Gas Phase UTE MRI of Propane and Propene

Abstract

¹H MRI of gases can potentially enable functional lung imaging to probe gas ventilation and other functions. In this work, ¹H MR images of hyperpolarized and thermally polarized propane gas were obtained using UTE (ultrashort echo time) pulse sequence. A 2D image of thermally polarized propane gas with ~0.9×0.9 mm² spatial resolution was obtained in less than 2 seconds, demonstrating that even non-hyperpolarized hydrocarbon gases can be successfully utilized for conventional proton MRI. The experiments were also performed with hyperpolarized propane gas and demonstrated acquisition of high-resolution multi-slice FLASH 2D images in ca. 510 s and non slice-selective 2D UTE MRI images in ca. 2 s. The UTE approach adopted in this study can be potentially used for medical lung imaging. Furthermore, the possibility to combine UTE with selective suppression of ¹H signals from one of the two gases in a mixture is demonstrated in this MRI study. The latter can be useful for visualizing industrially important processes where several gases may be present, e.g., gas-solid catalytic reactions.

Figure 1.

The schematic representation of the experimental setup for producing parahydrogen-induced polarization (PHIP) hyperpolarized (HP) propane via heterogeneous pairwise hydrogenation of propene with parahydrogen (A). An overlay of hydrogen 1 (¹H) magnetic resonance imaging (¹H MRI) FLASH image of hyperpolarized (HP) propane flowing into the 10-mm nuclear magnetic resonance (NMR) tube via 1/16″ OD Teflon capillary (B). The field of view (FOV) was 3.1 × 3.1 cm with 256 × 32 matrix size, and the total acquisition time was ∼510 seconds. Note that the NMR tube is schematically shown, and its length does not correlate with the actual scale of the 2-dimensional (2D) magnetic resonance (MR) image.

Figure 2.

Heterogeneous hydrogenation of propene to propane with pH₂ over Rh/TiO₂ catalyst with partial preservation of spin order of parahydrogen in the final HP product (A). ¹H NMR spectra of HP propane (blue) and thermally polarized propane (black) obtained in heterogeneous hydrogenation of propene with parahydrogen; the signal enhancement was ∼30-fold (B). ¹H MRI of HP propane flowing through a 1/16″ OD Teflon capillary inside a 10-mm NMR tube obtained with a FLASH pulse sequence (C). The FOV was 3.1 × 2.5 cm with 256 × 32 matrix size, and the total acquisition time was ∼510 seconds with slice thickness of ∼0.7 cm. The signal intensity color scale ranges from black (zero) to blue (maximum).

Figure 3.

MR images of 15-mm NMR tube filled with thermally polarized propane (A) and PHIP HP propane (the same experimental setup as for Figure 1) (B). The FOV was 3 × 3 cm with 32 × 32 matrix size, the total acquisition time was ∼2 seconds, and the slice thickness in the z direction was 10 mm.

Figure 4.

¹H MR image (A) and corresponding ¹H NMR spectrum of a sample comprising a 10-mm NMR tube with propylene placed inside a 15-mm tube with propane (B). ¹H MR image (C) and corresponding ¹H NMR spectrum (D) of the same phantom with application of selective suppression pulse for propane NMR signals. The FOV for both images was 5 × 5 cm with 128 × 128 matrix size and 10-mm slice thickness along the z axis; the total acquisition time was ∼8 seconds. The ¹H NMR spectrum was obtained using the same ultrashort echo time (UTE) pulse sequence as that for ¹H MRI but with the gradients switched off and the spectral bandwidth appropriately reduced.