Measuring diaphragmatic excursion using 4-dimensional ultrasound: a feasibility study

Four-dimensional ultrasound (4DUS) could provide more accurate characterisation of diaphragm function than existing M-mode ultrasound approaches. Therefore, the aim of this study was to investigate the feasibility of a novel method for tracking diaphragm excursion from 4DUS data. 4DUS was acquired from 12 participants who exhibited a range of breathing patterns. A custom algorithm tracked, reconstructed, and parameterized diaphragm movement using a sphere of time-varying radius. To validate the algorithm, a randomly selected slice of the sphere was sampled and compared to visual analysis. Agreement between the visual analysis and the algorithm was characterised using a Bland-Altman analysis. A root mean squared error (RMSE) metric was also calculated to quantify the fit between the ultrasound data and the spherical parametrisation. There was good agreement between the automated algorithm and visual analysis (bias, 0.09 cm; 95% limits of agreement, -0.44 to 0.25 cm). The RMSE metric was low (0.9-1.5 mm) across the 12 participants, demonstrating that the sphere was a good fit to the measured 4DUS data. This study demonstrates the feasibility of automated tracking of diaphragmatic excursion from 4DUS data using a sphere of time-varying radius. This technique may prove useful for diagnosing and monitoring breathing dysfunction.