Nuclear magnetic resonance imaging of airways in humans with use of hyperpolarized ³He

Abstract

The nuclear spin polarization of noble gases can be enhanced strongly by laser optical pumping followed by electron-nuclear polarization transfer. Direct optical pumping of metastable 3He atoms has been shown to produce enormous polarization on the order of 0.4-0.6. This is about 10(5) times larger than the polarization of water protons at thermal equilibrium used in conventional MRI. We demonstrate that hyperpolarized 3He gas can be applied to nuclear magnetic resonance imaging of organs with air-filled spaces in humans. In vivo 3He MR experiments were performed in a whole-body MR scanner with a superconducting magnet ramped down to 0.8 T. Anatomical details of the upper respiratory tract and of the lungs of a volunteer were visualized with the FLASH technique demonstrating the potential of the method for fast imaging of airways in the human body and for pulmonary ventilation studies.

In vivo NMR of hyperpolarized 3He in the human lung at very low magnetic fields

“…In-vivo studies on the human lung began in 1996 using conventional clinical systems operating at 0.8 T [4,5] and 1.5 T [6]. However, unlike conventional MRI, the strong field is not needed here to polarize the gas.…”

k‐Space filtering in 2D gradient‐echo breath‐hold hyperpolarized 3He MRI: Spatial resolution and signal‐to‐noise ratio considerations

“…In recent years MRI of inhaled hyperpolarized gases ( 3 He and 129 Xe) has shown much clinical promise in both static (1)(2)(3)(4)(5) and dynamic (6 -11) imaging of the lungs and airways. Through laser optical pumping techniques (12,13), levels of ensemble nuclear spin polarization up to 10 5 times greater than those attainable at thermal equilibrium at common imaging field strengths can be reached.…”

“…This hyperpolarization has made in vivo imaging of inhaled gas a realistic proposition in terms of detectable signal. There are two key constraints in the choice of an optimum imaging strategy with inhaled hyperpolarized 3 He gas. The first is the nonrecoverable hyperpolarization of the spin population once magnetization has been tipped in to the transverse plane by a B 1 radiofrequency (RF) excitation pulse.…”

“…This limits both the size of the flip angle and the number of applicable RF pulses (phase-encode views (n)) in the conventional Cartesian sampling of k-space. The second constraint results from the high diffusion coefficient of 3 He at atmospheric pressure in the lungs. This could potentially lead to significant signal dephasing as a function of the view-dependent MRI gradients.…”

“…The last approach is elegant in theory but requires precise calibration of the flip angle to achieve the best results, a feat that can be problematic in vivo. Moreover, the variable flip angle technique is restricted to 3 He MRI under static conditions because it uses up the complete polarization. In particular, it cannot be applied to dynamic imaging during inhalation and exhalation.…”