Over the past decade, I have been the main driver for the development, optimization and extension of a rapid MRI technique called GRASP (Golden-angle RAdial Sparse Parallel imaging, see above the roadmap of this project). In 2010, my colleagues Drs. Hersh Chandarana and Tobias Block demonstrated the clinical use of stack-of-stars sampling for free-breathing 3D MRI. The stack-of-stars trajectory was relatively new at that time and the so-called golden-angle sampling was not widely used yet. Since then, this trajectory has attracted substantial attention and interest in the MRI research community and has been implemented by different vendors now. I started involving in the project since 2011 and developed the GRASP technique by combining compressed sensing, parallel imaging and golden-angle stack-of-stars sampling. GRASP introduces a new and simple way of performing dynamic MRI. It enables free-breathing continuous data acquisition and employs sparse reconstruction that allows a flexible choice of temporal information, i.e., different reconstructions with user-defined parameters, such as temporal resolution and the number and position of temporal frames.
Like many iterative reconstruction techniques, the reconstruction time of GRASP is long and it is not practical to directly integrate into clinical MRI scanners. The development of the Yarra framework by Dr. Tobias Block was a milestone in the project. Yarra is a platform that can translate offline iterative reconstruction approaches into clinical MRI scanners in a smart way (see http://yarraframework.com). Since then, GRASP has been used in routine clinical MRI exams and has been evaluated for a wide range of clinical studies (see Feng L et al. JMRI 2017 Apr;45(4):966-987 for some examples). To date, GRASP MRI has been applied in >100,000 clinical patients.
I have led the extension of the GRASP framework to a number of improved variants. These include XD-GRASP (GRASP with eXtra Dimensions) to improve motion management, Koosh-ball-GRASP to combine GRASP with 3D golden-angle radial trajectories, variable-density stack-of-stars to improve the distribution of radial spokes in three dimensions, Unstreaking-GRASP to remove undersampling-induced residual streaking artifacts, RACER-GRASP (Respiratory-weighed and contrast enhancement-guide GRASP) to perform motion-weighted and dynamic contrast enhancement-guided reconstruction for capturing the most desired contrast phases, GROG-GRASP to improve reconstruction speed using the self-calibrating GRAPPA operator gridding (GROG) algorithm (see MRM 2008 Apr;59(4):930-5 for more details about GROG), and UTE-GRASP to combine GRASP with 3D golden-angle ultra-short TE radial sampling. These new developments have led to several novel applications, such as the 5D whole-heart sparse MRI, respiratory-resolved 4D whole-heart coronary MRA and 4D sparse lung MRI. In addition, stack-of-stars sampling has been extended to multi-echo acquisition (Dixon-RAVE: Dixon RAdial Volumetric Encoding) for free-breathing fat/water separation by my colleagues Dr. Tobias Block and Thomas Benkert. Dixon-RAVE has also been combined with the XD reconstruction strategy and has been tested for DCE-MRI.
During the past a couple of years, GRASP has been extended for further innovations. It has been extended to GRASP-Pro and XD-GRASP-Pro, which, as can be seen from the names, aim to boost the reconstruction performance of the original GRASP and XD-GRASP reconstruction. Recently, magnetization-prepared GRASP (MP-GRASP) and MP-Dixon-GRASP have been developed for rapid free-breathing 3D T1 mapping. We have also shown that MP-Dixon-GRASP enables fat-water separated T1 mapping to remove the influence of fat (and potentially iron as well).
The GRASP project represents a decade of innovation by our team consisting of imaging scientists, clinicians and our industry partners. The GRASP paper was announced as the third most-cited MRM paper at the 2017 ISMRM annual meeting, and the XD-GRASP paper was announced as the top most-cited MRM paper at the 2019 ISMRM annual meeting. With the rise of Artificial Intelligence in recent years, we are now aiming to integrate GRASP MRI with deep learning approaches to enable further improvement in reconstruction quality and speed, as well as new uses of this imaging framework. The initial feasibility of deep-learning-enabled golden-angle radial MRI has been demonstrated by us with a technique called SANTIS (see Deep Learning for Rapid MRI), and we are in the process of developing new quantitative imaging methods based on a combination of GRASP with deep learning.