REVIEW 5 major objections 5 minor 28 references
Bi-Directional ConvLSTM U-Net with Densley Connected Convolutions
T0 review · 5 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read A U-Net variant that fuses encoder and decoder features with a bi-directional ConvLSTM and uses dense bottleneck convolutions reports higher F1, accuracy, Jaccard, and AUC than earlier U-Net-based models on retinal, skin-lesion, and…
desk verdict A plausible U-Net extension whose headline numbers are mathematically impossible, so the state-of-the-art claim does not hold up. 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 load-bearing mechanism is the bi-directional ConvLSTM (BConvLSTM) placed in every skip connection: a layer made of two ConvLSTM stacks, ConvLSTM being a recurrent unit that replaces matrix multiplications with convolutions, one scanning the input sequence forward and one backward, whose hidden states are combined by a tanh layer. The sequence has two entries, the encoded feature map and the up-convolved decoded feature map, so each feature set can modulate the other through convolutional gates instead of just being concatenated. Two supporting mechanisms carry the rest of the argument: densely connected convolution blocks in the last encoding layer, where each block receives the concatenation of all earlier blocks' outputs, and batch normalization after each up-convolution, which normalizes activations by batch mean and variance and is credited with faster convergence.
What would settle it
Re-run the three experiments with U-Net, RU-Net, R2U-Net, and BCDU-Net from one code base under identical splits, preprocessing, patch sampling, and metric definitions; if the F1/Jaccard gaps reverse or vanish, the state-of-the-art claim fails. As a direct check on the reported ISIC numbers in Table 2, recompute F1 and Jaccard from the predicted masks: for binary segmentation Jaccard cannot exceed F1, so the table's Jaccard values (0.936, 0.937) above F1 (0.847, 0.851) would have to be resolved or reproduced before that comparison can stand.
Extended reading notes
Core claim
The central claim is that the way U-Net combines encoder and decoder feature maps in the skip connection is a bottleneck, and nonlinear bidirectional fusion removes it. In BCDU-Net, the high-resolution feature maps from a contracting level and the up-sampled, semantically richer feature maps from the adjacent decoding level are fed as a two-entry sequence into a bi-directional ConvLSTM; a tanh combination of the forward and backward hidden states produces the fused map that goes to the decoder. At the deepest encoding level, a chain of densely connected convolution blocks lets every block see the concatenation of all previous blocks' outputs, encouraging feature reuse. Evaluated on DRIVE, ISIC 2018, and the Kaggle lung dataset, the paper reports F1, accuracy, Jaccard, and AUC numbers above the previously published U-Net, RU-Net, and R2U-Net baselines, with the dense version consistently better than the version without dense connections.
Load-bearing premise
The headline comparison assumes that previously published baseline numbers were produced under the same data splits, patch sampling, preprocessing, and metric definitions as the authors' runs, and that the lung-region test phase can be done without ground-truth masks.
Editorial extensions
If this is right
- If the reported numbers hold, BCDU-Net with dense blocks is the best of the compared U-Net variants on all three datasets, not just one.
- On ISIC 2018, the reported Jaccard climbs from 0.592 for the best attention variant to 0.937, which would mean a large practical improvement for lesion boundary delineation.
- On the lung CT set, the reported F1 of 0.9904 and accuracy of 0.9972 would make the dense version the strongest of the compared models for this task.
- Batch normalization after up-convolutions would cut ISIC training epochs from about 200 to about 30, a direct practical saving whenever this architecture is retrained.
Reading between the lines
- A direct ablation that replaces BConvLSTM with a single ConvLSTM or an attention gate would identify whether the gain comes from bidirectionality or from learnable gating; the paper does not run that comparison.
- Since the BConvLSTM treats the two feature sets as a length-two sequence, one can read the mechanism as mutual spatial gating: each ConvLSTM state learns to encode information relevant to the other feature set; that interpretation is not spelled out in the paper.
- The same architectural recipe could be transplanted to other small-annotation segmentation tasks, such as microscopy or ultrasound, but the paper only evaluates fundus, dermoscopy, and CT images.
- For real-world lung screening, the training pipeline's surrounding-mask construction uses ground truth, so a deployable system would need an automatic way to estimate the same surrounding tissue at test time; the paper sketches the idea but does not specify that estimator.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes BCDU-Net, an extension of U-Net for medical image segmentation. The architecture inserts bi-directional ConvLSTM (BConvLSTM) layers into the skip connections and adds densely connected convolutional blocks in the deepest encoding layer, together with batch normalization after up-convolutions. The authors evaluate the method on three public benchmarks: DRIVE retinal vessel segmentation, ISIC 2018 skin lesion segmentation, and a Kaggle lung segmentation dataset, reporting F1-score, sensitivity, specificity, accuracy, Jaccard similarity, and AUC. The central claim is that BCDU-Net achieves state-of-the-art performance on all three datasets. The paper also includes an ablation study (Table 4) and a convergence-speed comparison with and without batch normalization.
Significance. If the reported improvements were valid, the architectural contribution would be of moderate interest to the medical image segmentation community: replacing concatenation in skip connections with BConvLSTM fusion and adding dense blocks are both plausible mechanisms for improving feature reuse, and the paper provides a public implementation link. However, the significance of the paper as submitted is undermined by internally inconsistent quantitative results. The reported F1 (Dice) values are numerically smaller than the reported Jaccard (IoU) values for the same predictions, which is mathematically impossible, and the Jaccard column appears to be a copy of the accuracy column for the proposed method. Since the headline contribution is an empirical state-of-the-art claim, these inconsistencies invalidate the central conclusion. The manuscript also leaves the lung test-time protocol underspecified because Algorithm 1 uses the ground-truth mask to construct the training surrounding mask, and the test-time analogue is not described.
major comments (5)
- [§4.2, Table 2 and §4.3, Table 3] The reported metrics violate a mathematical identity. For binary segmentation, F1 (Dice) is always greater than or equal to Jaccard (IoU) for the same set of predictions, with equality only in degenerate cases. Table 2 reports BCDU-Net (d=1) F1=0.847 and JS=0.936, and BCDU-Net (d=3) F1=0.851 and JS=0.937; both violate the inequality. Table 3 reports the same violation, for example BCDU-Net (d=3) F1=0.9904 and JS=0.9972. This indicates that the evaluation script or the reported numbers are unreliable, and it invalidates the state-of-the-art comparison that is the paper's central claim.
- [§4.2, Table 2 and §4.3, Table 3] The Jaccard column is identical to the Accuracy column for all BCDU-Net rows: Table 2 shows JS=0.936 and AC=0.936 for d=1 and JS=0.937 and AC=0.937 for d=3; Table 3 shows JS=0.9967 and AC=0.9967 for d=1 and JS=0.9972 and AC=0.9972 for d=3. Since Jaccard and accuracy are different quantities, this suggests the JS column was populated with accuracy values or the evaluation code computed the wrong metric. The authors should recompute all metrics from the raw test predictions and report the corrected tables before any performance claim can be assessed.
- [§4.4, Table 4 versus §4.1, Table 1] The DRIVE results are inconsistent between the two tables. Table 1 reports BCDU-Net (d=3) F1=0.8224, while Table 4 reports the same configuration as F1=0.8243. Likewise the U-Net accuracy values differ between Table 4 and Table 3 for the lung dataset (AC=0.9828 in Table 4 versus AC=0.9872 in Table 3). The authors need to explain which numbers come from which experimental protocol, and the final tables must report a single consistent set of test-set results, ideally with standard deviations over multiple runs.
- [§4.3, Algorithm 1] Algorithm 1 takes both the input X and the ground-truth mask GT Mask as inputs and constructs a surrounding mask by subtracting the ground truth. The text says this preprocessing is applied to the training set, but the test-time protocol is not specified. If the lung-region estimate at test time relies on the same surrounding-mask construction, it would require ground-truth masks on the test set, which makes the evaluation circular. The authors must describe how the surrounding mask is obtained for test images without ground-truth annotations and justify that the training and test protocols are aligned.
- [§4.1–§4.3, Tables 1–3] The state-of-the-art comparison is performed by quoting numbers from prior papers rather than by re-running baseline methods under the authors' exact training protocol, preprocessing, and split. Given that the metric errors above already show the authors' evaluation pipeline is not reliable, the cross-paper baseline comparisons cannot be accepted at face value. At minimum, the authors should re-implement U-Net, RU-Net, and R2U-Net under identical conditions, or clearly state the protocol differences and provide the uncertainty of each reported number.
minor comments (5)
- [Title] The title contains a typo: 'Densley' should be 'Densely'.
- [§4, Experimental Results] The text says 'Keras with TenserFlow backend'; 'TenserFlow' should be 'TensorFlow', and 'datase' in §4.1 should be 'dataset'.
- [§4.4, Discussion] The sentence 'ia able to encode relevant information' contains a typo: 'ia' should be 'is'.
- [Figure 3 caption] The caption of Figure 3 says 'Bi-directional ConvLSTM in CUA-Net', but the network is called BCDU-Net; the caption should be corrected.
- [§3.2.2, Bi-Directional ConvLSTM] The notation for the ConvLSTM equations is inconsistent: Equation (1) uses Wci, Wcf, and Wco with a Hadamard product in the cell-update and output-gate terms, whereas standard ConvLSTM formulations use convolution operations throughout; the authors should clarify the exact tensor operations and dimensions.
Circularity Check
No significant circularity: all load-bearing claims are empirical comparisons on public benchmarks against external baselines.
full rationale
The paper proposes an architectural modification of U-Net (BConvLSTM in skip connections, densely connected convolutions in the deepest encoding block, and batch normalization after up-convolutions) and supports its claims by direct experiments on three public datasets. There is no derivation from fitted parameters that is then renamed as a prediction: the reported F1, accuracy, Jaccard, sensitivity, specificity, and AUC values are measured outputs of trained networks, not functions of a fitted model. The BConvLSTM equations and dense-block description are presented as standard components from the cited literature; the paper does not claim to derive them from first principles, nor does it define any quantity in terms of its own output. The choice d=1 versus d=3 is a hyperparameter comparison reported on the same benchmarks, which is ordinary model selection rather than a circular prediction; the paper does not fit a parameter to a subset and then predict a closely related derived quantity. Baselines such as U-Net, RU-Net, R2U-Net, and Attention U-Net are taken from independently authored prior work, and the paper's citations of Alom et al. are not self-citations. Any concern about inconsistent evaluation protocols between the paper's runs and the quoted baselines is a correctness and reproducibility issue, not circularity, because the central claim's support is an external benchmark comparison rather than a self-referential derivation. The internally inconsistent F1 and Jaccard values in Tables 2 and 3 indicate an evaluation-script or reporting problem, not a case where the result is assumed in the input. Therefore, under the required evidentiary standard, no circular step can be exhibited.
Assumptions & free parameters
free parameters (1)
- Number of dense blocks (d) =
3
assumptions (3)
- domain assumption Baseline numbers from prior publications are directly comparable to the authors' runs.
- domain assumption Lung surrounding-tissue masks can be estimated at test time without ground-truth masks.
- domain assumption The reported F1 and Jaccard metrics are computed on the same binary predictions.
Cite this review
Pith. "Pith review of Bi-Directional ConvLSTM U-Net with Densley Connected Convolutions." pith.science (2026). https://pith.science/paper/4H5UF7AN
@misc{pith2026190900166,
author = {Pith},
title = {Pith review of: Bi-Directional ConvLSTM U-Net with Densley Connected Convolutions},
year = {2026},
howpublished = {\url{https://pith.science/paper/4H5UF7AN}},
note = {Machine review of arXiv:1909.00166}
}
read the original abstract
In recent years, deep learning-based networks have achieved state-of-the-art performance in medical image segmentation. Among the existing networks, U-Net has been successfully applied on medical image segmentation. In this paper, we propose an extension of U-Net, Bi-directional ConvLSTM U-Net with Densely connected convolutions (BCDU-Net), for medical image segmentation, in which we take full advantages of U-Net, bi-directional ConvLSTM (BConvLSTM) and the mechanism of dense convolutions. Instead of a simple concatenation in the skip connection of U-Net, we employ BConvLSTM to combine the feature maps extracted from the corresponding encoding path and the previous decoding up-convolutional layer in a non-linear way. To strengthen feature propagation and encourage feature reuse, we use densely connected convolutions in the last convolutional layer of the encoding path. Finally, we can accelerate the convergence speed of the proposed network by employing batch normalization (BN). The proposed model is evaluated on three datasets of: retinal blood vessel segmentation, skin lesion segmentation, and lung nodule segmentation, achieving state-of-the-art performance.
Figures
Figures from the paper (7 more)
Reference graph
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