This paper aims to solve a fundamental problem in intensity-
based 2D/3D registration, which concerns the limited capture range and
need for very good initialization of state-of-the-art image registration
methods. We propose a regression approach that learns to predict rota-
tion and translations of arbitrary 2D image slices from 3D volumes, with
respect to a learned canonical atlas co-ordinate system. To this end,
we utilize Convolutional Neural Networks (CNNs) to learn the highly
complex regression function that maps 2D image slices into their cor-
rect position and orientation in 3D space. Our approach is attractive
in challenging imaging scenarios, where significant subject motion com-
plicates reconstruction performance of 3D volumes from 2D slice data.
We extensively evaluate the effectiveness of our approach quantitatively
on simulated MRI brain data with extreme random motion. We further
demonstrate qualitative results on fetal MRI where our method is in-
tegrated into a full reconstruction and motion compensation pipeline.
With our CNN regression approach we obtain an average prediction er-
ror of 7mm on simulated data, and convincing reconstruction quality of
images of very young fetuses where previous methods fail. We further
discuss applications to Computed Tomography (CT) and X-Ray pro-
jections. Our approach is a general solution to the 2D/3D initialization
problem. It is computationally efficient, with prediction times per slice
of a few milliseconds, making it suitable for real-time scenarios.