Sonography Principles & Instrumentation (SPI) encompasses the technical and physical foundations of diagnostic ultrasound, including acoustic physics, transducer construction and beam characteristics, image formation, display modes, Doppler principles, artifacts, bioeffects and safety, quality assurance, and infection control. Mastery of SPI content is required for all ARDMS credentials. Sonography programs typically dedicate a full course to SPI preparation, and strong understanding of these principles is essential for optimizing image quality and recognizing artifacts in clinical practice.