Isotropic three-dimensional cardiac cine imaging at 0.55T using stack-of-spiral sampling and four-dimensional...

Isotropic three-dimensional (3D) cine imaging is an attractive one-stop-shop acquisition for cardiac MRI, as it can be arbitrarily resliced for the assessment of cardiac function and simplifies imaging workflows. Current free-breathing 3D cine approaches are hampered by long reconstruction times, and at lower-field strengths, by relatively long acquisition times. Here, we aim to maximize acquisition efficiency at 0.55T pairing two techniques; using a spiral acquisition with an optimized sampling distribution and a reconstruction incorporating data from all respiratory phases.

We implemented a 2 mm isotropic 3D cine approach on a prototype 0.55T scanner, using a 6 min stack-of-spiral balanced steady-state free precession (bSSFP) acquisition modified to use tiny-golden-angle in-plane rotations and distribute the k_z partition samples to a variable-density. The data were reconstructed with a modified iterative motion compensation reconstruction which resolved cardiac motion (denoted 4D iMoCo) and combined respiratory states using a navigator signal extracted from the acquired data. The proposed technique was compared to reference 2D free-breathing Cartesian volumetry of the left ventricle in 11 human subjects.

The 4D iMoCo reconstruction required 20 min. The proposed variable-density sampling distribution reduced image artifacts, compared to a common linear sampling approach, and improved apparent signal-to-noise with relative increase of 221 ± 99%. Measurements had good agreement with the 2D Cartesian reference data with a left-ventricular volume bias of -2.5 ± 6.2% and 2.6 ± 10.4% in diastole and systole, respectively, and an ejection fraction bias of -3.5 ± 8.8%.

We demonstrate an efficient free-breathing technique to produce 2 mm isotropic 3D cardiac images within a 6 min acquisition time and 20 min reconstruction time at 0.55T. Such a method could be a valuable clinical tool for cardiac imaging.