Salma Ajraoui

Compressed sensing in hyperpolarized 3He Lung MRI

In this work, the application of compressed sensing techniques to the acquisition and reconstruction of hyperpolarized 3He lung MR images was investigated. The sparsity of 3He lung images in the wavelet domain was investigated through simulations based on fully sampled Cartesian two‐dimensional and three‐dimensional 3He 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 3He with the levels of signal to noise ratio (SNR) (∼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 ADCcs ∼ mean ADCfull ∼ 0.16 cm2 sec−1). Prospective acquisition of 2‐fold undersampled two‐dimensional 3He 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 (SNRcs = 34, SNRfull = 19). Magn Reson Med 63:1059–1069, 2010.

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. Copyright

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 129Xe human lung magnetic resonance imaging was compared at 1.5 T and 3 T. Experiments were performed at both B0 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 129Xe 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 129Xe 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 129Xe gas in fully inflated lungs was measured to be 52 ms and 24 ms at 1.5 T and 3 T, respectively. Magn Reson Med, 2012.

Incorporation of prior knowledge in compressed sensing for faster acquisition of hyperpolarized gas images.

Adding prior knowledge to compressed sensing reconstruction can improve image reconstruction. In this work, two approaches are investigated to improve reconstruction of two‐dimensional hyperpolarized 3He lung ventilation images using prior knowledge. When compared against a standard compressed sensing reconstruction, the proposed methods allowed acquisition of images with higher under‐sampling factors and reduction of the blurring effects that increase with higher reduction factors when fixed flip angles are used. These methods incorporate the prior knowledge of polarization decay of hyperpolarized 3He and the mutual anatomical information from a registered 1H image acquired in the same breath. Three times accelerated two‐dimensional images reconstructed with compressed sensing and prior knowledge gave lower root‐mean square error, than images reconstructed without introduction of any prior information. When introducing the polarization decay as prior knowledge, a significant improvement was achieved in the lung region, the root mean square value decreased by 45% and from the whole image by 36%. When introducing the mutual anatomical information as prior knowledge, the root mean square decreased by 21% over the lung region and by 15% over the whole image. Magn Reson Med, 2013.

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

To compare susceptibility effects in hyperpolarized 3He lung MRI at the clinically relevant field strengths of 1.5T and 3T.

The influence of field strength on the apparent diffusion coefficient of 3He gas in human lungs

The 3He MR diffusion signal is sensitive to lung microstructure, but it is also affected by the presence of background field inhomogeneities induced by the magnetic susceptibility difference at the air‐tissue interface. These susceptibility‐induced gradients, which are dependent on field strength, have been assumed negligible in theoretical models used to extract airway morphometric information from 3He MR diffusion data at field strengths up to 4.7 T. In this work, the effect of susceptibility gradients on 3He apparent diffusion coefficient is demonstrated with experiments in healthy volunteers at two B0 field strengths: 1.5 and 3 T. Apparent diffusion coefficient values obtained at 3 T were systematically larger than at 1.5 T, demonstrating that susceptibility effects are statistically significant even at clinical field strengths (B0 ≤ 3 T) and introduce biases in the estimates of airway dimensions (e.g., mean linear intercept up to 17% larger at 3 T than 1.5 T). Susceptibility effects should be taken into account in the development of theoretical models of lung 3He MR diffusion and considered when interpreting 3He apparent diffusion coefficients obtained at different B0. Magn Reson Med, 2012.

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 3He, thus providing complementary information on lung function and physiology. The nuclear magnetic resonance signal of intrapulmonary hyperpolarized 3He 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 3He magnetic resonance imaging from the same dose of inhaled gas. The measured washout signal is corrected for T1 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 3He ventilation imaging sequence, thus providing additional information from a single dose of gas. Magn Reson Med, 2010.

Acquisition of 3He ventilation images, ADC, T2* and B1 maps in a single scan with compressed sensing

In imaging of human lungs with hyperpolarised noble gases, measurements of apparent diffusion coefficient (ADC) and relaxation time provide valuable information for the assessment of lung microstructure. In this work, a sequence was developed for interleaved acquisition of ventilation images, ADC, T2* and flip angle maps in a single scan from the human lungs with a single dose of inhaled 3He at 3 T. Spatially registered ventilation images with parametric maps were obtained. The total acquisition time was reduced by random undersampling of the k‐space and reconstruction using compressed sensing (CS). The gain in speed was used for an increase in spatial resolution. Mean ADC values from the fully sampled and undersampled CS data exhibit no statistically significant difference in a given subject. The mean T2* values, however, were found to differ significantly, which is attributed to the combined effect of low signal‐to‐noise ratio (SNR) of the fully sampled data and the smoothing effect inherent in CS reconstruction. Copyright

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

In hyperpolarised 3He 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. Copyright

Measurement of laser heating in spin exchange optical pumping by NMR diffusion sensitization gradients

This paper details pulsed gradient NMR measurements of the H3e diffusion coefficient in sealed cells during spin exchange optical pumping. The potential of ultra low field magnetic resonance imgaing (MRI) and NMR for noninvasive measurement of cell pressure is demonstrated. Diffusion sensitization gradients allow measurement of the H3e diffusion coefficient from which the pressure and/or temperature of the gas can be determined during optical pumping. The pressure measurements were compared with neutron time of flight transmission measurements. Good agreement was observed between the temperature/pressure measurements and predictions based on Chapman–Enskog theory. The technique had sufficient sensitivity to observe the diffusion coefficient increasing with temperature in a sealed cell. With this method, evidence for laser heating of the H3e during optical pumping was found. The results show that NMR diffusion measurements allow noninvasive measurement of the cell temperature and/or pressure in an optical ...