ISIC 2018 - Skin Lesion Segmentation using a LinkNet Derived Network


Introduction

The purpose of this document is the description of the architecture used in the 2018 ISIC Challenge for segmenting skin lesions present in images. We use a deep learning based solution for solving the problem. U-Net (Ronneberger, Fischer, and Brox 2015) derived architectures have been proven to be a very solid approach for this purpose. LinkNet (Chaurasia and Culurciello 2017) architecture allow the reduction of required parameters for solving the same task. LinkNet use the same approach that ResNets (He et al. 2016) for connecting encoder and decoder filters. Instead of concatenating their values, they sum them up, achieving state of the art results in semantic segmentation tasks.

Data and Preprocessing

The train set is formed by 12,342 images with their corresponding masks. A validation set with 1,436 images is used for hyper-parameter optimization. Data augmentation techniques are used to increment the diversity of input images and masks. Our data was extracted from the “ISIC 2018: Skin Lesion Analysis Towards Melanoma Detection” grand challenge datasets (Codella et al. 2017) (Tschandl, Rosendahl, and Kittler 2018).

Model

We used a LinkNet derived network for solving this challenge reducing the number of filters. Original ResNet architectures (ResNet18, ResNet34, etc.) are designed to perfom well in large classification problems like ImageNet(Deng et al. 2009). For the specific problem of disease characterization, the diversity of images is smaller than in such cases and enable the usage of narrower filter stacks, that’s why we limited the number of filters per layer to 64, allowing the usage of a model of only 600,000 parameters

High level description of the LinkNet model architecture (Chaurasia
and Culurciello
2017)[]{label="fig:linknet"}

Figure above shows a high level description of the model architecture. Network is constituted by two parts: an encoder and a decoder. The encoder is a residual net feature extractor. Decoder receives information from different feature maps of the encoder, summing up such values with their own.

Encoder block

Decoder block

Figures above show a diagram with a high level description of encoder and decoder blocks. Instead of concatenating decoder and encoder values at the output of each layer, LinkNet uses a residual network approach summing up the values, ie. reducing the number of parameters of the final network.

Training procedure

We split the training data in two sets: one with 13,000 images used for training and another one with 1,400 images for validation. We use the Adam optimizer with a learning rate of and Dice coefficient as a loss function. Model with higher performance in the validation set is chosen as a final model.

References

Chaurasia, Abhishek, and Eugenio Culurciello. 2017. “LinkNet: Exploiting Encoder Representations for Efficient Semantic Segmentation.” CoRR abs/1707.03718. http://arxiv.org/abs/1707.03718.

Codella, Noel C. F., David Gutman, M. Emre Celebi, Brian Helba, Michael A. Marchetti, Stephen W. Dusza, Aadi Kalloo, et al. 2017. “Skin Lesion Analysis Toward Melanoma Detection: A Challenge at the 2017 International Symposium on Biomedical Imaging (Isbi), Hosted by the International Skin Imaging Collaboration (ISIC).” CoRR abs/1710.05006. http://arxiv.org/abs/1710.05006.

Deng, J., W. Dong, R. Socher, L.-J. Li, K. Li, and L. Fei-Fei. 2009. “ImageNet: A Large-Scale Hierarchical Image Database.” In CVPR09.

He, Kaiming, Xiangyu Zhang, Shaoqing Ren, and Jian Sun. 2016. “Deep Residual Learning for Image Recognition.” In Proceedings of the Ieee Conference on Computer Vision and Pattern Recognition, 770–78.

Ronneberger, Olaf, Philipp Fischer, and Thomas Brox. 2015. “U-Net: Convolutional Networks for Biomedical Image Segmentation.” In International Conference on Medical Image Computing and Computer-Assisted Intervention, 234–41. Springer.

Tschandl, Philipp, Cliff Rosendahl, and Harald Kittler. 2018. “The HAM10000 Dataset: A Large Collection of Multi-Source Dermatoscopic Images of Common Pigmented Skin Lesions.” CoRR abs/1803.10417. http://arxiv.org/abs/1803.10417.

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