A technical note on variable inter-frame interval as a cause of non-physiological experimental artefacts in ultrasound

Ultrasound imaging is a well-recognised technique for the study of static tissues but its suitability for studying tissue dynamics depends upon accurate frame time information, which may not always be available to users. Here we present methods to quantify the inter-frame interval (IFI) variability, and evaluate different procedures for collecting temporal information from two ultrasound-imaging devices. The devices tested exhibited variable IFIs that could only be confirmed by direct measures of timing signals, available by means of electrical signals (triggers) and/or temporal information contained in the software used for the ultrasound data collection. Interpolating frame-by-frame measures of dynamic changes within image sequences using individual IFI values provided improved synchronisation between measures of skeletal muscle movement and activation; validating ultrasound as a valuable technique for the study of musculoskeletal tissue dynamics, when correctly implemented.