  PURPOSE To develop free-running multi-echo GRE for cardiac- and
respiratory-motion-resolved whole-heart fat fraction quantification. METHODS
Multi-echo readouts optimized for water-fat separation and quantification were
integrated within a non-ECG-triggered free-breathing 3D radial GRE acquisition.
Pilot Tone navigation was used to extract cardiac and respiratory motion
states. Following a XD-GRASP based image reconstruction of the separate echoes,
fat fraction, water fraction, R2star and B0 maps, as well as fat and water
images, were generated with a maximum likelihood fitting algorithm using graph
cuts. The acquisition, reconstruction and post-processing framework was tested
in 10 healthy volunteers at 1.5T and compared to a free-breathing ECG-triggered
5-echo acquisition. RESULTS The acquisition was successfully validated in vivo,
with motion compensation achieved over all collected echoes, in both
respiratory and cardiac dimensions. Pilot Tone navigation provided respiratory
and cardiac signals in good agreement (r=0.954 and r=0.783, respectively) with
self-gating signal extraction based on the MR data of the first echo. The
framework enabled pericardial fat imaging and quantification across the cardiac
cycle, revealing a decrease in apparent FF at systole across volunteers. 3D
motion-resolved fat fraction maps showed good correlation with reference
ECG-triggered measurements, as well as significant difference in measurements
performed with NTE = 4 and NTE = 8 echoes (P < 0.0001 in chest fat and P < 0.01
in pericardial fat). CONCLUSION Volunteer experiments at 1.5T demonstrated the
feasibility of a whole-heart free-running fat-fraction mapping technique for
cardiac MRI in a 6 min scan time, with a resolution of 2mm3 isotropic.
