Mine the moon for 3He MRI? Not yet.
Mine the moon for 3He MRI? Not yet.
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在月球上开采 3He MRI？
DOI：
10.1152/japplphysiol.00035.2013
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
Woods,JC
中科院分区：
文献类型：
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作者：
Woods,JC
最近肺成像界对3He的严重短缺议论不休，这是由于美国国土安全部的需求增加了10倍以上，需求量增加了10倍以上(2)。公共和私人资助的肺部MRI项目（除了那些涉及低温物理和中子探测的项目）戛然停止，因为科学工作中宝贵的超极化气体的可用性减少到零。许多人谴责这种情况，然而其他人，如罗伯茨研究所的研究小组，意识到有机会进一步研究129Xe，它是3He（1,3,4,7）的表亲，更重，自旋为1/2，极化程度更低。Robarts等人的结果令人振奋，因为129Xe磁共振图像显示的肺通气缺陷与3He分布相似，具有足够高的信噪比，可用于区域通气量化和表观扩散系数（ADC）的测量（4,7,8）。与3He相比，129Xe的通风缺陷更大，这常常引起人们对其原因的猜测，这可能与本质上较低的信号或较重气体的较大质量和粘度有关，也可能与之无关(6)。在本期中，Kirby等人对不同慢性阻塞性肺疾病（COPD）严重程度的受试者进行了3He和129Xe MRI的仔细比较(6)。这项研究的独特和重要之处在于，作者竭尽全力试图探索和解释在两种超极化气体混合物之间观察到的通风模式的一致差异。在他们和其他人的研究中，129Xe患者往往比3He患者有更大、更多的缺陷（6,8）。许多人非正式地推测，与3He相比，本质上较低的129Xe信号可能是129Xe被确定为通风缺陷的区域的原因，而另一些人则认为气体的物理性质更重要。Kirby研究通过对气体密度和粘度的精确估计，结合对个体COPD患者的两种气体图像的掩膜和感兴趣区域分析，开始更清楚地回答这个问题。本研究的目的是评估气肿严重程度较高地区的通气差异[通过更高的气体ADC和/或通过计算机断层扫描（CT）降低x射线衰减来测量]。尽管密度差异仍然很大（3He/129Xe为0.6/2.7 kg/m3），但仍试图匹配气体混合物的粘度（3He/129Xe分别为2.5/2.0 P）。综合起来，气体性质的差异应该导致3He在大导气管和小导气管中的流动阻力更低(5)，这确实是观察到的：129Xe混合物的通风缺陷更大。评估结构和功能的区域差异
THE PULMONARY IMAGING COMMUNITY was abuzz recently with the acute shortage of 3He, brought about by a more than 10-fold increase in demand from the US Department of Homeland Security, a demand increase more than 10-fold the annual supply (2). Public and privately funded pulmonary MRI projects ground to a screeching halt (in addition to those involving low-temperature physics and neutron detection), as the availability of the precious hyperpolarizable gas decreased to zero for scientific work. Many decried the situation, and yet others, like the group at Robarts Research Institute, realized an opportunity to further investigate 129Xe, the heavier, less-polarizable spin-1/2 cousin of 3He (1, 3, 4, 7). Results from Robarts and others have been heartening in the sense that 129Xe magnetic resonance images show pulmonary ventilation defects with similar distributions as 3He, with sufficiently high signal to noise for regional ventilation quantification and measurements of apparent diffusion coefficient (ADC)(4, 7, 8). Larger ventilation defects seen with 129Xe, compared with 3He, have often led to speculation about the causes, which may or may not relate to intrinsically lower signals or larger mass and viscosity of the heavier gas (6).In this issue, Kirby et al. present such a careful comparison of 3He and 129Xe MRI, in subjects with a range of chronic obstructive pulmonary disease (COPD) severities (6). What makes this study unique and important is the fact that the authors go to lengths to attempt to explore and explain the consistent differences in ventilation patterns observed between the two hyperpolarized gas mixtures. In their and others’ studies, patients have tended to have larger and more numerous defects with 129Xe than with 3He (6, 8). Many have speculated informally that the intrinsically lower 129Xe signal is likely responsible for regions identified as ventilation defects via 129Xe, compared with 3He, while others have argued that physical properties of the gases are more responsible. The Kirby study begins to answer this question more clearly with precise estimates of gas density and viscosity, combined with masked and region of interest analysis of images from the two gases in individual COPD patients. The stated purpose of this study was to evaluate these ventilation differences in regions of higher emphysema severity [measured by higher gas ADC and/or decreased X-ray attenuation via computed tomography (CT)]. An attempt was made to match viscosities of the gas mixtures (2.5/2.0 P for 3He/129Xe, respectively), although the density differences remained large (0.6/2.7 kg/m3 for 3He/129Xe). Combined, the gas-property differences should lead to lower flow resistance for 3He in both large-and small-conducting airways (5), and this is indeed what was observed: greater ventilation defects for the 129Xe mixture. To evaluate regional differences between structure and function