Restricted magnetic resonance diffusion of lung consolidation is not specific for respiratory exacerbation
Authors: Gaël Dournes, François Laurent
Journal: European Respiratory Journal (2017)

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/S) where S1 and S 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=S=0). 2) Due to additional noise, S1 and S may not be exactly equal to zero (as can also be seen in the figures of the article). Therefore S1=(s1+n1) and S=(s+n), where n represents the level of noise. Since noise is expected to be constant n≈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>