In this work, the application of compressed sensing techniques to the acquisition and reconstruction of hyperpolarized 3 He lung MR images was investigated. The sparsity of 3 He lung images in the wavelet domain was investigated through simulations based on fully sampled Cartesian two-dimensional and three-dimensional 3 He lung ventilation images, and the kspaces of 2D and 3D images were undersampled randomly and reconstructed by minimizing the L1 norm. The simulation results show that temporal resolution can be readily improved by a factor of 2 for two-dimensional and 4 to 5 for three-dimensional ventilation imaging with 3 He with the levels of signal to noise ratio (SNR) (~19) typically obtained. Hyperpolarized (HP) helium-3 gas MRI takes advantage of the nonequilibrium polarization achieved by optical pumping to provide high-resolution images of lung ventilation and function (1-3). The non-renewable longitudinal magnetization is depleted with application of radiofrequency (RF) excitations, and the relationship between the SNR, k-space sampling pattern, and the number of RF pulses is highly flip-angle dependent. Moreover, lung imaging requires rapid sequences to capture dynamic gas flow in the airways and ventilation volume during a breath hold (20 sec). Hence, HP 3 He MRI is a good candidate for undersampling schemes. In previous work, parallel RF encoding (4), radial (5), and spiral (6) methods have been used in human HP gas MRI of the lungs to accelerate the acquisition. These methods can be demanding on the hardware in that they require multiple receivers and implementation of robust non-Cartesian sequences. Recently, compressed sensing (CS) techniques have been applied to MRI acquisition and reconstruction. These methods were developed in the field of information theory by Donoho (7) and Candes et al. (8) and were recently applied to proton MRI (9) and spectroscopic imaging of HP 13 C (10) by Lustig and co-workers. The idea behind the theory is to reconstruct a subset of linear measurements, much smaller than the actual full data set, using a nonlinear method. With CS algorithms, the sparsity of MR images in a specific transform domain can be exploited in order to reconstruct images from undersampled k-space (9). The reconstruction relies on L1-norm minimization in a sparse transform space and the quality of reconstruction relies on the sparsity of the data.In this work, CS theory was applied to the reconstruction of subsampled Cartesian-encoded HP 3 He gas images of the lungs. The potential advantages and limitations of the method were investigated in the context of a potentially faster acquisition time with fewer RF pulses. First, the sparsity of lung images in the wavelet transform domain was investigated in order to validate the possibility of applying the CS method as a means of subsampling. Then, simulations were performed to investigate the feasibility of two-dimensional (2D) and three-dimensional (3D) Cartesian CS undersampling for 3 He lung MRI. The effect of reduction factor upon the quality of...

In this work, the production rate of a spin-exchange optical pumping 129Xe gas polarizer was optimized for routine generation of hyperpolarized 129Xe for in vivo lung MRI. This system uses a narrow (∼ 0.1 nm linewidth), tuneable external cavity laser (operating at ∼25 W) for SEOP of 3% gas mixtures of Xe inside a mid-pressure (2 bars) cell of 491 cm3 volume. Under this regime, theoretical and experimentally measured 129Xe polarizations were calculated to be 24% and 12%, respectively, for a gas flow rate of 300 sccm and a cell temperature of 373 K. The photon efficiency was evaluated, yielding theoretical and experimental values of 0.039 and 0.046, respectively. The theoretical efficiency was calculated from spin-exchange and spin-destruction cross sections and the experimental photon efficiency was measured under flow for a gas-cell residency time equal to an empirically determined spin-exchange time of 45 s. In addition, details of the Xe freeze-out process were analyzed with a model of polarization decay during Xe accumulation in the frozen phase, where a T1 of 87 ± 2 min was observed. To demonstrate the system's application, in vivo lung magnetic resonance images (signal-to-noise ratio ∼ 50 from a voxel of 15 mm× 4 mm× 4 mm) were acquired using modest volumes (<400 ml) of isotopically enriched (86% 129Xe) Xe gas polarized to >10%. Despite the experimental polarization being a factor of 2 lower than the predicted polarization for typical operating parameters, the system is close to the theoretical photon efficiency and the system has so far produced polarized gas for more than 100 in vivo 129Xe lung imaging studies.

Purpose:To compare susceptibility effects in hyperpolarized 3 He lung MRI at the clinically relevant field strengths of 1.5T and 3T. Materials and Methods:Susceptibility-related B 0 inhomogeneity was evaluated on a macroscopic scale by B 0 field mapping via phase difference. Subpixel susceptibility effects were quantified by mapping T * 2 . Comparison was made between ventilation images obtained from the same volunteers at both field strengths. Results:The B 0 maps at 3T show enhanced off-resonance effects close to the diaphragm and the ribs due to susceptibility differences. The average T * 2 from a voxel (20 ϫ 4 ϫ 4) mm 3 was determined as T * 2 ϭ 27.8 msec Ϯ 1.2 msec at 1.5T compared to T * 2 ϭ 14.4 msec Ϯ 2.6 msec at 3T. In ventilation images the most prominent effect is increased signal attenuation close to the intrapulmonary blood vessels at higher B 0 . Conclusion:Image homogeneity and T * 2 are lower at 3T due to increased B 0 inhomogeneity as a consequence of susceptibility differences. These findings indicate that 3 He imaging at 3T has no obvious benefit over imaging at 1.5T, as signal-to-noise ratio (SNR) was comparable for both fields in this work. IN MRI OF HYPERPOLARIZED NUCLEI the contribution of the Boltzmann polarization, which depends on the field strength (B 0 ) of the static magnetic field, to the longitudinal magnetization is negligible, with the nuclear polarization governed by the external (hyper)polarization physics. While in conventional proton MRI the signal-to-noise ratio (SNR) increases with higher B 0 field strength, the influence of B 0 on imaging of hyperpolarized species is less obvious (1), and imaging at low field strengths becomes feasible and possibly preferable due to longer transverse relaxation times (2-5).Nevertheless, the current trend for multinuclear MR development on whole-body scanners is toward field strengths Ն3T, driven by the need for improved SNR in new applications of low-abundance nuclei such as 13 C and 23 Na. As a consequence, imaging of hyperpolarized nuclei at 3T is emerging (6,7), and there is the need to evaluate the performance at this field strength.In addition to its influence on SNR (1,8), the B 0 field strength has an effect on the field homogeneity via localized magnetic susceptibility differences both on a microscopic (subpixel) and macroscopic (larger than pixel) length scale. The susceptibility difference between lung tissue and air is on the order of ⌬ Ϸ 9 ppm (9). Susceptibility-related field gradients have a bearing on image appearance and the local effective transverse relaxation time T * 2 measured over a given voxel size. Previously, susceptibility effects in 3 He lung MRI at different field strengths have been studied by ramping down a 1.5T scanner to 0.54T (10), and a method to compensate for susceptibility artifacts in gradient-echo imaging at 1.5T has been proposed (11).In this work the influence of susceptibility differences between tissue and gas spaces in 3 He lung imaging at 1.5T and 3T was studied on the macroscopic scale by empl...

The multinuclear technique is capable of providing registered lung images with mutually complementary functional and structural spatial information.

The development of hybrid medical imaging scanners has allowed imaging with different detection modalities at the same time, providing different anatomical and functional information within the same physiological time course with the patient in the same position. Until now, the acquisition of proton MRI of lung anatomy and hyperpolarised gas MRI of lung function required separate breath-hold examinations, meaning that the images were not spatially registered or temporally synchronised. We demonstrate the spatially registered concurrent acquisition of lung images from two different nuclei in vivo. The temporal and spatial registration of these images is demonstrated by a high degree of mutual consistency that is impossible to achieve in separate scans and breath holds.

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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.

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