REVIEW 4 major objections 5 minor 1 cited by
Solving Blind Inverse Problems: Adaptive Diffusion Models for Motion-corrected Sparse-view 4DCT
T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read This paper claims that a diffusion-model prior, combined with on-the-fly calibration of a surrogate-scaled deformation model, solves blind motion-corrected sparse-view 4DCT reconstruction, reporting artifact-free end-inhale images on XCAT…
desk verdict A sensible integration of diffusion priors and surrogate-driven motion estimation for sparse-view 4DCT, with promising phantom results but no validation of the estimated motion fields. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The adaptive forward model is the central object: $A_{\varphi,s} = [R \circ T_k \circ W_{s_k \varphi}]_k$, a per-time composition of the fan-beam line-integral operator $R$, a slice extractor $T_k$, and a deformation operator $W_{s_k \varphi}$ driven by a shared B-spline DVF scaled by scalar surrogates. The argument is carried by the alternating loop in Algorithm 1: a wavelet-domain diffusion network $v_\theta$ proposes a clean coefficient image, a short RMSprop step fits $(\varphi, s)$ to that proposal through the likelihood, and a data-consistency update in image space produces the next clean estimate before the DDIM step advances. The discrete wavelet transform $V$ is what makes 3-D diffusion tractable, and the jumpstart from a gated FBP image shortens the reverse process.
What would settle it
Run JRM-ADM on a dataset whose ground-truth motion includes spatially varying or inspiratory/expiratory hysteresis—for example, a XCAT phantom with different regional breathing amplitudes or a real patient 4DCT with irregular breathing—and compare end-inhale PSNR and SSIM against JRM-TV and gated-DPS. If JRM-ADM no longer exceeds the comparators or shows residual diaphragm motion artifacts, the central claim that adaptive diffusion models solve blind motion-corrected sparse-view 4DCT is refuted.
Extended reading notes
Core claim
JRM-ADM's central claim is that one can jointly reconstruct the image and estimate the motion by treating the forward model itself as an unknown that depends on a B-spline deformation vector field $\varphi$ and a surrogate vector $s$, with per-time deformations $\varphi_k = s_k \varphi$. During DDIM sampling in the wavelet domain, the method alternates between estimating the clean image from the diffusion network, calibrating $(\varphi, s)$ by minimizing the weighted negative log-likelihood against that estimate, and enforcing image-domain data consistency. The paper reports that this yields artifact-free, high-resolution end-inhale reconstructions under irregular breathing on XCAT phantom data, with higher PSNR and SSIM than gated-FBP, gated-DPS, and JRM-TV on all five test datasets.
Load-bearing premise
The method assumes every respiratory deformation is a single reference deformation field scaled by a scalar surrogate; if true breathing has spatially varying or hysteretic motion that this one-parameter scaling cannot represent, both the reconstructed image and the estimated motion will be biased.
Editorial extensions
If this is right
- The end-inhale phase can be reconstructed without phase gating, removing gating-induced noise amplification and irregular-breathing motion artifacts in the same pass.
- Surrogate signals need not be measured during acquisition; the scalar surrogate vector is recovered as part of the reconstruction.
- Diffusion priors give the resolution preservation that TV regularization lacks, while still suppressing streak and photon-counting noise.
- The Poisson statistical weighting in the data-consistency term carries over to low-dose photon-counting CT, where counts per view are small.
- Wavelet-domain diffusion keeps the computationally heavy 3-D problem within feasible memory and runtime budgets.
Reading between the lines
- The single-deformation-field scaling assumption, $\varphi_k = s_k \varphi$, is the part most likely to break on real patients; a natural extension would be to parameterize motion with two or more DVF components, for example separate inspiratory and expiratory fields, and let the surrogate vector choose their mixture.
- The same alternating calibration trick should transfer to other blind inverse problems in medical imaging where the unknown forward operator has a low-dimensional parameterization, such as joint activity and attenuation estimation in PET.
- Because the evidence is currently limited to XCAT phantoms that may conform to the motion model, the decisive test is a comparison on real 4DCT or on a phantom with spatially varying or hysteretic motion; the authors state they are working toward real CT volumes.
- The jumpstart strategy and five-step RMSprop calibration suggest a robustness question: how sensitive the final image is to the surrogate initialization and the number of calibration iterations could be checked with an ablation across datasets.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes JRM-ADM, a diffusion-model-based framework for joint reconstruction and motion estimation (JRM) in sparse-view 4DCT. Respiratory motion is modeled by a single deformation field φ scaled by a scalar surrogate signal s_k per time frame (φ_k = s_k·φ), and the forward model is calibrated adaptively during diffusion posterior sampling. The method operates in a wavelet latent space to reduce memory and computation, and it alternates between data-consistency updates of the image and RMSprop-based updates of (φ,s). Experiments on five XCAT phantoms report PSNR and SSIM for the end-inhale phase, showing improvements over gated-FBP, gated-DPS, and JRM-TV. The paper claims artifact-free, high-resolution reconstructions under irregular breathing conditions, while acknowledging the lack of real 4DCT validation.
Significance. If the results are substantiated, the paper makes a useful contribution by combining diffusion priors with motion-corrected 4DCT reconstruction and by addressing the computational cost through the wavelet-domain formulation. The pseudo-code and the explicit treatment of the blind inverse problem are helpful. However, the current evidence is limited to synthetic XCAT data, and the absence of any quantitative validation of the estimated deformation fields means the central 'motion-corrected' claim is not yet established. The reported image metrics alone cannot distinguish accurate motion estimation from the effect of the diffusion prior and multi-frame data consistency. The paper is clearly written and the computational direction is relevant, but the evaluation gaps are substantial enough to require revision before the findings can be considered reliable.
major comments (4)
- [Section 3.2 and Algorithm 1] The estimated deformation field φ and surrogate signal s are never compared to the ground-truth motion used to generate the five XCAT test datasets. Because Eq. (1) imposes a strong model (φ_k = s_k·φ) and Algorithm 1 (lines 7 and 13) performs only 5 RMSprop iterations for both the motion and image updates, the reported PSNR/SSIM gains cannot be attributed to accurate motion estimation. Please report quantitative motion errors (e.g., DVF endpoint error, surrogate correlation) for the test phantoms. The Section 4 limitation about XCAT training does not address this internal validation gap.
- [Section 3.1 and Table 1] The evaluation is restricted to a single end-inhale phase, and the paper never states how the end-inhale image was formed from the method's output (x̂0, φ̂, ŝ). If PSNR/SSIM are computed on W_{ŝ_k φ̂}(x̂0) for the end-inhale index k, this warping step should be described explicitly, and the same pipeline should be applied to multiple phases or a time-averaged error. Per-phase metrics are necessary to support the general claim of motion-corrected 4DCT reconstruction across the respiratory cycle.
- [Section 3.1 and Table 1] The most relevant baseline, Huang et al. [2] (surrogate-optimized JRM without diffusion), is cited and used as the basis of the motion model but is not included in the comparison. Without this baseline, the abstract's claim that the method 'outperforms existing techniques' overstates the evidence. Please add this baseline or clearly justify its omission.
- [Section 3.2 and Table 1] The comparison against JRM-TV does not isolate the contribution of the adaptive forward-model calibration, since JRM-TV uses the same all-frame data and the same motion model. An ablation that fixes (φ,s) to the gated estimates or runs the diffusion prior without the per-step calibration would demonstrate the value of the adaptive component, which is the paper's central novelty.
minor comments (5)
- [Section 2.2.2] The sentence 'performing the diffusion in a the wavelet domain' should read 'performing the diffusion in the wavelet domain.'
- [Section 3.2] 'This results are confirmed' should be 'These results are confirmed.'
- [Algorithm 1] The notation is incomplete: the Require line lists {α_t}_{t=1}^{T'} but the update in line 10 uses α_{t−δt}, and the relationship between δt and the α index is not defined.
- [Section 3.1] Hyperparameters that materially affect the method are not reported: the data-consistency weight schedule ζ_t, the number of RMSprop iterations for lines 7 and 13, the B-spline control-point grid, and the initial surrogate signal s. Providing these values is important for reproducibility.
- [Section 2.2.3] The phrase 'standard sinusoidal signal to initialize s' is vague; please specify the amplitude, frequency, and how the phase is aligned with the acquired data.
Circularity Check
No significant circularity: the motion model and forward-model fit are explicit assumptions from external work, and the XCAT train/test overlap is a generalization limitation, not a circular derivation; the sole self-citation is non-load-bearing.
full rationale
The derivation chain is self-contained and not circular. The motion model φ_k = s_k · φ is introduced as an explicit assumption in Eq. (1), borrowed from external work [1,2], and is not derived from the target reconstruction. Eq. (12) estimates (φ,s) from the current diffusion estimate x̂_{0|t}, and Eq. (13) enforces data consistency; this is a standard blind-inverse-problem fit, and the reported PSNR/SSIM are computed against XCAT ground truth, not against the fitted parameters. The diffusion prior is trained on XCAT phantoms and evaluated on separately generated XCAT morphologies; this is a train/test distribution match that can inflate apparent performance and limits claims about real CT, and the paper's Section 4 limitation ('our models were trained and evaluated on XCAT phantoms due to the limited availability of 4DCT datasets') is acknowledged. But it is not a circular derivation: the measurements y are still required, and the reconstruction is not identical to a training image by construction. The only self-citation, [17], appears in the Discussion to support 'generalizability to unseen data'; the paper's own held-out XCAT test already provides direct evidence for generalization to unseen phantom morphologies, so the self-citation is not load-bearing. No uniqueness theorem or ansatz is imported from the authors' own prior work. The absence of quantitative validation of the estimated motion fields is a substantive correctness/internal-validity gap, but it does not make the image-reconstruction claim circular.
Assumptions & free parameters
free parameters (5)
- Data consistency weight schedule ζ_t =
Not specified in the paper
- Number of RMSprop iterations for motion and data-consistency updates =
5
- Jumpstart timestep T' and DDIM step δt =
T'=300, δt=10
- Initial surrogate signal s =
Sinusoidal
- B-spline control point grid for φ =
Not specified
assumptions (5)
- standard math Beer-Lambert law with Poisson noise (Eq. 2-3)
- standard math Penalized weighted least squares approximation of the negative log-posterior (Eq. 5)
- domain assumption Surrogate-scaled single DVF motion model φ_k = s_k·φ (Eq. 1)
- domain assumption The DWT is bijective
- domain assumption The diffusion prior trained on XCAT generalizes to unseen XCAT morphologies
Cite this review
Pith. "Pith review of Solving Blind Inverse Problems: Adaptive Diffusion Models for Motion-corrected Sparse-view 4DCT." pith.science (2026). https://pith.science/paper/2RWCXTK6
@misc{pith2026250112249,
author = {Pith},
title = {Pith review of: Solving Blind Inverse Problems: Adaptive Diffusion Models for Motion-corrected Sparse-view 4DCT},
year = {2026},
howpublished = {\url{https://pith.science/paper/2RWCXTK6}},
note = {Machine review of arXiv:2501.12249}
}
read the original abstract
Four-dimensional computed tomography (4DCT) is essential for medical imaging applications like radiotherapy, which demand precise respiratory motion representation. Traditional methods for reconstructing 4DCT data suffer from artifacts and noise, especially in sparse-view, low-dose contexts. Motion-corrected (MC) reconstruction is a blind inverse problem that we propose to solve with a novel diffusion model (DM) framework that calibrates an adaptive unknown forward model for motion correction. Furthermore, we used a wavelet diffusion model (WDM) to address computational cost and memory usage. By leveraging the prior probability distribution function (PDF) from the DMs, we enhance the joint reconstruction and motion estimation (JRM) process, improving image quality and preserving resolution. Experiments on extended cardiac-torso (XCAT) phantom data demonstrate that our method outperforms existing techniques, yielding artifact-free, high-resolution reconstructions even under irregular breathing conditions. These results showcase the potential of combining DMs with motion correction to advance sparse-view 4DCT imaging.
Figures
Forward citations
Cited by 1 Pith paper
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Joint Reconstruction of Activity and Attenuation in PET by Diffusion Posterior Sampling in Wavelet Coefficient Space
A wavelet diffusion model combined with diffusion posterior sampling enables joint 3D activity-attenuation reconstruction in PET from emission data alone, outperforming MLAA on simulated TOF data.
Reference graph
Works this paper leans on
-
[2]
Resolving Variable Respiratory Motion From Unsorted 4D Computed Tomography
Y. Huang, B. Eiben, K. Thielemans, et al. “Resolving Variable Respiratory Motion From Unsorted 4D Computed Tomography”.International Conference on Medical Image Computing and Computer-Assisted Intervention. Springer. 2024, pp. 588–597
work page 2024
-
[1]
J. R. McClelland, M. Modat, S. Arridge, et al. “A generalized framework unifying image registration and respiratory motion models and incorporating image recon- struction, for partial image data or full images”.Physics in Medicine & Biology62.11 (2017), p. 4273
work page 2017
-
[3]
Diffusion posterior sampling for general noisy inverse problems
H. Chung, J. Kim, M. T. Mccann, et al. “Diffusion posterior sampling for general noisy inverse problems”.In Proceedings of the Eleventh International Conference on Learning Representations. ICLR, 2023.doi: 10.48550/arXiv.2209.14687
-
[4]
Diffusion models for medical image reconstruction
G. Webber and A. J. Reader. “Diffusion models for medical image reconstruction”. BJR| Artificial Intelligence1.1 (2024), ubae013.doi: 10.1093/bjrai/ubae013
-
[5]
ADOBI: Adaptive Diffusion Bridge For Blind Inverse Problems with Application to MRI Reconstruction
Y. Hu, A. Peng, W. Gan, et al. “ADOBI: Adaptive Diffusion Bridge For Blind Inverse ProblemswithApplicationtoMRIReconstruction”. arXivpreprintarXiv:2411.16535 (2024)
work page Pith review arXiv 2024
-
[6]
WDM: 3D wavelet diffusion models for high-resolution medical image synthesis
P. Friedrich, J. Wolleb, F. Bieder, et al. “WDM: 3D wavelet diffusion models for high-resolution medical image synthesis”.MICCAI Workshop on Deep Generative Models. Springer. 2024, pp. 11–21
work page 2024
-
[7]
Statistical image reconstruction for polyenergetic X-ray computed tomography
I. A. Elbakri and J. A. Fessler. “Statistical image reconstruction for polyenergetic X-ray computed tomography”.IEEE transactions on medical imaging21.2 (2002), pp. 89–99
work page 2002
-
[8]
Denoising diffusion probabilistic models
J. Ho, A. Jain, and P. Abbeel. “Denoising diffusion probabilistic models”.Advances inneuralinformationprocessingsystems 33(2020),pp.6840–6851. doi: 10.48550/ arXiv.2006.11239
Show all 18 references
-
[9]
Denoising diffusion implicit models
J. Song, C. Meng, and S. Ermon. “Denoising diffusion implicit models”.arXiv preprint arXiv:2010.02502(2020)
2020 arXiv
-
[10]
Blind inversion using latent diffusion priors
W. Bai, S. Chen, W. Chen, et al. “Blind inversion using latent diffusion priors”.arXiv preprint arXiv:2407.01027(2024)
2024 arXiv
-
[11]
Manifold preserving guided diffusion
Y. He, N. Murata, C.-H. Lai, et al. “Manifold preserving guided diffusion”.arXiv preprint arXiv:2311.16424(2023)
2023 arXiv
-
[12]
Denoising diffusion models for plug-and-play image restoration
Y. Zhu, K. Zhang, J. Liang, et al. “Denoising diffusion models for plug-and-play image restoration”.Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition. 2023, pp. 1219–1229
2023
-
[13]
Multi-Material Decomposition Using Spectral Diffusion Posterior Sampling
X. Jiang, G. J. Gang, and J. W. Stayman. “Multi-Material Decomposition Using Spectral Diffusion Posterior Sampling”.arXiv preprint arXiv:2408.01519(2024)
2024 arXiv
-
[14]
Torchradon: Fast differentiable routines for computed tomography
M. Ronchetti. “Torchradon: Fast differentiable routines for computed tomography”. arXiv preprint arXiv:2009.14788(2020)
2020 arXiv
-
[15]
4D XCAT phantom for multimodality imaging research
W. P. Segars, G. Sturgeon, S. Mendonca, et al. “4D XCAT phantom for multimodality imaging research”.Medical physics37.9 (2010), pp. 4902–4915.doi: 10.1118/1. 3480985
2010 doi
-
[16]
Learning Image Priors through Patch-based Diffusion Models for Solving Inverse Problems
J. Hu, B. Song, X. Xu, et al. “Learning Image Priors through Patch-based Diffusion Models for Solving Inverse Problems”.arXiv preprint arXiv:2406.02462(2024)
2024
-
[17]
Joint Reconstruction of the Activity and the Attenuation in PET by Diffusion Posterior Sampling: a Feasibility Study
C. Phung-Ngoc, A. Bousse, A. De Paepe, et al. “Joint Reconstruction of the Activity and the Attenuation in PET by Diffusion Posterior Sampling: a Feasibility Study”. arXiv preprint arXiv:2412.11776(2024)
2024 arXiv
-
[18]
CTrespiratorymotionsynthesisusingjoint supervised and adversarial learning
Y.-H.Cao,V.Bourbonne,F.Lucia,etal.“CTrespiratorymotionsynthesisusingjoint supervised and adversarial learning”.Physics in Medicine & Biology69.9 (2024), p. 095001
2024
Reviewed August 10, 2026 · model on record in the stance chip above.
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