Final answer:
The power and intensity of the ultrasound beam diminish as it travels through tissue primarily due to absorption, facilitated by the medium's viscosity, which converts sound wave energy into thermal energy. Additionally, reflection due to differences in acoustic impedance at media boundaries also affects the ultrasound intensity.
Step-by-step explanation:
The fundamental reason that the power and intensity of the ultrasound beam diminish as it travels through tissue is absorption. This occurs because the energy of the ultrasound wave is absorbed by the tissue and converted into thermal energy. This conversion process is facilitated by the viscosity of the medium the ultrasound passes through, with some heat transferring to the tissue during each compression of the ultrasound wave and even less heat transferring back during the rarefactions, thereby leading to a net transfer of thermal energy that diminishes the wave's organized energy. In addition to absorption, at the boundary between media of different acoustic impedances, some of the ultrasound wave energy is reflected and some is transmitted. The greater the difference in acoustic impedance, the greater the reflection and the smaller the transmission, further impacting the intensity of the ultrasound that continues forward.