Introduction

Dynamic contrast enhanced (DCE) MRI is an established technique for assessment of lung perfusion^1,2, but requires the administration of gadolinium-based intravenous contrast agents. These are reported to cause side effects like nephrogenic systemic sclerosis in patients with renal failure³. Furthermore, a breath-hold is required for DCE data acquisition. Therefore, validation of free-breathing contrast agent free 1H MRI postprocessing techniques like phase resolved functional lung (PREFUL) MRI⁴ is desirable. PREFUL MRI is based on the reconstruction of one respiratory and cardiac cycle to gain dynamic ventilation and perfusion information from one data set. A previous study already validated PREFUL MRI with DCE MRI in patients with chronic obstructive pulmonary disease (COPD)⁵. However, no feasibility study of PREFUL MRI across different centers was conducted. Therefore, we validated PREFUL MRI against DCE MRI in patients with cystic fibrosis (CF) in a dual center study using scanners from two different vendors.

Method

14 CF patients were included in this study (Center 1: 9 patients, Center 2: 5 patients). All patients underwent PREFUL and DCE MRI in the same imaging session.
Center 1: Data acquisition was performed on a 1.5T scanner (Siemens Avanto, Siemens Healthcare, Erlangen, Germany) using a spoiled gradient echo sequence (Field of view (FoV) = 500 x 500mm², matrix size = 128 x 128 (interpolated to 256 x 256), TE = 0.82ms – 0.88ms, TR = 3, flip angle 3°, temporal resolution 192ms, slice thickness 15mm) for PREFUL MRI and a 3D time-resolved angiography with stochastic trajectories (TWIST) sequence (FoV = 450 x 366mm², matrix size 256 x 146, TE = 0.80ms – 0.86ms, TR = 2.37ms – 2.47ms, temporal resolution 1.1s – 1.3s, slice thickness 5mm) for DCE MRI.
Center 2: Data was acquired on a 1.5T scanner (Signa HDxt, GE Healthcare, Milwaukee, WI) using a spoiled gradient echo sequence (FoV = 480 x 480mm², matrix size = 128 x 128 (interpolated to 256 x 256), TE = 0.8ms, TR = 2.5ms, flip angle 4°, temporal resolution 374ms, slice thickness 15mm) for PREFUL MRI and a 3D gradient echo sequence with view sharing (TRICKS) and parallel imaging (R=2) (FoV = 480 x 480mm², matrix size 120 x 80, TE = 0.69ms, TR = 2.1ms, temporal resolution 0.6s, slice thickness 10mm) for DCE MRI.
For PREFUL MRI four coronal slices, (two posterior slices, one tracheal slice centered at the aortic arch, one anterior slice) were acquired. Due to thinner slice thickness of DCE MRI compared to PREFUL MRI, compound DCE slices were calculated. PREFUL images were registered to DCE images. For further analysis, a perfusion-weighted PREFUL phase of the reconstructed PREFUL cardiac cycle and a corresponding perfusion-weighted DCE phase of the contrast agent pass of the DCE data set was selected. Then, perfusion defect percentage (QDP) maps for every slice and the whole lung were calculated using an individual threshold for each slice as previously descibed⁵. Values below this threshold were identified as perfusion defect. To assess the agreement between QDP-maps, the Dice coefficients of the defect and healthy regions were calculated on a voxel by voxel level. Furthermore, the spatial overlap between PREFUL and DCE QDP maps (i.e. percentage of voxels labeled as perfusion defect or healthy tissue with both methods) was computed and compared using a paired two-sided Wilcoxon test. The agreement of QDP was further assessed by Bland Altman analysis and Pearson correlation.

Results

Figure 1 shows representative perfusion-weighted PREFUL and DCE maps for both centers, the corresponding QDP-maps and maps illustrating the spatial overlap of the QDP-maps, with good visual agreement. In Table 1 QDP_PREFUL and QDP_DCE are compared for all slices. A median spatial overlap of 61% was found for the whole lung. Using Pearson correlation, a correlation coefficient of r = 0.70 (p = 0.005) was obtained between QDP_PREFUL and QDP_DCE for the whole lung. Bland Altman analysis for the whole lung is shown in Figure 2 (mean difference -5.4%, 95% confidence interval of mean difference [-12.8%-2.0%], mean difference ± 1.96*standard deviation [-37.5%-26.7%]).

Discussion

In accordance with a recent study⁵, good spatial agreement was found comparing QDP-maps. Minor differences between QDP_DCE and QDP_PREFUL could be explained by the lower resolution of PREFUL compared to DCE causing widening and blurring of vessels, which may contribute to imprecise estimation of perfused areas. In addition, the threshold used to calculate QDP-maps impacts the QDP values. Further, in this study data acquisition was performed with different temporal and spatial resolution across centers and imaging methods. Therefore, depending on the resolution, widening and blurring of vessels as well as partial volume effects are differently pronounced, affecting the calculated QDP-maps.

Conclusion

In this dual center study feasibility of PREFUL MRI was shown and validated with DCE MRI using scanners from two different vendors. This is an important step towards clinical implementation of PREFUL MRI.

Acknowledgements

No acknowledgement found.