Martin H. Deppe

Direct visualisation of collateral ventilation in COPD with hyperpolarised gas MRI

Background Collateral ventilation has been proposed as a mechanism of compensation of respiratory function in obstructive lung diseases but observations of it in vivo are limited. The assessment of collateral ventilation with an imaging technique might help to gain insight into lung physiology and assist the planning of new bronchoscopic techniques for treating emphysema. Objective To obtain images of delayed ventilation that might be related to collateral ventilation over the period of a single breath-hold in patients with chronic obstructive pulmonary disease (COPD). Methods Time-resolved breath-hold hyperpolarised 3He MRI was used to obtain images of the progressive influx of polarised gas into initially non-ventilated defects. Results A time-series of images showed that 3He moves into lung regions which were initially non-ventilated. Ventilation defects with delayed filling were observed in 8 of the 10 patients scanned. Conclusions A method for direct imaging of delayed ventilation within a single breath-hold has been demonstrated in patients with COPD. Images of what is believed to be collateral ventilation and slow filling of peripheral airspaces due to increased flow resistance are presented. The technique provides 3D whole-lung coverage with sensitivity to regional information, and is non-invasive and non-ionising.

Quantification of regional fractional ventilation in human subjects by measurement of hyperpolarized 3He washout with 2D and 3D MRI.

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.

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.

Slice profile effects in 2D slice-selective MRI of hyperpolarized nuclei.

This work explores slice profile effects in 2D slice-selective gradient-echo MRI of hyperpolarized nuclei. Two different sequences were investigated: a Spoiled Gradient Echo sequence with variable flip angle (SPGR-VFA) and a balanced Steady-State Free Precession (SSFP) sequence. It is shown that in SPGR-VFA the distribution of flip angles across the slice present in any realistically shaped radiofrequency (RF) pulse leads to large excess signal from the slice edges in later RF views, which results in an undesired non-constant total transverse magnetization, potentially exceeding the initial value by almost 300% for the last RF pulse. A method to reduce this unwanted effect is demonstrated, based on dynamic scaling of the slice selection gradient. SSFP sequences with small to moderate flip angles (<40 degrees ) are also shown to preserve the slice profile better than the most commonly used SPGR sequence with constant flip angle (SPGR-CFA). For higher flip angles, the slice profile in SSFP evolves in a manner similar to SPGR-CFA, with depletion of polarization in the center of the slice.

A flexible 32-channel receive array combined with a homogeneous transmit coil for human lung imaging with hyperpolarized 3He at 1.5 T.

Parallel imaging presents a promising approach for MRI of hyperpolarized nuclei, as the penalty in signal‐to‐noise ratio typically encountered with 1H MRI due to a reduction in acquisition time can be offset by an increase in flip angle. The signal‐to‐noise ratio of hyperpolarized MRI generally exhibits a strong dependence on flip angle, which makes a homogeneous B1+ transmit field desirable. This paper presents a flexible 32‐channel receive array for 3He human lung imaging at 1.5T designed for insertion into an asymmetric birdcage transmit coil. While the 32‐channel array allows parallel imaging at high acceleration factors, the birdcage transmit coil provides a homogeneous B1+ field. Decoupling between array elements is achieved by using a concentric shielding approach together with preamplifier decoupling. Coupling between transmit coil and array elements is low by virtue of a low geometric coupling coefficient, which is reduced further by the concentric shields in the array. The combination of the 32‐channel array and birdcage transmit coil provides 3He ventilation images of excellent quality with similar signal‐to‐noise ratio at acceleration factors R = 2 and R = 4, while maintaining a homogeneous B1+. Magn Reson Med, 2011.

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.

3He pO2 mapping is limited by delayed‐ventilation and diffusion in chronic obstructive pulmonary disease

Lung pO2 mapping with 3He MRI assumes that the sources of signal decay with time during a breath‐hold are radiofrequency depolarization and oxygen‐dependent T1 relaxation, but the method is sensitive to other sources of spatio‐temporal signal change such as diffusion. The purpose of this work was to assess the use of 3He pO2 mapping in patients with chronic obstructive pulmonary disease.

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.