ESTRO 37 Abstract book

S1173

ESTRO 37

A low IBV indicates high precision of reconstructed diaphragm position. Low RC and high IBV result in image artefacts, e.g. discontinuities between reconstructed slices, quantified by a low S. We tested for differences in parameters between strategies (Wilcoxon’s signed rank test; α=0.05). Results Strategy A reduced the underestimation of the reconstructed displacement significantly by on average 1.3% (range: 0%–4%) of the true motion amplitude, compared with strategy B. The average RC improved significantly by 1.3% from 94.2 % to 95.5 % when adopting strategy A; The average IBV remained the same, 1.6 mm. The average S was virtually the same for both strategies A (0.85) and B (0.86) (Figure 2).

Here, we aimed to validate and compare 2 amplitude binning strategies, assessing the accuracy of recons- tructed motion amplitude and 4DMRI quality. Material and Methods For each 4DMRI scan, 11 2D coronal slices were acquired repetitively (60 times, 6 minutes total), using a T2W TSE sequence (resolution: 1.3x1.6x5.0 mm 3 ). Prior to each slice acquisition, the position of the diaphragm was acquired using a 1D navigator. The 2D slices were sorted into 10 amplitude bins. For each bin/slice combination, the image with the median diaphragm position was selected for 4DMRI reconstruction, introducing an under- estimation in motion amplitude depending on bin size. We applied 2 amplitude binning strategies: (A) half a bin size for the end-inhale and end-exhale bins and (B) equal- sized amplitude bins (Figure 1), which is current clinical practice. Amplitude determination was assessed with dynamic phantom measurements, quantifying the reconstructed motion amplitude of a water vial moving with sin and cos 6 respiratory patterns with amplitudes of 1.5, 2 and 2.5 cm.

4DMRI quality was evaluated by in vivo measurements with 12 healthy volunteers and 2 abdominal cancer patients. To account for outliers, the images associated with the 5% most extreme diaphragm positions were discarded prior to binning. Binning strategies were compared based on the parameters: • * Reconstruction completeness (RC); fraction of the 110 (11 slices x 10 bins) bin/slice combinations with at least 1 image.

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* Intra-bin variation (IBV); interquartile range of the diaphragm positions within a bin/slice combination, averaged over 3 central slices. * Image smoothness (S); assessed by quantifying how well a parabola fits the diaphragm shape in a sagittal plane of the reconstructed 4DMRI, per bin (S = R 2 adj averaged over all bins). S ranges from 0 (discontinuous diaphragm shape; artefacts) to 1 (smooth shape; no artefacts).

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