We use a state-of-the-art CADe method for detecting sclerotic metastases candidates from CT volumes [9, 17]. The spine is initially segmented by thresholding at certain attenuation levels and performing region growing. Furthermore, morphological operations are used to refine the segmentation and allow the extraction of the spinal canal. For further information on the segmentation refer to [18]. Axial 2D cross sections of the spinal vertebrae are then divided into sub-segments using a watershed algorithm based on local density differences [19]. The CADe algorithm then finds initial detections that have a higher mean attenuation then neighboring 2D sub-segments. Because the watershed algorithm can cause over-segmentation of the image, similar 2D sub-segments detections are merged by performing an energy minimization based on graph cuts and attenuation thresholds. Finally, 2D detections on neighboring cross sections are combined to form 3D detections using a graph-cut-based merger. Each 3D detection acts as a seed point for a level-set segmentation method that segments the lesions in 3D. This step allows the computation of 25 characteristic features, including shape, size, location, attenuation, volume, and sphericity. A committee of SVMs [20] is then trained on these features. The trained SVMs further classify each 3D detection as ‘true’ or ‘false’ bone lesion. Example of bone lesions candidates using this detection scheme are shown in Fig. 1. Next, true bone lesions from this step are used as candidate lesions for a second classification based on CNNs as proposed in this paper. This is a coarse-to-fine classification approach somewhat similar to other CADe schemes such as [16].

A Region-of-Interest (ROI) in a CT image is extracted at each bone lesion candidate location (see Fig. 2). In order to increase the variation of the training data and to avoid overfitting analogous to the data augmentation approach in [11], each ROI is translated along a random vector in axial space. Furthermore, each translated ROI is rotated around its center times by a random angle . These translations and rotations for each ROI are computed times at different physical scales (the edge length of each ROI), but with fixed numbers of pixels. This procedure results in random observation of each ROI—an approach similar to [21]. Note that 2.5–5 mm thick-sliced CT volumes are used for this study. Due to this relative large slice thickness, our spatial transformations are all drawn from within the axial plane. This is in contrast to other approaches that use CNNs which sample also sagittal and/or coronal planes [13, 14]. Following this procedure, both the training and test data can be easily expanded to better scale to this type of neural net application. A CNN’s predictions on these random observations can then be simply averaged at each ROI to compute a per-candidate probability:
Here, is the CNN’s classification probability computed one individual image patch. In theory, more sophisticated fusion rules can be explored but we find that simple averaging works well. This proposed random resampling is an approach to effectively and efficiently increase the amount of available training data. In computer vision, translational shifting and mirroring of 2D image patches is often used for this purpose [11]. By averaging the predictions on random 2D views as in Eq. 1, the robustness and stability of CNN can be further increased as shown in Sect. 3.