Ultrasound clutter signal from vibrating muscles limits low velocity blood flow measurements

Skeletal muscle tension vibrates under sustained contraction, and will thereby generate low frequency clutter in the Doppler signal. Both muscles in the hand of the operator holding the ultrasound probe, and in the patient itself can give rise to the clutter. In this paper a model for the Doppler signal from vibrating muscles is presented. For small vibration amplitudes, the signal is a phase modulated signal, and the vibration amplitude can be estimated from the phase of the signal. Clutter rejection filters, which are commonly used to remove the low frequency components in the Doppler signal, will also remove the signal from low velocity blood flow. By comparing the model for the clutter signal with a previously presented model for the Doppler signal from moving blood, a theoretical minimum detectable blood velocity is found for different ultrasound frequencies. For 6 MHz Doppler this limit is 6.4 mm/s, which indicates that capillary blood perfusion is not measurable with conventional techniques.