PurposeTo help radiologists examine the growing number of computed tomography (CT) scans, automatic anomaly detection is an ongoing focus of medical imaging research. Radiologists must analyze a CT scan by searching for any deviation from normal healthy anatomy. We propose an approach to detecting abnormalities in axial 2D CT slice images of the brain. Although much research has been done on detecting abnormalities in magnetic resonance images of the brain, there is little work on CT scans, where abnormalities are more difficult to detect due to the low image contrast that must be represented by the model used.ApproachWe use a generative adversarial network (GAN) to learn normal brain anatomy in the first step and compare two approaches to image reconstruction: training an encoder in the second step and using iterative optimization during inference. Then, we analyze the differences from the original scan to detect and localize anomalies in the brain.ResultsOur approach can reconstruct healthy anatomy with good image contrast for brain CT scans. We obtain median Dice scores of 0.71 on our hemorrhage test data and 0.43 on our test set with additional tumor images from publicly available data sources. We also compare our models to a state-of-the-art autoencoder and a diffusion model and obtain qualitatively more accurate reconstructions.ConclusionsWithout defining anomalies during training, a GAN-based network was used to learn healthy anatomy for brain CT scans. Notably, our approach is not limited to the localization of hemorrhages and tumors and could thus be used to detect structural anatomical changes and other lesions.
Automatic detection of abnormalities to assist radiologists in acute and screening scenarios has become a particular focus in medical imaging research. Various approaches have been proposed for the detection of anomalies in magnetic resonance (MR) data, but very little work has been done for computed tomography (CT). As far as we know, there is no satisfactory approach for anomaly detection in CT brain images. We present a novel unsupervised deep learning approach to generate a normal representation (without anomalies) of CT head scans that we use to discriminate between healthy and abnormal images. In the first step, we train a GAN with 1000 healthy CT scans to generate normal head images. Subsequently, we attach an encoder to the generator and train the auto encoder network to reconstruct healthy anatomy from new input images. The auto encoder is pre-trained with generated images using a perceptual loss function. When applied to abnormal scans, the reconstructed healthy output is then used to detect anomalies by computing the Mean Squared Error between input and output image. We evaluate our slice-wise anomaly detection on 250 test images including hemorrhages and tumors. Our approach achieves an area under receiver operating characteristic curve (AUC) of 0.90 with 85.8% sensitivity and 85.5% precision without requiring large training data sets or labeled anomaly data. Therefore, our method discriminates between normal and abnormal CT scans with good accuracy.
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