  In large studies involving multi protocol Magnetic Resonance Imaging (MRI),
it can occur to miss one or more sub-modalities for a given patient owing to
poor quality (e.g. imaging artifacts), failed acquisitions, or hallway
interrupted imaging examinations. In some cases, certain protocols are
unavailable due to limited scan time or to retrospectively harmonise the
imaging protocols of two independent studies. Missing image modalities pose a
challenge to segmentation frameworks as complementary information contributed
by the missing scans is then lost. In this paper, we propose a novel model,
Multi-modal Gaussian Process Prior Variational Autoencoder (MGP-VAE), to impute
one or more missing sub-modalities for a patient scan. MGP-VAE can leverage the
Gaussian Process (GP) prior on the Variational Autoencoder (VAE) to utilize the
subjects/patients and sub-modalities correlations. Instead of designing one
network for each possible subset of present sub-modalities or using frameworks
to mix feature maps, missing data can be generated from a single model based on
all the available samples. We show the applicability of MGP-VAE on brain tumor
segmentation where either, two, or three of four sub-modalities may be missing.
Our experiments against competitive segmentation baselines with missing
sub-modality on BraTS'19 dataset indicate the effectiveness of the MGP-VAE
model for segmentation tasks.
