Multi-Parametric Artificial Neural Network Fitting of Phase-Cycled Balanced SSFP Data
Multi-parametric mapping techniques are an important tool in quantitative brain MRI to enable characterization of tissues and early diagnosis of pathologies within short acquisition times. Longitudinal and transverse relaxation times have proved to be a useful measure for monitoring brain tumors or neurodegenerative diseases, such as multiple sclerosi, Parkinson’s disease, or Alzheimer’s disease. Joint approaches, which allow quantifying multiple tissue parameters simultaneously based on the same acquisition scheme rather than performing sequential experiments for each, are particularly beneficial since they provide intrinsically co-registered maps.
In recent years, the concept of magnetic resonance fingerprinting (MRF) has become popular for simultaneous multi-parametric tissue characterization by generating signal evolutions, or fingerprints, thought to be unique for specific tissue types. Other novel approaches for combined T1 and T2 quantification utilize the mixed sensitivity to the longitudinal and transverse relaxation times of rapid non-balanced or balanced steady-state free precession (SSFP) sequences. Multi-pathway non-balanced SSFP imaging allows acquiring several steady-state configurations including higher order modes in a single scan, suited for simultaneous multi-parametric mapping. Phase-cycled balanced SSFP (bSSFP) acquisitions enable to sample the characteristic tissue-specific frequency profile, from which estimates for T1 and T2 can be derived.
The current phase-cycled bSSFP relaxometry techniques such as motion-insensitive rapid configuration relaxometry termed MIRACLE or the ellipse fitting approach termed PLANET are subject to a substantial mismatch between the estimated relaxation times of human brain tissues and measured gold standard reference values. This bias is especially prominent in white matter and likely linked to an asymmetric intra-voxel frequency content due to fiber tract geometry, in combination with multi-component relaxation, e.g. due to the presence of myelin. It has been reported that an asymmetric frequency distribution causes asymmetries in the bSSFP frequency profile. MIRACLE and PLANET fail to account for these anisotropies in the tissue microenvironment, but assume an idealized model for the bSSFP profile, i.e., a symmetric frequency response.
To eliminate this bias, in this work, a feedforward artificial neural network (ANN) is trained with phase-cycled bSSFP data acquired at 3 T in healthy volunteers as input and a multi-parametric output, i.e., T1, T2, transmit field, and off-resonance, using gold standard relaxation time measurements and reference field maps as target.
Correction of the cable-induced magnetic stray fields for SSFP-FID measurements (TR= 120 ms, N = 24) with multi-gradient-echo readouts in four subjects (no tissue current). The experiments were repeated twice, and the figure shows the results of the first experimental run. (a) Magnitude images. (b) Uncorrected magnetic field images showing the stray field generated by the current flow in the wire loop around the head. (c) Corrected magnetic field images, in which the stray field was calculated based on the reconstructed wire path and subtracted from the measured magnetic field. (d) magnetic field images of the control measurements performed without current injection.