Chapter 30: Medical Imaging

Medical imaging integrates fundamental physics principles to visualize internal human anatomy and physiological processes without invasive surgery. The chapter examines three major diagnostic modalities, each exploiting different physical phenomena to generate clinically useful images. X-ray imaging relies on electromagnetic radiation produced when high-speed electrons decelerate upon striking a metal target, converting kinetic energy into high-frequency photons. As these photons traverse tissue, their intensity diminishes exponentially according to the attenuation coefficient of the material, with bone absorbing significantly more radiation than soft tissue due to its higher atomic density. Image quality improves through intensifier screens that amplify weak signals and contrast media containing high atomic number elements that preferentially absorb X-rays, enhancing differentiation between similar tissues. Computerized tomography extends X-ray technology by rotating the radiation source around the patient and reconstructing detailed three-dimensional cross-sectional images from multiple angular projections. Ultrasound employs mechanical vibrations at frequencies above human hearing, generated and detected by piezoelectric crystals that convert electrical signals into sound waves and vice versa. The technique depends on acoustic impedance, the product of tissue density and sound velocity, which determines what fraction of energy reflects at tissue boundaries. An impedance-matching gel bridges the acoustic mismatch between air and skin, preventing nearly complete signal loss at the body surface. A-scans display one-dimensional echo patterns while B-scans combine multiple A-scans to construct two-dimensional anatomical representations. Positron emission tomography represents a fundamentally different approach by introducing radioactive tracers that concentrate in metabolically active regions. When the radioactive isotope undergoes beta-plus decay, the emitted positron immediately annihilates with an electron, converting their combined mass into two gamma-ray photons traveling in opposite directions. Detectors positioned in a ring around the patient identify these paired photons, and timing analysis pinpoints the precise annihilation location, creating images that reveal biochemical activity rather than mere anatomical structure.