Interstitial pneumonia of different tissue types, mainly reflected in differences in histological types, has a different prognosis. And imaging assumes an irreplaceable task in determining pneumonia as an inflammatory or fibrotic predominant lesion [3, 4].
Patients with IP are often treated with large amounts of glucocorticoids, but this approach is not universally effective, and there are unpredictable complications if antifibrotic drugs are started immediately without identifying the type of lesion.Therefore, the use of non-invasive imaging methods for the early diagnosis of lung lesions in IP patients can assist physicians in selecting the right clinical treatment plan while reducing the risk of physical injury to the patient.
DCE-MRI is a functional imaging technique in which MRI is performed uninterruptedly after rapid injection of contrast agent into the patient. The contrast agent enters the extravascular space from the vascular lumen through passive diffusion, changing the signal intensity of the tissue by altering the relaxation rate of water protons, and subsequently obtaining more visual quantitative parameters and pseudo-color images by circling the area of interest.The imaging is able to quantify the blood volume, blood flow, and vascular permeability of the lesion in the area of interest. By dynamically observing the enhancement map of the lesion area and the quantitative parameters, the blood flow and microvascular condition of the lesion can be assessed more accurately and dynamically.
The quantitative parameters are obtained by complex calculations based on DCE-MRI. The quantitative parameters Ktrans, Kep, Ve, and iAUC represent the transfer rate constant, the rate constant, the EES volume fraction, and the initial area under the time-concentration curve, respectively.
They represent the following meanings: Ktrans: the amount of contrast agent entering the extravascular extracellular space from the vessel per unit time, which is an absolute parameter of the perfusion rate and is also influenced by the tissue permeability. Kep: the contrast dose returning from the extravascular extracellular space to the vessel. Ve: the percentage of the extravascular extracellular space volume to the whole volume of the area of interest. iAUC: reflects the combined change of the above three indicators and is one of the semi-quantitative parameters.These parameters are based on the "Toft" model  and are divided into two zones based on the distribution of the contrast agent, the first zone being the central zone, which consists mainly of intravascular extracellular volume fraction (blood plasma), and the peripheral zone, which consists of extravascular extracellular volume fraction. The flow of contrast agent in this model is bidirectional, first flowing from the vascular lumen to the extracellular space outside the vessel, and after a period of time, contrast agent elutes back into the vasculature from the extracellular space outside the vessel. This continuous variation is well described by DCE-MRI, and the quantitative parameters are calculated by substituting the continuous image signal intensity variation into the model. On the dynamic enhancement pseudo-color map, red represents high perfusion, yellow represents slightly high perfusion, and blue represents low perfusion. So the closer the color to red, indicates the more adequate the blood supply, and the closer to blue, the poorer the blood supply.
There have been studies using DCE-MRI in the pancreas and breast [7, 8], but DCE-MRI is rarely used in interstitial pneumonia.
A total of 27 cases were enrolled in this study, including 10 cases in the UIP group and 17 cases in the NSIP group. In this study, we found that the Ktrans, iAUC values of UIP patients were lower than those of NSIP patients.The cause of this phenomenon is the abnormal accumulation of fibroblasts and inflammatory cells occurring in the lungs of UIP patients, replacing the normal interstitial structures of the lungs and causing damage to the alveolar epithelium, which is a stress point for pulmonary fibrosis, leading to damage to the interstitial capillaries and eventually causing irreversible distortion of the alveolar structures .Irreversible distortion of the alveolar structure hinders the "drainage" of the contrast agent from the lesion and is accompanied by difficulty in retreating or entering the contrast agent. Eventually, the lung tissue forming the honeycombed lung has lost its blood supply or receives limited blood supply, so the Ktrans, iAUC values are reduced, and the ROI of the UIP lesion is mainly blue-apochromatic on the corresponding dynamic enhancement pseudocolor image.
In contrast, NSIP-type lesions are mainly characterized by infiltration of inflammatory cells, widening of extravascular spaces and increased neovascularization , so the contrast agent enters the lesion without obstruction and elutes out of the lesion more rapidly, so the Ktrans and iAUC values are correspondingly higher, and then the color is mainly light yellow-light blue on the dynamic enhancement pseudo-color map. Moreover, Boesen et al used DCE-MRI and found that infectious osteoarthropathy leads to more neovascularization and increased perfusion in the lesion, which is also approximately the same as the results of the present study .
In conclusion, we found that quantitative parameters and dynamic enhancement pseudo-color maps have diagnostic significance in assessing IP, among which Ktrans and iAUC values of quantitative parameters have discriminatory significance in identifying UIP and NSIP, with Ktrans value as the greatest diagnostic efficacy. However, the sample size of this study is still small, and it is necessary to further expand the sample size to accurately identify the quantitative parameters of UIP and NSIP.