Although MRI has been shown to allow a multifaceted assessment of the cardiovascular system, basic constraints on MR imaging speed have resulted in comparatively long examination times for a typical clinical cardiac study. In recent years, various imaging techniques combining compressed sensing and golden-angle sampling have been proposed for rapid and continuous acquisitions in cardiac MRI. Such an imaging strategy entails more than just acceleration of existing imaging protocols and it could eventually lead to a “one-stop-shop” comprehensive cardiovascular MR workflow. This talk aims to summarize recent advances in rapid and continuous cardiac MRI and also highlight future directions.
For many years, the notion of “one-stop-shop” comprehensive cardiovascular examination with a single noninvasive imaging modality has been the “Holy Grail” of cardiovascular imaging. MRI is a promising modality to achieve such a goal. It offers superior soft-tissue characterization and flexible contrast parameters without radiation exposure, allowing for functional, hemodynamic and metabolic information as well as anatomical images with high resolution for a comprehensive exam. However, despite remarkable and diverse advances in the technology and methodology of MRI, its application for cardiovascular exam still remains relatively limited. Major challenges of cardiovascular MRI include: i) motion of the heart during the cardiac and respiratory cycles, ii) the complexity of cardiovascular anatomy, and iii) limitations of imaging speed. These factors have led to a complex and lengthy standardized clinical cardiac imaging workflow, where a sequential series of 2D imaging acquisitions are employed during multiple breath-holds. These acquisitions must be individually tailored and adjusted, which are highly dependent on operator experience and patients’ ability to hold their breath. This imaging workflow already represents decades of optimization, with complexity of clinical cardiac MR slowing or even obviating its uptake in many centers.
The development of fast imaging approaches, such as parallel imaging (1-3) and its different variants (4,5), have motivated a large body of research towards rapid and comprehensive cardiac MR examination. For example, previous studies have demonstrated the feasibility of a few-minute comprehensive cardiac MR examination with a short series of simple, breath-held and whole-heart acquisitions using parallel imaging (6). However, parallel imaging alone is not enough to ensure optimal imaging performance, as the acceleration is fundamentally limited by noise amplification in the reconstruction. Compressed sensing (7-9) is another powerful approach for rapid cardiac MRI; and appropriate combinations with parallel imaging have demonstrated further increase in imaging speed (10,11). Moreover, although tailored and punctuated acquisitions are often employed in conventional imaging setting, a non-repeating acquisition is more preferred in the era of sparsity for compressed sensing MRI, where increased dimensionality in the resulting datasets tends to provide increased correlations and sampling-pattern incoherence that can be exploited in image reconstruction to enable better performance. This fact has actually suggested a hint and tendency for a simple rapid, continuous and comprehensive imaging paradigm for MRI that can be achieved using a combination of compressed sensing, parallel imaging and the so-called golden-angle sampling (12). Such an imaging strategy entails more than just acceleration of existing imaging protocols and it could represent a shift of the day-to-day clinical workflow from conventional time-consuming and tailored acquisitions of carefully targeted slabs towards rapid, continuous and comprehensive volumetric acquisitions, with user-defined reconstructions that can be adapted retrospectively for different clinical needs.
In recent years, many research groups have proposed various novel rapid and continuous cardiovascular MRI techniques using different compressed sensing techniques and golden-angle sampling schemes. Examples of these studies range from 2D real-time cine imaging (13,14) and perfusion imaging (15,16) to higher dimensional (3D, 4D, 5D and up) cine imaging (17-19) and flow imaging (20). Moreover, many studies have also proposed techniques that enable simultaneous assessment of cardiac function and the cardiovascular anatomy (21-23). These advances have the potential to eventually lead to a new multifaceted cardiac MRI workflow, in which diverse information can stream in constantly and clinically useful information can be retrospectively captured from the continuously acquired dataset. This will not only provide new information to address numerous clinical applications, but it will also change the entire experience of MR imaging both for physicians and for patients.
This talk aims to describe the key components, including various golden-angle sampling schemes and compressed sensing techniques used for rapid and continuous MRI, and will also summarize their applications for various cardiovascular MR applications.
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