Constrained Ellipse Fitting for Efficient Parameter Mapping With Phase-Cycled bSSFP MRI

Abstract

Balanced steady-state free precession (bSSFP) imaging enables high scan efficiency in MRI, but differs from conventional sequences in terms of elevated sensitivity to main field inhomogeneity and nonstandard T 2 /T 1 -weighted tissue contrast. To address these limitations, multiple bSSFP images of the same anatomy are commonly acquired with a set of different RF phase-cycling increments. Joint processing of phasecycled acquisitions serves to mitigate sensitivity to field inhomogeneity. Recently phase-cycled bSSFP acquisitions were also leveraged to estimate relaxation parameters based on explicit signal models. While effective, these model-based methods often involve a large number of acquisitions (N ≈ 10-16), degrading scan efficiency. Here, we propose a new constrained ellipse fitting method (CELF) for parameter estimation with improved efficiency and accuracy in phase-cycled bSSFP MRI. CELF is based on the elliptical signal model framework for complex bSSFP signals; and it introduces geometrical constraints on ellipse properties to improve estimation efficiency, and dictionarybased identification to improve estimation accuracy. CELF generates maps of T 1 , T 2 , off-resonance and on-resonant bSSFP signal by employing a separate B 1 map to mitigate sensitivity to flip angle variations. Our results indicate that CELF can produce accurate off-resonance and bandingfree bSSFP maps with as few as N = 4 acquisitions, while estimation accuracy for relaxation parameters is notably limited by biases from microstructural sensitivity of bSSFP imaging.

Approximate B1+ scaling of the SSFP steady state

“…(The supplied code allows to investigate such cases.) Even the full bSSFP profile $$$ {m}_{\mathrm{bal}}\left(\vartheta \right) $$$, which can be estimated from phase cycled measurements 10,11 with variable $$$ {\vartheta}_j $$$, cannot resolve the issue: Due to equations (), () and (), $$$ {m}_{\mathrm{bal}} $$$ is fully determined by a small set of configurations $$$ {m}^{(n)} $$$, for example $$$ n\in \left\{-1,0,1\right\} $$$ 5 . The most obvious remaining option is therefore to include acquisitions with larger flip angles, but this increases the risk of systematic errors, caused by MT‐related signal loss 12,13 …”

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

“…Therefore, the resulting acquisition time of 20 min is significantly higher compared to the ME‐GRE acquisition of 3 min. Studies, similar as those performed in single compartments for quantitative mapping with PC‐bSSFP, 54 will need to be performed in order to define the minimum required phase‐cycled scans for water‐fat‐quantification. Furthermore, the effect of changing TR was not investigated.…”

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

“…14 The complex signal as a function of the RF phase increment is usually known as a "bSSFP profile." For banding artifact removal techniques such as geometric solution, 8 as well as for multi-parameter quantification, [9][10][11][12][13] the signal phase of the bSSFP profile is a crucial source of information. [8][9][10] In addition to previous studies on water-fat fraction mapping, 14 most quantitative mapping methods using bSSFP profiles [9][10][11][12][13] typically assume that a voxel has a single compartment with a distinct resonance frequency.…”

“…Balanced steady‐state free precession (bSSFP) MRI sequences provide images with a high SNR and T 2 / T 1 contrast 1 . Phase‐cycled bSSFP, which refers to multiple MRI acquisitions with different linear phase increments of the RF excitation pulse, can be used for banding artifact removal, 2–8 the quantification of the T 1 and T 2 relaxation times, 9–13 and the quantification of fat fraction within a voxel 14 . The complex signal as a function of the RF phase increment is usually known as a “bSSFP profile.” For banding artifact removal techniques such as geometric solution, 8 as well as for multi‐parameter quantification, 9–13 the signal phase of the bSSFP profile is a crucial source of information 8–10 …”

“…Balanced steady-state free precession (bSSFP) MRI sequences provide images with a high SNR and T 2 /T 1 contrast. 1 Phase-cycled bSSFP, which refers to multiple MRI acquisitions with different linear phase increments of the RF excitation pulse, can be used for banding artifact removal, [2][3][4][5][6][7][8] the quantification of the T 1 and T 2 relaxation times, [9][10][11][12][13] and the quantification of fat fraction within a voxel. 14 The complex signal as a function of the RF phase increment is usually known as a "bSSFP profile."…”