MRI of the lungs using hyperpolarized noble gases

Abstract

The nuclear spin polarization of the noble gas isotopes 3 He and 129 Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments. MRI has been extremely successful at diagnosing soft tissue disease since its discovery in 1972 (1). However, MRI is not as sensitive in comparison with other biomedical imaging techniques, such as CT, positron-emission tomography, or single-photon emission computed tomography. This is a consequence of a very small signal from a small population difference between nuclear energy states. For a spin-1/2 system, the "nuclear spin polarization", P N , is defined as:where N ϩ and N Ϫ denote populations with magnetic spin quantum numbers ϩ1/2 and Ϫ1/2, respectively. Typically, the thermal energy of the sample at temperature T exceeds the energy difference between the nuclear spin states in a magnetic field B 0 by several orders of magnitude ("hightemperature approximation") and the equilibrium polarization can be written as:where ␥ is the magnetogyric ratio, ប is Planck's constant divided by 2 , and k B is Boltzmann's constant. As an example, P N,0 Ϸ 5 ppm is predicted with Eq.[2] for protons ( 1 H) at body temperature (T ϭ 37°C) and B 0 ϭ 1.5T. In view of the inherent sensitivity problem, increasing the signalto-noise ratio (SNR) has been a field of continuous research since the discovery of NMR. Recently, the use of optically polarized noble gas isotopes 3 He and 129 Xe has attracted increasing interest for use in a variety of promising MR applications. These systems exhibit polarizations exceeding the thermal levels by several orders of magnitude. While the use of such "hyperpolarized" (HP) gases for MRI is a recent development, it is based on a solid foundation of work in atomic physics. The groundwork was laid by Kastler (2) more than 50 years ago by demonstrating transfer of angular momentum from circularly polarized light to the electron and nuclear spins of atoms, a process called "optical pumping" (OP). Since 1991, exploitation of OP as a means of enhancing signal initiated the development of a novel field in NMR (3,4). Research involving HP noble gases has been exceptionally fruitful in biomedical MRI as well as providing applications for investigation of materials (5-8).In the context of proton MRI, the lung is a particularly challenging area to study (9). Even at end expiration, the overall density is ...

q‐space analysis of lung morphometry in vivo with hyperpolarized3He spectroscopy

“…This is necessary because the q-space formalism assumes short gradient pulse approximation (16). For restricted diffusion, the signal decay as a function of the q value becomes smaller when the diffusion 1 The data failed the equal variance test, so a Mann-Whitney rank-sum test was performed on the data. The P-values reported are for this test.…”

“…Z n will be minimum for free diffusion (Eq. [1]) and maximum when the atoms are not diffusing along q. In the latter case, DPP(x)ϭ␦(x).…”

“…[1] and a delta function in the absence of diffusion. In the presence of confining walls or boundaries, the motion of a particle is restricted, resulting in a smaller X rms and a non-Gaussian DPP.…”

“…HYPERPOLARIZED 3 HE GAS (HHe) is an inhaled contrast agent that provides detailed information about lung function and structure in MRI (1)(2)(3)(4). Because no ionizing radiation is used in MRI and 3 He is a biologically inert gas, HHe MRI is a very safe imaging technique.…”

RF instrumentation for same‐breath triple nuclear lung MR imaging of 1H and hyperpolarized 3He and 129Xe at 1.5T

“…The two gases have distinct physical properties, which provide different but complementary functional information 7, 8, 9. The ability to image both nuclei in the same breath alongside the 1 H anatomical images adds further structural and functional sensitivity to the acquisition.…”