2.3 Interactions of Radiation with Matter

Interactions of Radiation with Matter

Quick Answer: For diagnostic NMT photons in tissue, Compton scatter dominates near Tc-99m (140 keV) and many SPECT energies; photoelectric effect matters more at lower energy and high Z (lead shielding, bone, CT contrast). HVL quantifies attenuation. Bremsstrahlung and characteristic x-rays describe how CT tubes make the x-ray beam used in SPECT/CT and PET/CT.

Why Interactions Matter

Every photon that leaves the patient (or scatters inside the patient) affects image quality, quantification, and radiation dose. Charged particles (β−, β+, α, conversion electrons) deposit energy locally — critical for therapy dosimetry and for understanding internal conversion. CNMT Domain I expects you to name the major photon interactions, know which dominates when, and define attenuation metrics used in shielding and CT.

Photoelectric Effect

In the photoelectric effect, the entire photon energy is absorbed by a bound orbital electron. The electron is ejected with kinetic energy E_photon − binding energy. A vacancy is filled by cascading electrons, producing characteristic x-rays and/or Auger electrons.

Probability increases strongly with atomic number (roughly ∝ Z³ to Z⁴ depending on energy regime) and decreases sharply as photon energy rises above absorption edges (roughly ∝ 1/E³ between edges). Practical consequences:

• Lead and tungsten shields stop γ rays largely via photoelectric absorption at diagnostic energies.
• Low-energy photons (e.g., Tl-201 mercury x-rays ~69–80 keV) have more photoelectric interaction in tissue and bone than 140 keV Tc-99m, affecting attenuation and scatter mix.
• CT contrast and bone stand out because iodine/calcium raise photoelectric probability relative to soft tissue.

Photoelectric absorption removes the primary photon completely (no scatter photon continues at reduced energy from that event). That removal contributes to true attenuation and to local dose.

Compton Scatter

In Compton scatter, the photon collides with a loosely bound (essentially free) electron, ejects it, and continues as a lower-energy photon deflected by a scatter angle. Energy sharing depends on angle: forward scatter retains more energy; large-angle scatter is softer.

Probability depends mainly on electron density (≈ physical density for soft tissue) and falls gradually with energy. In soft tissue from roughly 100 keV through several hundred keV, Compton is the dominant interaction for nuclear medicine imaging photons. That is why:

• Scatter degrades contrast on gamma-camera images unless rejected by energy windows and collimation.
• Scatter correction and energy resolution matter for quantification.
• Patient size and depth strongly affect detected scatter fraction.

Rule of thumb for exams:

• Photoelectric → low E, high Z
• Compton → mid E, tissue (most diagnostic NMT gammas)
• Pair production → high E (>1.022 MeV), high Z enhances it

Pair Production (Brief)

If photon energy exceeds 1.022 MeV (twice the electron rest energy 511 keV), the photon may convert near a nucleus into an electron–positron pair. Excess energy becomes kinetic energy of the pair. The positron later annihilates, producing two 511 keV photons.

Diagnostic Tc-99m (140 keV) and I-123 (159 keV) cannot undergo pair production. Even I-131’s 364 keV γ is below threshold. PET annihilation photons are 511 keV — still below 1.022 MeV — so the imaging photons themselves do not pair-produce; higher-energy gammas in some PET radionuclide cascades or therapy isotopes can. Know the threshold and that pair production is not the main interaction for standard diagnostic NMT imaging photons in tissue.

Attenuation and Half-Value Layer (HVL)

As a narrow monoenergetic beam passes through thickness x of material, intensity follows:

I = I₀ e^(−μx)

where μ is the linear attenuation coefficient (sum of photoelectric, Compton, pair, and coherent contributions as applicable).

Half-value layer (HVL) is the thickness that reduces intensity to half:

HVL = 0.693 / μ

(analogous to radioactive half-life). Tenth-value layer (TVL) reduces intensity to 10%. Clinical and safety uses:

• Syringe shields and vial shields are specified for a given isotope by attenuation performance (often stated as % reduction or lead thickness).
• Room shielding calculations use HVL/TVL of lead or concrete at the relevant energy.
• Patient attenuation in SPECT and PET requires correction (CT-based μ maps on hybrid systems).

Broad-beam vs narrow-beam: scatter buildup means real barriers may transmit more than simple narrow-beam HVL predicts; safety design accounts for buildup factors. For the exam, define HVL cleanly and know higher-energy photons generally need thicker HVL in the same material.

Approximate intuition (order of magnitude only — exact values depend on spectrum and geometry):

Charged-Particle Interactions (Context)

β particles lose energy by ionization/excitation and by radiative losses (Bremsstrahlung) when decelerated in high-Z material. α particles are densely ionizing with very short range. Internal conversion electrons behave like monoenergetic β particles. Therapy planning cares about range and LET; diagnostic imaging cares that β/α do not produce useful external images the way γ and annihilation photons do.

X-Ray Production: Bremsstrahlung and Characteristic X-Rays

Hybrid cameras include diagnostic CT. Tube x-rays arise two ways:

1. Bremsstrahlung (“braking radiation”): Electrons from the filament strike the anode and decelerate in the nuclear field, emitting a continuous x-ray spectrum up to eV_peak set by kVp. Most tube output is Bremsstrahlung.

2. Characteristic x-rays: Incident electrons eject inner-shell electrons from anode atoms (commonly tungsten). Outer electrons fill vacancies, emitting discrete energies characteristic of the anode element (tungsten K-characteristic lines near ~59–69 keV).

kVp sets maximum photon energy and influences beam hardness; mA and time (mAs) scale quantity (intensity). Filtration removes low-energy photons that dose the patient without improving image. For CNMT practice, you need enough CT physics to:

• Understand attenuation maps for SPECT/CT and PET/CT
• Recognize that CT contributes patient dose separate from the radiopharmaceutical
• Communicate safely around tube warm-up, technique factors, and shielding of the CT room

Scatter vs Absorption — Image Quality Link

• Absorbed photons (photoelectric) never reach the detector — pure attenuation.
• Scattered photons may still hit the crystal with wrong direction and reduced energy — if accepted in the window, they blur and reduce contrast.
• Energy discrimination (pulse-height analysis) rejects many Compton-scattered photons whose energy falls outside the photopeak window — a bridge to the next section on detectors.

Master dominance rules (Compton vs photoelectric), the pair-production threshold, HVL definition, and the two CT x-ray production mechanisms — that set covers nearly all Domain I interaction questions.