Faculty Bibliography

Recent studies have described some of the new opportunities that have arisen within the context of ultrafast two-dimensional imaging with the advent of spatiotemporal encoding methods. This article explores the potential of integrating these non-Fourier, single-scan, two-dimensional MRI principles, with multi-slice and phase-encoding schemes acting along a third dimension. In unison, these combinations enable the acquisition of complete three-dimensional images from volumes of interest within a 1-s timescale. A number of alternatives are explored for carrying out these very rapid three-dimensional acquisitions, including the use of two-dimensional, slice-selective, spatiotemporal encoding radiofrequency pulses, driven-equilibrium slice-selective schemes, and phase-encoded volumetric approaches. When tested under in vivo conditions, the 'hybrid' schemes combining spatiotemporal encoding with k-encoding imaging principles, proved to be superior to traditional schemes based on echo planar imaging. The resulting images were found to be less affected by field inhomogeneities and by other potential offset-derived distortions owing to a combination of factors whose origin is discussed. Further features, extensions and applications of these principles are also addressed.

Single-scan MRI underlies a wide variety of clinical and research activities, including functional and diffusion studies. Most common among these "ultrafast" MRI approaches is echo-planar imaging. Notwithstanding its proven success, echo-planar imaging still faces a number of limitations, particularly as a result of susceptibility heterogeneities and of chemical shift effects that can become acute at high fields. The present study explores a new approach for acquiring multidimensional MR images in a single scan, which possesses a higher built-in immunity to this kind of heterogeneity while retaining echo-planar imaging's temporal and spatial performances. This new protocol combines a novel approach to multidimensional spectroscopy, based on the spatial encoding of the spin interactions, with image reconstruction algorithms based on super-resolution principles. Single-scan two-dimensional MRI examples of the performance improvements provided by the resulting imaging protocol are illustrated using phantom-based and in vivo experiments.

An approach has been recently introduced for acquiring two-dimensional (2D) nuclear magnetic resonance images in a single scan, based on the spatial encoding of the spin interactions. This article explores the potential of integrating this spatial encoding together with conventional temporal encoding principles, to produce 2D single-shot images with moderate field of views. The resulting "hybrid" imaging scheme is shown to be superior to traditional schemes in non-homogeneous magnetic field environments. An enhancement of previously discussed pulse sequences is also proposed, whereby distortions affecting the image along the spatially encoded axis are eliminated. This new variant is also characterized by a refocusing of T(2)(*) effects, leading to a restoration of high-definition images for regions which would otherwise be highly dephased and thus not visible. These single-scan 2D images are characterized by improved signal-to-noise ratios and a genuine T(2) contrast, albeit not free from inhomogeneity distortions. Simple postprocessing algorithms relying on inhomogeneity phase maps of the imaged object can successfully remove most of these residual distortions. Initial results suggest that this acquisition scheme has the potential to overcome strong field inhomogeneities acting over extended acquisition durations, exceeding 100 ms for a single-shot image.