Pure balanced steady‐state free precession imaging (pure bSSFP)

Abstract

Purpose To show that for tissues the conspicuous asymmetries in the frequency response function of bSSFP can be mitigated by using a short enough TR. Theory and Methods Configuration theory indicates that bSSFP becomes apparently “pure” (i.e., exhibiting a symmetric profile) in the limit of TR →0. To this end, the frequency profile of bSSFP was measured as a function of the TR using a manganese‐doped aqueous probe, as well as brain tissue that was shown to exhibit a pronounced asymmetry due to its microstructure. The frequency response function was sampled using N=72 (phantom) and N=36 (in vivo) equally distributed linear RF phase increments in the interval [0,2π). Imaging was performed with 2.0 mm isotropic resolution over a TR range of 1.5–8 ms at 3 and 1.5 T. Results As expected, pure substances showed a symmetric TR‐independent frequency profile, whereas brain tissue revealed a pronounced asymmetry. The observed asymmetry for the tissue, however, decreases with decreasing TR and gives strong evidence that the frequency response function of bSSFP becomes symmetric in the limit of TR →0, in agreement with theory. The limit of apparently pure bSSFP imaging can thus be achieved for a TR ∼ 1.5 ms at 1.5 T, whereas at 3 T, tissues still show some residual asymmetry. Conclusion In the limit of short enough TR, tissues become apparently pure for bSSFP. This limit can be reached for brain tissue at 1.5 T with TR ∼ 1–2 ms at clinically relevant resolutions.

Getting the phase consistent: The importance of phase description in balanced steady‐state free precession MRI of multi‐compartment systems

“…In summary, working with a consistent phase description ($$$ \sigma =0 $$$) is crucial to correctly describe multi‐component and multi‐compartment bSSFP profiles, specifically to avoid possible systematic errors in the analysis of bSSFP profile asymmetries for example at higher field strengths, 26 for T 1 and T 2 quantification in gray matter, white matter, and in muscles, 9,11,20,21 fat‐water separation techniques 4,5,36 and fat quantification techniques using phase‐cycled bSSFP 14…”

High‐resolution neural network‐driven mapping of multiple diffusion metrics leveraging asymmetries in the balanced steady‐state free precession frequency profile

“…At lower field strengths (≤1.5 T), the bSSFP profile is reported to become approximately symmetric for sufficiently short TR. 56 However, here the aim was to use the bSSFP profile asymmetry as a contrast mechanism for multiparametric mapping and to explore its dependencies on metrics of interest like the WM fiber orientation (ϴ) with respect to B 0 , or the FA.…”

“…It is likely that the NN incorporates several other valuable features, which shape the characteristic bSSFP profiles, into its learning process, such as T 1 and T 2 relaxation times, proton density, and off‐resonances related to macroscopic B 0 field variations. At lower field strengths (≤1.5 T), the bSSFP profile is reported to become approximately symmetric for sufficiently short TR 56 . However, here the aim was to use the bSSFP profile asymmetry as a contrast mechanism for multiparametric mapping and to explore its dependencies on metrics of interest like the WM fiber orientation (ϴ) with respect to B 0 , or the FA.…”

SPARCQ: A new approach for fat fraction mapping using asymmetries in the phase‐cycled balanced SSFP signal profile

“…34 Such asymmetries were observed in gray matter, white matter, and muscle, [34][35][36] were stronger at high magnetic fields, 37 and can be avoided by reducing TR. 38 Profile asymmetries were used to obtain diffusion metrics, 39 and recent work suggests an interplay between the contributions of microstructural, chemical shift, and chemical exchange effects to the asymmetry of the bSSFP profile. 40 Although phase-cycled bSSFP was used for water-fat signal separation, [41][42][43][44][45] its potential to quantify fat by analyzing profile asymmetries has not been investigated.…”

“…Measurement of the bSSFP frequency response via phase‐cycling, that is phase‐cycled bSSFP, revealed profile asymmetries that indicated the presence of multiple tissue components (e.g., different types of tissue) within voxels 34 . Such asymmetries were observed in gray matter, white matter, and muscle, 34–36 were stronger at high magnetic fields, 37 and can be avoided by reducing TR 38 . Profile asymmetries were used to obtain diffusion metrics, 39 and recent work suggests an interplay between the contributions of microstructural, chemical shift, and chemical exchange effects to the asymmetry of the bSSFP profile 40 .…”