Running head: Compressed Sensing MRI Address correspondence to:

The sparsity which is implicit in MR images is exploited to significantly undersample k-space. Some MR images such as angiograms are already sparse in the pixel representation; other, more complicated images have a sparse representation in some transform domain – for example, in terms of spatial finite-differences or their wavelet coefficients. According to the recently developed mathematical theory of Compressed-Sensing (CS), images with a sparse representation can be recovered from randomly undersampled k-space data, provided an appropriate nonlinear recovery scheme is used. Intuitively, artifacts due to random undersampling add as noise-like interference. In the sparse transform domain the significant coefficients stand out above the interference. A non-linear thresholding scheme can recover the sparse coefficients, effectively recovering the image itself. In this paper, practical incoherent undersampling schemes are developed and analyzed by means of their aliasing interference. Incoherence is introduced by pseudo-random variable-density undersampling of phase-encodes. The reconstruction is performed by minimizing the ℓ 1 norm of a transformed image, subject to data fidelity constraints. Examples demonstrate improved spatial resolution and accelerated acquisition for multi-slice fast spin-echo brain imaging and 3D contrast enhanced angiography