Introduction

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive fibrosis disease of the lung with a poor prognosis and median survival of 3-5 years from diagnosis. High-resolution CT (HRCT) yields images with excellent anatomic detail of the lung and plays a key role in the management and differential diagnosis of IPF. However, radiation exposure is a significant concern in patients with IPF during longitudinal follow-ups. Thus, a radiation-free imaging tool is desirable. Recently, MRI with ultrashort echo time (UTE-MRI) sequences were applied for quantitative and qualitative assessment of lung parenchymal changes 1, 2. UTE-MRI image quality was comparable with that of HRCT 3. The aim of this study was to assess the utility of UTE-MRI for lung parenchymal imaging and compare the result with that of HRCT to diagnose IPF.

Methods

Following informed consent, a total of 36 patients (men: 34 (94.4%), women: 2 (5.6%); mean age: 62, age range: 43-78) were enrolled in the study. All patients underwent HRCT and MRI examinations. HRCT was obtained with multi-slice spiral CT scanners (Brilliance CT, Phillips Healthcare, Netherlands; Toshiba CT, Tokyo, Japan). UTE-MRI was performed with a 1.5T system (MAGNETOM Aera; Siemens Healthcare, Erlangen, Germany) during free-breathing. A prototypic 3D UTE sequence with a stack-of-spirals trajectory was used for data acquisition4 repetition time=3.85 msec; echo time=0.05 msec; flip angle=5°; voxel size=1.5 x 1.5 x 1.5 mm3). Free-breathing was enabled by prospective gating. Here, respiratory information was provided by a navigator and scanning is automatically stopped once sufficient data was acquired in end-expiratory state. Corresponding scan times depended on the breathing pattern and was within the range of 6-7 minutes. According to preoperative findings, the image quality of both HRCT and MR images was estimated using a 5-point scale. To compare the ability of UTE-MRI to assess lung parenchymal characteristics with HRCT, a 2-point visual scoring system was used to determine the presence of reticular and honeycombing patterns, traction bronchiectasis, and ground-glass opacities 5. The extent of interstitial lung changes was graded on a system according to Gay et al. 6. Two radiologists evaluated HRCT and MRI images independently for various morphologic abnormalities associated with IPF. The difference in image quality between HRCT and UTE-MRI was compared using a Wilcoxon-rank sum test. Kappa and weighted kappa analyses were used to measure intra- and interobserver, and inter-method agreements. Sensitivity and specificity were used to assess the performance of UTE-MRI for detecting pulmonary fibrosis.

Results

A representative case of IPF is shown in Figure 1. All interobserver agreements for image quality were statistically significant (HRCT: k=0.786; P＜0.0001; UTE-MRI: k=0.719; P＜0.0001). As Figure 2 shows, the mean image quality score for HRCT (4.9±0.3) was significantly higher than for UTE-MRI (4.1±0.7; P＜0.0001). Moreover, the inter-observer agreements of HRCT and UTE-MRI for the ability to detect the imaging characteristics of IPF were defined as substantial and excellent, respectively (HRCT: 0.727≤k≤1, P＜0.0001; UTE-MRI: 0.719≤k≤0.824, P＜0.0001). In addition, reticular (97.1%, k=0.654) and honeycombing (83.3%, k=0.625) patterns, and traction bronchiectasis (94.1%, k=0.640) were well visualized on UTE-MRI. For an IPF diagnosis, the sensitivity of UTE-MRI was 97.2%. The UTE-MRI protocol could diagnose IPF in 28 (90%) of 31 patients, with a total fibrosis score of 6-15, and 2 (40%) of 5 patients with a total fibrosis score >15. There was excellent interobserver agreement (HRCT, k=0.873, MRI, k=0.920).

Discussion & Conclusion

CT is considered an accurate tool to image lung parenchyma with excellent structural detail. However, the major disadvantage is ionizing radiation exposure; thus, an alternative radiation-free imaging modality is needed, particularly in certain patient groups, as well as in longitudinal studies. In this study, our results suggest that UTE-MRI is not significantly inferior to HRCT and has a similar potential to detect the radiologic characteristics of IPF and make a diagnosis. In addition, UTE-MRI is useful for sampling free induction decay using short repetition times that limit signal contaminations from long T1 signals and low proton density components, such as the lung parenchyma 7. In conclusion, UTE-MRI could be used in routine clinical practice as a radiation-free imaging modality in patients with IPF.

Acknowledgements

No acknowledgement found

References

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