Robust motion correction in CEST imaging exploiting low-rank approximation of the z-spectrum.

PURPOSE

To introduce and evaluate an image registration technique for robust quantification of CEST acquisitions corrupted by motion.

METHODS

The proposed iterative algorithm exploits a low-rank approximation of the z-spectrum (LRAZ), to gradually separate the contrast variation due to saturation at different off-resonance frequencies and accompanying motion. This registration method was first tested in a creatine CEST analysis of a phantom with simulated rigid motion. Subsequently, creatine CEST acquisitions in the human thigh during exercise were exemplarily corrected.

RESULTS

The z-spectrum obtained by applying LRAZ to the corrupted phantom series exhibited a normalized RMS error with respect to the noncorrupted gold standard series of less than 4%. The corresponding creatine map resulting from an asymmetry analysis of the registered data showed only little difference with regard to the noncorrupted determination, too. A comparable performance was observed exploiting LRAZ for the correction of nonrigid motion within the dynamic CEST acquisitions in skeletal muscles. While for the phantom simulations, high-quality registration was also possible by using a single reference image for the whole series and mutual information as similarity metric, this conventional approach resulted in inappropriate correction of the more complicated motion of the human thigh.

CONCLUSION

The newly introduced method allows for a robust registration of CEST image series, which are corrupted by rigid and nonrigid motion of the investigated organ. The technique therefore improves the diagnostic value in various applications of CEST.