Proseg

Proseg (probabilistic segmentation) is a cell segmentation method for spatial transcriptomics. Xenium, CosMx, MERSCOPE, and Visium HD platforms are currently supported, but it can be easily adapted to others.

Read the paper:

🗎 Jones, D.C., Elz, A.E., Hadadianpour, A. et al. Cell simulation as cell segmentation. Nat Methods (2025). https://doi.org/10.1038/s41592-025-02697-0

And the Research Brief:

🗎 Confronting the challenge of cell segmentation in spatial transcriptomics. Nat Methods (2025). https://doi.org/10.1038/s41592-025-02717-z

Table of Contents

• Installing
• Migrating to Proseg 3
• Usage
  • Spatialdata output format
  • General arguments
  • Output arguments
  • Model arguments
  • Xenium
  • Importing into Xenium Explorer
  • CosMx
  • MERSCOPE
  • VisiumHD
  • Initializing using Cellpose masks
• Getting help

Installing

Proseg can be built and installed with cargo by running.

cargo install proseg

The easiest way to install cargo for most is rustup.

From source

It can also be build manually from source, which is useful mainly if you want to try a specific revision or make changes

git clone https://github.com/dcjones/proseg.git
cd proseg
cargo build --release

Proseg can then be run with:

target/release/proseg

Limitations of the algorithm

Proseg segments cells based on the observed spatial distribution of transcripts. This generally works well at reducing spurious coexpression and transcript misassignment, but has some limitations to be aware of:

• Cells with very few observed transcripts will tend to have unreliable boundaries.

• Proseg relies on prior (usually image-based) segmentation to determine the number and approximate location of cells. It doesn't introduce new cells, so if the prior segmentation missed many cells, Proseg is not able to correct for that error.

• In highly homogeneous regions consisting of cells with very similar expression profiles, boundaries between cells will be much less reliable.

Migrating to Proseg 3

Proseg 3 has a few changes that users of earlier versions should be aware of.

1. By default proseg will output a spatialdata zarr directory that can be read by the spatiadata package in python. There are many other output options still, but these are disabled by default.
   • This is admittedly a little less convenience for R users. My recommendation would be to convert the AnnData part to h5ad in python like: python import spatialdata sdata = spatialdata.read_zarr("proseg-output.zarr") sdata.tables["table"].write_h5ad("proseg-anndata.h5ad") then read this h5ad file into R with zellkonverter.
   • The proseg-to-baysor command now operates on these zarr directories.
2. By default, count matrices generated by Proseg 3 are integer point estimates, not continuous expected counts, which should make some downstream analysis simpler.
3. Some simplifications were made to the model and sampling procedure. Now the sampling schedule is controlled with these four arguments: --burnin-samples, --samples giving the number of iterations, and --burnin-voxel-size and --voxel-size giving the x/y size of the voxels in microns. The burn in voxel size must be an integer multiple of the final voxel size.
4. The --nbglayers arguments has been removed. There is now just one --voxel-layers argument controlling how many voxels are stacked on the z-axis.
5. The voxel morphology prior has been changed. Instead of --perimeter-bound and --perimeter-eta, there is one --cell-compactness argument, where smaller numbers lead to more compact (equivalently, more circular) cells.

Usage

Proseg is run on a table of transcript positions which in some form must include preliminary assignments of transcripts to nuclei or cells. Xenium, CosMx, and MERSCOPE all provide this out of the box in some form.

Proseg is invoked from the command line like:

proseg [arguments...] /path/to/transcripts.csv.gz

The method is general purpose. There are command line arguments to tell it which columns in the csv file to use, and how they should be interpreted, but typically one of the presets --xenium, --cosmx, --merfish, or --visiumhd are used for common platforms.

Proseg is a sampling method, and in its current form in non-deterministic. From run to run, results will vary slightly, but not in a way that would seriously affect the interpretation of the data.

To see a list of command line arguments, run

proseg --help

Most of these can be disregarded. The most relevent ones will be described below:

Spatialdata output format

The spatialdata zarr output that proseg generates can be read with

import spatialdata
sdata = spatialdata.read_zarr("proseg-output.zarr")

This object contains: * Transcript positions and metadatai in sdata.points["transcripts"]. (This can use significant space, so can be excluded if not needed with --exclude-spatialdata-transcripts). * Cell polygons in sdata.shapes["cell_boundaries"] * Cell level information in AnnData format in sdata.tables["table"], which contains: * Cell metadata in obs * Gene metadata in var * Sparse cell-by-gene count matrix in X * Cell centroids in obsm["spatial"] * Some information about the proseg run in uns["proseg_run].

General arguments

• --nthreads N sets the number of threads to parallelize across. By default proseg will use all available CPU cores, which may be a bad idea on a shared machine.
• --output-spatialdata output.zarr: Proseg will output a spatialdata zarr directory which can be read by the spatialdata python package and contains all metadata, count matrix, and cell geometry.
• --overwrite: An existing zarr directory will not be overwritten unless this argument is passed.
• --voxel-layers N: Number of layers on the z-axis to model 3D cells.
• --samples N: Run the sampler for this N iterations.
• --burnin-samples N: Run the sampler for a preliminary N samples at a lower resolution.
• --voxel-size S: Voxel size in microns on the x/y axis.
• --burnin-voxel-size S: Larger voxel size to use for the burn-in phase. (This must be an integer multiple of the final voxel size).

Output arguments

In addition to the spatialdata zarr output, results can be written to separate number of tables, which can be either gzipped csv files or parquet files, and GeoJSON files giving cell boundaries.

• --output-counts counts.mtx.gz: Output a cell-by-gene count matrix in gziped matrix market format. (Which can be read with e.g. mmread in scipy).
• --output-expected-counts expected-counts.mtx.gz: Output an expected count matrix, where the counts are non-integer estimates from taking the mean over multiple samples.
• --output-cell-metadata cell-metadata.csv.gz: Cell centroids, volume, and other information.
• --output-transcript-metadata transcript-metadata.csv.gz: Transcript ids, genes, revised positions, assignment probability, etc.
• --output-gene-metadata gene-metadata.csv.gz: Per-gene summary statistics
• --output-rates rates.csv.gz: Cell-by-gene Poisson rate parameters. These are essentially expected relative expression values, but may be too overly-smoothed for use in downstream analysis.

Cell boundaries can be output a number of ways:

• --output-cell-polygons cell-polygons.geojson.gz: 2D consensus polygons for each cell in GeoJSON format. These are flattened from 3D, which each xy position assigned to the dominant cell.
• --output-cell-polygon-layers cell-polygons-layers.geojson.gz: Output a separate, non-overlapping cell polygon for each z-layer, preserving 3D segmentation.

Model arguments

A number of options can alter assumptions made by the model, which generally should not need

• --ncomponents 10: Cell gene expression is a modeled as a mixture of negative binomial distributions. This parameter controls the number of mixture components. More components will tend to nudge the cells into more distinct types, but setting it too high risks manifesting cell types that are not real.
• --no-diffusion: By default Proseg models cells as leaky, under the assumption that some amount of RNA leaks from cells and diffuses elsewhere. This seems to be the case in much of the Xenium data we've seen, but could be a harmfully incorrect assumption in some data. This argument disables that part of the model.
• --diffusion-probability 0.2: Prior probability of a transcript is diffused and should be repositioned.
• --diffusion-sigma-far 4: Prior standard deviation on repositioning distance of diffused transcripts.
• --diffusion-sigma-near 1: Prior standard deviation on repositioning distance of non-diffused transcripts..
• --nuclear-reassignment_prob 0.2: Prior probability that the initial nuclear assignment (if any) is incorrect.
• --cell-compactness 0.03: Larger numbers allow less spherical cells.

General advice

• If the dataset consists of multiple unrelated tissue sections, it's generally safer to split these and segment them separately. This avoids potential sub-optimal modeling due to different z-coordinate distributions.
• If proseg crashes without any kind of error message, or if it suddenly starts running extremely slowly, that's usually a sign that it ran out of memory. The best option would be to run on a system with more memory, but increasing the voxel size, lowering the number of voxel layers, and disabling diffusion will all reduce the memory usage, typically at the cost of lower transcript assignment accuracy.
• Proseg is aided by doing local repositioning transcripts (I call this "transcript repo" or "transcript diffusion"). If your skeptical that transcripts leak or diffuse, it's still useful to allow some amount of repositioning to overcome limitations with voxel resolution. Setting --diffusion-probability 0.0 will still let transcripts make small scale adjustments, unlike --no-diffusion which will completely disable that part of the model.

Xenium

Proseg will work on Xenium transcript tables in either csv.gz or parquet format. The latter will be slightly more efficient to read.

proseg --xenium transcripts.csv.gz
# or
proseg --xenium transcripts.parquet

Importing into Xenium Explorer

After segmenting with Proseg, the data can be converted back to format readable by Xenium Explorer.

1. First convert the proseg output to a format matching Baysor's using the included proseg-to-baysor command: sh proseg-to-baysor proseg-output.zarr \ --output-transcript-metadata proseg-to-baysor-transcript-metadata.csv --output-cell-polygons proseg-to-baysor-cell-polygons.geojson
2. This can then be converted to a xenium bundle with Xenium Ranger. sh xeniumranger import-segmentation \ --id sample_id \ --xenium-bundle /path/to/xenium/bundle \ --transcript-assignment proseg-to-baysor-transcript-metadata.csv \ --viz-polygons proseg-to-baysor-cell-polygons.geojson \ --units=microns where /path/to/xenium/bundle is the original xenium bundle.
3. Xenium Explorer should be able to read the resulting xenium bundle which will be written to a directory named whatever is passed to --id.

Known issues: * Problems importing can arise if transcripts with qv scores below 20 are not filtered out. This is done by default in Proseg, but lowering this cutoff could cause issues. * Earlier versions of Xenium Ranger/Explorer tend to mangle polygons generated by Proseg. Since Proseg is voxel based, any cell boundary that isn't axis aligned (i.e. composed of vertical and horizontal line segments), is due to Xenium software mis-rendering them.

CosMx

Proseg works on CosMx transcript table with

proseg --cosmx sample_tx_file.csv

Legacy CosMx data

Earlier versions of CosMx did not automatically provide a single table of global transcript positions. To work around this, we provide a Julia program in extra/stitch-cosmx.jl to construct a table from the flat files downloaded from AtoMx.

To run this, some dependencies are required, which can be installed with ```sh