MRI of helium‐3 gas in healthy lungs: Posture related variations of alveolar size

Abstract

Purpose:To probe the variation of alveolar size in healthy lung tissue as a function of posture using diffusionweighted helium-3 hyperpolarized gas imaging. Materials and Methods:Measurements of the helium-3 apparent diffusion coefficient (ADC) were made on six healthy subjects. These were used to show the variation of alveolar size between the lowermost dependent regions of the lung compared to the uppermost regions of the lung in four postures: supine, prone, left-lateral decubitus, and right-lateral decubitus. Results:The distribution of acinar size in the lungs was found to be heterogeneous, and influenced by lung orientation. In nearly all postures, the ADC was significantly higher in the non-dependent uppermost regions of the lung compared to the dependent lowermost regions of the lung; the greatest variation was found in the left-lateral decubitus position. The difference in ADC between uppermost and lowermost regions was on average 0.012 cm 2 second -1 , which represents 20% of the average ADC value for the whole lung. A systematic decrease in ADC from the apex of the lung to the base was also found, which corresponds to an inherent gradient in alveolar size. Conclusion:The posture dependent variations in ADC were attributed to compression of the parenchyma under its own weight and the mass of the heart. OVER THE PAST DECADE, advances in polarizing technology have allowed imaging of the human lungs using the noble gases helium-3 and xenon-129 as inhaled contrast agents (1). A promising aspect of this technology is the ability to probe the lung microstructure with diffusion sensitive measurements using pulsed gradient sequences (2). Diffusion of the gas atoms in the presence of these gradients results in an attenuation of the magnetic resonance imaging (MRI) signal, which can be used to calculate the apparent diffusion coefficient (ADC). Helium has a very low atomic mass and consequently a very high diffusion coefficient. In the lungs, the dimensions of the acini are smaller than the average distances traversed by freely diffusing helium atoms during the time it takes to measure diffusion during a typical MRI experiment. This "restriction" in gas diffusion leads to a measured ADC that is much smaller in healthy lungs than for diffusion in free-space. Typical MRI measurements reveal that the helium-3 ADC is between 0.1 cm 2 second -1 and 0.2 cm 2 second -1 in healthy lungs (3-5), whereas in free-space it is 0.88 cm 2 second -1 when mixed with room air.The restricted nature of diffusion in the lungs means that the ADC can be used to probe the lung microstructure. For example, in emphysematous lung, destruction of alveolar infrastructure leads to an enlargement of the respiratory airways within the acini, which results in a significantly higher ADC measurement (4,5). Thus, the ADC's sensitivity to the confining length scales can provide an indirect measure of acinar size, despite the fact that the alveolar dimensions are microscopic compared to the size of image pixels. This property of the ADC can be exploi...

Hyperpolarized 3He MR Imaging of the Lung: Effect of Subject Immobilization on the Occurrence of Ventilation Defects

“…The heart and abdomen also contribute to the greater compression of the posterior lung regions in the supine position (23). Gravitational-dependent atelectasis is a common finding in the dependent portions of the lung using computed tomography (8,12,14,17,18) and has been investigated in animal studies and patients using a variety of other modalities including H 3 He MR (7)(8)(9)(10)(12)(13)(14)18,19).…”

“…Although there is generally homogeneous distribution of the MR signal throughout the lung in healthy subjects following inhalation of the gas, small peripheral ventilation defects in the dependent areas of the lungs can been seen and are believed to be caused by collapse of small portions of the lung (3,5). The purpose of this study was to investigate the effect of patient positioning and time on the frequency, size and location of these defects, in order to better understand their impact and significance in the diagnosis of pulmonary disease when using H 3 He MR. Our findings were compared with earlier work done in posture-dependence of lung function, in animal studies and patients, using a variety of other modalities (7)(8)(9)(10)(12)(13)(14)18,19).…”

A system for open‐access 3He human lung imaging at very low field

“…A major limitation in this area is the lack of a method to obtain quantitative, high-resolution maps of lung ventilation and perfusion with the subject in different orientations. Despite minimal orientation possible within the confines of a traditional human MRI scanner, some initial studies have shown that posture changes from supine (face up) to prone (face down) and to decubitus (sideways) affect the lung physiology in a way that can be clearly probed by 3 He MRI (18). Very-low-field 3 He MRI incorporating novel, open-access magnet designs can provide this area of pulmonary physiology with a powerful tool for studying orientational and posture-related effects.…”

Feature analysis of hyperpolarized helium‐3 pulmonary MRI: A study of asthmatics versus nonasthmatics

“…Much of the development for intensity inhomogeneity correction has been applied to MRI of neuroanatomy, where many techniques require a classification or segmentation step identifying the gray matter, white matter, and cerebrospinal fluid regions (16), a scenario that does not apply to helium-3 lung image data. With helium-3 image data, in addition to the amplitude of radiofrequency field, there are other potential contributions to intensity nonuniformity, such as the anatomic diffusion gradient (17) and posturerelated dependencies (18). In general, we tend to observe a low-frequency intensity variation toward the posterior of the lung and, although we do not know the relative contribution of these factors to the overall nonuniformity, we have found that application of our bias-correction algorithm improves image processing performance.…”