The research, led by Ali Abidalkareem, Ali K. Ibrahim, and Hanqi Zhuang of Florida Atlantic University, tackles three stubborn problems that have plagued medical image classifiers for years: the scarcity of expert-annotated fundus photographs, the severe imbalance between common and rare disease categories, and the tendency of models to falter when moved from one imaging environment to another. The team’s earlier system, a Fused Ensemble Convolutional Inception Network, had reached 93.2 percent accuracy by randomly generating and fusing multiple inception-style networks, but it relied only on simple geometric data augmentation and processed images at a modest 100 by 100 pixel resolution. The new framework extends that foundation with three complementary innovations: a generative diffusion model that manufactures synthetic retinal images, a self-supervised contrastive learning protocol that extracts meaning from unlabeled data, and a five-model ensemble fused by a learned neural network.

The first innovation addresses the imbalance problem directly. In the APTOS 2019 dataset used for training, the healthy No-DR category contains 1,805 images, roughly 49.3 percent of the corpus, while the Severe DR category contains just 193 images, about 5.3 percent, a greater than nine-fold disparity. To correct this, the researchers trained a class-conditional latent denoising diffusion probabilistic model, or DDPM, a generative architecture that learns to create images by first progressively corrupting training photographs with Gaussian noise over many timesteps and then learning to reverse that corruption. Because the model operates in the compressed latent space of a variational autoencoder rather than in raw pixel space, the computational cost drops substantially while perceptually important structure is preserved. A technique called classifier-free guidance, applied with a scale of 3.0, sharpens the class conditioning during sampling so that generated images faithfully reflect the requested severity level. For each cross-validation fold, the team generated 5,000 synthetic images for Mild DR and 3,000 each for Severe and Proliferative DR, approximately balancing the underrepresented categories against the Moderate class.

Crucially, the synthetic images are never handed to the classifier as labeled training examples. Instead, they enter the pipeline at exactly one point: the unsupervised pre-training stage. The second innovation adapts SimCLR, a contrastive self-supervised learning framework, to the retinal domain. SimCLR works by taking each image, creating two differently augmented views of it, and training an encoder network to maximize agreement between representations of the same image while minimizing agreement with views of different images. The loss function, known as NT-Xent, depends only on the identity of augmented pairs within a minibatch and never references a severity label. This means a synthetic image conditioned on the Severe DR class is never paired with a Severe DR ground-truth label anywhere in the pipeline; the conditioning label is used only to steer generation and is discarded immediately afterward. The fundus-specific augmentation distribution includes full 360-degree rotations, random flips, resized crops covering 80 to 100 percent of the field of view, color jittering, Gaussian blur, and simulated illumination gradients to mimic vignetting, while deliberately excluding transformations that would produce anatomically implausible images. After 200 epochs of pre-training with a ResNet-50 encoder, the projection head is discarded and the learned weights initialize the supervised classification network.

The third innovation is an ensemble of five architecturally diverse convolutional networks: EfficientNet-B3, ResNet-50, VGG16, VGG19, and DenseNet-121. Each backbone is fine-tuned independently on real, expert-labeled images using a class-weighted loss to counter residual imbalance, and their softmax outputs are concatenated and fed into a learned fusion network called Pattern-Net, a three-layer multilayer perceptron trained on validation-split predictions. Architectural diversity matters because different network families extract different feature patterns; combining them through a learned fusion layer rather than simple averaging allows the system to exploit complementary strengths. The researchers emphasize that this design replaces the random parameter initialization of their earlier work with pre-trained, heterogeneous models, yielding richer representations while retaining the ensemble diversity principle.

Evaluated under 5-fold stratified cross-validation on APTOS 2019, the integrated framework achieved 95.1 percent accuracy, a quadratic weighted kappa of 0.928, an F1-score of 0.91, and an area under the receiver operating characteristic curve of 0.97. The quadratic weighted kappa, the primary metric for ordinal severity scales, rewards predictions that are close to the true grade even when not exactly correct. The class-wise breakdown reveals where the gains matter most clinically: Severe DR accuracy jumped from 61.8 percent with the EfficientNet-B3 baseline to 85.6 percent, a gain of 23.8 percentage points, while Mild DR rose from 71.2 to 88.9 percent, a gain of 17.7 points. Proliferative DR improved by 16.7 points to 92.3 percent, and Moderate DR by 10.9 points to 94.3 percent. Because overall accuracy can be misleading when most samples belong to the majority class, these per-class improvements on the clinically critical minority categories are the most meaningful measure of progress.

An ablation study disentangled the contribution of each component. The multi-architecture ensemble alone contributed the largest single gain, adding 5.3 percentage points over the baseline. SimCLR contrastive pre-training added another 1.1 points, and enriching the pre-training corpus with diffusion-generated images added a further 0.8 points, bringing the total to 95.1 percent. The authors note that these effects combine super-additively, particularly for minority-class performance, suggesting that generative augmentation and self-supervised representation learning reinforce one another rather than merely stacking. Statistical validation strengthened the claims: paired McNemar tests on out-of-fold predictions, adjusted with the Holm-Bonferroni procedure for multiple comparisons, showed that the full framework significantly outperformed every comparator, with Holm-adjusted p-values below 0.001 against the baseline and ensemble, and 0.012 against the SimCLR-only configuration. Paired bootstrap confidence intervals for the differences in kappa, macro F1, and macro AUC all excluded zero.

To test generalization under domain shift, the team evaluated the fully trained framework in a strict zero-shot setting on EyePACS, an independent dataset of fundus images acquired with different cameras under varied conditions. No EyePACS image was used for training, diffusion modeling, pre-training, or hyperparameter selection, and quality filtering removed 5,269 ungradable images, leaving 29,857 for evaluation. The result was an accuracy of 78.4 percent with a kappa of 0.809, a drop of 16.7 percentage points relative to internal validation. The degradation was steepest for the minority classes, with Mild DR falling to 61.7 percent and Severe DR to 57.2 percent, consistent with the greater difficulty of detecting subtle pathological signs across differing camera hardware, illumination protocols, and patient demographics. The authors frame this gap honestly as a quantification of expected real-world performance reduction rather than a failure, noting that the zero-shot result remains within the range of reported cross-dataset DR classification studies.

The synthetic images themselves were assessed through three complementary checks. The latent diffusion model achieved a Fréchet Inception Distance of 24.3 between synthetic and real fundus distributions, indicating close statistical similarity. In a Turing-style test, two ophthalmologists reviewing 100 generated images per class misidentified them as real 41.3 percent of the time on average, close to the 50 percent rate expected by chance. A separately trained classifier agreed with the intended conditioning label 89.4 percent of the time. The researchers are careful, however, to state that these evaluations establish visual plausibility rather than lesion-level clinical equivalence, since blinded lesion-by-lesion annotations and inter-observer agreement statistics were not collected. Gradient-weighted Class Activation Mapping, or Grad-CAM, was also applied to correctly classified images from all five severity grades, showing that activation patterns concentrate within the retinal field rather than the surrounding background, with localized responses for moderate and severe cases and broader spatial responses for proliferative disease.

The authors position the framework as a research prototype rather than a clinically validated decision-support system, and they outline the road ahead: domain adaptation or multi-site training to close the EyePACS gap, prospective multi-site clinical validation, blinded lesion-level grading of synthetic images, quantitative explainability checks against expert masks, and extension to diabetic macular edema through multi-task learning. Still, the study demonstrates a principled, data-efficient recipe for imbalanced medical image classification, one that could bring scalable automated retinal screening closer to the resource-limited settings where it is needed most. With diabetes rates climbing worldwide, the ability of a machine to conjure rare disease images from thin air, learn from them without labels, and then catch the earliest signs of blindness may prove to be more than an academic curiosity.

Subject of Research: A deep learning framework combining class-conditional diffusion models, SimCLR contrastive pre-training, and a multi-architecture ensemble for five-class diabetic retinopathy severity classification from fundus images.

Article Title: Diffusion-augmented contrastive learning for diabetic retinopathy severity classification

Article References: Diffusion-augmented contrastive learning for diabetic retinopathy severity classification. (n.d.). https://doi.org/10.3389/frai.2026.1932259

Image Credits: AI Generated

DOI: 10.3389/frai.2026.1932259

Keywords: diabetic retinopathy, diffusion models, contrastive learning, SimCLR, ensemble learning, deep learning, medical imaging, class imbalance, domain shift, fundus images, APTOS 2019, EyePACS