  Intra-frame motion blurring, as a major challenge in free-breathing dynamic
MRI, can be reduced if high temporal resolution can be achieved. To address
this challenge, this work proposes a highly-accelerated 4D (3D+time) real-time
MRI framework with sub-second temporal resolution combining standard
stack-of-stars golden-angle radial sampling and tailored GRASP-Pro
(Golden-angle RAdial Sparse Parallel) reconstruction. Specifically, 4D
real-time MRI acquisition is performed continuously without motion gating or
sorting. The k-space centers in stack-of-stars radial data are organized to
guide estimation of a temporal basis, with which GRASP-Pro reconstruction is
employed to enforce joint low-rank subspace and sparsity constraints. This new
basis estimation strategy is the new feature proposed for subspace-based
reconstruction in this work to achieve high temporal resolution (e.g.,
sub-second/3D volume). It does not require sequence modification to acquire
additional navigation data, is compatible with commercially available
stack-of-stars sequences, and does not need an intermediate reconstruction
step. The proposed 4D real-time MRI approach was tested in abdominal motion
phantom, free-breathing abdominal MRI, and dynamic contrast-enhanced MRI
(DCE-MRI). With the ability to acquire each 3D image in less than one second,
intra-frame respiratory blurring can be intrinsically reduced for body
applications with our approach, which also eliminates the need for motion
detection and motion compensation.
