Room: AAPM ePoster Library
An RF pulse sequence is designed and implemented onto scanner to produce 3D sparse sampling chemical-shift-imaging (CSI) for hyperpolarized 13C metabolic MRI. The 3D CSI is reconstructed with a 4D compressed sensing algorithm.
The sparse sampling patterns are firstly generated with higher density around the center of the 3D k-space. The acquisition starts from the center of the k-space and roll out to the edge of the 3D k-space for optimum MR signal efficiency. The acquisition of k-space voxels is encoded according to the sparse pattern. A variable-flip-angle scheme is implemented to compensate the loss of high frequency signal. At reduction rate of 8 or 16 (R=8 or 16), a CSI matrix of 16x16x16 can be completed in 40 and 20 seconds respectively which are within the hyperpolarized MR signal time frame (typically T1=20s to 40s). The CSI reconstruction of the sparsely acquired data is accomplished with a 4D compressed sensing algorithm with adaptive stepping regularization weighting. The sequence is implemented on a 13C enabled GE MR750W scanner and tested with a phantom of four chambers filled with C1-labeled 13C metabolites lactate, alanine, formic acid and bicarbonate respectively.
The sequence has operated successfully on the scanner in real-time sparse acquisition with reduction rates R=2 to 32 (50% to 3.125% sampling). The reconstructed CSI is of high quality. At R=16 (6.25% sampling) the normalized root-of-mean-square-error nRMSE=2.34x10^-3 and the structural-similarity-index SSIM=0.977. Even at R=32, it produced an nRMSE=3.66x10^-3 and SSIM=0.949. Both acquisition time and image quality reached our goals and made the hyperpolarized 3D metabolic MR CSI possible.
The CSI with full-length Free-Induced-Decay (FID) acquisition can cover a wider spectrum band comparing to partial FID methods. Sparse phase encoding CSI acquisition combining with 4D compressed sensing is the practically possible method for hyperpolarized metabolic 3D MR CSI.
Data Acquisition, Reconstruction, Spectroscopic Imaging