CHARACTERISTICS OF INTERACTIONS

In a radiation interaction, the radiation and the material with which it interacts may be considered as a single system. • When the system is compared before and

after the interaction, certain quantities will be found to be invariant.

• Invariant quantities are exactly the same before and after the interaction.

• Invariant quantities are said to be conserved in the interaction.

One quantity that is always conserved in an interaction is the total energy of the system, with the understanding that mass is a form of energy.

Other quantities that are conserved include momentum and electric charge.

Some quantities are not always conserved during an interaction. • For example. the number of particles may not

be conserved because particles may be • fragmented,

• fused,

• created (energy converted to mass), or

• destroyed (mass converted to energy).

Interactions may be classified as either • elastic or

• inelastic.

An interaction is elastic if the sum of the kinetic energies of the interacting entities is conserved during the interaction. • If some energy is used to free an electron or

nucleon from a bound state, kinetic energy is not conserved and the interaction is inelastic.

• Total energy is conserved in all interactions, but kinetic energy is conserved only in interactions designated as elastic.

ENTC 4390

DIRECTLY IONIZING RADIATION

When an electron is ejected from an atom, the atom is left in an ionized state. • Hydrogen is the element with the smallest

atomic number and requires the least energy (binding energy of 13.6 eV) to eject its K-shell electron.

Radiation of energy less than 13.6 eV is termed nonionizing radiation because it cannot eject this most easily removed electron.

Radiation with energy above 13.6 eV is referred to as ionizing radiation.

If electrons are not ejected from atoms but merely raised to higher energy levels (outer shells), the process is termed excitation, and the atom is said to be “excited.”

Charged particles such as electrons, protons, and atomic nuclei are directly ionizing radiations because they can eject electrons from atoms through charged-particle interactions.

Neutrons and photons (x and rays) can set charged particles into motion, but they do not produce significant ionization directly because they are uncharged. • These radiations are said to he indirectly

ionizing.

Energy transferred to an electron in excess of its binding energy appears as kinetic energy of the ejected electron. • An ejected electron and the residual positive ion

constitute an ion pair. An average energy of 33.85 eV termed the W-

quantity or W, is expended by charged particles per ion pair produced in air. • The average energy required to remove an electron

from nitrogen or oxygen (the most common atoms in air) is much less than 33.85 eV.

The W-quantity includes not only the electrons binding energy but also the average kinetic energy of the ejected electron and the average energy lost as incident particles excite atoms, interact with nuclei, and increase the rate of vibration of nearby molecules. • On the average. 2.2 atoms are excited per ion

pair produced in air.

ENTC 4390MEDICAL IMAGING

Interactions of Radiation

Particle Interactions

Particles of ionizing radiation include charged particles, such as:• alpha particles

• protons

• electrons,

• beta particles

• positrons, and

• neutrons.

The behavior of heavy charged particles is different from that of lighter charged particles such as electrons and positrons.

Excitation, Ionization, and Radiative Losses

Energetic charged particles all interact with matter by electrical forces and lose kinetic energy via:• excitation,

• ionization, and

• radioactive losses.

Excitation and ionization occur when charged particles lose energy by interacting with orbital electrons.• Excitation is the transfer of some of the

incident particle’s energy to electrons in the absorbing material, promoting them to electron orbits farther from the nucleus.• Higher energy levels.

In excitation, the energy transferred to an electron does not exceed its binding energy.• The electron doesn’t leave the atom.

Following excitation, the electron will return to a lower energy level,• With the emission of the excitation energy in the

form of• Electromagnetic radiation or

• Auger electrons.

Atomic Emissions

Photon or bundle of em-radiation

Secondelectronejected from atom

K

L

K

L

Characteristic Radiation Auger Electron

Note: 20181Tl → 201

80Hg by absorbing an orbital electron.We primarily image the K X-rays of 201

80Hg.

Characteristic X-Rays

Electron transitions between atomic shells results in the emission of radiation in the visible, ultraviolet, and x-ray portions of the electromagnetic (EM) spectrum.• Characteristic x-rays are named according to

the orbital in which the vacancy occurred.

Electromagnetic radiation

Electromagnetic Radiation

Electromagnetic radiation consists of oscillating electric and magnetic fields.

An electromagnetic wave requires no medium for propagation, • That is, it can travel in a vacuum as well as

through matter.

The wavelength of an electromagnetic wave is depicted as the distance between adjacent crests of the oscillating fields. • The constant speed of electromagnetic

radiation in a vacuum is the product of the frequency and the wavelength of the electromagnetic wave.

c =

Often it is convenient to assign wavelike properties to electromagnetic rays. • At other times it is useful to regard these

radiations as discrete bundles of energy termed photons or quanta.

• The two interpretations of electromagnetic radiation are united by the equation

E = h• where E represents the energy of a photon and

represents the frequency of the electromagnetic wave. The symbol h represents Planck’s constant, 6.62 x 10 -34 J-sec.

The frequency is

= c

and the photon energy may be written as

E = (hc

The energy in keV possessed by a photon of wavelength, in nanometers is

E = 1.24/

• Electromagnetic waves ranging in energy from a few nano-electron volts up to gigaelectron volts make up the electromagnetic spectrum.

Ultraviolet Light

Ultraviolet (UV) light is usually characterized as nonionizing.• UV light is used to

• sterilize medical instruments,

• destroy cells,

• produce cosmetic tanning, and

• treat certain dermatologic conditions

Visible Light

Visible light is the part of the EM spectrum to which the retina is most sensitive.

Infrared

Infrared is the energy released as heat by materials near room temperature.• Infrared sensitive devices can record heat

signatures.

• To date, they have not found a good medical imaging application.

The radiation resulting from a vacancy in the K shell is called a K-characteristic x-ray.• The radiation resulting from a vacancy in the

L shell is called a L-characteristic x-ray.

If the vacancy in one shell is filled by the adjacent shell it is identified by a subscript alpha.

• L K transition = Kand

• M L transition = L

If the electron vacancy is filled from a nonadjacent shell it is identified by a subscript beta.

• M K transition = K

The energy of the characteristic x-ray is the difference between the electron binding energies (Eb) of the respective shells.

–

–

– –

–Vacant

-2.5 keV

-11 keV

-69.5 keV -67 keV K

Characteristic X-Ray

K

L

M

Auger Electrons

An electron cascade does not always result in a characteristic x-ray.• A competing process that predominates in low

Z elements is Auger electron emission.• The energy released is transferred to an orbital

electron, typically in the same shell as the cascading electron.

–

–

–

–

–Vacant

-2.5 keV

-11 keV

-69.5 keV 64.5 keV Auger electron

K

L

M

If the transferred energy exceeds the binding energy of the electron, ionization occurs, whereby the electron is ejected from the atom.• The result of ionization is an ion pair consisting

of the ejected electron and the positively charged atom.• Sometimes the ejected electrons possess enough

energy to produce further ionizations called secondary ionization.

The term interaction may be used to describe • the crash of two automobiles

• (an example in the macroscopic world) or

• the collision of an x ray with an atom • (an example in the submicroscopic world).

Interactions in both macroscopic and microscopic scales follow fundamental principles of physics such as

(a) the conservation of energy and

(b) the conservation of momentum.

Neutrons are uncharged particles.• Neutrons do not interact with electrons and do

not directly cause excitation and ionization.• They do, however, interact with atomic nuclei,

sometimes liberating charged particles or nuclear fragments.

• Neutrons may also be captured by atomic nuclei.

In some cases the neutron is reemitted. In other cases the neutron is retained,

converting the atom to a difference nuclide.• In this case, the binding energy my be emitted via

spontaneous gamma-ray emission:

HnH 211

MeVE 22.2

–

–

–

–

–

–

–

–

–

––

–

–

–

–

––

–

–

–

–

–

–

–

–

+++++ ++++++++

Neutron

+

Pair production only occurs when the energies of x-ray or gamma ray exceed 1.02 MeV.

•An x-ray or gamma ray interacts with the electric field of the nucleus of an atom.

The photon’s energy is transformed into an electron-positron pair.

–

–

–

–

–

–

–

–

–

–

––

–

–

–

–

––

–

–

–

–

–

–

–

–

+++++ ++++++++

–

annihilation

0.51 MeV

0.51 MeV

When the positron (a form of antimatter) comes to rest, it interacts with a negatively charged electron, resulting in the formation of two oppositely directed 0.511 MeV annihilation photons.• Positrons are important in imaging of positron emitting

radiopharmaceuticals in which the resultant annihilation photon pairs emitted from the patient are detected by positron emission tomography (PET) scanners.

Ionizing Radiation