Hyperpolarized (129)Xe MRI: a viable functional lung imaging modality?

Abstract

The majority of researchers investigating hyperpolarized gas MRI as a candidate functional lung imaging modality have used (3)He as their imaging agent of choice rather than (129)Xe. This preference has been predominantly due to, (3)He providing stronger signals due to higher levels of polarization and higher gyromagnetic ratio, as well as its being easily available to more researchers due to availability of polarizers (USA) or ease of gas transport (Europe). Most researchers agree, however, that hyperpolarized (129)Xe will ultimately emerge as the imaging agent of choice due to its unlimited supply in nature and its falling cost. Our recent polarizer technology delivers vast improvements in hyperpolarized (129)Xe output. Using this polarizer, we have demonstrated the unique property of xenon to measure alveolar surface area noninvasively. In this article, we describe our human protocols and their safety, and our results for the measurement of the partial pressure of pulmonary oxygen (pO(2)) by observation of (129)Xe signal decay. We note that the measurement of pO(2) by observation of (129)Xe signal decay is more complex than that for (3)He because of an additional signal loss mechanism due to interphase diffusion of (129)Xe from alveolar gas spaces to septal tissue. This results in measurements of an equivalent pO(2) that accounts for both traditional T(1) decay from pO(2) and that from interphase diffusion. We also provide an update on new technological advancements that form the foundation for an improved compact design polarizer as well as improvements that provide another order-of-magnitude scale-up in xenon polarizer output.

Figure 1

Polarization is plotted against output rate for the UNH polarizer and several other polarization technologies. Highest magnetization, indicated by the figure-of-merit contours, is available at lowest values of polarization.

Figure 2

The laser light enters the polarization column along a direction opposite the flow of gas

Figure 3

Prof Hersman with Xemed's compact clinical HXe polarizer “Bell” in January 2007.

Figure 4

Blood pressure at baseline and 10 minutes after each breath hold experiment for human subject 4 (HS4). This data was acquired from 5/4/2005 to 7/14/2007.

Figure 5

Schematic of Chemical Shift Saturation Recovery method. (a) Idealized 1D semi-infinite phases. (b) A more realistic model with a finite width septum. Net diffusion from gas to dissolved phase after time t_diff is shown in purple.

Figure 6

Experimentally measured F(t) shown with blue dots as a function of √t. Lung volume was fixed and in the example shown here, it is at total lung capacity (TLC). Also shown is a fit to the early time data using Eqn [1] (red line).

Figure 7. SB-XTC raw and processed data

Coronal projection images were acquired from subject HS6 in the supine position after 1L of 86% enriched ¹²⁹Xe was inhaled (47% TLC). (a-c) Example of the three serially acquired single breath XTC (SB-XTC) gradient echo images and the information obtained from them. (a) First image showing ¹²⁹Xe ventilation, (b) second “control” image that was acquired after image (a) and then an XTC exchange sensitization (RF pulses off resonance) so that the only attenuation between image (a) and (b) is due to RF depletion and T₁ decay from pO₂, (c) third “XTC” image acquired after image (b) and an XTC exchange sensitization (RF pulses applied on resonance) so that the attenuation between image (b) and (c) is due to RF depletion, T₁ decay from pO₂, and ¹²⁹Xe interphase diffusion. Voxels in image (c) have been scaled up by the attenuation factor measured between (a) and (b) so that the attenuation observed between (b) and (c) is solely due to interphase diffusion. Also shown are (d) the resulting F_XTC and (e) pO₂-equiv maps, and their respective histograms (f) and (g).

Figure 8. SB-XTC raw and processed data

Coronal projection images were acquired from subject HS6 in the supine position after 1.8L of 86% enriched ¹²⁹Xe was inhaled (63% TLC). Description for subfigures a-g is identical to Figure 7.

Figure 9

Comparison of apex to base (a) <F_XTC> and (b) <pO₂-equiv> averaged right/left as a function of apex to base (superior to inferior) position. Note that the region in the immediate vicinity of the diaphragm for the <pO₂-equiv> plot at 63% lung volume was inconsistent with the remainder of the lung. Artifacts due to diaphragm motion are suspected to be the cause. This region was excluded from the plot below because of excessive variability of <pO₂-equiv> in that region.