ABSTRACT
I. Introduction 151
II. Hardware Requirements 154
III. 3He MR Imaging 155
IV. 129Xe MR Imaging 157
V. Conclusions 158
References 158
I. Introduction
Magnetic resonance imaging (MRI) using hyperpolarized noble gases, such as
helium-3 (3He) and xenon-129 (129Xe), has been a novel addition to the
arsenal of pulmonary diagnostic tests. It has enabled the development of MRI
in the chest, which was traditionally hampered by the low number of protons
and magnetic field inhomogeneities that exist at the boundaries of air/softtissues of the lung parenchyma. Hyperpolarized noble gas MRI makes use of
the inherent properties of these nonradioactive isotopes, which have a nuclear
spin of 1/2. This makes them sensitive to nuclear magnetic resonance (NMR) methodology, albeit that this would only be marginally detectable at normal
thermal equilibrium. In a strong magnetic field, the atoms can exist in two ground
states which are parallel or antiparallel to the magnetic field. It is the introduction
of large quantities of energy using laser light in a process termed “optical
pumping,” which will allow one population of the ground states to grow (1-3).
This imbalance of the normal distribution of ground states is called hyperpolar-
ization, and the percentage of the population is what is generally described as the
level of polarization. Thus, at 50-80% polarization, the signal that is introduced
into the lungs by breathing in a small amount of gas is in the order of.100,000 at thermal equilibrium, and more than sufficient to allow for detailed images of
the lungs to be obtained.
