Probing lung physiology with xenon polarization transfer contrast (XTC)

Abstract

One of the major goals of hyperpolarized‐gas MRI has been to obtain 129Xe dissolved‐phase images in humans. So far, this goal has remained elusive, mainly due to the low concentration of xenon that dissolves in tissue. A method is proposed and demonstrated in dogs that allows information about the dissolved phase to be obtained by imaging the gas phase following the application of a series of RF pulses that selectively destroy the longitudinal magnetization of xenon dissolved in the lung parenchyma. During the delay time between consecutive RF pulses, the depolarized xenon rapidly exchanges with the gas phase, thus lowering the gas polarization. It is demonstrated that the resulting contrast in the 129Xe gas image provides information about the local tissue density. It is further argued that minor pulse‐sequence modifications may provide information about the alveolar surface area or lung perfusion. Magn Reson Med 44:349–357, 2000. © 2000 Wiley‐Liss, Inc.

Imaging alveolar–capillary gas transfer using hyperpolarized 129 Xe MRI

“…Their presence would indicate a signal from the gas. Their absence verifies that Xe gas is not significantly absorbed by these conducting airways (19). Third, when lung tissue density, xenon solubility, and image acquisition parameters are taken into consideration, the dissolved-phase image resolution (1.25 ϫ 1.25 mm 2 ) and signal-to-noise ratio (6.8 Ϯ 2) achieved are exactly as expected given the achievable airspace image resolution (0.31 ϫ 0.31 mm 2 ) and SNR (9.1 Ϯ 2).…”

“…By contrast, micrometer-scale thickening of alveolar septa is enough to completely extinguish the RBC image intensity on XACT. Similarly, we expect XACT to be more sensitive than XTC imaging (14,19), which was developed to measure tissue thickness. Because the size of the XTC effect is proportional to total tissue volume, micrometer-scale thickening might increase the XTC effect by only tens of a percent.…”

“…An alternate imaging method that retains higher spatial resolution while indirectly probing the gas-exchange process is called xenon polarization transfer contrast (XTC). This method uses the attenuation of airspace 129 Xe signal after rf irradiation of the dissolved phase 129 Xe frequencies to indirectly map 129 Xe gas exchange between airspace and dissolved phase (19). XTC has been shown to be sensitive to increases in tissue density resulting from atelectasis, for example (14), but does not distinguish the 129 Xe signal originating from the barrier and RBC compartments.…”

Development of a Functionalized Xenon Biosensor

“…72,73 The exchange properties of the biosensor system presented here allow for the implementation of novel signal enhancement techniques using modified pulse sequences. The application of selective pulses is accomplished easily due to the large separation (~120 ppm) between xenon resonances in solution and in cages.…”

Simultaneous magnetic resonance imaging of ventilation distribution and gas uptake in the human lung using hyperpolarized xenon-129

“…In particular, xenon's relatively high solubility in biological tissues (2), along with its exquisite sensitivity to its environment that results in an enormous range of chemical shifts upon solution (3), make hyperpolarized Xe129 particularly attractive for exploring certain characteristics of lung function, such as gas exchange and uptake (4,5), not accessible with the use of hyperpolarized He3. Upon inhalation, Xe129 gives rise to multiple MRI spectral peaks in the lung (6)(7)(8)(9), each associated with a physically different compartment.…”