medtokenizers

medtokenizers compresses 2D and 3D medical images into latents or discrete token grids. It provides one encoder-decoder backbone with interchangeable quantization heads (VQ, LFQ, FSQ, Residual FSQ, VAE, AE), so the quantizer can be treated as a controlled variable rather than a fixed preprocessing choice. It also provides shared reconstruction metrics and dataset tokenization utilities.

This library accompanies the paper Tokenizer-Generator Coupling in Medical Image Generation, which uses it for the tokenizer half of a factorial study. See Citation.

Install

pip install medtokenizers

Requires Python 3.10+ and PyTorch 2.0+. Tested on Linux and macOS. A CUDA GPU is needed for 3D training, but everything below runs on CPU.

For a CPU-only install, take PyTorch from its CPU index first so torch and torchvision come from the same build:

pip install torch torchvision --index-url https://download.pytorch.org/whl/cpu
pip install medtokenizers

To work on the library itself, install from a checkout instead:

git clone https://github.com/liamchalcroft/medtokenizers.git
cd medtokenizers
pip install -e ".[test]"

Run it

Swap the quantizer, keep everything else fixed:

import torch

from medtokenizers import DiscreteTokenizer

x = torch.randn(1, 1, 64, 64)

for quantizer in ["VQ", "FSQ", "RESFSQ"]:
    model = DiscreteTokenizer(
        dim=2, quantizer=quantizer, resolution=64, spatial_compression=8
    ).eval()
    with torch.no_grad():
        tokens = model.tokenize(x)  # (1, 8, 8) integer codes
        recon = model.detokenize(tokens)  # (1, 1, 64, 64)

LFQ is the one head that needs two further arguments, because its codebook is defined by its binary dimension rather than inferred:

model = DiscreteTokenizer(
    dim=2,
    quantizer="LFQ",
    resolution=64,
    spatial_compression=8,
    codebook_size=1024,
    codebook_dim=10,
)

The continuous heads live on a separate class, selected by formulation instead of quantizer, and return a real-valued latent rather than indices:

from medtokenizers import ContinuousTokenizer

model = ContinuousTokenizer(
    dim=2, formulation="VAE", resolution=64, spatial_compression=8
).eval()
with torch.no_grad():
    latent = model.tokenize(x)  # (1, 4, 8, 8)
    recon = model.detokenize(latent)

At evaluation time ContinuousTokenizer.encode returns the posterior mean, so latents extracted with model.eval() are deterministic.

Set dim=3 for volumes; the same calls take (B, C, H, W, D) tensors.

Volumetric inference

A simulated T1-weighted brain volume ships inside the package (from the BrainWeb Simulated Brain Database, with provenance in src/medtokenizers/assets/README.md), so the example runs without a download and works from an installed wheel. example_volume_path() resolves it. Random initialisation is enough for a smoke test; load your own weights for real reconstructions.

python examples/inference_on_brain.py --model-type maisi

Large volumes are reconstructed with Gaussian-weighted sliding windows:

import medrs
import torch

from medtokenizers import MAISITokenizer, example_volume_path

img = medrs.load(str(example_volume_path()))
volume = img.to_torch_with_dtype_and_device(dtype=torch.float32)
volume = volume.unsqueeze(0).unsqueeze(0)  # (H, W, D) -> (1, 1, H, W, D)
volume = (volume - volume.min()) / (volume.max() - volume.min() + 1e-8)

model = MAISITokenizer().eval()
with torch.no_grad():
    recon = model.reconstruct(volume, roi_size=(96, 96, 64), overlap=0.5)

Sliding-window reconstruction over a full 1 mm volume is memory-hungry. Reduce roi_size or overlap if you are not on a GPU.

Tokenizers

Class	Selected by	Output
DiscreteTokenizer	quantizer="VQ" | "FSQ" | "LFQ" | "RESFSQ"	integer token grid
ContinuousTokenizer	formulation="VAE" | "AE"	real-valued latent
MAISITokenizer	fixed MAISI configuration	real-valued latent
TiTokTokenizer	1D transformer tokenizer	fixed-length sequence of K tokens
RAETokenizer (experimental)	frozen foundation-model encoder	real-valued latent

Adding a quantizer means subclassing BaseQuantizer (two abstract methods, forward and indices_to_codes) and wiring it into DiscreteTokenizer. The encoder-decoder and the tokenization I/O are unchanged by the choice.

TiTok

TiTok encodes an image or volume into a fixed-length 1D sequence of K tokens. The encoder concatenates patch tokens with K learnable latent tokens and keeps only the latter; the decoder reconstructs from quantized latents plus mask tokens. Input shape must equal resolution, and each spatial dimension must be divisible by patch_size.

from medtokenizers import TiTokTokenizer

tokenizer = TiTokTokenizer(
    dim=2,
    in_channels=1,
    out_channels=1,
    resolution=64,
    patch_size=16,
    num_tokens=32,
    num_embeddings=1024,
    hidden_dim=256,
)
indices, codes, quant_loss = tokenizer.encode(torch.randn(2, 1, 64, 64))
recon = tokenizer.decode(codes)

TiTok's original two-stage proxy-code warmup and decoder fine-tuning are not wired into the training scripts; implement that externally if you want the published recipe.

Evaluation

from medtokenizers import compute_lpips, compute_psnr, compute_ssim

psnr = compute_psnr(reference, reconstruction)
ssim = compute_ssim(reference, reconstruction)
lpips = compute_lpips(reference, reconstruction)

TokenizerEvaluator runs these over a loader, together with perplexity and codebook utilisation for discrete models:

from medtokenizers import TokenizerEvaluator, load_tokenizer

model = load_tokenizer("./path/to/checkpoint")
results = TokenizerEvaluator(model, device="cuda").evaluate(test_loader)

No pretrained checkpoints ship with this repository.

Ecosystem

medtokenizers is one of three packages:

• medrs: medical-image I/O (NIfTI/DICOM) and preprocessing primitives.
• medtokenizers: tokenizers that compress scans into latents or token grids (this package).
• medlatents: generative models (autoregressive, MaskGIT, diffusion, flow matching, Bayesian flow) that learn over those tokens.

images -> medrs (load) -> medtokenizers (tokenize) -> medlatents (generate)
       -> medtokenizers (detokenize) -> images

scripts/tokenize_dataset.py writes a directory of per-split .npz files plus metadata.json, with key codes (int16) for discrete models and latents (float16) for continuous ones.

Contributing

See CONTRIBUTING.md for development setup and the pull-request workflow, and CODE_OF_CONDUCT.md for community standards.

Citation

@article{chalcroft2026coupling,
  author = {Chalcroft, Liam},
  title  = {Tokenizer--Generator Coupling in Medical Image Generation},
  year   = {2026},
  note   = {arXiv preprint, to appear}
}

@software{chalcroft_medtokenizers,
  author  = {Chalcroft, Liam},
  title   = {{medtokenizers}: Continuous and discrete tokenizers for volumetric medical imaging},
  url     = {https://github.com/liamchalcroft/medtokenizers},
  version = {0.1.0}
}

Machine-readable metadata is in CITATION.cff.

License

MIT, see LICENSE.

Some modules derive from third-party projects: CompVis (Stable Diffusion, taming-transformers) and lucidrains vector-quantize-pytorch under MIT, and NVIDIA Cosmos-Tokenizer and MONAI/MAISI under Apache-2.0. Their terms are preserved in THIRD_PARTY_NOTICES.md and NOTICE. Two things are outside the MIT licence: NVIDIA MAISI pretrained weights, if you use them, remain under NSCLv1; and the bundled bundled BrainWeb volume is data carrying its own citation requirement.