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, 3He and 129Xe, 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 3He and 129Xe.

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 3He3 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.

NN

= exp ( )

2 p B0

kBT

Bo B

EnergyE = p B0

E = - p B0

o = B0

(Gyromagnetic ratio)

= 42.58 MHz /T for proton

M = p =

N N-

N N+

N p2 B0

kBT

M

B0

Magnetization

τ =

= μ × B

d L

d t

= B

x

B0

y

z

μ

z’

x’

y’

μθB1(rf)

T1 ( longitudinal ) relaxation

T2 ( transverse ) relaxation

xy

z

Mz

B0

M0

M0 ( 1- e-1)

T1

eMxy

T2

Relaxation

z’

x’ y’

B0

Mxy

Free induction decay ( FID )

z

x y

B0

Detection coil

Mxy

Inhomogeneous B0 field z

x y

B0

fs

dephase 1 / T2* = 1/T2 + 1/T2 inhomo

Spin-echo sequence

z

x y

B0

fs

180o pulse

z

x y

B0

f

s

z

x y

B0

echo

Figure 1 NMR and MRI imaging instrument

Figure 2 NMR image of (a) water at 4.7 T, (b) laser polarized noble 3He 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 3He gas, (d) polarized 3He 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 3He 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 mm2 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