Hyperpolarized Nuclei for NMR Imaging and Spectroscopy - Lunds ...

2.4.1 T 1 relaxation mechanisms for hyperpolarized gases
Several mechanisms contribute to the relaxation rate (1 T 1)
of hyperpolarized
noble gases in vitro (e.g., in a transport container) and in vivo, including
surface relaxation, oxygen-induced relaxation, gradient-induced relaxation,
and dipolar relaxation.
The surface relaxation mechanisms of 3 He and 129 Xe are complex
(Fitzsimmons et al. 1969, Driehuys et al. 1995), but can be outlined as
12
1 1 S
=
T η V
1,
surface
where SV is the surface-to-volume ratio of the gas container and η is a coefficient
dependent on the surface material, the temperature, and the magnetic
field strength. In iron-free glass cells, the T 1 of hyperpolarized 3 He can
be ∼100 h, and several 100 h with additional coating of cesium or rubidium
(Heil et al. 1995, Wolf 2000, Gentile et al. 2001). In Pyrex cells coated with
sol-gel, T 1 as long as 340 h has been measured (Hsu et al. 2000). In the porcine
lung, the T 1,surface for 3 He has been measured to be > 4 h, corresponding
to a value of η of > 22 h cm –1 (Deninger et al. 1999), which is better than
many uncoated and coated glass surfaces. For 129 Xe, a T 1 value of 3 h has
been reported for gas contained in a 7.5 cm diameter quartz cell (Chann et
al. 2002), corresponding to η ≈ 3.8 h cm –1 .
Paramagnetic molecular oxygen is a very potent source of relaxation. The
oxygen-induced relaxation rate has been empirically determined for 129 Xe
(Jameson et al. 1988) and 3 He (Saam et al. 1995) according to
1
T
1,
O2
1
T
1,
O2
003 .
O −1
129
p 2 273 ⎛ 300⎞
= 0. 388 ⎜ ⎟
1. 013 T ⎝ T ⎠
042 .
O −1
3
p 2 273 ⎛ 299⎞
= 045 .
⎜ ⎟
1. 013 T ⎝ T ⎠
s ( Xe)
s ( He)
[9]
[10]
where T is the temperature [K] and p O2 is the oxygen partial pressure [bar].
Eq. [10] is valid in the temperature range from 200 K to 400 K. At room
temperature (293 K), the relaxation rates can thus be expressed as