Abstract
Purpose/Introduction: MREIT is an emerging method to measure
the ohmic tissue conductivities, with several potential biomedical applications. Its sensitivity depends on the magnitude of the applied current, which is limited to 1–2 mA in the human brain [1, 2]. This renders in vivo applications challenging. Here, we aim to analyze and
optimize the efficiency of two MREIT pulse sequences for in vivo brain imaging.
Subjects and Methods: The electrical current injected into the subject creates an additional magnetic field (DBz,c) that can be detected from the phase of the magnetization [3]. Multi-Echo Spin Echo (MESE; Fig. 1a) and Steady-State Free Precession Free Induction Decay (SSFP-FID; Fig. 1b) are two sensitive MREIT pulse sequences. The efficiencies of MESE and SSFP-FID DBz,c measurements (gDBz,c) are defined as the signal-to-noise ratios (SNRs) per square
root measurement time as in equations (1, 2), and given by
Necho, c, SNRn, TES, sp, sp/2, l, and Ttot are total number of echoes, gyromagnetic ratio, SNR of the nth echo, echo spacing, RF pulse widths, transverse magnetization, and total measurement time, respectively [4, 5]. The MESE efficiency is simulated, considering T1, T2, and T2*
relaxation in the SNRn. The simulations are experimentally validated for 0.5 mA injection current Ic in a doped saline filled spherical homogenous phantom, 10 cm in diameter (T1 = 1 s, T2 = 100 ms). The efficiency of SSFP-FID3, the most sensitive of the three variants, is simulated and experimentally validated for 1 mA in the same phantom. All simulations are performed using rotation matrices, and
cross-checked with the analytical equations.
Results: MESE simulations and experiments are compared in Fig. 2. The simulation results for three SSFP-FID variants (Fig. 1b; first two as in [4]; additional SSFP-FID3 with current injection in the entire TR period) are shown in Fig. 3a. SSFP-FID3 simulations and experiments are compared in Figure 3b-f.
Discussion/Conclusion: The measured and simulated efficiency maps for the MESE and SSFP-FID experiments are in good agreement. The most efficient regions for the MESE and SSFP-FID3 are Necho = [2, 3], TES = [60–100] ms, and TE = [60–90] ms, TR = [120–180] ms for a = 20°, respectively. For single echo acquisitions, B0 inhomogeneities and the low bandwidth per pixel at these long TES and TR create geometric image distortions. This can be fixed by multi-echo
summation with a slight decrease in efficiency. Both sequences are promising for testing in vivo applications.
