presurfer-box

MATLAB-free MP2RAGE preprocessing for presurfer, using SPM Standalone in a Docker or Apptainer container.

presurfer-box makes the presurfer steps (bias correction, MPRAGEising, strip masks, and brain masks) available as a Python package and command-line tool. You do not need MATLAB, a MATLAB licence, or a local SPM installation. SPM runs inside the official SPM Standalone container, and Python prepares the files and runs the container for you.

Status: This version uses SPM25 Standalone. Results have not yet been numerically validated against the historical SPM12/MATLAB pipeline.

Table of contents

• Installation
• Quick start
• Choosing a workflow
• Python API
• Outputs
• Example
• Legacy MATLAB implementation
• Reproducibility and validation
• Licence
• Changelog
• Citation

Installation

The command is presurfer. The older name presurfer-box also works.

presurfer-box requires Python 3.11+ and one container runtime: Docker, or Apptainer. It is a standard pyproject.toml package, so install it with whichever of these you already use.

venv

python3.11 -m venv .venv
source .venv/bin/activate
pip install presurfer
presurfer --help

conda / mamba

presurfer comes from PyPI, not conda-forge, so pip install does the actual install inside a conda or mamba environment:

conda create -n presurfer python=3.11
conda activate presurfer
pip install presurfer
presurfer --help

(mamba is a drop-in replacement for conda in both commands above.)

From a source checkout

git clone https://github.com/srikash/presurfer-box.git
cd presurfer-box
pip install .            # or: pip install -e . for an editable install

The legacy MATLAB source is optional. Fetch its pinned submodule only if you need to inspect or run the historical implementation:

git submodule update --init --recursive

uv

Included because this project's development and CI use uv. It is not required for everyday use.

# from PyPI, into an isolated tool environment
uv tool install --python 3.11 presurfer
presurfer --help

# from a source checkout
cd presurfer-box
uv sync
uv run presurfer --help

The SPM container

The Python package does not include SPM. Download the official container once, then check that presurfer can use it.

Docker

docker pull ghcr.io/spm/spm-docker:docker-matlab-25.01.02
presurfer --check

Apptainer (for example on an HPC cluster)

apptainer pull --name spm.sif \
  oras://ghcr.io/spm/spm-docker:singularity-matlab-25.01.02

presurfer --engine apptainer --sif spm.sif --check

A successful check prints the runtime, the image, and the SPM version.

Quick start

# Create a bias-corrected image and bias field
presurfer biascorrect IMAGE.nii

# Create MPRAGEised UNI
presurfer MPRAGEise INV2.nii UNI.nii

# Create an INV2-derived strip mask
presurfer stripmask INV2.nii

# Create UNI tissue classes, brain mask, and white-matter mask
presurfer brainmask UNI.nii

# Do all of the above for a FreeSurfer run in one go
presurfer recon-prep INV2.nii UNI.nii

Inputs can be .nii or .nii.gz. Docker is the default runtime. To use a local Apptainer image, pass --engine apptainer --sif IMAGE before the command:

presurfer --engine apptainer --sif spm.sif stripmask INV2.nii

To use a different Docker image, pass --image IMAGE instead. --image and --sif cannot be combined.

Container engine

--engine chooses between docker (the default) and apptainer. Apptainer needs a local SIF file, so --engine apptainer must be used with --sif. Passing --sif alone selects Apptainer automatically.

presurfer --engine apptainer --sif spm.sif brainmask UNI.nii

Threads

The MATLAB Runtime inside the SPM container starts with a single thread, so SPM would otherwise use one core however many the machine has. presurfer sets the thread count for every SPM step. By default it uses the number of available cores, but never fewer than 8 and never more than 16, because extra threads stop helping beyond a certain point. Choose a number yourself with --threads:

presurfer --threads 12 brainmask UNI.nii

In Python, pass threads=12 to any workflow function.

The thread count can change results in the last digits, although a fixed count always gives identical results. spm.log records the container, image, and thread count of every step, so you can reproduce a run by passing the same --threads value.

Choosing a workflow

MPRAGEise bias-corrects INV2 in a temporary directory, rescales the corrected INV2 to the range 0 to 1, and multiplies it voxel by voxel with UNI. It does not run the strip-mask workflow, so run stripmask separately if you need the mask.

recon-prep runs the whole preparation with two SPM segmentations instead of three. It segments INV2 (which gives the strip mask and a bias-corrected INV2), reuses that bias-corrected INV2 to MPRAGEise UNI, and then segments the MPRAGEised UNI to get the brain and white-matter masks. It checks that INV2 and UNI share the same voxel grid before it starts SPM.

Because recon-prep takes the bias correction from the strip-mask segmentation (2 mm sampling) instead of the separate 3 mm bias correction that MPRAGEise runs, its MPRAGEised image is not the same as the one MPRAGEise makes. In one test subject, about 40% of voxels differed, the mean difference was 0.3% of the intensity range, and the largest was 18%. Over the brighter half of the voxels the two images correlated at 0.988.

This is a deliberate trade. recon-prep saves one segmentation, about 13 to 15 minutes per subject on a 12-thread machine, in exchange for a slightly different MPRAGEised image. Whether that difference matters for your FreeSurfer surfaces is worth checking on your own data. Use MPRAGEise followed by brainmask when you need the result of the original MATLAB pipeline.

Python API

from presurfer import (
    presurf_biascorrect,
    presurf_mprageise,
    presurf_recon_prep,
    presurf_seg,
    presurf_stripmask,
)

presurf_biascorrect("INV2.nii")
presurf_mprageise("INV2.nii", "UNI.nii")
presurf_stripmask("INV2.nii")
presurf_seg("UNI.nii")
presurf_recon_prep("INV2.nii", "UNI.nii")

Every function accepts the optional keywords image=, runtime=, and threads=. For Apptainer, pass the local SIF path as image=:

presurf_stripmask("INV2.nii", image="spm.sif", runtime="apptainer")

presurf_biascorrect, presurf_stripmask, and presurf_seg return the path of the presurf_* output directory. presurf_mprageise returns the path of the *_MPRAGEised.nii image. presurf_recon_prep returns the run directory that holds presurf_INV2, presurf_MPRAGEise, and presurf_UNI. All of them raise RuntimeError if the container is unavailable or SPM fails.

Outputs

Each run creates a new UTC timestamped directory beside its input, containing one presurf_* directory for the workflow. The biascorrect, stripmask, and brainmask workflows also save the SPM output as spm.log, with a short header for each step that gives the container, the image, and the MATLAB thread count:

YYMMDDThh:mm:ss_presurfer/
├── spm.log
└── presurf_INV2/      # or presurf_UNI, presurf_biascorrect, presurf_MPRAGEise

For example, 250101T12:34:56_presurfer/presurf_INV2/. If two runs start in the same second, the second directory gets a numeric suffix (-2). MPRAGEise uses a temporary tmp_ directory for its bias-correction step and removes it afterwards.

recon-prep writes all three workflows into one run directory:

YYMMDDThh:mm:ss_presurfer/
├── spm.log
├── presurf_INV2/        # strip mask and bias-corrected INV2
├── presurf_MPRAGEise/   # MPRAGEised UNI
└── presurf_UNI/         # brain mask and white-matter mask, from the MPRAGEised UNI

Compressed .nii.gz inputs are supported. The source file is never changed, and the workflow writes an uncompressed working copy into its output directory.

Example

Prepare an MP2RAGE for FreeSurfer

One command prepares INV2 and UNI for FreeSurfer:

presurfer recon-prep INV2.nii UNI.nii

It runs the INV2 segmentation first, reuses its bias correction to MPRAGEise UNI, and then segments the MPRAGEised UNI. Everything goes into one run directory:

YYMMDDThh:mm:ss_presurfer/
├── presurf_INV2/        # INV2_stripmask.nii
├── presurf_MPRAGEise/   # UNI_MPRAGEised.nii
└── presurf_UNI/         # UNI_MPRAGEised_brainmask.nii, UNI_MPRAGEised_WMmask.nii

The strip mask, from INV2:

The MPRAGEised UNI. MPRAGEising is better than background removal ('denoising').

The brain mask, from the MPRAGEised UNI:

Then use the strip mask to remove the non-brain parts of the MPRAGEised UNI, and run recon-all on the stripped image:

RUN=YYMMDDThh:mm:ss_presurfer
fslmaths $RUN/presurf_MPRAGEise/UNI_MPRAGEised.nii \
  -mul $RUN/presurf_INV2/INV2_stripmask.nii MPRAGEised_stripped.nii

Here is a fully automated segmentation with presurfer and FreeSurfer, including laminar surfaces:

Step by step

Run the steps separately when you only want some of the outputs, or need to work on an image in between.

presurfer stripmask INV2.nii            # strip mask from INV2
presurfer MPRAGEise INV2.nii UNI.nii    # MPRAGEised UNI
presurfer brainmask UNI_MPRAGEised.nii  # brain mask from the MPRAGEised UNI

The step-by-step route needs three SPM segmentations instead of two, because MPRAGEise runs its own bias correction.

To run only the SPM bias correction, use presurfer biascorrect.

Legacy MATLAB implementation

The original MATLAB implementation lives in srikash/presurfer and remains available here as a pinned Git submodule:

src/matlab/presurfer

For MATLAB use:

addpath('src/matlab/presurfer/func')

Update the submodule only when intentionally adopting a newer original presurfer commit (unlikely to happen).

Reproducibility and validation

• Default image: ghcr.io/spm/spm-docker:docker-matlab-25.01.02
• SPM runtime: SPM25 Standalone
• Historical implementation: SPM12 with MATLAB
• Numerical equivalence with the historical pipeline has not yet been validated against reference NIfTI data.
• The MPRAGEise multiplication is reimplemented with NumPy and NiBabel. It follows the original mat2gray(INV2) .* UNI calculation but also needs validation on representative data.
• Bias correction (biascorrect and the first step of MPRAGEise) samples at 3 mm, as the original MATLAB code does. The strip-mask and brain-mask segmentations sample at 2 mm.
• Tidying the code did not change any numbers. On one test subject, a single-thread bias correction from this version was byte-identical to the one from the earlier code, and the MPRAGEise calculation was byte-identical before and after it moved into its own module.
• A fixed --threads value gives identical results every time. Different values can change results slightly, and recon-prep differs from MPRAGEise as described above.

Licence

MIT. See LICENSE.

Changelog

See CHANGELOG.md for what changed in each version.

Citation

See CITATION.cff.