  Compressed sensing has shown great potential in reducing data acquisition
time in magnetic resonance imaging (MRI). Recently, a spread spectrum
compressed sensing MRI method modulates an image with a quadratic phase. It
performs better than the conventional compressed sensing MRI with variable
density sampling, since the coherence between the sensing and sparsity bases
are reduced. However, spread spectrum in that method is implemented via a shim
coil which limits its modulation intensity and is not convenient to operate. In
this letter, we propose to apply chirp (linear frequency-swept) radio frequency
pulses to easily control the spread spectrum. To accelerate the image
reconstruction, an alternating direction algorithm is modified by exploiting
the complex orthogonality of the quadratic phase encoding. Reconstruction on
the acquired data demonstrates that more image features are preserved using the
proposed approach than those of conventional CS-MRI.
