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Magnetic resonance imaging (MRI) reveals fine brain details, but spotting a tumor still needs expert eyes. Deep neural networks have changed the game, yet they demand huge labeled datasets, which are costly and delicate to produce. That’s why researchers are turning to semi‑supervised learning, a strategy that mixes a few certified examples with many unlabeled images.

A popular trick is pseudo‑labeling: after an initial training, the network assigns provisional labels to unlabeled scans, then uses them to fine‑tune itself. Consistency regularization pushes the model to give similar outputs even when the image is slightly altered (noise, rotations). GANs (Generative Adversarial Networks) generate synthetic tumor images, enriching the training pool. Contrastive learning teaches the model to tell apart similar and different regions, sharpening its edge detection. Holistic approaches blend several of these ideas into one framework.

Recent studies show that with only 10‑20 % of labeled data, these methods match or surpass fully supervised networks. The outcome is faster diagnosis, less reliance on specialists, and a path toward more accessible clinical decision‑support tools.

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Semi‑supervised learning leverages latent representation theory: networks map MRI scans into a vector space where tumor structures form distinct clusters. Pseudo‑labeling relies on a confidence threshold; if the predicted probability exceeds 0.9, the label is accepted as true. Consistency regularization uses a Mean Squared Error loss between predictions of augmented versions of the same image, reducing model variance. GANs involve a generator G that synthesizes tumor images and a discriminator D that assesses realism; training proceeds as a zero‑sum game, enhancing data diversity. Contrastive learning employs an InfoNCE loss, maximizing similarity between positive pairs (same image with different transforms) and minimizing it for negative pairs (different images). Holistic methods, such as multi‑view co‑training, combine multiple networks with different architectures (U‑Net, Transformer) and exchange pseudo‑labels to reduce bias. Common evaluation metrics include Dice coefficient, Hausdorff distance, and sensitivity/specificity, showing 3‑5 % gains over traditional supervised models. Clinical adoption demands multi‑center cohort validation and integration with PACS systems to ensure traceability and regulatory compliance.

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Authors:
Chengcheng JinTheam Foo NgHaidi Ibrahim
Date Published:
Original Article www.mdpi.com
Fact-check verified on 23/05/2026
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Creative Commons License CC BY-NC-SA 4.0

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