Purpose: This study aimed to clarify the correlations between values of diaphragmatic movement as respiratory muscle activity measured by ultrasonography (US) and magnetic resonance imaging (MRI) under quiet breathing (QB) and forced abdominal breathing (FAB) to select the optimal evaluation method according to the purpose of physical therapy, such as breathing instruction.

Materials and Methods: Twenty healthy young-adult volunteers participated and performed both QB and FAB. During breathing, the distance of diaphragmatic movement and diaphragm thickness was measured by US, followed by measurements of lung lengths in the sagittal and coronal planes by MRI. Correlations between parameter values measured by US and MRI were analyzed using the Pearson product-moment correlation coefficient.

Results: Under QB, significant positive correlations were found between the distance of diaphragmatic movement (Ddi/Ht) measured by US and the length of the mid-axillary line in the sagittal plane (SM/Ht) measured by MRI (r=0.602, p=0.005) and between the Ddi/Ht and posterior length in the sagittal plane (SP/Ht) (r=0.592, p=0.006). A significant correlation was also found between the length of the SM and tidal volume (r=0.591, p=0.012) during FAB. However, no significant correlation was found between US and MRI during FAB.

Conclusions: US is suitable for assessing narrow-range diaphragmatic activity, such as QB. However, for forced breathing, which increases respiratory muscle activity, MRI with a wider observation range that correlates with the ventilation volume is also indicated as an excellent evaluation tool. In conclusion, these findings might provide reference values for physical therapy investigation because understanding the mechanism of respiratory muscle movements is necessary for physical therapy, evaluation, and treatment of breathing disorders.

Diaphragm, Respiratory muscle, Ultrasonography, MRI, Physiotherapy