T1 Maps From Shifted Echoes and Stimulated Echoes

Theory The proposed sequence is shown in Figure 1. The basic idea is to obtain a spin echo (SE) and a stimulated echo (STE) with different T1 weighting for T1 calculation. In contrast to the FastPACE technique, both echoes were acquired separately. Then, only magnitude images are required which allows for exploiting the Half Fourier technique and using phased array coils. Acquisition of the true STE is not possible since it occurs at time TE/2. Data sampling is therefore shifted to the left resulting in a “stimulated gradient echo” (4) with additional T2* weighting. The acquisition of the SE has to be shifted in parallel to introduce identical T2* weighting. This new sequence layout offers several additional advantages: (i) No phase alternating is required which allows for halving the acquisition time. (ii) The second 90° RF pulse also acts as a reset pulse flipping the full STE back into the longitudinal direction. This effect is expected to gain higher precision particularly for higher T1 values. (iii) Setting the first RF pulse to 90° results in a higher signal intensity for SE and STE, which can, however, be offset by a stronger signal attenuation due to T2* decay and a limited acquisition bandwidth. Considering Mz as the sequence steady state magnetization, the signal equations for the particular echoes are given by: