INTERACTIONS OF RADIATION WITH MATTER

Transcription

1 INTERACTIONS OF RADIATION WITH MATTER Renée Dickinson, MS, DABR Medical Physicist University of Washington Medical Center Department of Radiology Diagnostic Physics Section Outline Describe the various ways particles (e.g. electrons), gamma-rays and x-rays interact with and are attenuated in matter Describe the energy dependence of these interactions Describe the various quantitative parameters used to characterize x-ray attenuation a copy of this lecture may be found at: Describe qualitatively x-ray beam effective energy and beam quality 2 Particle Interactions Excitation, Ionization, & Radiative Losses Energetic charged particles interact via electrical forces Lose kinetic energy through excitation, ionization and radiative losses Incoming charged particles interact with orbital electrons Excitation: imparted energy is less than the binding energy (E < E b ) Orbital electron excited to higher energy (outer orbit), then emits EM (characteristic x-ray) or Auger electrons (de-excitation) when electron returns to lower energy Approximately 70% of electron energy deposition leads to non-ionizing excitation Excitation & Ionization Bremsstrahlung (Radiative Losses) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p UW and Renee L Butler, M.S., DABR 1

2 Excitation, Ionization, & Radiative Losses Energetic charged particles interact via electrical forces Lose kinetic energy through excitation, ionization and radiative losses Incoming charged particles interact with orbital electrons Excitation, Ionization, & Radiative Losses Energetic charged particles interact via electrical forces Lose kinetic energy through excitation, ionization and radiative losses Incoming charged particles interact with orbital electrons Ionization: imparted energy is greater than the binding energy (E > E b ) Orbital electron is ejected Creates an ion pair ejected electron & positively charged atom Sometimes electrons with enough kinetic energy to produce further ionizations (secondary ionizations) Such electrons are called delta rays At very low energy (~40 ev), probability of excitation and ionization about equal. Incident particles (e.g. electrons) with higher energies have increased probability of ionization over excitation. Low energy particles are more likely to excit atoms. Example: a 10 kev incident electron results in over 450 secondary electrons with energies ranging from 10- to 70 ev. c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p Particle Range and LET Heavier particles have more direct (strait path), lighter particles scatter. Particle Range and LET LET is proportional to incident particle charge and inversely proportional to the incident particle energy High LET radiation: alpha particles, protons Low LET radiation: electrons, β - and β +, x-rays or γ-rays High LET >> damaging than low LET radiation c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.36. Linear energy transfer (LET) amount of energy deposited per unit length (ev per cm) Describes the energy deposition density which largely determines the biologic consequence of radiation exposure 7 8 UW and Renee L Butler, M.S., DABR 2

3 Excitation, Ionization, & Radiative Losses Bremsstrahlung (x-ray production lecture), also known as radiative losses or radiative interactions Particle Interactions Recall, excitation and ionization involves incident electrons interacting with orbital electrons of an atom REVIEW: Excitation & Ionization: free electron interactions w/matter Bremsstrahlung (Radiative Losses): x-ray production Radiative losses involve incident electrons interacting with the nucleus of the atom More in x-ray production lecture c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p X-ray Interactions with Matter X-ray Absorption and Scatter Four major interactions are of importance to diagnostic radiology and nuclear medicine, each characterized by a probability (or cross-section ) of interaction Classical (Rayleigh or elastic) scattering Compton scattering Photoelectric effect Rayleigh Scattering Photoelectric Effect Pair Production Pair production (not in diagnostic energy range, incident x-ray must have energy > 1.02 MeV) E γ = energy of incident electron E BE = binding energy of orbital electron UW and Renee L Butler, M.S., DABR 3

4 Classical (Rayleigh or elastic) Scattering E γ << E BE Incident photon interacts with and excites the total atom (electrons in the atom oscillate in phase) Occurs at very low energies (15 to 30 kev), increases in probability with decreasing energy. No ionization takes place and no loss of energy The photon is scattered (re-emitted) in a range of different directions, but close to that of the incident photon Detection of scattered photo has negative effect on image quality Relatively infrequent probability (~ 5%) for energies > 70 kev (diagnostic energy range) Accounts for ~10% of measured photons at 30 kev (mammography energy range) Dominant interaction of x-rays with soft tissue in the diagnostic range and beyond (approx 30 kev - 30MeV) Interaction of incident photon and an outer shell electron Results in ionization of the atom, a scattered photon, and the ejected electron Ejected (Compton) electron loses its energy via excitation and ionization c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p (outer shell electron interaction) Original photon deflected which in turn decreases subject contrast (outer shell electron interaction) Conservation of energy & momentum: E o = E sc + E e As incident E 0 increases, the photon & electron are scattered in more forward direction, increasing likelihood of being measured by the imaging detector (decreased contrast) Compton scatter is greatest contributor of scattered x-rays in diagnostic imaging E o = incident photon energy E sc = scattered photon energy E e = ejected electron energy At diagnostic energies ( kev) most energy transferred to the scattered photon c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.40. c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p UW and Renee L Butler, M.S., DABR 4

5 (outer shell electron interaction) Probability of Compton interaction increases w incident photon energy and electron density In tissue, the number of electrons per gram is fairly constant, therefore the probability is roughly proportional to material density (ρ) Interaction of incident photon with inner shell electron Results in a photoelectron and characteristic x-ray (or Auger electron) All incident photon energy (E o ) is transferred to the ejected photoelectron (E e ) E e = E o E BE c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.40. c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p (inner shell electron interaction) (inner shell electron interaction) The empty electron shell immediately fills with an electron from an outer orbital resulting in the emission of characteristic x-rays (or Auger electron) The energy of the characteristic x-ray emitted (E ) is the difference in the binding energies of the orbitals c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.42. Probability of photoelectric absorption proportional to Z 3 /E 3 Due to the absorption of the incident x-ray without scatter, maximum subject contrast arises with a PE interaction Explains why contrast decreases as higher energy x-rays are used in the imaging process (in addition Compton scatter decreasing contrast); as E o is increased, the probability of PE is decreased eight-fold Increased probability of photoelectric absorption just above the inner shell E BE ( absorption edges ) UW and Renee L Butler, M.S., DABR 5

6 Use in Radiology Contrast Agents Imaging Detectors Element Symbol (inner shell electron interaction) Atomic # (Z) k edge Energy Iodine I kev Barium Ba kev Gadolinium Gd kev Cesium (DR) Cs kev Causes discontinuities in the attenuation profiles K-edge is important in diagnostic imaging K-edges become significant factors for higher Z materials as the E BE are in the diagnostic energy range Use in Radiology Contrast Agents Imaging Detectors Element Symbol (inner shell electron interaction) Atomic # (Z) k edge Energy Iodine I kev Barium Ba kev Gadolinium Gd kev Cesium (DR) Cs kev Increased absorption probabilities improve subject contrast and quantum detective efficiency At photon E << 50 kev, the PE plays an important role in imaging soft tissue, amplifying small differences in tissues of slightly different Z and improving subject contrast (e.g., in mammography) Note: PE is also the dominant interaction contributing to dose to the patient c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p Pair Production E γ > 1.02 MeV Only occurs if incident photon has energy exceeding 1.02 MeV (beyond diagnostic & NM energy range) Interaction with the nucleus, creating an electron-positron pair (both lose their energy via excitation/ionization) When positron comes to rest, it interacts with an electron and produces two 511-keV photons. Question If a technologist were to stand 2 meters away from a patient during fluoroscopy (outside the primary beam) the dose received by the technologist would be mainly due to: A. Compton electrons. B. Photoelectrons. C. Compton scattered photons. D. Characteristic x-rays generated in the patient. E. Coherent scatter. c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p UW and Renee L Butler, M.S., DABR 6

7 X-ray Interactions with Matter X-ray Attenuation & Beam Quality REVIEW: Rayleigh Scattering: image degradation (~5% at 70 kev) : image degradation Photoelectric Effect: increased contrast/dose Pair Production: outside diagnostic energy range c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.44. Linear Attenuation Coefficient (μ) Mass Attenuation Coefficient (σ) Half Value Layer (HVL) & Beam Hardening X-ray Attenuation X-ray Attenuation Removal of photons from the primary beam as it passes through matter Caused by both absorption and scattering At low energies (<26 kev), photoelectric absorption dominates in soft tissue Most interaction in soft tissue (b/c of photon energy and low-z of soft tissue) are Compton Cross section is a measure of the probability of an x-ray interaction Linear attenuation coefficient (μ) Mass attenuation coefficient (σ) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.44. Interaction probability is related to the thickness of material and the number of atoms a photon encounters per unit distance Linear attenuation coefficient (μ) describes the fraction of incident photons that will be attenuated per unit thickness Strongly dependent on photon energy As E o increases, μ decreases Units: cm -1 Recall: photon interaction is strongly dependent on density (ρ) if you divide μ by ρ UW and Renee L Butler, M.S., DABR 7

8 X-ray Attenuation Interaction probability is related to the thickness of material and the number of atoms a photon encounters per unit distance Mass attenuation coefficient (μ/ρ) normalization of the linear attenuation coefficient for standard unit density Units: cm 2 /g 1 X-ray Attenuation Interaction probability is related to the thickness of material and the number of atoms a photon encounters per unit distance Example: water, ice, and vapor μ water > μ ice >> μ vapor μ/ρ water = μ/ρ ice = μ/ρ vapor Μ total = Μ Rayleigh + Μ Compton + Μ PE + Μ PP c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p Beam Quality Beam Quality Half Value Layer (HVL) Defined as the thickness of material required to reduce the primary photon intensity to one-half the initial value. Indirect measure of photo energies in the beam (or beam quality) primary x-ray beams used in radiology have a spectrum of energies Half Value Layer (HVL) Defined as the thickness of material required to reduce the primary photon intensity to one-half the initial value. HVL is related to the material μ ln For diagnostic x-ray beams, we express HVL in mmal Example: photons transmitted through 5 HVLs of material the fraction of photons remaining is 1 3.1% 2 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.51, UW and Renee L Butler, M.S., DABR 8

9 X-ray Attenuation & Beam Quality REVIEW: Linear attenuation coefficient (μ) and mass attenuation coefficient (μ/ρ) are highly dependent on photon energy Half value layer (HVL) = 0.693/ μ c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 3 rd ed., p.46. Summary: Radiation Interaction w/ Matter Particle interactions excitation & ionization (free electron absorption/scatter in tissue), and radiative losses (Bremsstrahlung, x-ray production in radiology) Photon interactions Rayleigh, Compton (dominant interaction, primary contribution of scattered photons for a wide-range of E o ), Photoelectric effect (PE, improved contrast b/c of k-edges, higher probability at low E o ), and Pair Production (n/a in radiology) Linear (μ) and mass (μ/ρ) attenuation coeifficients quantitatively describe x-ray attenuation and energy dependence in matter Half value layer (HVL) quantitatively describes a photon beam spectrum and how it relates to μ UW and Renee L Butler, M.S., DABR 9