Constructing a single-objective oblique plane microscope (OPM) for fast, multi-colour, high-resolution volumetric fluorescence imaging

This paper presents a detailed protocol for constructing and characterizing a single-objective oblique plane microscope using commercially available components to enable fast, high-resolution, multi-colour volumetric fluorescence imaging while overcoming the complex alignment challenges of traditional designs.

The Gist

Imagine you want to take a high-resolution 3D video of a tiny, living city inside a cell. You want to see how the buildings (proteins) move and interact without burning the city down with too much light.

This paper is essentially a "DIY Instruction Manual" for building a very special, high-tech camera called an Oblique Plane Microscope (OPM).

Here is the breakdown of what the paper does, using simple analogies:

1. The Problem: The "Two-Headed" Monster

Traditional high-speed microscopes (Light Sheet Microscopes) are like trying to take a photo of a fish in a tank using two separate cameras:

• One camera shines a thin sheet of light from the side to illuminate the fish.
• A second camera, placed at a 90-degree angle, takes the picture.

The Catch: This setup is bulky, requires the fish to be glued in a weird gel cylinder, and it's hard to fit into standard lab dishes. It's like trying to photograph a painting by having one person shine a flashlight from the left and another person take the photo from the top, while the painting is stuck to a wall.

2. The Solution: The "One-Eyed" Wizard (OPM)

The authors built a microscope that uses only one objective lens (one "eye") to do both jobs: shining the light and taking the picture.

• The Trick: They shine the light at a slanted angle (oblique) through the lens. The light hits the sample, and the glowing fluorescence comes back through the same lens.
• The Magic Mirror: Because the light sheet is tilted, the image coming back is also tilted (like a slanted slice of bread). To fix this, they use a clever system of mirrors and lenses (called a Remote-Refocusing System) that acts like a "digital straightener." It catches that slanted image, rotates it, and flattens it out so the camera sees a perfect, straight picture.

Analogy: Imagine looking at a tilted mirror in a funhouse. The reflection is skewed. This microscope has a special set of lenses that acts like a "smart glass" that automatically straightens the reflection so you see the object exactly as it is, even though the light hit it at a weird angle.

3. Why Build It Yourself? (The "IKEA" Factor)

Until now, these microscopes were like custom-made, million-dollar sculptures. Only a few labs in the world had them because they were incredibly hard to build. If you messed up the alignment by a hair's breadth, the whole thing was blurry.

This paper is the "IKEA Manual" for scientists.

• It lists every single part you need to buy (like buying screws and wood for a bookshelf).
• It gives step-by-step instructions on how to align the lasers and mirrors.
• It explains how to test if your microscope is working correctly using tiny glowing beads (like checking if a new TV has dead pixels).

4. How It Works (The "Slicing" Analogy)

To get a 3D video, you don't need to move the sample up and down (which is slow and shaky).

• The Old Way: Imagine slicing a loaf of bread by moving the bread up and down against a stationary knife. Slow and wobbly.
• The OPM Way: You keep the bread still, but you move the knife back and forth very quickly.
• In this microscope, a tiny vibrating mirror (a Galvo mirror) moves the light sheet back and forth across the sample. The camera snaps pictures as the "knife" moves. This is so fast it can capture living cells moving in real-time without blurring.

5. What Can It See?

The authors tested their new microscope on two very different things:

1. Heart Cells: They took 3D videos of rat heart cells, showing the intricate "muscle fibers" that make the heart beat.
2. Diatoms: These are tiny, glass-like algae with intricate 3D shells. The microscope could see inside their glass houses to see their DNA and internal organs.

The Big Takeaway

This paper democratizes a superpower. It takes a complex, expensive, "black box" technology and turns it into a set of instructions that any well-equipped biology lab can follow.

In short: They built a "one-lens" microscope that is fast, gentle on living cells, and fits on a standard lab bench, and they wrote a guide so you can build one too. This allows scientists to watch the 3D movies of life happening inside cells with unprecedented clarity.

Technical Summary

1. Problem Statement

While Light-Sheet Fluorescence Microscopy (LSFM) offers superior speed, resolution, and reduced phototoxicity compared to traditional wide-field or confocal microscopy, conventional LSFM setups face significant limitations:

• Sample Mounting Constraints: Most LSFM configurations (e.g., iSPIM, OTLS) require two orthogonally aligned objectives, necessitating custom sample holders (e.g., gel cylinders) and making them incompatible with standard biological containers like Petri dishes or multi-well plates.
• Mechanical Instability: Volumetric imaging often requires physically translating the sample or objectives, which introduces mechanical vibration, drift, and limits acquisition speed.
• Accessibility Barrier: Oblique Plane Microscopy (OPM) was developed to overcome these issues by using a single objective for both illumination and detection. However, OPM construction is hindered by its complex optical design, particularly the Remote-Refocusing System (RFS) required to correct the tilted image plane. The lack of commercial off-the-shelf solutions and detailed, step-by-step construction protocols has made OPM inaccessible to most biology laboratories.

2. Methodology

The authors present a comprehensive protocol for constructing a compact, single-objective OPM system using commercially available components. The system is built on a standard inverted microscope body (Nikon Eclipse TE2000-U) and features a two-layer optical design:

• Optical Architecture:
  • Excitation Path (Bottom Layer): Uses a 100× high-NA silicone oil immersion objective (O1) to generate an oblique Gaussian light sheet. The sheet is formed by a cylindrical lens and scanned laterally across the sample using a 1D galvanometric (Galvo) mirror.
  • Emission Path (Top Layer): Fluorescence collected by O1 is relayed through a 4f system to a secondary objective (O2, 40× air). The core innovation is the Remote-Refocusing System (RFS), which creates an undistorted intermediate image. This image is then re-imaged by a tertiary, glass-tipped objective (O3, AMS-AGY v1.0) mounted at a 35° tilt to correct the oblique angle, projecting the image onto an sCMOS camera.
• Design & Calculation: The paper provides theoretical frameworks and an Excel tool ("OPM_Easy2Use") to calculate critical parameters, including:
  • Matching the light sheet waist to the Depth of Field (DOF) of the primary objective.
  • Calculating the Rayleigh length and imaging depth based on the tilt angle.
  • Determining the magnification ratio between O1 and O2 to satisfy the refractive index matching condition ($M = n_1/n_2$) for aberration-free refocusing.
• Alignment Procedure: The protocol details a rigorous, step-by-step alignment process (~15 hours total) utilizing a "Two-Iris Alignment Technique" to define optical axes. Key steps include:
  • Establishing a reference laser path.
  • Aligning the common path (Galvo and dichroic mirrors).
  • Constructing the emission path with precise 4f relay systems.
  • Calibrating the RFS by measuring lateral magnification stability across a ~82 µm refocusing range.
  • Characterizing the light sheet using fluorescein solutions and fluorescent microspheres.

3. Key Contributions

• Standardization of OPM: The paper provides the first detailed, open-access protocol for building a single-objective OPM from scratch using off-the-shelf components, removing the "black box" nature of previous custom builds.
• Compatibility: The design is compatible with standard biological sample containers (e.g., multi-well plates, coverslips) and commercial inverted microscope bodies, bridging the gap between high-end LSFM and routine cell biology.
• Performance Optimization: The use of a glass-tipped tertiary objective (O3) eliminates working distance losses and improves collection efficiency, while the RFS ensures diffraction-limited performance without mechanical sample translation.
• Educational Resource: The inclusion of theoretical calculations, alignment photos, troubleshooting guides, and open-source code (LabVIEW control and Python reconstruction scripts) lowers the technical barrier for researchers.

4. Results

The constructed system was rigorously characterized and demonstrated on biological samples:

• Resolution: The system achieved near-diffraction-limited performance with an average lateral (xy) resolution of ~374 nm and an axial (z) resolution of ~660 nm (measured using 100 nm fluorescent microspheres at 488 nm excitation).
• Volumetric Imaging: The system successfully performed fast, multi-color volumetric imaging. The Galvo mirror scanning allowed for volume acquisition rates limited only by the camera frame rate, enabling real-time observation of dynamic processes.
• Biological Applications:
  • Cardiomyocytes: High-resolution 3D reconstruction of adult rat ventricular cardiomyocytes, visualizing membrane structures (WGA-Alexa 488) and actin cytoskeleton (CellMask Deep Red) with clear myofibrillar architecture.
  • Diatoms: Imaging of Coscinodiscus radiatus diatoms, resolving intracellular structures within rigid silica frustules using SYTO® 9 staining and chlorophyll autofluorescence.
• Multi-Color Capability: The system successfully demonstrated simultaneous two-color imaging, proving its utility for co-localization studies.

5. Significance

This work democratizes access to advanced light-sheet microscopy. By providing a reproducible, step-by-step guide, the authors enable life science and medical researchers to construct their own high-performance OPM systems without needing specialized optical engineering expertise. The resulting capability to perform fast, high-resolution, multi-color 3D imaging on standard sample formats significantly advances the study of spatiotemporal organization in live cells, tissues, and organoids, while minimizing phototoxicity and photobleaching. Furthermore, the principles outlined here offer a blueprint for implementing Remote-Refocusing Systems in other LSFM modalities.