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

3 Methods
3.1 Hyperpolarization equipment
Three hyperpolarized nuclei were investigated in this thesis: the noble gases
129 Xe (Papers I and V) and 3 He (Papers III and IV), and a liquid 13 C substance
(Paper II). The noble gases were polarized by means of optical
pumping, whereas a different principle (DNP) was used for polarization of
13 C (see section 2.2).
3.1.1 Polarization and handling of 3 He and 129 Xe
129 Xe was polarized using a prototype commercial polarizer (IGI. 9800, Amersham
Health, Durham, NC, USA). In this system, a continuously flowing
gas mixture at ∼5 bar pressure (1% 129 Xe, 10% N 2, and 89% 4 He) passes an
optical cell containing ∼1 g of Rb. When the cell is heated to 160–180 °C,
small amounts of Rb evaporate and get optically pumped by a circularly polarized
795-nm laser beam illuminating the cell. Via collisions between Xe
and Rb, the polarization of the Rb electrons is transferred to the Xe nuclei.
After leaving the optical cell, the gas mixture passes a cold-finger trap, cooled
by liquid nitrogen, where the hyperpolarized 129 Xe is frozen, while the other
gases are ventilated out into the atmosphere. When a sufficient amount of
Xe has accumulated (typically within half an hour), the cold finger is rapidly
heated with water and the thawed Xe is dispensed into a reservoir.
Another prototype commercial polarizer (IGI. 9600, Amersham Health)
was used for polarization of 3 He. Similar to the Xe polarizer, an optical cell
containing Rb is heated to 160–180 °C and illuminated with a 795-nm circularly
polarized laser beam, but the gas mixture (99% 3 He, 1% N 2) is filled
into the cell once and remains inside the closed cell during the polarization
procedure. The optical cell is pressurized to ∼9 bar, corresponding to a hyperpolarized
gas volume of ∼1.1 liter at atmospheric pressure. The polarization
process usually runs overnight (typically during 15–18 h).
Once sufficient polarization levels were reached, both the 129 Xe and the
3 He gas were dispensed into a plastic bag (Tedlar ® , Jensen Inert, Coral
Springs, FL, USA) with low surface relaxation rate and transported ∼40 m
from the polarizer to the MRI scanner (duration of transport less than
5 min). Using a dedicated measurement equipment (Amersham Health), the
actual polarization level in the Tedlar bag was determined before and after
the NMR experiments. The measurement principle is based on the detec-
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