利用动脉自旋标记技术测量肺血流分布的特征。
Characterizing pulmonary blood flow distribution measured using arterial spin labeling.
机构信息
Department of Medicine, Division of Physiology, University of California, San Diego, La Jolla, CA 92093-0931, USA.
出版信息
NMR Biomed. 2009 Dec;22(10):1025-35. doi: 10.1002/nbm.1407.
The arterial spin labeling (ASL) method provides images in which, ideally, the signal intensity of each image voxel is proportional to the local perfusion. For studies of pulmonary perfusion, the relative dispersion (RD, standard deviation/mean) of the ASL signal across a lung section is used as a reliable measure of flow heterogeneity. However, the RD of the ASL signals within the lung may systematically differ from the true RD of perfusion because the ASL image also includes signals from larger vessels, which can reflect the blood volume rather than blood flow if the vessels are filled with tagged blood during the imaging time. Theoretical studies suggest that the pulmonary vasculature exhibits a lognormal distribution for blood flow and thus an appropriate measure of heterogeneity is the geometric standard deviation (GSD). To test whether the ASL signal exhibits a lognormal distribution for pulmonary blood flow, determine whether larger vessels play an important role in the distribution, and extract physiologically relevant measures of heterogeneity from the ASL signal, we quantified the ASL signal before and after an intervention (head-down tilt) in six subjects. The distribution of ASL signal was better characterized by a lognormal distribution than a normal distribution, reducing the mean squared error by 72% (p < 0.005). Head-down tilt significantly reduced the lognormal scale parameter (p = 0.01) but not the shape parameter or GSD. The RD increased post-tilt and remained significantly elevated (by 17%, p < 0.05). Test case results and mathematical simulations suggest that RD is more sensitive than the GSD to ASL signal from tagged blood in larger vessels, a probable explanation of the change in RD without a statistically significant change in GSD. This suggests that the GSD is a useful measure of pulmonary blood flow heterogeneity with the advantage of being less affected by the ASL signal from tagged blood in larger vessels.
动脉自旋标记（ASL）方法提供的图像中，每个图像体素的信号强度理想情况下与局部灌注成正比。对于肺灌注研究，肺段内 ASL 信号的相对分散度（RD，标准差/平均值）被用作流量异质性的可靠测量指标。然而，由于 ASL 图像还包含来自较大血管的信号，如果血管在成像期间充满标记的血液，则这些信号可能反映血液体积而不是血流，因此，肺内 ASL 信号的 RD 可能与灌注的真实 RD 系统地不同。理论研究表明，肺脉管系统的血流呈对数正态分布，因此异质性的适当衡量标准是几何标准差（GSD）。为了测试 ASL 信号是否呈对数正态分布用于肺血流，确定较大的血管是否在分布中起重要作用，并从 ASL 信号中提取与生理相关的异质性度量，我们在六位受试者中定量分析了干预（头低位倾斜）前后的 ASL 信号。与正态分布相比，ASL 信号的分布更适合用对数正态分布来描述，这将均方误差降低了 72%（p < 0.005）。头低位倾斜显著降低了对数正态尺度参数（p = 0.01），但未改变形状参数或 GSD。倾斜后 RD 增加，且仍显著升高（增加 17%，p < 0.05）。测试案例结果和数学模拟表明，RD 比 GSD 对较大血管中标记血液的 ASL 信号更敏感，这可能是 RD 变化而 GSD 无统计学意义变化的原因。这表明 GSD 是肺血流异质性的有用衡量标准，其优点是受较大血管中标记血液的 ASL 信号的影响较小。
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