Final answer:
Unstable cavitation during ultrasound therapy can be caused by high intensity, a stationary soundhead, lack of a coupling medium, and a high Beam Non-uniformity Ratio (BNR).
Step-by-step explanation:
The risk factors for unstable cavitation when applying ultrasound include the following: a high intensity of the ultrasound, which can lead to excessive energy absorption by tissues resulting in potential damage; having the soundhead remaining stationary during treatment, which may concentrate the energy in one area and increase the likelihood of cavitation; and the absence of a coupling medium, which is necessary to transmit ultrasound energy effectively and safely from the transducer to the skin. Additionally, a high Beam Non-uniformity Ratio (BNR) indicates uneven distribution of the ultrasound beam, which can cause hot spots and increase the risk of cavitation.