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REVIEW 3 major objections 4 minor 18 references

Fast Dynamic Perfusion and Angiography Reconstruction using an end-to-end 3D Convolutional Neural Network

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read A 3D Dense-UNet with multi-level SSIM loss reconstructs 4D perfusion and 4D angiography from interleaved half-sampled crushed and non-crushed Hadamard te-pCASL data, preserving image quality while halving scan time.

desk verdict A well-executed in-silico proof-of-concept for 3D CNN reconstruction of perfusion and angiography from half-sampled Hadamard te-pCASL, but the abstract's clinical claim outruns the evidence. read the letter →

arxiv 1908.08947 v2 pith:PPEOLTG6 submitted 2019-08-24 eess.IV cs.CV

classification eess.IVcs.CV
keywords Hadamardtime-encodedASLpseudo-continuousarterialspinlabeling4DMRAperfusion3DconvolutionalneuralnetworkDense-UNetmulti-levelSSIMlossMRIacceleration
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper tries to establish that a single 3D convolutional network can replace a second MRI acquisition. In Hadamard time-encoded pCASL, dynamic perfusion and dynamic angiography are normally acquired separately, one with flow-crushing and one without, which doubles scan time. The authors propose an end-to-end 3D Dense-UNet that takes interleaved half-sampled crushed and non-crushed data and outputs both 4D perfusion and 4D angiography at seven time-points. Training data are generated synthetically from a generalized kinetic model using in vivo arrival-time maps and simulated brain anatomy. On 313 test datasets the network reaches SSIM 97.3 ± 1.1 for perfusion and 96.2 ± 11.1 for angiography, arguing the factor-of-two scan-time penalty is avoidable.

What carries the argument

The load-bearing device is a 3D Dense-UNet with a multi-level loss. Dense blocks with $3\times3\times3$ and $1\times1\times1$ convolutions improve feature propagation and gradient flow; up-sampling uses a constant trilinear resize kernel to avoid checkerboard artifacts. The multi-level loss ML-SSIM weights the SSIM loss at three network levels, chosen because SSIM outperformed MSE, VGG-16 perceptual loss, and single-scale SSIM for perfusion reconstruction. The training data come from a generalized Buxton kinetic model (Eqs. 3 and 4) that combines an intravascular arterial signal and an extravascular tissue signal parameterized by AAT, BAT, and CBF; fully sampled Hadamard decoding of the generated scans provides the ground truth.

What would settle it

Acquire real crushed and non-crushed Hadamard-8 te-pCASL scans on a cohort, form full-sampled ground truth by the standard decoding and subtraction, then feed the interleaved 50-percent-sampled subset through the trained network; if the SSIM against real full-sampled output falls substantially below 97 percent for perfusion or 96 percent for angiography, the central claim fails. A cheaper check is to compare the kinetic model's predicted arterial and tissue signals to measured signals in voxels with known transit times.

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Extended reading notes

Core claim

The central claim is that a 3D Dense-UNet trained with a multi-level SSIM loss can decode 4D perfusion and 4D angiography directly from interleaved 50-percent-sampled crushed and non-crushed Hadamard te-pCASL scans, without a separate full acquisition. The network uses an 8-row Hadamard encoding, takes four crushed and four non-crushed patches as input, and outputs 14 patches, seven perfusion and seven angiography time-points. With the ML-SSIM loss it achieves SSIM of 97.3 ± 1.1 on perfusion and 96.2 ± 11.1 on angiography for 313 test datasets, and reconstructs all time-points in about 205 ms. The paper also presents a data-generation framework based on the generalized Buxton kinetic model to create realistic training data from in vivo arterial arrival time, bolus arrival time, and simulated CBF maps.

Load-bearing premise

The synthetic te-pCASL data produced by the kinetic model from in vivo arrival-time maps and simulated anatomies is representative enough of real crushed and non-crushed Hadamard te-pCASL acquisitions; if real signals contain effects the model omits, the trained network's outputs may not match the reported SSIM.

Editorial extensions

If this is right

  • If the central claim holds, simultaneous 4D MRA and perfusion can be acquired in one interleaved scan rather than two, halving the te-pCASL scan time.
  • The network reconstructs all seven time-points of both output types in roughly 205 ms per patch set, suggesting a speed compatible with clinical workflow.
  • The ML-SSIM loss gives a statistically significant improvement over MSE loss for perfusion ($p<0.05$), while matching the SSIM loss for angiography; this identifies the loss design as a key factor.
  • The synthetic data-generation framework extends to higher-rank Hadamard matrices and other time-encoded ASL schemes, since the kinetic model equations generalize beyond Hadamard-8.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The interleaved half-sampling idea could transfer to other dual-acquisition protocols where two scans are subtracted, with the network learning the unmixing directly from data.
  • Because angiography is intrinsically sparse and dominated by elongated structures, explicit sparsity-aware or structure-aware loss terms could push early time-point angiography quality beyond the reported SNR.
  • The synthetic-to-real gap is the largest open risk; fine-tuning on a small set of real in vivo crushed and non-crushed pairs would test whether the reported SSIM translates to clinical acquisitions.
  • With inference under a quarter of a second, the network could be embedded into a real-time reconstruction loop, displaying both perfusion and angiography immediately after acquisition.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. Hadamard time-encoded pCASL can provide both dynamic perfusion and 4D angiography when acquired with and without flow-crushing, but the two acquisitions double scan time. This paper proposes a 3D Dense-UNet that inputs interleaved 50%-sampled crushed and non-crushed Hadamard te-pCASL images (four of eight encodings each) and outputs seven-timepoint perfusion and angiographic volumes, achieving a factor-two acceleration relative to full sampling. Training and test data are generated entirely from a simulator built on a generalized Buxton kinetic model (Eqs. 3-4) using 6 BAT maps, 4 AAT maps, and 20 BrainWeb anatomies. The authors compare MSE, VGG perceptual, SSIM, and their proposed multi-level SSIM losses and report best perfusion SSIM of 97.3 +/- 1.1 and angiography SSIM of 96.2 +/- 11.1 on 313 synthetic test datasets. The paper claims this obviates the additional scan time in practice.

Significance. The contribution is potentially useful: an end-to-end 3D CNN with dense connectivity and a multi-level SSIM loss is a reasonable architecture for recovering both smooth perfusion and sparse vascular structures from undersampled Hadamard-encoded ASL, and the loss-function comparison with Wilcoxon tests is a strength. The data-generation pipeline is clearly specified and makes the method reproducible. The significance is conditional because the entire evaluation is in-distribution with respect to the authors' own simulator: no real crushed/non-crushed te-pCASL data are used for validation, and the metrics therefore estimate performance on synthetic signals that follow Eqs. (3)-(4), not on clinical acquisitions with dispersion, motion, partial volume, or B1/B0 effects. The abstract's clinical claim of 'negating the additional scan time' is stronger than the evidence supports.

major comments (3)
  1. [§2.3, Eqs. (3)-(4); Abstract; Conclusion] All training, validation, and test examples are generated by the authors' simulator from 6 BAT maps, 4 AAT maps, and 20 BrainWeb subjects; the only corruption added is Gaussian white noise with standard deviation between 0 and 5 (Section 3). The kinetic model omits transit-time dispersion, partial-volume effects, motion, B1/B0 inhomogeneities, T1/T2 variation, and labeling-efficiency errors. Because the network is trained and tested on this same distribution, the reported SSIM values (97.3 +/- 1.1 perfusion, 96.2 +/- 11.1 angiography) do not by themselves support the abstract's statement that the method 'negat[es] the additional scan time' in real acquisitions. The paper itself acknowledges this in the Conclusion: 'A further step of this study is enriching the training and validation datasets with in vivo data.' I ask the authors to either add an in vivo or at least a domain-shift experiment (e.g., realistic noise/artifact model or a held-out set from a different simulator/scanner) and to temper the abstract and conclusion so the claim is limited to the synthetic setting.
  2. [§2.3, final sentence] The sentence 'To evaluate the generated data, the signal evolution pattern was validated by the Buxton curve model [15]' is self-referential, because Eqs. (3)-(4) are themselves a generalization of the Buxton model. Matching the Buxton curve therefore only checks internal consistency of the generator, not agreement with real te-pCASL signals. This sentence should be removed or replaced by a comparison with measured crushed/non-crushed te-pCASL data.
  3. [§1 and §4] The paper states the goal of 'accurate CBF quantification' but no quantitative CBF accuracy is reported. Table 1 and Fig. 3 contain only image-space metrics (SSIM, MSE, SNR, pSNR) on reconstructed perfusion volumes; there is no comparison of estimated CBF values (e.g., bias and limits of agreement in mL/100 g/min) against the known simulation parameters or against a reference method. Either add such an analysis or remove the CBF-quantification claim from the introduction and conclusion.
minor comments (4)
  1. [Table 1, Fig. 3] For the angiography rows with high variance (SSIM and ML-SSIM), the reported mean +/- SD is strongly skewed (ML-SSIM SSIM mean 96.2, median 99.7, SD 11.1); report median and IQR and state how many of the 313 test datasets drive the low mean.
  2. [Figure 1] The formula 'w1 x Loss level1 + w2 x Loss level1 + w3 x Loss level3' appears to have a typo; the second term should likely be 'w2 x Loss level2'.
  3. [Introduction, Reference [1]] Reference [1] (Ferlay et al., cancer epidemiology) does not correspond to the first sentence about ASL being non-invasive; please replace it with the intended ASL review or methods citation.
  4. [§2.3, Section 3] The Gaussian noise augmentation is specified as 'random standard deviation between 0 and 5' without stating the units or the signal scale to which it is applied; please clarify (e.g., as a percentage of the mean signal or in arbitrary intensity units).

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the reconstruction target is full-sampled Hadamard decoding, which is independent of the network and the subsampled input.

full rationale

The paper's claimed derivation chain is the following: fully sampled crushed and non-crushed Hadamard te-pCASL data are decoded by the standard Hadamard decoding/subtraction operation M (Eq. 1) to produce ground-truth 4D perfusion and angiography; the network M' is trained to reproduce those same ground-truth outputs from interleaved 50%-subsampled inputs (Eq. 2). The target is therefore defined independently of the network, by the closed-form decoding of full data, and the subsampled inputs are strict subsets of those full data. This is a legitimate supervised regression task rather than a self-definitional or fitted-input-called-prediction loop: no parameter is fitted to the test outputs, and no uniqueness theorem or ansatz is imported from the authors' prior work to force the result. The only self-referential elements are minor: the synthetic generator is built from the Buxton kinetic model and then checked against the Buxton curve model, which is an internal consistency check rather than load-bearing evidence; and the decoding method is cited to the authors' own prior work, but that method is a standard published technique used as a fixed preprocessing operator, not a self-citation invoked to justify the paper's conclusion. The genuine limitation is external validity, not circularity: all training and test data come from the same in-silico generator, so reported SSIM estimates performance on simulator-like signals. The paper acknowledges this in the Conclusion: "A further step of this study is enriching the training and validation datasets with in vivo data." This is a correctness and generalization risk, not a circular-derivation risk. Therefore no circular step is exhibited.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The central claim rests on the validity of the synthetic signal generator (Eqs 3-4), on the linear superposition of arterial and tissue signals, and on the assumption that in-distribution synthetic performance predicts real-world behavior. The loss weights and kinetic scale parameters (aCBV, CBF, M0a, T1a) are taken from literature or chosen by hand and are mostly unreported, so a reader cannot reproduce the training data exactly.

free parameters (4)
  • GM/WM/CSF CBF values = not stated (literature values assigned)
    Tissue CBF values from the literature are assigned to BrainWeb segmentation labels to generate the synthetic tissue signal in Eq (4); exact values are not reported.
  • aCBV (arterial cerebral blood volume) = not stated
    Scales the arterial signal in Eq (3); the value is not reported, yet it controls the angiography signal amplitude in the synthetic data and thus the training targets.
  • ML-SSIM loss weights w1, w2, w3 = not stated
    The multi-level loss in Fig. 1 requires three hand-chosen weights; the paper never reports their values, so the training objective is not fully specified.
  • Arterial and tissue relaxation times T1a/T1b and M0a normalization = not stated
    Kinetic model constants in Eqs (3)-(6) that determine signal timing and amplitude; values are not given and Eq (3) uses T1b in place of the defined T1a.
assumptions (4)
  • domain assumption Buxton general kinetic model for ASL signal
    Section 2.3 builds the tissue and arterial signal model on the Buxton model [15] without re-deriving its validity for Hadamard encoding.
  • domain assumption Voxel signal is the linear sum of tissue and arterial contributions
    Section 2.3 states 'For voxels containing large arteries, the pCASL signal can be computed by S_voxel = S_tissue + S_artery'; this superposition may not hold with exchange or partial volume effects.
  • domain assumption Synthetic data distribution matches real te-pCASL acquisitions
    The paper states in Section 2.3 that real data is difficult to acquire and proposes modeling; the unstated premise that Eqs (3)-(4) capture in vivo signal behavior is load-bearing for clinical claims.
  • standard math Hadamard decoding is the correct ground-truth estimator
    Ground truth is formed by fully-sampled Hadamard decoding [2,3,18]; this assumes the standard linear decoding model is exact for the synthetic signals.

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Cite this review

Pith. "Pith review of Fast Dynamic Perfusion and Angiography Reconstruction using an end-to-end 3D Convolutional Neural Network." pith.science (2026). https://pith.science/paper/PPEOLTG6

@misc{pith2026190808947,
  author       = {Pith},
  title        = {Pith review of: Fast Dynamic Perfusion and Angiography Reconstruction using an end-to-end 3D Convolutional Neural Network},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/PPEOLTG6}},
  note         = {Machine review of arXiv:1908.08947}
}
abstract

Hadamard time-encoded pseudo-continuous arterial spin labeling (te-pCASL) is a signal-to-noise ratio (SNR)-efficient MRI technique for acquiring dynamic pCASL signals that encodes the temporal information into the labeling according to a Hadamard matrix. In the decoding step, the contribution of each sub-bolus can be isolated resulting in dynamic perfusion scans. When acquiring te-ASL both with and without flow-crushing, the ASL-signal in the arteries can be isolated resulting in 4D-angiographic information. However, obtaining multi-timepoint perfusion and angiographic data requires two acquisitions. In this study, we propose a 3D Dense-Unet convolutional neural network with a multi-level loss function for reconstructing multi-timepoint perfusion and angiographic information from an interleaved $50\%$-sampled crushed and $50\%$-sampled non-crushed data, thereby negating the additional scan time. We present a framework to generate dynamic pCASL training and validation data, based on models of the intravascular and extravascular te-pCASL signals. The proposed network achieved SSIM values of $97.3 \pm 1.1$ and $96.2 \pm 11.1$ respectively for 4D perfusion and angiographic data reconstruction for 313 test data-sets.

Figures

Figures reproduced from arXiv: 1908.08947 by the authors.

Figure 1
Figure 1. Proposed network with single- and multi-level loss functions. The training [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Data generator framework for one subject, the inputs of the framework [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Boxplots for different metrics and CNNs for perfusion and angiographic [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Qualification results, 4D a) MRA and b) perfusion and error at multi [PITH_FULL_IMAGE:figures/full_fig_p010_4.png]

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Reference graph

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Reviewed August 14, 2026 · model on record in the stance chip above.