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...

CSSR with hyperpolarized Xe is sensitive to pathology-induced degradation of lung structure/function and shows promise for quantification of disease severity and monitoring treatment response. Magn Reson Med 74:196-207, 2015. © 2014 Wiley Periodicals, Inc.

Multiple-breath washout hyperpolarized (3)He MRI was used to calculate regional parametric images of fractional ventilation (r) as the ratio of fresh gas entering a volume unit to the total end inspiratory volume of the unit. Using a single dose of inhaled hyperpolarized gas and a total acquisition time of under 1 min, gas washout was measured by dynamic acquisitions during successive breaths with a fixed delay. A two-dimensional (2D) imaging protocol was investigated in four healthy subjects in the supine position, and in a second protocol the capability of extending the washout imaging to a three-dimensional (3D) acquisition covering the whole lungs was tested. During both protocols, subjects were breathing comfortably, only restricted by synchronization of breathing to the sequence timings. The 3D protocol was also successfully tested on one patient with cystic fibrosis. Mean r values from each volunteer were compared with global gas volume turnover, as calculated from flow measurement at the mouth divided by total lung volume (from MRI images), and a significant correlation (r = 0.74, P < 0.05) was found. The effects of gravity on R were investigated, and an average decrease in r of 5.5%/cm (Δr = 0.016 ± 0.006 cm(-1)) from posterior to anterior was found in the right lung. Intersubject reproducibility of r imaging with the 2D and 3D protocol was tested, and a significant correlation between repeated experiments was found in a pixel-by-pixel comparison. The proposed methods can be used to measure r on a regional basis.

PurposeTo demonstrate three‐dimensional (3D) multiple b‐value diffusion‐weighted (DW) MRI of hyperpolarized 3He gas for whole lung morphometry with compressed sensing (CS).MethodsA fully‐sampled, two b‐value, 3D hyperpolarized 3He DW‐MRI dataset was acquired from the lungs of a healthy volunteer and retrospectively undersampled in the k y and k z phase‐encoding directions for CS simulations. Optimal k‐space undersampling patterns were determined by minimizing the mean absolute error between reconstructed and fully‐sampled 3He apparent diffusion coefficient (ADC) maps. Prospective three‐fold, undersampled, 3D multiple b‐value 3He DW‐MRI datasets were acquired from five healthy volunteers and one chronic obstructive pulmonary disease (COPD) patient, and the mean values of maps of ADC and mean alveolar dimension (Lm D) were validated against two‐dimensional (2D) and 3D fully‐sampled 3He DW‐MRI experiments.ResultsReconstructed undersampled datasets showed no visual artifacts and good preservation of the main image features and quantitative information. A good agreement between fully‐sampled and prospective undersampled datasets was found, with a mean difference of +3.4% and +5.1% observed in mean global ADC and Lm D values, respectively. These differences were within the standard deviation range and consistent with values reported from healthy and COPD lungs.ConclusionsAccelerated CS acquisition has facilitated 3D multiple b‐value 3He DW‐MRI scans in a single breath‐hold, enabling whole lung morphometry mapping. Magn Reson Med 77:1916–1925, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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 (&lt;400 ml) of isotopically enriched (86% 129Xe) Xe gas polarized to &gt;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.

In hyperpolarized noble gas (HNG) magnetic resonance (MR) imaging, the available polarization is independent of magnetic field strength and for large radiofrequency (rf) coils, such as those used for chest imaging, the body noise becomes the primary noise source making signal-to-noise ratio (SNR) largely frequency independent at intermediate field strengths (0.1-0.5 T). Furthermore, the reduction in the transverse relaxation time, T2, of HNG in lungs with increasing field strength, results in a decrease in the achievable SNR at higher fields. In this work, the optimum field strength for HNG MR imaging was theoretically calculated in terms of both SNR and spatial resolution. SNR calculations used the principle of reciprocity and included contributions to the noise arising from both coil and sample losses in a chest-sized coil for lung imaging. The effects of susceptibility differences, transverse relaxation time, and diffusion were considered in the resolution calculations. The calculations show that the optimum field strength for HNG MR imaging of human lungs is between 0.1 and 0.6 T depending on gas type (helium or xenon) and sample size. At the field strengths currently used by conventional clinical proton MR imaging systems (1-3 T), the predicted SNR are 10%-50% lower than at the optimum field with only slightly worse spatial resolution (10%-20%). At higher fields (>3 T), however, the SNR degrades considerably reducing the achievable spatial resolution. Although HNG of the lung is still feasible at very low field strengths (<50 mT), the available SNR is much lower than at optimum fields and this reduces the achievable spatial resolution. These findings suggest that HNG imaging may be optimally performed at much lower field strengths (0.1-0.6 T) than conventional clinical proton MR imaging systems. This could considerably decrease cost, improve patient access, and reduce chemical shift and susceptibility artifacts and rf heating.

Detection of early subclinical lung disease in children with cystic fibrosis by lung ventilation imaging with hyperpolarised gas MRI ABSTRACT Hyperpolarised 3 He ventilation-MRI, anatomical lung MRI, lung clearance index (LCI), low-dose CT and spirometry were performed on 19 children (6-16 years) with clinically stable mild cystic fibrosis (CF) (FEV 1 > −1.96), and 10 controls. All controls had normal spirometry, MRI and LCI. Ventilation-MRI was the most sensitive method of detecting abnormalities, present in 89% of patients with CF, compared with CT abnormalities in 68%, LCI 47% and conventional MRI 22%. Ventilation defects were present in the absence of CT abnormalities and in patients with normal physiology, including LCI. Ventilation-MRI is thus feasible in young children, highly sensitive and provides additional information about lung structure-function relationships.

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...