Mesoscopy: Innovations in high-resolution and large-field imaging
Mesoscopy refers to imaging methodologies that provide a field of view (FOV) ranging from several millimeters to centimeters while achieving cellular or even subcellular resolution (Figure 1). This technological framework employs specially designed large-scale objective lenses to correct aberrations across extended FOVs, synchronized with light-field acquisition modalities through either scanning point detection or large-format array detection. Conventional microscopes, constrained by the limitations of objective lenses, exhibit a trade-off between the FOV and resolution. To achieve both high resolution and a large FOV, common approaches such as FOV stitching and Fourier ptychography were employed. However, these methods were extremely slow and imposed numerous constraints on samples. In 2016, a mesoscopic objective lens was introduced to address these challenges, achieving a 6 mm FOV and 0.7 μm resolution, thereby increasing the imaging throughput of conventional objective lenses by orders of magnitude.1 In the same year, this technology was recognized as one of the top ten physics breakthroughs worldwide by Physics World. Since then, mesoscopic imaging technology has gradually gained momentum and has been applied in various fields.
Key components
The objective lens serves as the core component of mesoscopy. The initial design of mesoscopic objective lenses typically involves enlarging traditional objective lens elements. However, this enlargement led to increased aberrations, particularly at the lens periphery. The design of a mesoscopic objective requires the critical aspect of meticulous aberration optimization, such as quasi-aplanatic architectures for dual aberration suppression, curvature-engineered high-order balancing, and freeform/diffractive corrective elements. Cutting-edge mesoscopic objectives show outstanding multimodal abilities. Representative designs achieve broadband performance from 400 to 1,000 nm, with sub-micron resolution over an 8 mm FOV at a numerical aperture (NA) of 0.5. More specialized variants allow three-photon imaging in near-infrared (NIR)-II, with better penetration depth in a 3.5 mm FOV at an NA of 0.75.
As another core component, detectors are functionally categorized into two operational paradigms: point-scanning detectors and wide-field array detectors. (1) Point-scanning detectors are predominantly represented by photomultiplier tubes (PMTs). In most point-scanning mesoscopic imaging systems, conventional PMTs with a sensor diameter of less than 5 mm are used. Nevertheless, some studies have proposed large-target PMT (with a diameter exceeding 10 mm) to collect fluorescence emitted at larger angles. (2) Wide-field array detectors: mesoscopic imaging necessitates large-format sensors with high-pixel densities. Among current commercial cameras, only a few industrial cameras meet this requirement. These cameras are relatively slow, and their parameters, such as noise and sensitivity, are slightly inferior. To improve the imaging effect and speed, some research has pieced together multiple scientific-grade cameras as the detector, but the cost is extremely high. With the development of high-pixel, large-target-surface, scientific-grade cameras, mesoscopic imaging parameters will be enhanced.
