Bioeffects refer to the biological effects of ultrasound energy on living tissue. The two main mechanisms are thermal effects (heating from sound absorption, relevant particularly in Doppler modes) and mechanical effects (primarily cavitation — the formation and oscillation or collapse of microbubbles). The Thermal Index (TI) and Mechanical Index (MI) displayed on ultrasound machines provide real-time estimates of these risks. The ALARA (As Low As Reasonably Achievable) principle guides sonographers to use the lowest output settings necessary to obtain diagnostic information, minimizing bioeffects exposure.