Leveraging Virtual Reality for Immersive Segmentation and Analysis of Cryo-Electron Tomography Data

Summary

Cryo-electron tomography (cryo-ET) enables 3D visualization of cellular ultrastructure at nanometer resolution, but manual segmentation remains time-consuming and complex. We present a novel workflow that integrates advanced virtual reality software for segmenting cryo-ET tomograms, showcasing its effectiveness through the segmentation of mitochondria in mammalian cells.

Abstract

Cryo-electron tomography (cryo-ET) is a powerful technique for visualizing the ultrastructure of cells in three dimensions (3D) at nanometer resolution. However, the manual segmentation of cellular components in cryo-ET data remains a significant bottleneck due to its complexity and time-consuming nature. In this work, we present a novel segmentation workflow that integrates advanced virtual reality (VR) software to enhance both the efficiency and accuracy of segmenting cryo-ET datasets. This workflow leverages an immersive VR tool with intuitive 3D interaction, enabling users to navigate and annotate complex cellular structures in a more natural and interactive environment. To evaluate the effectiveness of the workflow, we applied it to the segmentation of mitochondria in retinal pigment epithelium (RPE1) cells. Mitochondria, essential for cellular energy production and signaling, exhibit dynamic morphological changes, making them an ideal test sample. The VR software facilitated precise delineation of mitochondrial membranes and internal structures, enabling downstream analysis of the segmented membrane structures. We demonstrate that this VR-based segmentation workflow significantly improves the user experience while maintaining accurate segmentation of intricate cellular structures in cryo-ET data. This approach holds promise for broad applications in structural cell biology and science education, offering a transformative tool for researchers engaged in detailed cellular analysis.

Introduction

Cryo-electron tomography (cryo-ET) has revolutionized our ability to visualize cellular components in their near-native state at high-resolution^1,2. This powerful technique allows researchers to characterize cellular ultrastructure, providing unprecedented insights into cellular architecture and function. However, cryo-ET is not without its limitations, chief among them is the requirement that samples be thin enough to be electron transparent (typically <0.5 µm) for imaging in standard cryo-capable transmission electron microscopes. Recent advancements in cryo-focused ion beam milling have enabled the thinning of thick samples for cryo-ET analysis^3,4.

Typical cryo-ET workflows begin with the collection of a tilt-series, where the sample is imaged at various angles, ranging from +60° to -60°. These images are then computationally aligned and backprojected to create a three-dimensional (3D) volume or tomogram^5,6,7. This tomogram serves as a detailed 3D map of the cellular landscape, offering both spatial and temporal information about cellular structures. Further refinement through subtomogram averaging, where multiple copies of the same structure are aligned and averaged, can push resolution limits even further, sometimes achieving sub-nanometer resolution^7,8,9,10.

A crucial step in extracting meaningful biological information from these tomograms is segmentation. This process involves annotating specific cellular structures, such as membranes, within the 3D volume. Segmentation enables advanced analyses, including the calculation of intermembrane distances and membrane curvature, providing valuable insights into cellular processes^11,12. While several software packages are available for this task, including Dragonfly, Amira, MemBrain, EMAN2, and tomomemsegtv^{13,14,15,16,17}, the segmentation process remains a significant bottleneck in cryo-ET data analysis. It is often a labor-intensive and time-consuming manual process, potentially taking weeks to months to complete. Many of these packages offer automatic segmentation features but frequently require extensive manual correction to remove false positives, a process that can be laborious and unintuitive when performed slice-by-slice or in 3D.

We propose an alternative approach by leveraging virtual reality (VR) technology to address these challenges. VR offers an immersive and interactive method for data visualization, allowing users to navigate through the tomographic volume as if inside the cellular environment itself. This approach also provides a valuable platform for science education and scientific exploration and discovery of in situ cryo-ET data by providing an immersive and uniquely engaging experience. In this work, we present a protocol for cryo-ET data segmentation using syGlass¹⁸, VR software designed for scientific visualization. This software provides a comprehensive toolkit for cryo-ET data analysis, including manual segmentation, refinement of automatically generated segmentations, and even particle picking within tomograms. Our study demonstrates the viability of VR as a powerful tool for cleaning segmentations, particle picking, and manual segmentation in cryo-ET data analysis.

To illustrate the utility of the VR software for cryo-ET segmentation, we focus on the analysis of mitochondrial morphology in retinal pigment epithelium (RPE1) cells. Mitochondria serve as an excellent test case for segmentation due to their complex structure and the presence of readily quantifiable features, such as the distance between outer and inner mitochondrial membranes. These features can be accurately measured using surface morphometrics analysis tools¹², providing robust metrics for assessing segmentation quality. This protocol provides step-by-step instructions for segmenting cryo-ET data using syGlass, demonstrating its utility within the cryo-ET segmentation pipeline. By incorporating VR-based segmentation into the cryo-ET workflow, we aim to improve both the efficiency and accuracy of manual structural analysis in cellular biology.

Protocol

1. Preparing cryo-ET data for segmentation

1. Convert raw Cryo-ET tomograms into a data format compatible with syGlass, such as TIFF stacks. Set the signal so that the particles are white on black, and perform histogram equalization with ImageJ¹⁹.
   NOTE: To enhance the signal-to-noise ratio, it is recommended that the tomograms be denoised with software such as Warp²⁰, Topaz Denoise²¹, cryoCARE^22,23, or IsoNet²⁴ before being imported into the software.
2. Launch the VR software on the computer. Navigate to the File menu and select Create Project.
3. Click on Create New Project | Add Files. Navigate to the location where the TIFF files are saved, and import them into the software.
4. When prompted to confirm if the files are a time series, click No.
5. Name the project, then click Save to create a new project under the project list.
6. Double-click the project to open the tomogram and load it into the software interactive virtual reality environment.

2. Setting up virtual reality (VR)

1. Connect the VR headset and hand controllers to the computer.
2. Follow the onscreen instructions to calibrate the VR environment.
   NOTE: Calibration refers to the three-dimensional space where the segmentation will be done. The parameters (Length, width, and height) of the working space need to be defined before the VR projection is displayed. The VR projection will only be visible in these parameters.
3. Segmentation will be done only within the defined working three-dimensional space.Ensure the field of view contains the area desired to be worked on in the VR environment.

3. Optimizing 3D visualization

1. Click on the Visualization button in the software interface. Adjust various visualization options such as contrast, windowing, brightness, and threshold sliders to enhance the signal and minimize noise.
2. Use the hand controllers to pull the tomogram closer or push it away as needed.
3. Activate the Cut tool using the left-hand controller to inspect different slices within the tomogram visually.

4. Segmentation process

1. Navigate through the tomogram to the desired slice where segmentation will begin.
2. Activate the Region of Interest (ROI) option under the annotation menu using the hand controllers. A green box will appear within the tomogram.
3. Adjust the size and position of the green box, moving it to the area to be segmented.
4. Lock the ROI using the left-hand controller and initiate segmentation. After locking the ROI, the tool automatically switches to a paint mode that allows segmentation of the volumetric data.
5. Zoom in or out of the tomogram for precise segmentation.
6. Adjust the paint brush size by rotating clockwise or counterclockwise for optimal control.
7. Carefully segment the ROI (e.g., mitochondrial membranes) within the selected 3D area. If an error occurs during segmentation, press and hold the secondary controller trigger to engage erase mode, then erase the error using the same motion as segmenting.
8. Repeat this process region by region until the entire tomogram is segmented.

5. Exporting segmented data and analysis

1. After completing the segmentation, select the project by clicking on it to ensure it is highlighted.
2. Click the Projects tab in the upper-left corner, then select ROIs.
3. Choose to export either the entire volume or a specific ROI. Specify the export location for the segmented data.
4. Load and analyze the segmented data using the software of choice and generate figures for publication.

6. Importing binary mask into the software for cleanup

1. Perform steps 1.1-1.6.
2. Right-click on the project and click Add mask data. Then, navigate to where the initial segmentation is saved and import it under the same project.
3. Engage ROI annotation to make edits to the initial segmentation.
4. Add/erase segmentation to clean up the initial segmentation.
5. Adjust Mask ROIs under the ROI option in the masks tab. To change the mask ROI from one to the other, turn on the only overwrite mask option and go to paint mode with the hand controller. This mode will only overwrite mask data that is visible.
6. Export the adjusted mask ROIs utilizing the steps described in steps 5.1-5.3.

7. Picking particles coordinates using the VR software

1. Create a project using tomogram (TIFF stack) utilizing the steps described in section 1.
2. Click on Annotation | Count, which opens a window with manual or automated options.
   1. For the manual option, choose a color for the counts, and start marking coordinates.
   2. For the automated option, choose a cell size and input the resolution. Click on Run auto counting, edit the counts (keep good picks and discard bad ones), and save the progress.
3. Export the count coordinates by clicking on the project tab | annotations.
   1. For the annotation origin, choose Bottom Left Corner and for Annotation Units, choose Voxels.
   2. Click on the counting points cell to highlight the cell.
   3. Click on Export selected to save the coordinates of the selection in a .csv file.
      NOTE: This file can be used to extract subtomograms from the tomogram for subtomogram averaging

Discussion

In this work, we demonstrated how VR, specifically using syGlass, can be integrated into the cryo-ET pipeline to effectively segment cellular structures. Although our focus was primarily on membranes, there is no inherent limitation preventing the segmentation of other cellular structures, such as filaments or ribosomes, as one can adjust the brush shape and size, and thresholding to ensure that only voxels corresponding to the desired cellular objects are marked. One of the major advantages of VR in segmentation is the intuitive, hands-on interaction with the data, thereby allowing users to visualize and manipulate volumes as if they were physically inside them. Traditional methods of manual segmentation or cleaning initial segmentation masks typically involve working directly on a computer, which can be less immersive and slower to annotate.

By incorporating VR into the cryo-ET workflow, users can not only rapidly interact with segmentation masks generated by other software, but also use VR to guide the segmentation of partially segmented structures and efficiently clean up false positives. Currently, manual segmentation is still required for most annotation use cases, and the presented workflow enables users to generate segmentations that are suitable for downstream analysis with improved ease and speed. For this study we used the HTC Vive VR headset, but the software is compatible with devices that have SteamVR or OculusVR support.

For optimal application of this protocol, the cryo-ET data should meet specific criteria, such as high-quality tomograms with a high signal-to-noise ratio (SNR). The SNR is an essential parameter, as the clarity of structural features directly influences the effectiveness of manual segmentation in VR. Preprocessing steps such as missing wedge correction and denoising are key parts of the workflow; in this protocol, we utilized IsoNet for these purposes. The tomograms should be reconstructed at a suitable resolution -- using a voxel size that provides sufficient structural detail and enough separation between cellular structures to enable effective segmentation between the structures while further maintaining manageable data sizes for VR visualization. Additionally, inverted contrast with cellular structures appearing white on a black background and histogram equalization should be applied to enhance the visibility of membranes and other structures within the syGlass environment.

Several critical points must be considered to ensure the success of this protocol. First, during data preprocessing, accurate application of contrast inversion and histogram equalization is vital; improper adjustments can result in suboptimal visualization in VR, making segmentation challenging. Second, within the VR software, appropriate adjustments of windowing, brightness, and threshold settings are essential for optimal visualization of structures. Users should experiment with these settings to achieve the best results for their specific datasets and VR setups.

When it comes to troubleshooting, users may encounter issues such as VR system performance limitations, especially when handling large tomogram datasets. If the VR environment becomes laggy or unresponsive, consider downsampling the data or segmenting the tomogram in smaller sections by adjusting the bounding box in the ROI tools in syGlass and moving it along the tomogram as segmentation is being performed. The performance slider in the visualization menu can also be adjusted to reduce lagging during the segmentation process. Additionally, to mitigate motion sickness, users can adjust VR settings to reduce motion effects or take regular breaks during segmentation sessions.

Our VR-based segmentation protocol offers significant benefits to current cryo-ET segmentation workflows. Conventional manual segmentation methods often involve 2D annotation on individual slices, which can be time-consuming and may not fully capture three-dimensional continuity¹³. In addition, VR-based segmentation introduces a much more immersive view of the cellular structures, aiding in visualization. Automated machine-learning segmentations are emerging as a powerful method to obtain segmentations from cellular tomograms^14,16,17, though the high noise levels and complex structures present in cryo-ET data lead to gaps and false positives that require manual intervention. This protocol offers an alternative approach to manually segment cryo-electron tomograms, to generate initial segmentations that may potentially be used as training data for other neural network software, or to clean up initial segmentations generated from other automated approaches.

In conclusion, this study highlights VR-based segmentation as a promising tool for cryo-ET data analysis and education, offering enhanced efficiency and a more immersive user experience. With further development, VR technology has the potential to revolutionize the way we interpret and disseminate scientific discoveries of complex cellular structures in cryo-ET datasets, providing a valuable alternative to traditional segmentation and education methods.

Materials

Name	Company	Catalog Number	Comments
CryoET Data			Format:TIFF-stack, TIFF
HTC VIVE Cosmos	HTC	99HARL000-00	https://www.vive.com/sea/product/vive-cosmos/features/
Intel(R) Core(TM) i7-10870H CPU @ 2.20 GHz 2.21 GHz	Intel		https://ark.intel.com/content/www/us/en/ark/products/208018/intel-core-i7-10870h-processor-16m-cache-up-to-5-00-ghz.html
NVIDIA GeForce RTX 3070 Laptop GPU	NVIDIA		https://www.nvidia.com/en-us/geforce/laptops/compare/30-series/
syGlass Software	syGlass		syGlass Software installed on a compatible Windows PC
VIVE Cosmos Hand Controllers	HTC	99HAFR001-00	https://www.vive.com/us/accessory/cosmos-controller-right/
Windows 11 Home	Microsoft		Microsoft Windows 11 Home