Design of Gradient-Echo Sequences
Rahel Heule1,2,3
1Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany, 2High Field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 3Center of MR Research, University Children's Hospital Zurich, Zurich, Switzerland
1Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany, 2High Field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 3Center of MR Research, University Children's Hospital Zurich, Zurich, Switzerland
Synopsis
Signal generation in magnetic resonance imaging is driven by either of two fundamental mechanisms: spin echoes or gradient echoes. This lecture introduces the basic concepts and properties of gradient echo formation in comparison to spin-echo imaging. The influence of acquisition parameters on the produced contrast is described and typical artifacts are discussed. Special focus is on the design of rapid gradient-echo sequences based on the principle of steady-state free precession (SSFP), including three main classes: radiofrequency-spoiled gradient-echo, nonbalanced SSFP, and balanced SSFP imaging.
Target audience
Researchers and clinicians interested in the design of rapid gradient-echo MR sequences.Outline of educational talk
Gradient echo sequences with short repetition times are well suited for volumetric 3D acquisitions and other applications, which necessitate rapid data sampling. In steady-state free precession (SSFP) imaging, steady states for the transverse and/or the longitudinal magnetization will be established after a certain number of consecutive radiofrequency pulses (cf. Fig. 1). Overall, SSFP sequences can broadly be classified as follows:· Incoherent (spoiled): The transverse magnetization components are disrupted (spoiled) prior to the next RF pulse.
· Coherent: The transverse coherences are preserved.
The second class can further be subdivided into nonbalanced and balanced SSFP sequences. The term ‘nonbalanced’ is used to indicate that in this type of sequence, dephasing gradients are present, i.e. the net total gradient moment along at least one axis is not zero. The term ‘balanced’ indicates that the gradient moments are fully rephased within TR along all axes.
Acknowledgements
No acknowledgement found.References
No reference found.Figures
Figure 1. Illustration of the transverse steady state in short-repetition time coherent
gradient-echo sequences (TR ≤ T2). Just after each RF pulse, a free induction decay (FID)-like
signal forms (SSFP-FID). Just before each RF pulse, a time-reversed FID signal
occurs (SSFP-Echo).