  Medical image analysis suffers from a shortage of data, whether annotated or
not. This becomes even more pronounced when it comes to 3D medical images.
Self-Supervised Learning (SSL) can partially ease this situation by using
unlabeled data. However, most existing SSL methods can only make use of data in
a single dimensionality (e.g. 2D or 3D), and are incapable of enlarging the
training dataset by using data with differing dimensionalities jointly. In this
paper, we propose a new cross-dimensional SSL framework based on a pseudo-3D
transformation (CDSSL-P3D), that can leverage both 2D and 3D data for joint
pre-training. Specifically, we introduce an image transformation based on the
im2col algorithm, which converts 2D images into a format consistent with 3D
data. This transformation enables seamless integration of 2D and 3D data, and
facilitates cross-dimensional self-supervised learning for 3D medical image
analysis. We run extensive experiments on 13 downstream tasks, including 2D and
3D classification and segmentation. The results indicate that our CDSSL-P3D
achieves superior performance, outperforming other advanced SSL methods.
