Fluorinated gas MRI is a promising method for pulmonary ventilation imaging that does not require additional polarization equipment. To date, short echo time spoiled gradient echo sequences with long repetition times relative to T1 have been employed for lung ventilation imaging in humans. Here, we present an optimization of steady state free precession sequences for imaging of C3F8 gas in lungs, and demonstrate that image signal-to-noise ratio may be improved by exploiting the short T1, and relatively long T2.
Simulations: PFP steady state transverse magnetization was simulated for SSFP4 and for SPGR5 sequences. Values of T1 and T2 of PFP gas mixed with 21% O2 were taken to be 16ms3. The value of T2* in the PFP phantom was 3.4ms from previous experimental measurements. T2 of PFP likely reduces in a similar manner to T1, as molecular collisions are the predominant relaxation mechanism3,6. Global specific absorption rate (SAR) was calculated conservatively as the input power to the coil being completely deposited into a 70kg patient, while local SAR was approximated as 15 times higher than global SAR, a ratio similar to that presented for transceive arrays7.
Experiments: An in-house 4-element fixed phase/amplitude vest transceiver coil8 was used for imaging of 19F nuclei on a 1.5T GE Signa HDx scanner. The phantom consisted of an outer container (30ℓ) filled with 3.6g/ℓ NaCl and 1.96g/ℓ CuSO4⋅5H2O solution and inner container (8ℓ) filled with C3F8 gas. FA maps in Figure 1 were calculated by fitting signal according to a sinusoidal relation with increasing input power using a gradient echo sequence. SPGR and SSFP sequence performance was evaluated using 3D sequences with the following parameters: matrix=64×32×24, bandwidth=20kHz, FOV=40cm, slice thickness=10mm and 16 averages. For SPGR; hard pulse width (PW)=312µs, TR=2.4ms and TE=0.7ms, while for SSFP; PW=608µs, TR=3ms and TE=1.1ms. For in-vivo comparison of the two sequences, 6 averages were acquired for a 14s breath-hold and 60° FA was used for SSFP. The SPGR sequence was modified for a similar breath-hold time and near SNR-optimal FA (TR=2.8ms, TE=1ms and PW=258µs, FA=25°).
1. S. J. Kruger, S. K. Nagle, M. J. Couch, Y. Ohno, M. Albert, and S. B. Fain, "Functional imaging of the lungs with gas agents," Journal of Magnetic Resonance Imaging, 2015;43:295-315
2. M. J. Couch, I. K. Ball, T. Li, M. S. Fox, S. L. Littlefield, B. Biman, et al., "Pulmonary ultrashort echo time 19F MR imaging with inhaled fluorinated gas mixtures in healthy volunteers: feasibility," Radiology, 2013;269:903-909
3. Y. V. Chang and M. S. Conradi, "Relaxation and diffusion of perfluorocarbon gas mixtures with oxygen for lung MRI," Journal of Magnetic Resonance, 2006;181:191-198
4. B. A. Hargreaves, S. S. Vasanawala, J. M. Pauly, and D. G. Nishimura, "Characterization and reduction of the transient response in steady-state MR imaging," Magnetic Resonance in Medicine, 2001;46:149-158
5. S. C. L. Deoni, "High-resolution T1 mapping of the brain at 3T with driven equilibrium single pulse observation of T1 with high-speed incorporation of RF field inhomogeneities (DESPOT1-HIFI)," Journal of Magnetic Resonance Imaging, 2007;26:1106-1111
6. N. L. Adolphi and D. O. Kuethe, "Quantitative mapping of ventilation-perfusion ratios in lungs by 19F MR imaging of T1 of inert fluorinated gases," Magnetic Resonance in Medicine, 2008;59:739-46
7. B. Guérin, M. Gebhardt, P. Serano, E. Adalsteinsson, M. Hamm, J. Pfeuffer, et al., "Comparison of simulated parallel transmit body arrays at 3 T using excitation uniformity, global SAR, local SAR, and power efficiency metrics," Magnetic Resonance in Medicine, 2015;73:1137-1150
8. A. Maunder, M. Rao, F. Robb, and J. Wild, "RF coil design for multi-nuclear lung MRI of 19F fluorinated gases and 1H using MEMS," Proc. Intl. Soc. Mag. Reson. Med., 2016;24:3504
9. U. Wolf, A. Scholz, C. P. Heussel, K. Markstaller, and W. G. Schreiber, "Subsecond fluorine-19 MRI of the lung," Magnetic Resonance in Medicine, 2006;55:948-951
10. R. E. Jacob, Y. V. Chang, C. K. Choong, A. Bierhals, D. Zheng Hu, J. Zheng, et al., "19F MR imaging of ventilation and diffusion in excised lungs," Magnetic Resonance in Medicine, 2005;54:577-585
11. U. Wolf, A. Scholz, M. Terekhov, K. Muennemann, K. Kreitner, C. Werner, et al., "Fluorine-19 MRI of the lung: first human experiment " Proc. Intl. Soc. Mag. Reson. Med., 2008;16:3207
12. 60601-2-33, "Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis," in Medical electrical equipment - Part 2-33, ed, 2015.