Lisa Leroi, Vincent Gras, Nicolas Boulant, Mathilde Ripart, Emilie Poirion, Mathieu D. Santin, Romain Valabregue, Franck Mauconduit, Lucie Hertz-Pannier, Denis Le Bihan, Ludovic de Rochefort and Alexandre Vignaud, “Simultaneous PD, T1, T2 and flip angle mapping of the brain at 7T using multi-parametric 3D SSFP imaging and parallel transmission universal pulses.” in Magnetic Resonance in Medicine 2020
Performing simultaneous quantitative MRI at ultra-high field is challenging, as B0 and B1+ heterogeneities as well as Specific Absorption Rate increase. Too large deviations of flip angle from the target can induce biases and impair signal-to-noise ratio in the quantification process. In this work, we use calibration-free parallel transmission, a dedicated pulse sequence parameter optimization and signal fitting to recover 3D proton density, flip angle, T1 and T2 maps over the whole brain, in a clinically suitable time.
Eleven optimized contrasts were acquired with an unbalanced Steady-State Free Precession sequence by varying flip angle amplitude and radiofrequency phase cycling increment, at a 1.0×1.0x3.0mm3 resolution. Acquisition time was of 16min36sec for the whole brain. Parallel transmission and Universal Pulses were used to mitigate B1+ heterogeneity to improve the results’ reliability over six healthy volunteers (3 females/males, age 22.6±2.7 years-old). Quantification of the physical parameters was performed by fitting acquired contrasts to the simulated ones using the Bloch-Torrey equations with a realistic diffusion coefficient.
Whole-brain 3D maps of effective flip angle, PD and relaxation times were estimated. Parallel transmission improved the robustness of the results at 7T. Results were in accordance with literature and with measurements from standard methods.
These preliminary results show robust PD, FA, T1 and T2 map retrieval. Other parameters, such as Apparent Diffusion Coefficient, could be assessed. With further optimization in the acquisition, scan time could be reduced and spatial resolution increased to bring this multi-parametric quantification method to clinical research routine at 7-tesla.
Keywords : quantitative MRI, T1, T2, relaxometry, Parallel transmission, Universal Pulses
Partners : CEA NeuroSpin, CENIR (Center for NeuroImaging Research) – ICM (Institut du Cerveau et de la Moelle épinière), Inserm, CNRS, AMU, CRMBM