SDLFormer: A Sparse and Dense Locality-enhanced Transformer for
  Accelerated MR Image Reconstruction
Rahul G. S. Sriprabha Ramnarayanan Mohammad Al Fahim Keerthi Ram Preejith S. P Mohanasankar Sivaprakasam
Transformers have emerged as viable alternatives to convolutional neural
networks owing to their ability to learn non-local region relationships in the
spatial domain. The self-attention mechanism of the transformer enables
transformers to capture long-range dependencies in the images, which might be
desirable for accelerated MRI image reconstruction as the effect of
undersampling is non-local in the image domain. Despite its computational
efficiency, the window-based transformers suffer from restricted receptive
fields as the dependencies are limited to within the scope of the image
windows. We propose a window-based transformer network that integrates dilated
attention mechanism and convolution for accelerated MRI image reconstruction.
The proposed network consists of dilated and dense neighborhood attention
transformers to enhance the distant neighborhood pixel relationship and
introduce depth-wise convolutions within the transformer module to learn
low-level translation invariant features for accelerated MRI image
reconstruction. The proposed model is trained in a self-supervised manner. We
perform extensive experiments for multi-coil MRI acceleration for coronal PD,
coronal PDFS and axial T2 contrasts with 4x and 5x under-sampling in
self-supervised learning based on k-space splitting. We compare our method
against other reconstruction architectures and the parallel domain
self-supervised learning baseline. Results show that the proposed model
exhibits improvement margins of (i) around 1.40 dB in PSNR and around 0.028 in
SSIM on average over other architectures (ii) around 1.44 dB in PSNR and around
0.029 in SSIM over parallel domain self-supervised learning. The code is
available at https://github.com/rahul-gs-16/sdlformer.git