Controlling diffusion of 3He by buffer gases: a structural contrast agent in lung MRI.

PURPOSE: To study the influence of admixing inert buffer gases to laser-polarized (3)He in terms of resulting diffusion coefficients and the consequences for image contrast and resolution. MATERIALS AND METHODS: The diffusion coefficient of (3)He was altered by admixing buffer gases of various molecular weights ((4)He, N(2), and SF(6)). The influence of the pulse sequence and the diffusion coefficient on the appearance of MRI of (laser-polarized) gases was analyzed by comparison of basic theoretical concepts with demonstrative experiments. RESULTS: Excellent agreement between theoretical description and observed signal in simple gradient echoes was observed. A maximum signal gain can be predicted and was experimentally validated. Images acquired under such conditions revealed improved resolution. The nature and concentration of the admixed gas defines a structural threshold for the observed apparent diffusion coefficient (ADC) as demonstrated with diffusion-weighted MRI on a pig's lung flooded with suitable gas mixtures. CONCLUSION: A novel procedure is proposed to control the diffusion coefficient of gases in MRI by admixture of inert buffer gases. Their molecular mass and concentration enter as additional parameters into the equations that describe structural contrast. This allows for setting a structural threshold up to which structures contribute to the image. For MRI of the lung this enables images of very small structural elements (alveoli) only, or in the other extreme, all airways can be displayed with minimal signal loss due to diffusion.

[Microstructure of the lung: diffusion measurement of hyperpolarized 3Helium]. / Mikrostruktur der Lunge: Untersuchung mittels Diffusionsmessung von hochpolarisiertem 3Helium.

Imaging methods to study the lung are traditionally based on x-ray or on radioactive contrast agents. Conventional magnetic resonance imaging (MRI) has only limited applications for lung imaging because of the low tissue density of protons concentration of hydrogen atoms, which are usually the basis for the imaging. The introduction of hyperpolarized noble gases as a contrast agent in MRI has opened new possibilities for lung diagnosis. The present paper describes this new technique. Diffusion-weighted MRI for assessment of the lung microstructure is presented here as an example of the new possibilities of functional imaging. Studies to determine the sensitivity of the diffusion measurement and regarding the correlation with traditionally established methods are also presented, along with results of the measurement of the reproducibility determined in a clinical pilot study on healthy volunteers and patients. Furthermore, a pilot measurement of the 3He diffusion tensor in the lung is presented.

[Hyperpolarized (3)helium gas for functional magnetic resonance imaging of the lung]. / Funktionelle MRT der Lunge unter Verwendung von hyperpolarisiertem 3Helium-Gas.

Lung imaging is traditionally done using X-ray-based methods, since MRI is limited by low proton density as well as inherent magnetic field inhomogeneities of the lung tissue. After introduction of MRI using hyperpolarized noble gases, a totally new field of MRI of the chest has rapidly evolved. These techniques reveal new functional information of the lungs, which could not be obtained before. The first part of this review describes the underlying MR technology explaining distribution of static ventilation, dynamic distribution of ventilation, lung microstructure (apparent diffusion coefficient [ADC]), measurement of oxygen partial pressure (pO(2)), and safety. The clinical potential is afterwards demonstrated in the second part. Therefore, the effort in normal lungs and the mainly focused diseases chronic obstructive pulmonary disease (COPD), smoker's lung, cystic fibrosis, asthma, lung transplantation, and pulmonary embolism are reported.

Assessment of lung microstructure with magnetic resonance imaging of hyperpolarized Helium-3.

Magnetic resonance imaging of the apparent diffusion coefficient (ADC) of hyperpolarized Helium-3 is a new technique for probing pulmonary microstructure in vivo. The aim of this study was the assessment of potential sources of systematic errors of the ADC measurement. The influence of macroscopic motion was determined by measurements at two different delays after initiating the breath-hold, and before and after cardiac arrest. An intercentre comparison was performed in two age- and lung function-matched groups of lung-healthy volunteers at two research sites. Moreover, measurements of diffusion anisotropy were performed. We found no dependency of the ADC as a function of the delay after stop of inspiration. The influence of cardiac motion was less than 10%. In the intercentre comparison study, an excellent agreement between the two sites was found. First measurements of the diffusion tensor of intrapulmonary Helium-3 are shown.

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

PURPOSE: To determine the reproducibility of several parameters of the ADC measurement by calculating the scan-to-scan intrasubject variability. MATERIALS AND METHODS: Measurements were performed using a gradient-echo sequence with a bipolar gradient for diffusion weighting (b=3.89 sec/cm2). Five patients with pulmonary emphysema, and six healthy-lung volunteers were included in the study. Images were acquired after inspiration of 3He during a single inspiratory breath-hold. To assess the reproducibility, the measurement was performed twice (time between measurements=20 minutes) without repositioning the subjects. Analysis was performed on the basis of region-of-interest (ROI) analysis and global lung ADC histograms. RESULTS: The mean ADC of a ROI varied by 5.1% between two measurements for volunteers and by 6.1% for patients. In the global evaluation, the 75th percentile demonstrated the best reproducibility (2%), while other parameters showed variations up to 12%. Only the variation of the standard deviation (SD) and the measure of homogeneity of the ADC map showed a significant difference between patients and volunteers. CONCLUSION: Diffusion-weighted imaging (DWI) is a well-reproducible method for assessing the lung microstructure.