Single-breath clinical imaging of hyperpolarized (129)Xe in the airspaces, barrier, and red blood cells using an interleaved 3D radial 1-point Dixon acquisition

Abstract

Purpose: We sought to develop and test a clinically feasible 1-point Dixon, three-dimensional (3D) radial acquisition strategy to create isotropic 3D MR images of (129)Xe in the airspaces, barrier, and red blood cells (RBCs) in a single breath. The approach was evaluated in healthy volunteers and subjects with idiopathic pulmonary fibrosis (IPF).

Methods: A calibration scan determined the echo time at which (129)Xe in RBCs and barrier were 90° out of phase. At this TE, interleaved dissolved and gas-phase images were acquired using a 3D radial acquisition and were reconstructed separately using the NUFFT algorithm. The dissolved-phase image was phase-shifted to cast RBC and barrier signal into the real and imaginary channels such that the image-derived RBC:barrier ratio matched that from spectroscopy. The RBC and barrier images were further corrected for regional field inhomogeneity using a phase map created from the gas-phase (129)Xe image.

Results: Healthy volunteers exhibited largely uniform (129)Xe-barrier and (129)Xe-RBC images. By contrast, (129)Xe-RBC images in IPF subjects exhibited significant signal voids. These voids correlated qualitatively with regions of fibrosis visible on CT.

Conclusions: This study illustrates the feasibility of acquiring single-breath, 3D isotropic images of (129)Xe in the airspaces, barrier, and RBCs using a 1-point Dixon 3D radial acquisition.

Keywords: 1-point Dixon; 3D Radial; Hyperpolarized xenon.

© 2015 Wiley Periodicals, Inc.

Figures

Figure 1

The phase calibration scan shown in (A) acquires dissolved-phase spectra at 4 echo times (275, 375, 475, 575 μs) with a NEX of 50. Crusher gradients are played out on all three gradient axes to de-phase any off-resonant gas-phase signal. The last 25 dissolved-phase FIDs are averaged together and fit to a sum of Lorentzian and Dispersive functions to extract the phase difference between the RBC and barrier resonances. These phase differences are plotted as a function of echo time (B) and fit to a line to calculate the TE90.

Figure 2

Phase correcting the dissolved-phase images. (A) The ratio of the sum of the real and imaginary channels as a function of receiver phase applied. The inset shows the approach to the spectroscopic RBC:barrier ratio. For this subject, a receiver phase offset of −87.7° makes the ratio of real and imaginary channel signals equal to the spectroscopic ratio R=0.43. (B) Magnitude images of the real and imaginary channel after applying the calculated receiver phase offset. As shown by the white arrow, this subject exhibits a low intensity stripe at the base of the lung in the real channel. This may be partly caused by the abrupt change in the regional phase as shown by the difference phase map (C). Once the regional phase inhomogeneity correction was applied, this apparent defect was removed (D).

Figure 3

Representative single breath images of 129Xe in the gas-phase (grayscale), barrier tissues (green), and RBCs (red). In the healthy volunteer distribution of 129Xe in all three resonances was largely homogeneous. In the IPF subject gas-phase and barrier images exhibit largely homogeneous intensity, but the RBC image contains widespread gas-transfer defects.

Figure 4

The 129Xe-RBC images show numerous gas-transfer defects (yellow) that qualitatively correlate with regions of fibrosis seen on CT (blue arrows). Gas-transfer defects were also found in regions that had no visible fibrosis on CT. These could point to regions of subtle inflammation or early fibrosis that may respond to therapy.

Figure 5

Ratios of the 129Xe-barrier:gas, 129Xe-RBC:gas, and the RBC:barrier images. Unlike the 129Xe-RBC images seen in figure 3, compared to the healthy volunteer, the signal intensity of the RBC:gas images are dramatically different in the IPF subject. Similarly, the RBC:barrier maps in the IPF subject show dramatically reduced 129Xe uptake by the RBCs.