Point-of-care ultrasound (POCUS) uses high-frequency sound waves (2-15 MHz) generated by piezoelectric crystals that convert electrical energy to mechanical energy and vice versa. The transducer emits ultrasound pulses that travel through tissue; when the acoustic impedance changes at tissue interfaces (e.g., fluid-to-solid, tissue-to-air), some energy is reflected back as echoes. The time delay and intensity of returning echoes are processed to create real-time 2D images. Key physics principles: higher frequency probes (10-15 MHz linear) provide better resolution but less penetration depth (ideal for superficial structures, vascular access, soft tissue); lower frequency probes (2-5 MHz curvilinear or phased array) penetrate deeper but with lower resolution (ideal for abdominal, cardiac, thoracic imaging). Fluid appears anechoic (black), solid organs are hypoechoic to hyperechoic (gray to white), bone and air create strong reflection artifacts. The FAST exam (Focused Assessment with Sonography for Trauma) evaluates four windows for free fluid: subxiphoid (pericardial effusion), right upper quadrant/Morrison's pouch (most dependent space in supine patient), left upper quadrant/splenorenal recess, and suprapubic/pelvis. Cardiac POCUS evaluates four views: parasternal long axis (PLAX), parasternal short axis (PSAX), apical four-chamber (A4C),...