealing with samples that vary in height, as FPM required refocusing even for variations of a few tens of microns.
Given that computational microscopy often involves stitching together over 100 lower-resolution images to create a larger field of view, APIC’s ability to maintain focus across varying sample heights significantly accelerates the imaging process and reduces the potential for human error.

Implications for AI in medical imaging

The development of APIC aligns with broader research efforts in Yang’s lab to optimise image data for artificial intelligence (AI) applications in medicine. Yang notes: “Recently, my lab showed that AI can outperform expert pathologists at predicting metastatic progression from simple histopathology slides from lung cancer patients. That prediction ability is exquisitely dependent on obtaining uniformly in-focus and high-quality microscopy images, something that APIC is highly suited for.”

Future applications

Cao emphasises the potential for APIC beyond microscopy: “We have developed a framework to correct for the aberrations and also to improve resolution. Those two capabilities can be potentially fruitful for a broader range of imaging systems.”

1. Cao, R., Shen, C., & Yang, C. (2024). High-resolution, large field-of-view label-free imaging via aberration-corrected, closed-form complex field reconstruction. Nature Communications, 15.