Abstract
The aim of this work was to built an apparatus for producing of
hyperpolarized 129Xe gas of sufficient amount and
polarization and to use this gas as contrast agent for in vivo
measurements at the 3 Tesla magnetic resonance (MR) tomograph of
the Physikalisch-Technische-Bundesantsalt (PTB) Berlin. With the
129Xe-polarization of PXe~20 % finally achieved
in nat. Xe gas volumes of ~0.5 l it was possible to obtain lung
images of a healthy volunteer and to get the first 129Xe
spectroscopic MR images of the human brain. Furthermore, a completely
different field of application of hyperpolarized Xe gas was
demonstrated: the SQUID-detection of free spin-precession in magnetic
fields of a few nT.
In the beginning an apparatus was built that worked in a cyclic
mode to hyperpolarize 129Xe gas by optically spin exchange
pumping using rubidium vapor and xenon gas in natural abundance (nat.
Xe). Later on this set-up was replaced by an apparatus in which the
gas was continuously driven through the pumping cell to reduce losses
in the 129Xe polarization. By the use of an on-line
129Xe-NMR experiment working at 120 kHz resonance
frequency (B0~10 mT) set up directly at the optical
pumping cell it was possible to implement a method for characterizing
the efficiency of the optical pumping process. The sensitivity of the
on-line NMR experiment was improved such that the production
of the nuclear spin-polarization could be quantitatively tracked.
Non adiabatic processes were utilized to initiate free precession
of the 129Xe nuclear magnetization of hyperpolarized Xe
gas at ultra low magnetic fields within a magnetically shielded room
at the PTB Berlin. Due to the very low ambient field of Bamb~4.5
nT within the shielded room Larmor-precession times TL=1/fL
of up to 18 s and T2-relaxation times up to 8000 s were
observed. By varying the total pressure inside the glass cell
containing the hyperpolarized Xe gas the T2-rates were
measured in the motional narrowing regime. A theory of Cates et al.
(PRA Vol. 37, 1988, S. 2877) which was not directly tested in this
pressure regime before was successfully used to describe the measured
T2-rates.
The human lung images obtained in conjunction with the 3 Tesla
MR-tomograph using hyperpolarized Xe gas as contrast agent have shown
that first clinical studies are now possible. By the use of MR
transmit-receive coils for 129Xe specially built at the
PTB Berlin a spatial resolution of 4x4x20 mm3 in the lung
imaging was obtained. The results of the spectroscopic measurements
of hyperpolarized 129Xe dissolved in the human brain
demonstrated considerable improvements in comparison to the published
measurements mostly performed on animals. By means of time resolved
spectroscopic measurements the dynamic of the signal intensity in the
brain was studied. Furthermore, first slice selective 1D chemical
shift images of hyperpolarized 129Xe in the human brain
were obtained. | 
