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arxiv: 2604.14800 · v1 · submitted 2026-04-16 · 📡 eess.IV · cs.CV· physics.med-ph

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Generative Modeling of Complex-Valued Brain MRI Data

Jens Kleesiek, Jens Weingarten, Jessica Mnischek, Kevin Kr\"oninger, Lukas T. Rotkopf, Marco Schlimbach, Moritz Rempe

Authors on Pith no claims yet

Pith reviewed 2026-05-10 09:18 UTC · model grok-4.3

classification 📡 eess.IV cs.CVphysics.med-ph
keywords complex-valued MRIgenerative modelingphase preservationsynthetic data augmentationbrain abnormality detectionvariational autoencoderflow matchingfastMRI dataset
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The pith

A generative model for complex-valued brain MRI produces synthetic scans that let classifiers detect abnormalities more accurately than training on real data alone.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper sets out to show that magnitude and phase can be modeled together in a generative system for brain MRI instead of throwing away phase as standard pipelines do. It pairs a conditional variational autoencoder that keeps phase coherence above 0.997 with a flow-matching generator to create new samples. Downstream classifiers trained only on these synthetic images reach an AUROC of 0.880 for normal-versus-abnormal detection on fastMRI, beating the 0.842 score from real data, and the edge remains on an external biopsy-labeled set. A reader would care because real MRI datasets are small and costly, while phase is known to carry tissue information that could sharpen diagnosis if it can be used reliably.

Core claim

The framework uses a conditional variational autoencoder to compress complex-valued MRI into latent codes while preserving phase coherence above 0.997, then applies a flow-matching model to generate new complex scans. Real-versus-synthetic classifiers give AUROC scores between 0.50 and 0.66, showing the outputs are nearly indistinguishable from real data. Classifiers trained entirely on the synthetic data achieve an AUROC of 0.880 for normal-versus-abnormal classification on the fastMRI dataset, surpassing the 0.842 baseline obtained from real data, and this performance advantage carries over to an independent external test set with biopsy-confirmed labels.

What carries the argument

Conditional variational autoencoder that preserves phase coherence in complex MRI, paired with a flow-matching generative model that samples new magnitude-plus-phase images.

If this is right

  • Synthetic complex-valued MRI can augment scarce real datasets for training diagnostic models without loss of performance.
  • Phase data in MRI contains features useful for distinguishing normal from abnormal tissue that magnitude images alone miss.
  • The same generative approach can supply labeled training examples for rare conditions where collecting real scans is difficult.
  • Generated samples maintain utility across different scanners and institutions, supporting multi-site model development.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If phase truly improves detection, reconstruction software could be redesigned to output and store complex data rather than magnitude only.
  • The same joint magnitude-phase modeling could be tested on other modalities that acquire complex signals, such as certain CT or ultrasound techniques.
  • A controlled ablation that removes phase from the synthetic data and measures the resulting drop in classifier AUROC would isolate how much of the gain comes from phase versus the generative process.

Load-bearing premise

The phase information encoded and regenerated by the model carries genuine pathology signals that improve classification without introducing spurious patterns that happen to work only on the chosen test sets.

What would settle it

Retraining the downstream normal-versus-abnormal classifier on a larger, multi-institution real dataset and finding that its AUROC exceeds the synthetic-data AUROC of 0.880 would show the reported advantage does not generalize.

Figures

Figures reproduced from arXiv: 2604.14800 by Jens Kleesiek, Jens Weingarten, Jessica Mnischek, Kevin Kr\"oninger, Lukas T. Rotkopf, Marco Schlimbach, Moritz Rempe.

Figure 1
Figure 1. Figure 1: Pipeline to generate raw MRI patches. Complex-valued MRI patches (2 [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Two samples of training data after ESPIRiT combination, IFT and patching for a normal [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Autoencoder reconstruction examples for an abnormal sample (AXT1POST, left) and a [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Generated and real samples from the Stage 1 flow matching model. Synthetic samples [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Samples generated by the Stage 2 flow matching model, conditioned on normal (left) and [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Downstream classification AUROC on the fastMRI test set as a function of real data [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Downstream classification AUROC on the external test set as a function of real data [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Downstream classification AUROC when synthetic data is progressively added to the full [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
read the original abstract

Objective. Standard Magnetic Resonance Imaging (MRI) reconstruction pipelines discard phase information captured during acquisition, despite evidence that it encodes tissue properties relevant to tumor diagnosis. Current machine learning approaches inherit this limitation by operating exclusively on reconstructed magnitude images. The aim of this study is to build a generative framework which is capable of jointly modeling magnitude and phase information of complex-valued MRI scans. Approach. The proposed generative framework combines a conditional variational autoencoder, which compresses complex-valued MRI scans into compact latent representations while preserving phase coherence, with a flow-matching-based generative model. Synthetic sample quality is assessed via a real-versus-synthetic classifier and by training downstream classifiers on synthetic data for abnormal tissue detection. Main results. The autoencoder preserves phase coherence above 0.997. Real-versus-synthetic classification yields low AUROC values between 0.50 and 0.66 across all acquisition sequences, indicating generated samples are nearly indistinguishable from real data. In downstream normal-versus-abnormal classification, classifiers trained entirely on synthetic data achieve an AUROC of 0.880, surpassing the real-data baseline of 0.842 on a publicly available dataset (fastMRI). This advantage persists on an independent external test set from a different institution with biopsy-confirmed labels. Significance. The proposed framework demonstrates the feasibility of jointly modeling magnitude and phase information for normal and abnormal complex-valued brain MRI data. Beyond synthetic data generation, it establishes a foundation for the usage of complete brain MRI information in future diagnostic applications and enables systematic investigation of how magnitude and phase jointly encode pathology-specific features.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 1 minor

Summary. The manuscript proposes a generative framework that combines a conditional variational autoencoder (to compress complex-valued brain MRI while preserving phase) with a flow-matching model to synthesize realistic magnitude-and-phase data. It reports phase coherence above 0.997, real-versus-synthetic AUROCs of 0.50–0.66, and downstream normal-versus-abnormal classification AUROCs of 0.880 (synthetic) versus 0.842 (real) on fastMRI, with the advantage persisting on an external biopsy-labeled dataset from a different institution.

Significance. If the central empirical claims hold after verification, the work would be significant for demonstrating that joint modeling of magnitude and phase can produce synthetic data that improves downstream diagnostic performance beyond magnitude-only baselines. The use of a public dataset (fastMRI) and an independent external test set with biopsy confirmation strengthens the evaluation.

major comments (3)
  1. [Downstream evaluation] Downstream evaluation section: the AUROC gain (0.880 synthetic vs. 0.842 real) is presented as evidence that phase information contributes diagnostically relevant features, yet no ablation comparing complex-valued synthetic data against magnitude-only synthetic data is reported. This comparison is required to establish that the observed lift is attributable to phase modeling rather than other aspects of the generative pipeline.
  2. [External validation] External validation paragraph: the persistence of the performance advantage on the independent biopsy-labeled set is a key claim, but the manuscript provides no quantitative assessment of distribution shift (e.g., scanner parameters, acquisition protocols) between the generative training distribution and the external data. Without this, it remains possible that the reported edge reflects unmodeled covariates rather than genuine pathology encoding.
  3. [Methods] Methods, model architecture and training: the description of the conditional VAE latent dimension, conditioning variables, flow-matching network capacity, and training schedule is insufficient to reproduce the reported phase coherence >0.997 or to diagnose whether the downstream gains arise from faithful pathology encoding or from modeling artifacts.
minor comments (1)
  1. [Abstract] The abstract states real-versus-synthetic AUROC values as a range (0.50–0.66) without per-sequence breakdowns or reference to a supplementary table; adding this detail would improve clarity.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive comments, which highlight important aspects for strengthening our claims on phase-aware generative modeling of complex-valued MRI. We address each major comment below and will revise the manuscript to incorporate the requested analyses and details.

read point-by-point responses
  1. Referee: [Downstream evaluation] Downstream evaluation section: the AUROC gain (0.880 synthetic vs. 0.842 real) is presented as evidence that phase information contributes diagnostically relevant features, yet no ablation comparing complex-valued synthetic data against magnitude-only synthetic data is reported. This comparison is required to establish that the observed lift is attributable to phase modeling rather than other aspects of the generative pipeline.

    Authors: We agree that the current evidence would be strengthened by directly isolating the contribution of phase. In the revised manuscript we will add an ablation in which a magnitude-only generative model (trained and sampled without phase) is compared head-to-head with the complex-valued model on the same downstream abnormality-classification task. This will clarify whether the reported AUROC advantage arises from joint magnitude-phase modeling. revision: yes

  2. Referee: [External validation] External validation paragraph: the persistence of the performance advantage on the independent biopsy-labeled set is a key claim, but the manuscript provides no quantitative assessment of distribution shift (e.g., scanner parameters, acquisition protocols) between the generative training distribution and the external data. Without this, it remains possible that the reported edge reflects unmodeled covariates rather than genuine pathology encoding.

    Authors: We acknowledge the need for explicit quantification of distribution shift. In the revision we will add a dedicated paragraph reporting scanner field strengths, sequence parameters, and statistical measures of image-level shift (intensity histograms, contrast-to-noise ratios, and Kolmogorov-Smirnov tests) between the fastMRI training distribution and the external biopsy-labeled cohort. This analysis will help substantiate that the observed advantage is attributable to pathology encoding rather than acquisition covariates. revision: yes

  3. Referee: [Methods] Methods, model architecture and training: the description of the conditional VAE latent dimension, conditioning variables, flow-matching network capacity, and training schedule is insufficient to reproduce the reported phase coherence >0.997 or to diagnose whether the downstream gains arise from faithful pathology encoding or from modeling artifacts.

    Authors: We appreciate the call for greater reproducibility. The revised methods section will specify the cVAE latent dimension, the complete set of conditioning variables, the exact architecture and parameter count of the flow-matching network, and the full training schedule with all hyperparameters. These additions will enable independent reproduction of the phase-coherence results and allow readers to assess whether downstream gains stem from pathology encoding or modeling artifacts. revision: yes

Circularity Check

0 steps flagged

No circularity detected; results are empirical evaluations on held-out data

full rationale

The paper describes an empirical pipeline (conditional VAE + flow-matching) for generating complex-valued MRI and evaluates it via direct metrics: phase coherence >0.997, real-vs-synthetic AUROC 0.50-0.66, and downstream normal-vs-abnormal AUROC of 0.880 (synthetic-trained) vs 0.842 (real baseline) on fastMRI held-out data plus an external biopsy-labeled set. No mathematical derivation chain exists that reduces these quantities to fitted parameters by construction. No self-definitional steps, fitted-input predictions, or load-bearing self-citations are present in the reported results. The external test set and held-out real data provide independent benchmarks, keeping the evaluation self-contained without circular reduction.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of variational autoencoders and flow-matching models plus the unstated premise that the training data distribution is representative of real clinical variability. No new physical axioms or invented entities are introduced.

free parameters (2)
  • latent dimension and conditioning variables
    Standard hyperparameters of the cVAE that control compression and phase preservation; their specific values are not reported in the abstract.
  • flow-matching network capacity and training schedule
    Hyperparameters of the generative model whose tuning affects sample quality and downstream AUROC.
axioms (1)
  • domain assumption The complex-valued MRI signal can be meaningfully compressed while preserving phase coherence for downstream pathology tasks.
    Invoked in the autoencoder design and phase-coherence metric; not derived from first principles.

pith-pipeline@v0.9.0 · 5606 in / 1399 out tokens · 32763 ms · 2026-05-10T09:18:31.762975+00:00 · methodology

discussion (0)

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