  It can be challenging to identify brain MRI anomalies using supervised
deep-learning techniques due to anatomical heterogeneity and the requirement
for pixel-level labeling. Unsupervised anomaly detection approaches provide an
alternative solution by relying only on sample-level labels of healthy brains
to generate a desired representation to identify abnormalities at the pixel
level. Although, generative models are crucial for generating such anatomically
consistent representations of healthy brains, accurately generating the
intricate anatomy of the human brain remains a challenge. In this study, we
present a method called masked-DDPM (mDPPM), which introduces masking-based
regularization to reframe the generation task of diffusion models.
Specifically, we introduce Masked Image Modeling (MIM) and Masked Frequency
Modeling (MFM) in our self-supervised approach that enables models to learn
visual representations from unlabeled data. To the best of our knowledge, this
is the first attempt to apply MFM in DPPM models for medical applications. We
evaluate our approach on datasets containing tumors and numerous sclerosis
lesions and exhibit the superior performance of our unsupervised method as
compared to the existing fully/weakly supervised baselines. Code is available
at https://github.com/hasan1292/mDDPM.
