Hyperpolarized 129Xe gas lung MRI-SNR and T2* comparisons at 1.5 T and 3 T.

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.

K-space filter deconvolution and flip angle self-calibration in 2D radial hyperpolarised 3He lung MRI.

In hyperpolarised (3)He lung MRI with constant flip angles, the transverse magnetisation decays with each RF excitation imposing a k-space filter on the acquired data. For radial data acquired in an angularly-sequential order, this filter causes streaking, angular shading and loss of spatial resolution in the images. The main aim of this work was to reduce the effects of the RF depletion k-space filter in radial acquisitions. Two approaches are presented; (i) retrospective deconvolution of the k-space filter for sequentially-acquired data and (ii) golden angle acquisition order. Radial trajectories sample the centre of k-space with every projection, thereby self-tracking signal decay. The inverse of the signal decay function was used to retrospectively deconvolve RF depolarisation k-space filter effects and the method was demonstrated in 2D radial imaging in phantoms and human lungs. A golden angle radial acquisition was shown to effectively suppress artefacts caused by the RF depletion k-space filter. In addition, the average flip angle per slice was calculated from the signal decay and the values were found to correspond with conventional flip angle maps, providing a means of flip angle self-calibration.

Combined measurement of pulmonary inert gas washout and regional ventilation heterogeneity by MR of a single dose of hyperpolarized 3He.

Washout of inert gases is a measure of pulmonary function well-known in lung physiology. This work presents a method combining inert gas washout and spatially resolved imaging using hyperpolarized (3) He, thus providing complementary information on lung function and physiology. The nuclear magnetic resonance signal of intrapulmonary hyperpolarized (3) He is used to track the total amount of gas present within the lungs during multiple-breath washout via tidal breathing. Before the washout phase, 3D ventilation images are acquired using (3) He magnetic resonance imaging from the same dose of inhaled gas. The measured washout signal is corrected for T(1) relaxation and radiofrequency depletion, converting it into a quantity proportional to the apparent amount of gas within the lungs. The use of a pneumotachograph for acquisition of breathing volumes during washout, together with lung volumes derived from the magnetic resonance imaging data, permits assessment of the washout curves against physiological model predictions for healthy lungs. The shape of the resulting washout curves obtained from healthy volunteers matches the predictions, demonstrating the utility of the technique for the quantitative assessment of lung function. The proposed method can be readily integrated with a standard breath-hold (3) He ventilation imaging sequence, thus providing additional information from a single dose of gas.

Synchronous acquisition of hyperpolarised 3He and 1H MR images of the lungs - maximising mutual anatomical and functional 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.

Compressed sensing in hyperpolarized 3He lung MRI.

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 k-spaces 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) (approximately 19) typically obtained. The feasibility of producing accurate functional apparent diffusion coefficient (ADC) maps from undersampled data acquired with fewer radiofrequency pulses was also demonstrated, with the preservation of quantitative information (mean ADC(cs) approximately mean ADC(full) approximately 0.16 cm(2) sec(-1)). Prospective acquisition of 2-fold undersampled two-dimensional (3)He images with a compressed sensing k-space pattern was then demonstrated in a healthy volunteer, and the results were compared to the equivalent fully sampled images (SNR(cs) = 34, SNR(full) = 19).

Susceptibility effects in hyperpolarized (3)He lung MRI at 1.5T and 3T.

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 T2. 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 T2 from a voxel (20 x 4 x 4) mm(3) was determined as T2 = 27.8 msec +/- 1.2 msec at 1.5T compared to T2 = 14.4 msec +/- 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 T2 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.