Single-shot pulse sequences are a class of sequences that can rapidly traverse k-space in a single repetition time (TR), producing sufficient k-space data for image reconstruction. In this lecture, we will highlight four single-shot pulse sequences, including echo planar imaging (EPI), turbo-spin echo (TSE) or fast spin echo (FSE), gradient and spin echo (GRASE), and spiral. For each pulse sequence, we will describe the spin physics involved, discuss several “tricks” in practical implementation, present strategies to mitigate image artifacts, and provide examples in clinical and research applications.
Target Audience
Scientists and clinicians who are interested in understanding single-shot pulse sequences with rapid k-space traversal and efficient utilization of magnetization.Objectives
This lecture has four objectives: (a) to understand the principles governing single-shot pulse sequences; (b) to learn several practical strategies to implement these sequences, (c) to develop skills in mitigating image artifacts, and (d) to identify major clinical and research applications of single-shot pulse sequences.Scope of the Lecture
Increasing the temporal resolution has been a major motivation for developing MRI technologies. Since MRI signals are acquired in k-space, rapid k-space traversal has been one of the most effective and popular approaches to increase the temporal resolution. Single-shot pulse sequences are a class of sequences that rapidly traverse k-space in a single repetition time (TR), producing sufficient k-space data for image reconstruction (1,2). In this lecture, we will highlight four single-shot pulse sequences (4-6), including echo planar imaging (EPI), turbo-spin echo (TSE) or fast spin echo (FSE), gradient and spin echo (GRASE), and spiral. For each pulse sequence, we will describe the spin physics involved, discuss several “tricks” in practical implementation, and provide examples in clinical and research applications. In single-shot pulse sequences, the MRI signal following an excitation maximally extends its k-space coverage. This often requires rapid and frequent switching of spatially-encoding gradients, exacerbating eddy currents. In addition, single-shot pulse sequences are particularly sensitive to off-resonance effects arising from magnetic susceptibility variations, static B0-field inhomogeneity, or chemical shift. Moreover, signals in single-shot pulse sequences often experience substantial T2 or T2* decays, leading to image blurring. Together, these adverse effects make single-shot pulse sequences particularly vulnerable to many kinds of image artifacts. Examples of how to recognize and manage these artifacts will be discussed.1. Bernstein MA, King KF, Zhou XJ. Handbook of MRI Pulse Sequences. San Diego, California: Elsevier Academic Press; 2004.
2. Brown RW, Cheng Y-CN, Haacke EM, Thompson MR, Venkatesan R. Magnetic resonance imaging : physical properties and sequence design. New York: Wiley Publishing; 2014.
3. Mansfield P. Echo-Planar Imaging. In: Encyclopedia of Magnetic Resonance. Chichester, UK: John Wiley & Sons, Ltd; 2007.
4. Hennig J, Nauerth A, Friedburg H. RARE imaging: A fast imaging method for clinical MR. Magn. Reson. Med. 1986; 3: 823–833.
5. Oshio K, Feinberg, DA. GRASE (Gradient‐and Spin‐Echo) imaging: A novel fast MRI technique. Magn. Reson. Med. 1991; 20,344-349.
6. Ahn CB, Kim JH, Cho ZH. High-Speed Spiral-Scan Echo Planar NMR Imaging-I. IEEE Trans. Med. Imaging 1986; 5: 2-7.