Hyperpolarized ¹²⁹Xe gas lung MRI–SNR and T₂^* comparisons at 1.5 T and 3 T

Abstract

In this study, the signal-to-noise ratio of hyperpolarized (129)Xe human lung magnetic resonance imaging was compared at 1.5 T and 3 T. Experiments were performed at both B(0) fields with quadrature double Helmholtz transmit-receive chest coils of the same geometry with the same subject loads. Differences in sensitivity between the two field strengths were assessed from the signal-to-noise ratio of multi-slice 2D (129)Xe ventilation lung images obtained at the two field strengths with a spatial resolution of 15 mm × 4 mm × 4 mm. There was a systematically higher signal-to-noise ratio observed at 3 T than at 1.5 T by a factor of 1.25. Mean image signal-to-noise ratio was in the range 27-44 at 1.5 T and 36-51 at 3 T. T 2* of (129)Xe gas in the partially inflated lungs was measured to be 25 ms and 18 ms at 1.5 T and 3 T, respectively. T 2* of (129)Xe gas in fully inflated lungs was measured to be 52 ms and 24 ms at 1.5 T and 3 T, respectively.

Dose and pulse sequence considerations for hyperpolarized 129Xe ventilation MRI

“…For example, Xu et al (16) reported mean SNR of 36, and 44 for 129 Xe ventilation MRI at 1.5 T and 3.0 T. They employed a voxel volume of 0.24 ml and an estimated DE ∼0.36 ml. This suggests that they achieved an SNR n of 4.2 ml -2 at 1.5 T, and 5.0 ml -2 at 3.0 T. Even after accounting for their use of a lower 4 kHz bandwidth, their normalized SNR values are 50-86% higher than those we report here.…”

“…However, this slope was much smaller or even presented as a descending trend in the GRE acquisition (Figure 6). This is likely because GRE images were acquired from the anterior-to-posterior slice order, which attenuates the true signal enhancement in the better-ventilated posterior slices because they experience greater T1 and RF decay before being encoded (16,34). By contrast the 3D-radial acquisition is a truly 3D scan that preserves the physiologically dictated SNR distribution.…”

Feasibility of human lung ventilation imaging using highly polarized naturally abundant xenon and optimized three‐dimensional steady‐state free precession

“…NAXe MR is particularly promising at 3 T, wherein the increased SNR compared with 1.5 T—arising from a combination of differences in Larmor frequency, field inhomogeneity, coil sensitivity and the “system” (e.g., transmission line and receiver noise figure) at the two field strengths—reflects previous observations with spoiled gradient echo imaging . This SNR benefit, coupled with the possibility of using receiver array coils rather than standard transmit–receive designs as applied here, may facilitate NAXe lung imaging with 3D isotropic resolutions in future.…”

“…The T 2, CPMG of 129 Xe at 1.5 T or 3 T has not been reported, although measurements at low‐field (0.2 T) in humans , and high‐field (4.7 T) in rats have suggested a value of ∼300 ms in the lungs. However, because the field dependence of T 2, CPMG is not known, and measurement of this parameter is challenging in the lungs with xenon due to the finite inter‐echo times attainable and diffusion of the gas causing incomplete refocusing, a range of T 2, CPMG values between 300 ms and 25 ms (the 129 Xe T 2 * in partially inflated human lungs at 1.5 T ) was considered in the following simulations. The longitudinal relaxation time of 129 Xe in human lungs was taken to be T 1 = 20 s .…”

Quantitative analysis of hyperpolarized129Xe gas transfer MRI

“…Note that, for this work, TE 90 values were measured from the center of the RF pulse, rather than from its end; thus, TE 90 values presented here are longer than those indicated previously). The dissolved‐phase components were back extrapolated to t = 0, using T2* = 2 ms, whereas the gas‐phase was corrected to t = 0 using T2* = 50 ms . In addition, the 129 Xe gas‐phase image intensity was increased by sin(20°)/sin(0.5°) = 39 to account for the higher flip angle applied to the dissolved versus gas‐phase images.…”