Electromagnetic Radiation & Radiation Concepts - Quick Notes

Radiographer Role

• Be familiar with different types of radiation

• Be able to answer questions and educate patients

• Understand how both ends of the electromagnetic spectrum are used in medical imaging

• Explain the nature of ionizing radiation, as well as risks and benefits

• Be the patient’s advocate in discussions of radiation with other professionals

• Safely use radiation for medical imaging purposes

Electromagnetic Radiation: Nature and Characteristics

• Electromagnetic radiation: electric and magnetic disturbance traveling at the speed of light

• All spectrum components share the same velocity: $c \approx 3\times 10^{8} \ \text{m s}^{-1}$

• Vary in energy, wavelength, and frequency

• EM radiation can exist apart from matter and travel through vacuum; originates from atoms

Electromagnetic Radiation: Course of Travel and Intensity

• EM radiation travels as divergent rays from a source; intensity spread over a larger area

• Intensity is energy flow per second (photon flux); greatest at the center

• Intensity diminishes with distance; follows inverse relationships

Distance Formula and Inverse Square Law

• Inverse square law for intensity: $I<em>{2} = I</em>{1} \left(\dfrac{d<em>{1}}{d</em>{2}}\right)^2$

• I1: initial intensity, d1: initial distance, d2: final distance, I2: final intensity

Spectrum: Key Relationships

• Electromagnetic spectrum from lowest to highest energy: radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays

• Wavelength range: $\lambda \in [10^{-16},\ 10^{6}]\ \text{m}$

• Frequency range: $f \in [10^{0},\ 10^{24}]\ \text{Hz}$

• Velocity and wavelength-frequency relation: $c = f\lambda$; $f = \dfrac{c}{\lambda},\quad \lambda = \dfrac{c}{f}$

• Velocity relation: $v = f\lambda$ (for EM radiation, $v=c$)

Energy and Wave-Particle Duality

• EM radiation exhibits wave-particle duality

• Energy relates to frequency: $E = h f$

• Planck’s constant (approximate): $h = 4.15\times 10^{-15}\ \text{eV s}$

• Energy range of photons: $E \in [10^{-12},\ 10^{10}]\ \text{eV}$

Rest of the Spectrum and Ionization.

• 

  Ionization status (as per summary):

  • Radio waves: No

  • Microwaves: No

  • Infrared: No

  • Visible light: No

  • Ultraviolet: No

  • X-rays: Yes

  • Gamma rays: Yes

X-Rays and Gamma Rays: Similarities and Differences

• Similarities:

  • Exhibit wave-particle characteristics; high energy; can burn skin

  • Intensity follows inverse square law; can ionize matter

• Differences:

  • Gamma rays originate from atomic nuclei (nuclear transitions)

  • X-rays originate from interactions between electrons and atoms

Particulate Radiation

• Particulate radiation includes alpha and beta particles

• Capable of ionizing matter; more common in nuclear medicine or radiation therapy

Alpha and Beta Particles

• Alpha particles:

  • The nucleus: two protons and two neutrons

  • Positive charge; short range; cannot penetrate many materials

• Beta particles:

  • Electrons emitted from unstable nuclei; originate in nucleus (not electron shell)

  • Lighter than alpha; may ionize along their path

• Beta particles can be negative (beta minus) or positive (beta plus, a positron)

Radioactivity

• Radioactivity: decay of unstable nuclei emitting gamma, alpha, or beta particles to reach stability

• Decay transforms into new elements

• Half-life: time for half of atoms to decay

Sources of Exposure

• Natural/background and manmade sources

• Subcategories: cosmic, terrestrial, internal, medical

• Total dose varies by geographic location

Interaction with Matter: Reflection, Transmission, Absorption, Attenuation

• Energy determines how EM radiation interacts with matter

• Can be reflected, transmitted, absorbed, or attenuated by tissues

Radiopaque vs Radiolucent (Practical Imaging Concept)

• Ra diopaque materials (bone) absorb X-rays (appear white)

• Radiolucent materials (soft tissue) transmit more X-rays (appear darker)

Quick Reference: Core Takeaways

• EM radiation properties: speed, wavelength, frequency, energy; wave-particle duality

• Core equations:

• Spectrum uses and ionization tendency: X-rays & Gamma rays ionize; others do not (per summary)

• Alpha/beta particles: particulate radiation with distinct properties

• Radioactivity and half-life: decay, stability, time scales

• Exposure sources: natural vs manmade; medical contributions

• Interactions with matter: reflection, transmission, absorption, attenuation

• Imaging relevance: radiopaque vs radiolucent materials