Single-Breathhold 3D MR Elastography in the Liver, With Simultaneous R2* and PDFF Mapping

To develop a sequence for the rapid acquisition of MR elastography (MRE) parameters in 3D, with simultaneous measurement of proton-density fat fraction (PDFF) and R₂* for multiparametric assessment of liver disease.

The proposed sequence uses an interleaved motion-encoding scheme to acquire 3D volumes of all motion encodings and wave offsets from a single series of readouts with constant repetition time. This leads to a highly time-efficient acquisition, which together with incoherent k-space undersampling permits all MRE data to be acquired in a single breathhold. The undersampled volumes are reconstructed using compressed sensing with model-based temporal regularisation. The sequence can be combined with a multi-echo readout to allow 3D PDFF and R₂* maps to be reconstructed from the reference volume of the MRE acquisition. Validation was performed in phantoms and in eight healthy volunteers.

Phantom measurements of all viscoelastic parameters, PDFF, and R₂* corresponded closely with reference measurements. Average liver tissue parameter values acquired with the combined MRE, PDFF and R₂* sequence at 3 T were 1.36 $\pm$ 0.15 m/s for shear-wave speed, 1.78 $\pm$ 0.41 kPa for the magnitude of the complex shear modulus, 0.70 $\pm$ 0.16 kPa for the loss modulus, 3.6% $\pm$ 1.0% for PDFF, and 46.9 $\pm$ 7.3 s^-1 for R₂*.

Single-breathhold 3D MRE in the liver was achieved with a novel acquisition ordering. The sequence offers 3D elastograms from a 16-s breathhold, or 3D elastograms with co-registered PDFF and R₂* maps from a 21-s breathhold.