Session Item

MR-Linacs have opened up possibilities not only in terms of precise tumor tracking, dose delivery and adapted treatment planning, but also for functional lung imaging in the clinical workflow for lung cancer treatment response monitoring without prolonging treatments. Non-uniform Fourier Decomposition (NuFD) is one of the techniques developed for non-contrast enhanced lung MRI at 1.5 T and allows to assess ventilation (V) and perfusion (Q). Since low-field MRI has been shown to be advantageous for lung imaging due to low susceptibility artefacts, we aim at demonstrating the feasibility and potential of NuFD for a 0.35 T MR-Linac and propose two signal normalization strategies for enhancing reproducibility of the results in the presence of breathing amplitude variations.

Ten healthy volunteers (five female/male) were scanned with ethics approval at a 0.35 T MR-Linac (MRIdian, ViewRay Inc., Cleveland, Ohio) using an optimized balanced steady-state free precession (bSSFP) sequence with flip angle=70°, TR/TE=2.42/1.02 ms, FOV=500x500x20 mm³, matrix=128x128 and frame rate=3.68 images/s. Two coronal slices, referred to as ‘aorta’ and ‘lung’, were selected for each volunteer. Image series (240 images) were acquired in normal free-breathing with breaks inside and outside the scanner as well as in deep and shallow breathing. After image registration using ANTs and segmentation, the average lung signal was filtered into signals corresponding to V and Q. The NuFD was performed pixel-wise and V- and Q-maps were generated from the peak-value in the respective Fourier spectrum. For intra-volunteer V-map reproducibility, a normalization factor was defined based on the linear correlation of V-signal and diaphragm position of each scan as well as the diaphragm motion amplitude of the reference scan. This allows to correct the signal’s dependence on the diaphragm motion amplitude, which varies with breathing pattern. The second strategy normalizes the V-maps with the average V-signal within a selected ROI and therefore eliminates the dependency on the signal amplitude. Six different ROI locations within the lung have been investigated.

Due to a clear signal variation with frequencies corresponding to the breathing frequency and the heart beat, the NuFD algorithm was successfully applied to all volunteer scans and V- and Q-maps were generated. Comparing the normalization results of all volunteer scans (Fig. 1 and Tab. 1), median deviations of 7.8%/7.0%/12.3% (normalization factor/best ROI/worst ROI) from the reference scan were achieved for combined slices, which outperforms the uncorrected median deviation of 32.4%.

Non-contrast enhanced functional lung MRI concepts can be transferred to a 0.35 T MR-Linac and integrated into clinical treatment workflows with standard equipment and short acquisitions. For the comparison of intra-volunteer scans, two signal normalization strategies have been successfully introduced and demonstrated comparable performance.