Biophysics & Medical Physics — 60 Terms Defined

Absorbed Dose Biophysics & Medical Physics Gy (Gray = J/kg) The physical quantity measuring the amount of energy deposited by ionizing radiation per unit mass of matter. D = dE / dm, 1 Gy = 1 J / kg = 100 rad

Action Potential Biophysics & Medical Physics mV (Amplitude ~100 mV) & m/s (Conduction Speed) A rapid, self-propagating electrical voltage impulse along the membrane of excitable cells (neurons and muscle cells). v_conduction ∝ √(d) (Unmyelinated), v_conduction ∝ d (Saltatory Myelinated)

Active Transport Biophysics & Medical Physics Ions/sec or μmol/min The energy-requiring transport of ions or molecules across a biological cell membrane against their concentration gradient. 3 Na⁺_out + 2 K⁺_in per ATP Hydrolyzed (Sodium-Potassium Pump)

ALARA Principle Biophysics & Medical Physics Radiation Safety Protocol A fundamental radiation protection safety principle stating that all radiation exposures must be kept 'As Low As Reasonably Achievable' (ALARA). Dose Optimization: Minimize Time, Maximize Distance, Optimize Shielding

Audiometry Biophysics & Medical Physics dB HL (Decibels Hearing Level) & Hz The clinical evaluation and quantitative measurement of human hearing sensitivity across pure tone sound frequencies. Hearing Level dB HL = 20 log₁₀(P_measured / P_reference)

Bio-optics Biophysics & Medical Physics Scattering Coefficient μ_s (cm⁻¹) & Absorption μ_a (cm⁻¹) The study of light interaction with biological tissues and the development of optical diagnostic and therapeutic instruments. Radiative Transport Equation (RTE) in Turbid Tissues

Bioenergetics Biophysics & Medical Physics kJ/mol (Gibbs Free Energy) The branch of biophysics and biochemistry studying energy transformation, transfer, and coupling mechanisms in living organisms. ΔG = ΔH - T ΔS, ΔG'° = -R T ln K_eq

Bioimpedance Biophysics & Medical Physics Ω (Ohms) The measure of a biological tissue's opposition to the flow of electric alternating current. Z = R + i X_c = √(R² + X_c²)

Biological Transducers Biophysics & Medical Physics mV / pA (Receptor Signal Output) Specialized sensory receptor cells that convert external physical or chemical stimuli into graded bioelectrical signals. Transduction: Physical Stimulus Energy → Receptor Potential (mV)

Bioluminescence Biophysics & Medical Physics Photons/s (Bioluminescent Flux) The enzymatic production and emission of light by living organisms through chemical reactions converting ATP into photon energy. Luciferin + O₂ + ATP → Oxyluciferin + AMP + PPi + h ν (Light)

Biomechanics of Gait Biophysics & Medical Physics m/s (Walking Speed) & Ground Reaction Force (N) The quantitative analysis of human locomotion mechanics, muscle force coordination, and ground reaction forces during walking or running. Center of Mass Inverted Pendulum Energy Exchange (T ↔ V)

Bone Mechanics Biophysics & Medical Physics MPa / GPa (Tensile & Compressive Strength) The study of mechanical structural behavior, anisotropy, and remodeling of bone tissue under physical loads. Stress σ = F / A, Young's Modulus E_cortical ≈ 18 GPa

Brachytherapy Biophysics & Medical Physics Gy/h (High or Low Dose Rate) An internal radiation therapy technique where sealed radioactive sources are placed directly inside or adjacent to a tumor. Inverse Square Dose Falloff: D(r) ∝ 1 / r²

Cardiac Biomechanics Biophysics & Medical Physics kPa (Myocardial Wall Stress) & mmHg (Pressure) The application of solid and fluid mechanics principles to model heart chamber contraction, blood flow dynamics, and myocardial wall stress. Wall Stress σ = (P × r) / (2 h) (Law of Laplace for Spheres)

Computed Tomography (CT Scan) Biophysics & Medical Physics Hounsfield Units (HU, Water = 0, Air = -1000, Bone = +1000) A diagnostic medical imaging modality combining rotating X-ray tubes and digital detectors with filtered back-projection algorithms to produce 3D cross-sectional tissue images. Hounsfield Unit HU = 1000 × (μ_tissue - μ_water) / (μ_water - μ_air)

Cytoskeleton Mechanics Biophysics & Medical Physics pN·μm² (Bending Stiffness) The study of the mechanical properties, viscoelasticity, and force generation of the intracellular filament network (actin, microtubules, intermediate filaments). Flexural Rigidity EI = E_young × I_area_moment

DNA Biophysics Biophysics & Medical Physics nm (Persistence Length) & pN (Stretching Force) The physical study of mechanical stiffness, elasticity, topology, and base-pairing thermodynamics of double-stranded DNA. Persistence Length A_p ≈ 50 nm, Contour Length L

Effective Dose Biophysics & Medical Physics mSv (Millisieverts) A radiation protection quantity representing the overall stochastic health risk to the whole body by weighting organ equivalent doses by tissue sensitivity factors w_T. E = ∑ H_T × w_T = ∑ D_T × w_R × w_T

Electro-osmosis Biophysics & Medical Physics m/s (Electro-osmotic Velocity) The motion of a liquid containing mobile ions through a porous biological membrane or microchannel driven by an applied electric field. v_eo = (ε_r ε₀ ζ / η) E (Smoluchowski Equation)

Electrocardiography (ECG) Biophysics & Medical Physics mV (Amplitude ~1 mV) & bpm (Heart Rate) A non-invasive diagnostic technique recording the spatial bioelectric potential changes produced by heart muscle depolarization and repolarization. Cardiac Dipole Vector P(t), Einthoven's Triangle Lead Vectors

Electroencephalography (EEG) Biophysics & Medical Physics μV (Microvolts ~10–100 μV) & Hz A non-invasive neurophysiological technique measuring electrical brain activity using surface scalp electrodes. Frequency Bands: Delta (1–4Hz), Theta (4–8Hz), Alpha (8–12Hz), Beta (12–30Hz)

Electroporation Biophysics & Medical Physics kV/cm (Electric Field Strength) A biophysical technique applying short, high-voltage electric field pulses to cells to temporarily increase cell membrane permeability by creating nanoscale aqueous pores. Critical Transmembrane Breakdown Voltage V_c ≈ 0.5–1.0 V

Entropy in Biological Systems Biophysics & Medical Physics J/K (Entropy) The thermodynamic concept describing how open biological systems maintain high internal structural order by expelling heat and entropy to their environment. dS_system + dS_environment ≥ 0 (Schrödinger: 'Living on Negative Entropy')

Equivalent Dose Biophysics & Medical Physics Sv (Sievert = J/kg) or rem A radiation protection dose quantity weighing absorbed dose by the specific biological effectiveness factor w_R of the radiation type. H = D × w_R (Radiation Weighting Factor w_R)