Simultaneous T2 and T2' mapping is highly desirable in applications investigating changes in blood oxygenation, iron content and bone mineral density. Simultaneous T2 and T2' mapping has been traditionally achieved using hybrid gradient echo/spin echo sequences sampling the descending and ascending portions of a spin echo with a train of gradient echoes [1-3]. However, such techniques can be sensitive to the performance of the employed refocusing pulses in the presence of B1 inhomogeneities [3]. Gradient echo imaging using adiabatic T2 preparation has enabled T2 mapping in the presence of inhomogeneous B1 fields [4]. In addition, both water and fat components can be present in many body tissues,which has to be considered in the extraction of T2 and T2' parameters [5]. The simultaneous quantification of the proton-density fat fraction (PDFF) can be also of particular interest (e.g. in the liver and in bone marrow). Multi-echo gradient echo imaging can separate water and fat components and quantify PDFF. Therefore, the purpose of the present work was to introduce a novel method for simultaneous T2, T2' and PDFF mapping, relying on an adiabatic T2-preparation combined with a time-interleaved multi-echo gradient echo acquisition (TIMGRE).

Pulse sequence:

A magnetization preparation module was played out, consisting of a saturation pulse, a delay and a dynamic adiabatic T2-preparation (T2prep) (Figure 1). The adiabatic T2prep was based on a BIR4 pulse extended by ΔT2prep / 2 between the pulse segments to dynamically increase T2-weighting. Furthermore, the effective flip angle of the BIR4 module could be adjusted by setting the phase jump $$$\Delta\Phi_\alpha$$$ of the second segment, e.g. to π or 3/2 π to achieve a flip angle of 0 or 180 degrees, respectively. Each T2prep time was acquired twice, once with an effective flip angle of 0 and once with 180 degrees in order compensate for T1 recovery effects before the readout [7]. The consecutive readout was based on a 3D TIMGRE acquisition scheme [6], while the flip angle was kept constant.

Signal model:

The standard water-fat signal model, including $$$M$$$ fat peaks, a field map $$$f_B$$$ and a common T2* for water and fat [9], was extended by the T2 of water ($$$T_{2w}$$$) and fat ($$$T_{2f}$$$) and the common $$$T’_2$$$. With $$$t_i$$$ referring to the gradient echo time accounting for T2* relaxation during the TIMGRE readout and $$$TE_j$$$ referring to the T2prep weighting time, the signal is given by:

$$S\left(t_i,TE_j\right)=\left[\rho_wexp\left(-\frac{TE_j+t_i}{T_{2w}}\right)+\rho_fexp\left(-\frac{TE_j+t_i}{T_{2f}}\right)\sum_{m=1}^{M}\alpha_m{}exp\left(i2\pi\Delta{}f_mt_i\right)\right]exp\left(i2\pi{}f_Bt_i\right)exp\left(-\frac{t_i}{T'_2}\right)$$

An algorithm was implemented which, in an initial step, estimated complex water, fat, field map and T2* [9] based on the first dynamic with T2prep = 0 ms. It then used them as constrained input to the full model and solved for $$$T_{2w}$$$, $$$T_{2f}$$$ and $$$T’_2$$$.

Phantom and in vivo measurements:

All studies were performed on a 3T scanner (Ingenia, Philips Healthcare). Agar based water-fat phantoms [8] with nominal fat fractions of 0, 5, 10, 15 and 100 % and the spine of a healthy volunteer (36y, male) were measured with a conventional TIMGRE sequence and the T2prep-TIMGRE sequence. (parameters in Table 3) Single-voxel STEAM spectroscopy was also performed during the phantom (voxel size = 14 x 14 x 14 mm³) and in vivo (L3/4 vertebrae, voxel size = 18 x 24 x 14 mm³) experiment for validation with the following parameters: TR = 6000ms (in vivo: 5000ms), TE = 12/16/20/24 ms, TM = 18 ms, BW = 3000 Hz, samples = 4096.

Figure 2 shows the results, including measured fat fraction (a+c), T2* (b) and T2 of water (d) maps, of the performed phantom experiments using the conventional TIMGRE and the T2prep-TIMGRE. The results of the in vivo experiment are shown in Figure 3. Furthermore, all measured fat fraction and T2 of water values (± standard deviation) are given in Table 1 and 2, respectively. The phantom and in vivo experiments showed good agreement for measuring fat fraction between MRS, TIMGRE and T2prep-TIMGRE (Table 1, Figure 2) and for the T2 of water between MRS and T2prep-TIMGRE (Table 2, Figure 2). This study has also some limitations. First, although considerably small, the T1-weighting during the T2prep has not been considered in the signal model, as the BIR4 pulse flips the magnetization for a short period during excitation into the longitudinal axis. Second, $$$T_{2f}$$$ cannot be correctly estimated due to the limited off-resonance insensitivity of the BIR4 pulse. In conclusion, a novel T2prep-TIMGRE sequence was developed for simultaneous measurement of T2 of water, T2′ and PDFF, and the feasibility of measuring the above parameters in the spine was demonstrated.