Alternating Look-Locker for quantitative T₁ , T_1ρ and B₁ 3D MRI mapping

Purpose: To develop a new MRI method, entitled alternating Look-Locker (aLL), for quantitative $T_1$ , $T_{1\rho }$ , and $B_1$ 3D mapping.

Methods: A Look-Locker scheme that alternates magnetization from +Z and -Z axes of the laboratory frame is utilized in combination with a 3D Multi-Band Sweep Imaging with Fourier Transformation (MB-SWIFT) readout. The analytical solution describing the spin evolution during aLL, as well as the correction required for segmented acquisition were derived. The simultaneous $B_1$ and $T_1$ mapping are demonstrated on an agar/saline phantom and on an in-vivo rat head. $T_{1\rho }$ relaxation was achieved by cyclically applying magnetization preparation (MP) modules consisting of two adiabatic pulses. $T_{1\rho }$ values in the rat brain in-vivo and in a gadobenate dimeglumine (Gd-DTPA) phantom were compared to those obtained with a previously introduced steady-state (SS) method.

Results: The accuracy and precision of the analytical solution was tested by Bloch simulations. With the application of MP modules, the aLL method provides simultaneous $T_1$ and $T_{1\rho }$ maps. Conversely, without it, the method can be used for simultaneous $T_1$ and $B_1$ mapping. $T_{1\rho }$ values were similar with both aLL and SS techniques. However, the aLL method resulted in more robust quantitative mapping compared to the SS method. Unlike the SS method, the aLL method does not require additional scans for generating $T_1$ maps.

Conclusion: The proposed method offers a new flexible tool for quantitative mapping of $T_1$ , $T_{1\rho }$ , and $B_1$ . The aLL method can also be used with readout schemes different from MB-SWIFT.