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Editorial for “Application of Simultaneous 18F‐FDG PET With Monoexponential, Biexponential, and Stretched Exponential Model‐Based Diffusion‐Weighted MR Imaging in Assessing the Proliferation Status of Lung Adenocarcinoma”

Editorial for “Application of Simultaneous 18F-FDG PET With Monoexponential, Biexponential, and Stretched Exponential Model-Based Diffusion-Weighted MR Imaging in Assessing the Proliferation Status of Lung Adenocarcinoma” The apparent diffusion coefficient (ADC) generated from diffusion-weighted imaging (DWI) is now widely utilized for the quantitative assessment of various malignant lesions. ADC on DWI yields comparable quantitative ability to standardized uptake value (SUV) on positron emission tomography (PET) without radiation exposure. Recent developments in the intravoxel incoherent motion (IVIM) technique with multiple b values provides the true diffusion coefficient (D), pseudo-diffusion coefficient (D*), and perfusion fraction (f ) and can distinguish molecular diffusion from microcircular perfusion, which is also considered a promising biomarker of malignant lesions on magnetic resonance imaging (MRI). DWI-derived metrics, however, require reproducibility in the same datasets as well as test–retest repeatability for clinical purposes, such as estimation of therapeutic effect and prediction of prognosis. Moreover, the assessment of these quantitative metrics in the lung should be performed with special caution because of its respiratory motion and susceptibility artifacts derived from its air–tissue interface. The reproducibility (intraand interobserver repeatability) for ADC and D of the lung lesions is relatively maintained and has been estimated as within coefficients of variation (WCV) at 5.0%–6.6%. However, reproducibility for D* and f was higher than that for ADC and D and was estimated to be over 20% (21%–35%). The test–retest repeatability of D* and f (WCV at 37%–73%) was also higher than that of ADC and D (WCV at 11%–13%). It is therefore considered that the published data showed that DWI metrics of ADC and D could be reliable biomarkers in terms of reproducibility and repeatability. However, the repeatability of D* and f requires further assessment and discussion for possible use in clinical purposes. The present study showed the correlations of DWIderived ADC, D, D*, and f as well as PET-derived SUV, metabolic tumor volume (MTV), and total lesion glycolysis with the Ki-67 proliferation index (PI) in patients with lung adenocarcinoma who underwent simultaneous PET/MRI scan. The authors concluded that the combination of D, SUVmax, and MTV might predict Ki-67 PI status and might aid in the development of an individualized treatment plan for patients with lung adenocarcinoma. It is encouraging to know that PET/MRI could provide useful parameters for predicting Ki-67 status owing to the simultaneous acquisition of PET and MRI; however, attention should be paid to the lesion size and location in the lung before applying the results to a clinical setting. The reproducibility and repeatability of DWI-derived metrics could deteriorate when the lesion size is less than 2 cm in diameter and/or the location of the lesion is in the lower lung lobe, because small-sized lesions increase the partial volume effect, and respiratory motion is larger in the lower lung field. In this regard, the robustness of DWIderived metrics for quantitative analysis in the lung is still not guaranteed and needs to be improved. Similarly, the repeatability of PET metrics is also highly affected by the lesion size and respiratory motion in the chest, suggesting the need for further development of high-resolution and respiratorycompensated imaging methods for both DWI and PET to acquire robust metrics for quantitative assessment of lung lesions. Another drawback of DWI-derived metrics for PET/MRI is the significant distortion of the images due to the drawback of single-shot echo-planar imaging, which does not allow voxel-by-voxel analysis of PET and MRI despite their simultaneous acquisition. The application of less