Low–field NMR (or MRI) Images of Laser polarized Noble Gas

MRI is a minimally invasive imaging technique with inormalous impact biomedical and physical science. Conventional MRI employs large magnetic fields to in induce observed thermally Boltzmann polarization in nuclear spin of liquids such as water. Examples include diagonostic clinical medicine, biological research, such as imaging of brain function, material science, soft condensed matter physics, such as imaging of foams. However, the large imaging field of conventional MRI require cumbersome ( 笨重 ) and expensive equipment and limit the scientific application.

At low magnetic field near room temperature, the thermally polarized nuclear magnetization such as 1H in water is extremely weak (polarization is 10-8), requiring extensive signal averaging to obtain resolvable NMR signal and making MRI impractical for with conventional method The greatly increased nuclear spin polarization of the noble gas, 3 He and 129 Xe, provided by optical pumping (laser polarization) enable efficient gas phase MRI in low magnetic field (10 G).

. With laser polarization, however angular momentum is transferred from photon to nuclei, and a large nonequilibrium nuclear spin (>10 %) can be created in the spin-1/2 noble gas 3 He and 129 Xe. Laser polarized noble gas can be stored in specially prepared container for several hours before the spin polarization decays back to thermal equilibrium.

NMR images of laser polarized 3He 3 gas were obtained at 21 G (Gausses) using a simple, home built 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 system for diagnosis of lung diseases, as well as imaging of voids in porous media and within metallic systems.

N N = exp ( ) 2  p B 0 kBTkBT BoBo B Energy E =  p B 0 E = -  p B 0  o =  B 0  (Gyromagnetic ratio) = MHz /T for proton M =  p = NN- NN+ N  p 2 B 0 kBTkBT M B0B0 Magnetization

τ = = μ × B d L d t  =  B x  B0B0 y z μ z’ x’ y’ μ θ B 1(rf)

T 1 ( longitudinal ) relaxation T 2 ( transverse ) relaxation x y z MzMz B0B0 M0M0 M 0 ( 1- e -1 ) T1T1 e M xy T2T2 Relaxation z’ x’ y’ B0B0 M xy

Free induction decay ( FID ) z x y B0B0 Detection coil M xy Inhomogeneous B 0 field z x y B0B0 ff ss dephase 1 / T 2 * = 1/T 2 + 1/T 2 inhomo

Spin-echo sequence z x y B0B0 ff ss 180 o pulse z x y B0B0 ff ss z x y B0B0 echo

Figure 1 NMR and MRI imaging instrument

Figure 2 NMR image of (a) water at 4.7 T, (b) laser polarized noble 3 He gas at 21 G, (b) water at 21 G, (d) polarized 3He gas inside a H-shape glass cell at 21 gauss.

Figure 3 NMR images of (a) water in w-shaped Plexglas cell at 4.7 T, (b) water with high susceptibility materials are nearby, (c) polarized 3 He gas, (d) polarized 3 He gas with high permeability materials nearby.

(a) Water in cylindrical glass cell at 4.7 Tesla, (b) water in cylindrical brass cell at 4.7 Tesla, (c) laser polarized gas in gas cell (d) laser polarized gas 3 He gas in brass cell.

Low field Noble gas MRI is a power diagonotic technique with novel application in physical and biomedical science. A simple low field MRI apparatus system that provide laser polarized He3 gas images at 21 G a few seconds, with a two-dimensional spatial resolution of ~1 mm 2 for slice thickness of 1 cm, comparable to the resolution at high magnetic field provided by commercial MRI instruments.

Phys. Rev. Lett. 81, 3785 (1998). Low–field MRI of Laser polarized Noble Gas