Exploring pixel difference noise floor in tongue ultrasound data

Pixel Difference (PD) is a change metric for image sequences used to detect tongue movement in ultrasound data. It measures overall change by treating each frame as a vector and calculating vector norms between consecutive images. The PD curves have a substantial noise floor but it is unclear to what extent this noise is caused by physiological processes like the pulse and tensing and relaxation of muscle fibres. To evaluate the contribution of physiological processes to the PD noise floor, we measured one human participant at rest and an excised bovine tongue using the same ultrasound imaging parameters. The mean PD ± standard deviation for human tongue was 93,851 ± 8409 in rest and 274,348 ± 207,791 in movement. For the bovine tongue we obtained five different locations between the root and the tip. The mean PD range was (109,156, 176,433) and standard deviation range was (986, 1273). In the bovine samples, mean PD correlates with standard deviation (r = 0.9972) in line with the multiplicative noise model. The data from human at rest differ from this pattern due to a higher standard deviation. This suggests that the PD standard deviation—not the PD noise floor—reflects physiological processes in resting tongue ultrasound data.