Low-Field MRI of Laser Polarized Noble Gas

Abstract

NMR images of laser polarized 3He gas were obtained at 21 G using a simple, homebuilt instrument. At such low fields magnetic resonance imaging (MRI) of thermally polarized samples (e.g., water) is not practical. Low-field noble gas MRI has novel scientific, engineering, and medical applications. Examples include portable systems for diagnosis of lung disease, as well as imaging of voids in porous media and within metallic systems.

A system for open‐access 3He human lung imaging at very low field

“…Shown in Fig. 7(a) is an image of the phantoms that had been used previously in bench-top low-field MRI experiments (13). The images are undistorted, indicating true gradient linearity across this smaller FOV, as well as a SNR of ~30.…”

“…Such hyperpolarization, which for 3 He can be 10,000 times greater than the polarization obtained by thermal equilibrium at 1 T, yields a magnetization that is similar to those found in water and other liquids when placed in such large external magnetic fields. Additionally, because the 3 He spins are polarized prior to the imaging procedure, a large applied magnetic field B 0 is not required for high-resolution MRI (13). Therefore, laser-polarized 3 He MRI can be performed at applied magnetic fields substantially lower than in clinical scanners, potentially <10 mT (100 G), with obtainable SNR and resolution for lung imaging approaching that obtained using clinical scanners.…”

“…We initially demonstrated the feasibility of low-field 3 He MRI using a solenoid magnet operating at approximately 2 mT (20 G) (13,19) and obtained 3 He images of sealed glass cells (13) and excised rat lungs (19). This system achieved image resolution comparable to that obtained with clinical scanners, although with much smaller samples, while the reduction of susceptibility-induced gradients was seen in both the cells and rat lungs.…”

An open-access, very-low-field MRI system for posture-dependent 3He human lung imaging

“…Note that the NMR signal voltage from a hyperpolarized sample scales linearly with B 0 [41], nonetheless, sufficient SNR for high-resolution MRI can be achieved at B 0 ~ 10 -100 mT due to reduced sample (tissue) noise, and increased T 2 and because of the reduced effect of susceptibility-induced background field gradients. Thus, as we have shown previously [39,40,42,43,31], hyperpolarized 3 He MRI can be performed at B 0 substantially lower than in clinical scanners, ~ 10 mT (100 G), with SNR and resolution for lung imaging approaching that obtained in clinical scanners. This fact enables us to exploit dramatic simplifications in magnet technology in the very-low-field regime (~ 10 mT), such as open-access electromagnets, that enable a walk-in, open scanner where a subject can sit, stand, lie horizontal or recline at any angle [42,31].…”

“…This enhanced polarization, which can be ~ 10,000 times greater than that obtained via thermal equilibrium at 1 T, yields a magnetization of the same order as that found in water when placed in a large magnetic field. Furthermore, as the noble gas spins are polarized via the optical procedure prior to imaging, a large applied magnetic field, B 0 , is not required for high-resolution MRI [39,40]. Note that the NMR signal voltage from a hyperpolarized sample scales linearly with B 0 [41], nonetheless, sufficient SNR for high-resolution MRI can be achieved at B 0 ~ 10 -100 mT due to reduced sample (tissue) noise, and increased T 2 and because of the reduced effect of susceptibility-induced background field gradients.…”