Interactions with Matter ( RADIATION PROTECTION) *Chapter 3

Peak kVp (exposure factor)

The highest energy level of photons

in the X-ray beam, equal to the highest voltage

established across the x-ray tube

 Controls the quality, penetrating power of the

photons and to some degree the quantity

Milliampere-second (mAs) (Exposure factor)

the product of electron tube current and the amount of time in seconds that the x-ray tube is activated

 How much radiation is directed toward a patient

 Quantity only

Absorption in biologic tissue

If x-ray enters human tissue, they may do the following :

1. Interact with the atoms of the biologic material in the patient and be absorbed

2. Interact with the atoms of the biologic material and be scattered

3. Pass through without interaction

Absorption

when some or all of the electromagnetic energy is transferred from the x-ray to the atoms in the biologic material

Absorbed dose (D)

The amount of energy absorbed per unit mass

 Unit mGyt

X RAY TUBE

Beam Production

X-ray beam is produced when a stream of energetic electrons bombards a positively charged target in a highly evacuated glass tube.

 The target is also known as the anode

• When the electrons hit the target, X-ray photons are produced

• X-ray photons exit from the tube through the glass window

BEAM

When the photons are created, they have a broad range of energies, not all of which are useful

• Low energy photons will only cause dose, best if filtered out before striking the patient

Glass Window

acts as a filter by removing very-low energy x-rays

Collimators

Contain aluminum to intercept the emerging rays before they reach the patient

• Aluminum “hardens” the beam by removing low-energy components that would only increase the patient dose

Permanent inherent Filtration

x-ray glass wall and added aluminum

Primary Radiation

The filtered X ray Photon Beam

Photon Energy

Photons have a multiple energies, but the most energetic photon can have no more energy that the electrons that bombarded the target

• The energy of the electrons inside the tube is specified in terms of electrical voltage (eV),

 in diagnostic radiology it is express in kilovolts (kV)

• The voltage that travels across the tube fluctuates, and is termed kilovoltage peak value (kVp

Energy

• Electron with the potential difference of 1 volt = energy of 1 eV (electron volt)

• Technique of 100 kVp = 100,000 eV, or 100keV

• KvP refer to the voltage on the x-ray tube

• KeV refers to the energy of specific x-rays

Attenuation

Attenuation is the reduction in the number of primary photons in the x-ray beam through absorption and scatter as the beam passes through the patient

• Non-interacting x-ray photons reach the radiographic image receptor- direct transmission

Direct Transmission

when primary x-ray photons traverse the patient without interacting  Optimal images come from direct transmission

Indirect Transmisson

When photons are scattered or deflected with potential loss of energy but may still traverse the patient and strike the IR

 Compton interaction

 Coherent interaction

Primary Photons

the photons “beam” before it strikes the patient

Exit, image-formation, photons

The photons that emerge from the patient and strike the detector below  Non-interacting and small-angle photons

Attenuated

any process decreasing the intensity of the primary photon beam directed toward a particular path

• If the intended path is the detector and the photon did not strike the predefined location, then it is said to be attenuated

• Attenuation = both absorption and scatter processes

Scatter

Small angle and Radiographic fog

Small angle

when the photon bends from original path but not so much that it misses the target  Degrades the image by blurring the sharp outlines of dense structures

Radiographic fog

Undesirable additional darkness on a completed radiographic image caused by scatted radiation reaching the IR. It interferes with the radiologist’s ability to accurately read a radiograph

• Reduce scatter by reducing the field size “amount of body being radiated”, collimate the beam

Interactions with matter

1. Coherent scatter

2. Photoelectric absorption

3. Compton scatter**

4. Pair production

5. Photodisintegration

Coherent Scattering

Also known as:

• Classical scattering

• Elastic scattering

• Unmodified Scattering

• Thompson scattering (Bushong)

• Process that results in no loss of energy as x-rays scatter

• Low energy photon will interact with an atom causing it to vibrate. The vibration causes energy in the form of an electromagnetic wave, which is released as a scattered wave or photon

• NO energy has been absorbed

• Small change in direction of the emitted photon, < 20 degrees

Photoelectric Absorption

Most important interaction between x-ray photon and the patient for producing useful images (provides contrast)

• It is the interaction between an x-ray photon and an inner shell electron (K-shell or L-shell)

• The incoming x-ray photon surrenders all of its energy to the inner shell electron, ejecting it from orbit, and the incoming photon no longer ceases to exist

Photoelectric Absorption

The unbound orbital electron is called a photoelectron and possesses kinetic energy equal to the energy of the incident photon minus the binding energy of the electron shell

 The photoelectron can interact with other atoms in the vicinity causing excitation or ionization

• The vacancy causes the atom to become unstable

• The instability is alleviated by filling the vacancy in the inner shell with electrons from the outer shells falling down into the openings

• When the electrons drop down closer to the nucleus, an energy loss occurs

• The energy released is in the form of a photon called a characteristic photon, characteristic x-ray

Photoelectric interaction- Auger effect

Auger “awzhay” effect:

• Inner electron is removed from an atom, causing an inner shell vacancy. As the vacancy is filled and energy is released, instead of the energy emerging as a characteristic radiation it transfers its energy to another electron in the atom.

• The newly stuck electron is ejected an auger electron

Probability of Photoelectric absorption

Depends on the energy of the incident x-ray photon and the atomic number of the atoms comprising of the irradiated object

• Decrease in energy = increase in probability

• Increase in atomic number = increased probability

Destiny

increases = increased probability

Thickness

is directly related= increased thickness will have increased absorption

Attenuation

• Less attenuation = darker image

• More attenuation = lighter image

Compton Scattering

Also known as:

• Incoherent scattering

• Inelastic scattering

• Modified Scattering

• Responsible for most of the scatter radiation produced during radiologic procedures

 May be forward small angle scatter

 Backscatter

 Lateral side scatter •

Scatter is responsible for occupational dose

Compton Scattering (Part two)

• Incoming x-ray photon interaction with a loosely bound outer electron of an atom •

• The incoming x-ray photon surrenders a portion of its energy when dislodging the electron from orbit

• • Disrupting the atom makes it Ionizing

Compton Scattered Electron

The freed electron is called a Compton scattered electron, or secondary, or recoil electron

 Possesses excess kinetic energy

which can ionize near by atoms

• The incident x-ray photon only surrendered some of its energy. It continues on its way but in a new direction. It is called the Compton scattered photon

Pair Production

Only occurs if the incident x-ray photon is at least 1.022 MeV

- Not in the diagnostic radiology range

• An incoming x-ray photon strongly interacts with the electric field surrounding the nucleus of an atom and disappears

• Photon energy is absorbed and transformed into matter composed of two particles: a negatron and positron

Photodisintegration

Interactions that occur with energies exceeding 10 MeV

-Not in diagnostic range

• The high-energy incident photon collides with the nucleus, which directly absorbs all the photon’s energy

• The excess energy in the nucleus is then released by ejecting a neutron from the nucleus

• Altering the nucleus makes the nucleus radioactive