UncertaINR: Uncertainty Quantification of End-to-End Implicit Neural Representations for Computed Tomography
Francisca Vasconcelos*1,2,
Bobby He*1,
Nalini Singh3,
Yee Whye Teh1
1University of Oxford, 2UC Berkeley, 3MIT, *denotes equal contribution
Published in the Transactions on Machine Learning Research (April 2023).
Paper | Talk | Slides | OpenReview
This is the official implementation of the paper "UncertaINR: Uncertainty Quantification of End-to-End Implicit Neural Representations for Computed Tomography". The repository is based off the SIREN codebase.
Implicit neural representations (INRs) have achieved impressive results for scene reconstruction and computer graphics, where their performance has primarily been assessed on reconstruction accuracy. As INRs make their way into other domains, where model predictions inform high-stakes decision-making, uncertainty quantification of INR inference is becoming critical. To that end, we study a Bayesian reformulation of INRs, UncertaINR, in the context of computed tomography, and evaluate several Bayesian deep learning implementations in terms of accuracy and calibration. We find that they achieve well-calibrated uncertainty, while retaining accuracy competitive with other classical, INR-based, and CNN-based reconstruction techniques. Contrary to common intuition in the Bayesian deep learning literature, we find that INRs obtain the best calibration with computationally efficient Monte Carlo dropout, outperforming Hamiltonian Monte Carlo and deep ensembles. Moreover, in contrast to the best-performing prior approaches, UncertaINR does not require a large training dataset, but only a handful of validation images.
You can set up a conda environment with all dependencies like so:
conda create -n uinr python=3.8
conda activate uinr
pip install -r requirements.txt
For HMC experiments, we use NumPyro which relies on JAX. More details on installing JAX for your CUDA setup can be found here.
You should also set up a wandb account and set your wandb account as an environment variable from command line e.g.
export WANDB_API_KEY=example_key
The top-level scripts organized as follows:
_utils.pycontains utility functions, including saving results and plotting images.dataio.pyloads training and testing data.loss_functions.pycontains loss functions for our different experiments.modules.pycontains layers and full neural network modules combined with uncertainty methods.numpyro_port.pycontainstraining.pycontains a generic training routine.
The folders are organized as follows:
./experiment_scripts/contains scripts to reproduce experiments in the paper../conf/contains nested Hydra configurations, useful for managing experiment folders and setting configs for different training procedures../example_configs/contains some example wandb sweep configs with tuned hyperparameters../data/contains data from the AAPM 2016 Low-Dose CT Grand Challenge./checkpoints/contains pre-trained checkpoints for both UINRs and Grid of Pixels on AAPM to initialize HMC runs.
To start training an UINRs with MCDropout on AAPM:
python experiment_scripts/run_uncertainr.py uncertainty=mc_dropout reg.coeff=0.008 model.omega_0=40 num_epochs=15000 data.test_set=True
which should take around an hour to run on an 12GB NVIDIA GeForce_GTX_2080Ti GPU.
To start running inference on an UINR with HMC in NumPyro on AAPM:
python experiment_scripts/run_hmc_numpyro.py uncertainty=hmc model.width=200 model.depth=4 reg.coeff=0.03 model.omega_0=60 model.use_checkpoint=True data.test_set=True uncertainty.num_samples=500
which should take around 10 hours to run on an 24GB NVIDIA Titan RTX GPU. These hyperparameters are tuned and match those found in Table 8 of our paper.
We focused on the problem of image reconstruction in the medical setting of computed tomography. Here, we are interested in reconstructing 2D images, represented as functions
UncertaINR leverages an end-to-end INR, which learns a functional form of the reconstructed image. Sampling this function across muliple pixel coordinates and performing a Radon transform on the sampled image generates a "learned" sinogram, which is used to calculate a loss relative to the true sinogram data. In addition to hypertuning our models, crucial to achieving competitive model performance was: Random Fourier Features (RFFs), image-padding, and total-variation (TV) regularization.
UncertaINR is a Bayesian framework, achieving uncertainty quantification via inference over the INR parameters and/or image output, enabling prediction of an image distribution (as opposed to a singular image). We tested several uncertainty quantification approaches, namely: Bayes-by-Backpropogation (BBB), Hamiltonian Monte Carlo (HMC), Monte Carlo Dropout (MCD), and Deep Ensembles (DEs). BBB was implemented using Blitz and HMC was implemented via the No-U-Turn-Sampler sampling scheme in NumPyro, which is based in JAX.
We performed our ablation study on a dataset of 256x256 2D artificially-generated Shepp-Logan phantom images, implemented using phantominator (validation scan ids: 1-5, test scan ids: 6-10). We performed model comparison tests on a dataset of real 512x512 2D reconstructed scan images from the AAPM 2016 Low-Dose CT Grand Challenge (validation and test images included in the data/ folder). Note that artificial Gaussian noise was added to the AAPM images (to achieve a 40dB SNR).
We compared UncertaINR to several baselines. Classical medical imaging reconstruction techniques (FBP, CGLS, EM, SIRT, and SART) were implemented using TomoPy. State-of-the-art CT image reconstruction baseline results (FISTA-TV, FBP CoIL, FISTA-TV CoIL, FBP Unet, GM-RED, FBP Unet COIL, GM-RED COIL) were taken from the 2021 paper by Sun et al., "CoIL: Coordinate-based Internal Learning for Imaging Inverse Problems".
Finally, as an uncertainty baseline, we implemented our architecture with a Grid-of-Pixels (GOP), replacing the INR. Given that this pixel-based representation did not leverage a neural network encoding, BBB, MCD, and DEs were not applicable. Thus, uncertainty quantification was only possible via HMC.
In this work, we proposed UncertaINR: a Bayesian reformulation of INR-based image reconstruction. In the high-stakes and well-motivated application of CT image reconstruction, UncertaINR attained calibrated uncertainty estimates without sacrificing reconstruction quality relative to other classical, INR-based, and CNN-based reconstruction techniques on realistic, noisy, and underdetermined data. In the context of INR UQ, contrary to common intuition, we found that simple and efficient MCD rivaled (even outperformed) the popular DEs and the sophisticated, yet computationally-expensive HMC methods. UncertaINR's strategic use of INRs outperformed classical reconstruction approaches, while alleviating key challenges faced by state-of-the-art deep learning methods -- namely generalizability and small-scale medical datasets. In addition to informing doctor diagnoses, UncertaINR's well-calibrated uncertainty estimates could pave the way for reduced healthcare costs, via methods like automated triage, and reduced patient radiation exposure, via methods like active learning.
If you find this work useful, please cite:
@article{vasconcelos2023uncertainr,
title={UncertaINR: Uncertainty Quantification of End-to-End Implicit Neural Representations for Computed Tomography},
author={Francisca Vasconcelos and Bobby He and Nalini Singh and Yee Whye Teh},
journal={Transactions on Machine Learning Research},
issn={2835-8856},
year={2023},
url={https://openreview.net/forum?id=jdGMBgYvfX},
}
