RASER: A new ultrafast magnetic resonance imaging method

Abstract

A new MRI method is described to acquire a T(2)-weighted image from a single slice in a single shot. The technique is based on rapid acquisition by sequential excitation and refocusing (RASER). RASER avoids relaxation-related blurring because the magnetization is sequentially refocused in a manner that effectively creates a series of spin echoes with a constant echo time. RASER uses the quadratic phase produced by a frequency-swept chirp pulse to time-encode one dimension of the image. In another implementation the pulse can be used to excite multiple slices with phase-encoding and frequency-encoding in the other two dimensions. The RASER imaging sequence is presented along with single-shot and multislice images, and is compared to conventional spin-echo and echo-planar imaging sequences. A theoretical and empirical analysis of the spatial resolution is presented, and factors in choosing the spatial resolution for different applications are discussed. RASER produces high-quality single-shot images that are expected to be advantageous for a wide range of applications.

Super‐resolved spatially encoded single‐scan 2D MRI

“…It follows that spatially encoded methods do not require FT processing for delivering their imaging information: the signal's magnitude, |S(t)|, is the image being sought. We and others have discussed elsewhere how this property, which in turn is closely linked to experiments put forward by Kunz and Pipe decades ago (17-19), allows spatially encoded MRI to cope efficiently with field inhomogeneities and to deal simultaneously with multiple sites possessing different chemical shifts (14)(15)(16). These advantages, however, were also found to materialize at a decreased efficiency in terms of the spatial encoding's use of the acquisition time variable, which usually results in spatial resolution penalties.…”

“…In the present study, we relied on a straightforward ''hybrid'' 2D implementation of the single-scan acquisition (13,15,16), whereby spatial encoding and conventional k-encoding act along orthogonal y and x axes, respectively (Fig. 1e).…”

“…Central in the development of these new experiments is the spatiotemporal manipulation of the spin interactions, a concept that originated from a search for methods capable of delivering arbitrary multidimensional MR spectra in a single scan (10,11). The generality of the ensuing approach eventually led to several new routes for executing single-scan multidimensional MRI (12)(13)(14)(15)(16). Contrary to EPI, these new MRI methods were found to be local in nature in the sense that, at each instant, the spins' signal S(t) becomes proportional to the density profile rðrÞ within a limited region of the volume of interest.…”

In vivo single-shot 13C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding

“…Although further optimizations could yield improved spectral and spatial resolution as well as added sensitivity, the current results already show that SPEN-based sequences can be utilized as a modality for fast dynamic imaging. Further developments are in progress to exploit additional benefits of spatiotemporal encoding, such as a built-in restricted FOV capability [24,26], multi-echo experiments and parallel imaging acquisition [34]. Representative metabolic images from a rat following injection of hyperpolarized [1][2][3][4][5][6][7][8][9][10][11][12][13] …”

“…First introduced for the acquisition of 2D NMR spectra in a single scan [19,20], spatiotemporal encoding (SPEN) utilizes a chirped pulse combined with a gradient for a sequential manipulation of the spins that can directly monitor their density in real space [21,22]. In an imaging context, SPEN uses such a manipulation along the low-bandwidth domain in combination with a conventional k-space readout, leading to a so-called hybrid acquisition mode [23,24]. Hybrid SPEN has a higher robustness against magnetic field inhomogeneities than EPI, reflecting the larger bandwidths it can access for the spatially sequential excitation and detection.…”