We read with interest the recent publication by Ciet et al. [1] related to the diagnosis of respiratory tract exacerbation (RTE) in cystic fibrosis (CF) using diffusion-weighted magnetic resonance imaging (DWI). RTE is a difficult diagnosis and their results highlight the usefulness of magnetic resonance imaging (MRI) in radiation-free management of CF. In this study, the visual analysis of a DWI score had good diagnostic accuracy to discriminate between controls and RTE's and the authors must be congratulated for this result. However, in their study, the total acquisition time in the supine position to complete both T2 and DWI sequences was 30 min, which is very long in the clinical context of RTE if we are to obtain artefact-free images. In addition, only the largest restricted consolidations in the RTE group were selected for analysis, meaning that bronchi potentially thickened and mucus-filled during RTE were excluded. Last but not least, in order to assess the value of DWI as compared to a standard T2 sequence for diagnosing RTE, it is necessary to demonstrate whether or not the DWI “hotspots” could correspond to an artefact called the T2-shine-through effect [2]. Indeed, T2 has already been demonstrated as a biomarker for acute lung inflammation [3]. To solve this, the authors performed quantitative measurements of the apparent diffusion coefficient (ADC) in both controls and subjects undergoing RTE. In controls, ADC measurements were performed randomly inside the lung parenchyma when no hotspots were visible. Surprisingly, the receiver operating characteristic (ROC) curves of the ADC indicated 100% specificity for the diagnosis of RTE towards the lowest ADC values. This crucial point is unfortunately not discussed though it is a feature that corresponds neither to the literature [4–6] nor to the physical principle of this measurement. Indeed, the general mathematical formula to calculate an ADC value is: ADC= −1/b1·Ln(S1/S0) where S1 and S0 correspond to the DWI signals at two b-values, where b is the gradient factor. Therefore, there are two mathematical possibilities under normal conditions: 1) Owing to the very short decay time of the lung signal due to susceptibility artefacts, there is a need for ultra-short echo times (of a microsecond order of magnitude) in order to obtain any signal from the lungs [7, 8]. The DWI echo time was 54 ms and thus the lung signal was a null value at all b-values. In agreement with this, the figures in the article display no vessel and no signal inside the lung (and, as a consequence, S1=S0=0). 2) Due to additional noise, S1 and S0 may not be exactly equal to zero (as can also be seen in the figures of the article). Therefore S1=(s1+n1) and S0=(s0+n0), where n represents the level of noise. Since noise is expected to be constant n0≈n1 and, therefore, the mathematical formula for ADC becomes: ADC= −1/b·Ln(1)=0. Restriction of lung magnetic resonance diffusion is not specific for RTE but can correspond to a normal parenchyma http://ow.ly/wUgk30fDGQ8

中文翻译：

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肺实变的受限磁共振扩散不是呼吸恶化的特异性

我们饶有兴趣地阅读了 Ciet 等人最近发表的文章。[1] 使用弥散加权磁共振成像 (DWI) 诊断囊性纤维化 (CF) 中的呼吸道恶化 (RTE)。RTE 是一项困难的诊断，其结果突出了磁共振成像 (MRI) 在 CF 无辐射管理中的有用性。在这项研究中，DWI 评分的视觉分析在区分对照和 RTE 方面具有良好的诊断准确性，必须祝贺作者的这一结果。然而，在他们的研究中，在仰卧位完成 T2 和 DWI 序列的总采集时间为 30 分钟，如果我们要获得无伪影的图像，这在 RTE 的临床环境中是非常长的。此外，仅选择 RTE 组中最大的受限合并进行分析，这意味着排除了 RTE 期间可能增厚和充满粘液的支气管。最后但并非最不重要的是，为了评估 DWI 与用于诊断 RTE 的标准 T2 序列相比的价值，有必要证明 DWI“热点”是否可以对应于称为 T2 透光效应的人工制品[2]。事实上，T2 已被证明是急性肺部炎症的生物标志物 [3]。为了解决这个问题，作者对对照组和接受 RTE 的受试者的表观扩散系数 (ADC) 进行了定量测量。在对照组中，当没有可见热点时，在肺实质内随机进行 ADC 测量。令人惊讶的是，ADC 的受试者工作特征 (ROC) 曲线表明，对于最低 ADC 值的 RTE 诊断具有 100% 的特异性。遗憾的是，这个关键点没有被讨论，尽管它既不符合文献 [4-6] 也不符合该测量的物理原理。实际上，计算 ADC 值的通用数学公式为： ADC= -1/b1·Ln(S1/S0) 其中 S1 和 S0 对应于两个 b 值处的 DWI 信号，其中 b 是梯度因子。因此，在正常情况下有两种数学可能性：1）由于磁敏感伪影导致肺信号的衰减时间非常短，因此需要超短回波时间（微秒级），以便从肺部获取任何信号 [7, 8]。DWI 回波时间为 54 ms，因此肺信号在所有 b 值下均为空值。与此一致的是，文章中的数字显示肺内没有血管和信号（并且，因此，S1=S0=0）。2) 由于额外的噪声，S1 和 S0 可能不完全等于零（也可以在文章的图中看到）。因此 S1=(s1+n1) 和 S0=(s0+n0)，其中 n 代表噪声水平。由于预期噪声为常数 n0≈n1，因此，ADC 的数学公式变为：ADC= -1/b·Ln(1)=0。肺磁共振扩散的限制不是 RTE 特有的，但可以对应于正常的实质 http://ow.ly/wUgk30fDGQ8