Wolfgang G. Schreiber

Functional MRI of the lung using hyperpolarized 3‐helium gas

Lung imaging has traditionally relied on x‐ray methods, since proton MRI is limited to some extent by low proton density in the lung parenchyma and static field inhomogeneities in the chest. The relatively recent introduction of MRI of hyperpolarized noble gases has led to a rapidly evolving field of pulmonary MRI, revealing functional information of the lungs, which were hitherto unattainable. This review article briefly describes the physical background of the technology, and subsequently focuses on its clinical applications. Four different techniques that have been used in various human investigations are discussed: ventilation distribution, ventilation dynamics, and small airway evaluation using diffusion imaging and oxygen uptake assessment. J. Magn. Reson. Imaging 2004;20:540–554.

Dynamic contrast‐enhanced myocardial perfusion imaging using saturation‐prepared TrueFISP

To develop and test a saturation‐recovery TrueFISP (SR‐TrueFISP) pulse sequence for first‐pass myocardial perfusion imaging.

A multicenter measurement of magnetization transfer ratio in normal white matter

To assess the importance of intercenter variations when measuring magnetization transfer ratio (MTR) in the brain, six European centers measured MTR in normal white matter. MTR ranged from 9 to 51 percent units (25 sequences). The effective flip angle of the saturating pulse divided by the pulse repetition time (ENRsat degrees/msec) was a good predictor of MTR (MTR = 3.25 ENRsat).J. Magn. Reson. Imaging 1999; 9:441–446.

Assessment of a single-acquisition imaging sequence for oxygen-sensitive 3He-MRI

MRI of the lungs using hyperpolarized helium‐3 (3He) allows the determination of intrapulmonary oxygen partial pressures (pO2). The need to separate competing processes of signal loss has hitherto required two different imaging series during two different breathing maneuvers. In this work, a new imaging strategy to measure pO2 by a single series of consecutive scans is presented. The feasibility of the method is demonstrated in three healthy human volunteers. Maps and histograms of intrapulmonary pO2 are calculated. Changes in the oxygen concentration of the inhaled gas mixture are well reproduced in the histograms. Monte Carlo (MC) simulations of the temporal evolution of 3He hyperpolarization within the lungs were performed to evaluate the accuracy of this measurement technique, and its limitations. Magn Reson Med 47:105–114, 2002.

Diffusion-weighted MRI of the lung with hyperpolarized helium-3: a study of reproducibility.

To determine the reproducibility of several parameters of the ADC measurement by calculating the scan‐to‐scan intrasubject variability.

Pulmonary ventilation imaged by magnetic resonance: at the doorstep of clinical application

Over the past few years, magnetic resonance imaging (MRI) has emerged as an important instrument for functional ventilation imaging. The aim of this review is to summarize established clinical methods and emerging techniques for research and clinical arenas. Before the advent of MRI, chest radiography and computed tomography (CT) dominated morphological lung imaging, while functional ventilation imaging was accomplished with scintigraphy. Initially, MRI was not used for morphological lung imaging often, due to technical and physical limitations. However, recent developments have considerably improved anatomical MRI, as well as advanced new techniques in functional ventilation imaging, such as inhaled contrast aerosols, oxygen, hyperpolarized noble gases (Helium-3, Xenon-129), and fluorinated gases (sulphur-hexafluoride). Straightforward images demonstrating homogeneity of ventilation and determining ventilated lung volumes can be obtained. Furthermore, new image-derived functional parameters are measurable, such as airspace size, regional oxygen partial pressure, and analysis of ventilation distribution and ventilation/perfusion ratios. There are several advantages to using MRI: lack of radiation, high spatial and temporal resolution and a broad range of functional information. The MRI technique applied in patients with chronic obstructive pulmonary disease, emphysema, cystic fibrosis, asthma, and bronchiolitis obliterans, may yield a higher sensitivity in the detection of ventilation defects than ventilation scintigraphy, CT or standard pulmonary function tests. The next step will be to define the threshold between physiological variation and pathological defects. Using complementary strategies, radiologists will have the tools to characterize the impairment of lung function and to improve specificity.

3He-MRI-based measurements of intrapulmonary pO2 and its time course during apnea in healthy volunteers: first results, reproducibility, and technical limitations

We applied a recently developed method of following the time course of the intrapulmonary oxygen partial pressure pO2(t) during apnea by 3He MRI to healthy volunteers. Using two imaging series with different interscan times during two breathholds (double acquisition technique), relaxation of 3He due to paramagnetic oxygen and depolarization by RF pulses were discriminated. In all four subjects, the temporal evolution of pO2 was found to be linear, and was described by an initial partial pressure p0 and a decrease rate R. Also, regional differences of both p0 and R were observed. A correlation between p0 and R was apparent. Finally, we discuss limitations of the double acquisition approach. Copyright

3He MRI in healthy volunteers: preliminary correlation with smoking history and lung volumes

MRI with hyperpolarized helium‐3 (3He) provides high‐resolution imaging of ventilated airspaces. The first aim of this 3He‐study was to compare observations of localized signal defects in healthy smokers and non‐smokers. A second aim was to describe relationships between parameters of lung function, volume of inspired 3He and signal‐to‐noise ratio. With Ethics Committee approval and informed consent, 12 healthy volunteers (seven smokers and five non‐smokers) were studied. Imaging was performed in a 1.5 T scanner using a two‐dimensional FLASH sequence at 30V transmitter amplitude (TR/TE/α = 11 ms/4.2 ms/<10°). Known amounts of 3He were inhaled from a microprocessor‐controlled delivery device and imaged during single breath‐holds. Images were evaluated visually, and scored using a prospectively defined ‘defect‐index’. Signal‐to‐noise ratio of the images were correlated with localization, 3He volumes and static lung volumes. Due to poor image quality studies of two smokers were not eligible for the evaluation. Smokers differed from non‐smokers in total number and size of defects: the ‘defect‐index’ of smokers ranged between 0.8 and 6.0 (median = 1.1), that of non‐smokers between 0.1 and 0.8 (median = 0.4). Intraindividually, an anteroposterior gradient of signal‐to‐noise ratio was apparent. Signal‐to‐noise ratio correlated with the estimated amount of hyperpolarization administered (r = 0.77), but not with static lung volumes. We conclude that 3He MRI is a sensitive measure to detect regional abnormalities in the distribution of ventilation in clinically healthy persons with normal pulmonary function tests. Copyright

k-Space filtering in 2D gradient-echo breath-hold hyperpolarized 3He MRI: Spatial resolution and signal-to-noise ratio considerations

In this work some of the factors that can influence the signal‐to‐noise ratio (SNR) and spatial resolution in MR images of inhaled hyperpolarized gases are systematically addressed. In particular, the effects of RF depletion of longitudinal polarization and image gradient diffusion dephasing were assessed in terms of their contribution to a k‐space filter. By means of theoretical simulations and a novel method of experimental validation using a variable transverse magnetization of the 1H signal, systematic quantitative and qualitative investigations of the effects of k‐space filtering intrinsic to imaging of hyperpolarized gas were made. A 2D gradient‐echo image is considered for a range of flip angles with centric, sequential, and half‐Fourier Cartesian phase‐encoding strategies, and the results are assessed in terms of SNR and spatial resolution in the reconstructed images. Centric phase encoding was found to give the best SNR at higher flip angles, with a trade‐off in spatial resolution compared to sequential phase encoding. A half‐Fourier approach potentially offers increased SNR through the use of higher flip angles without compromising the spatial resolution, which is comparable to that achieved with sequential encoding. Magn Reson Med 47:687–695, 2002.

Functional Evaluation of Emphysema Using Diffusion-weighted 3helium-magnetic Resonance Imaging, High-resolution Computed Tomography, and Lung Function Tests

Purpose:To assess the emphysematous enlargement of distal airspaces and concomitant large and small airway disease using diffusion-weighted 3Helium-magnetic resonance imaging (MRI), high-resolution computed tomography (HRCT), and lung function tests (LFT). Methods:Seven patients were examined after single lung transplantation (LTx) and 1 before double LTx for various forms of emphysema. Five patients after double LTx served as controls. Patients were assessed by 3Helium-MRI (apparent diffusion coefficient [ADC]), HRCT (mean lung density [MLD], emphysema index [EI]), and LFT. Results:Transplanted lungs: mean ADC = 0.17 cm2/s, MLD = −848 H, EI = 22%. Emphysematous lungs: mean ADC = 0.33 cm2/s, MLD = −922 H; EI = 54%. Good correlations were found between ADC and MLD (r = 0.6), EI (r = 0.8), intrathoracic gas volume (r = 0.7), forced expiratory volume in 1 second (r = 0.7), and forced expiratory flows (r = 0.7). In contrast, HRCT only provided moderate correlations with LFT (EI: r = 0.5; MLD: r [le] 0.4). Conclusion:In this initial study, 3He-MRI yield good correlations with HRCT and agrees better than HRCT with the functional characterization of emphysema regarding hyperinflation, large and small airway disease as provided by LFT.