Final answer:
The American Institute of Ultrasound in Medicine indicates that no bioeffects are observed at SPTA intensity levels below 100 mW/cm² for unfocused ultrasound beams.
Step-by-step explanation:
The American Institute of Ultrasound in Medicine's statement on mammalian biological effects of ultrasound in vivo suggests that no bioeffects have been noted for unfocused ultrasound beams when the spatial pulse temporal average (SPTA) intensity is less than 100 mW/cm². This threshold is a precautionary standard to ensure patient safety during diagnostic procedures that use ultrasound.
Diagnostic intensities of ultrasound are typically around 10-2 W/m², which is far below the threshold that could cause thermal damage. Ultrasound has been used for many decades with detailed follow-up studies showing no evidence of ill effects, in contrast to the risks associated with X-rays.
It is worth mentioning that when ultrasound is focused to much higher intensities, in the range of 10³ to 10µ W/m², it can cause significant bioeffects and is used for therapeutic purposes such as shattering gallstones or treating cancerous tissues through mechanisms like cavitation.