Imaging: Atomic Structure, X-Ray Interactions (Scatter), and Beam Restriction

Protons, Neutrons, and Electrons

The 3 fundamental particles of an atom

Nucleus

Protons & Neutrons are found in the:

Positive

What kind of charge do protons have?

Orbiting the nucleus in electron shells

Electrons are found:

Neutral

What kind of charge do neutrons have?

Negative

What kind of charge do electrons have?

Atomic number

the number of protons in the nucleus

Z number

The atomic number is also known as the:

Atomic mass number

the number of protons and neutrons in the nucleus of an atom

Atomic weight

weight (mass) of all particles in the atom

-Centrifugal Force
-Attractive Electrostatic Force

The 2 forces keeping electrons in orbit:

Centrifugal force

caused by the orbit of the electron = pushes outward

Attractive Electrostatic force

+ charged nucleus, - electron = the two want to meet

Orbital forces (info)

Centrifugal force and Attractive electrostatic force are balanced (cancel each other out), keeping the electron in orbit

the amount of energy required by an electron to stay in orbit

Electron's energy level:

K-Q

The electron orbital shells are lettered:

Electron binding energy

Energy needed to eject electron from atom

Increases

Electron binding energy (Eb) _________ as atomic number increases

An atom adding or losing an electron

An ion is created by:

Subtracting an electron

Positive ion results from:

Adding an electron

Negative ion results from:

Isotope

An atom with the same number of protons but with different numbers of neutrons

Changes the element

Changing the Z# (# or protons)

Attenuation

Reduction in the number of x-ray photons as a result of interacting with matter

photons interact with matter, change directions, and may lose energy

The process of scattering occurs when:

At the atomic level

Where do photons interact with matter

1. Transmission without any interaction (no change in direction)
2. Total absorption (= patient dose)
3. Penetration with loss of energy (= compton scatter = occupational dose)

Processes x-rays can undergo as they reach the patient:

photon energy level (kVp)

The type of interaction the x-ray undergoes depends on:

Electron kinetic energy

dynamic energy of electron due to its relative position in the electron orbital cloud

Electrons in inner shells

Higher binding energies but lower kinetic energy

Electrons in outer shells

Lower binding energies but higher kinetic energy

1. Coherent Scattering
2. Photoelectric (PE) Absorption
3. Compton Scattering
4. Pair Production
5. Photodisintegration

The basic interactions between x-rays and matter are:

Pair production & photodisintegration

very high energy levels - more for radiation therapy/therapeutic use (interactions)

-Photoelectric (PE) Absorption
-Compton Scatter

What 2 interactions impact diagnostic radiology?

Photoelectric effect

The product of combining and x-ray photon and an electron

When an incident x-ray photon interacts with an inner-shell electron

When does photoelectric absorption occur?

Characteristic cascade

The result of a vacancy with an inner shell electron

Characteristic cascade electron

undergoes change in energy level when transferring shells

Secondary radiation energy

Radiation that originates from irradiated material outside x-ray tube

Characteristic cascade fills the void in inner shell and sheds its excess

Secondary radiation is released within an atom that has undergone PE absorption when:

-Incident photon interacts with an inner orbit electron
-gives all its energy to the electron
-ejects electron from orbit
-photon is "absorbed"

PE Absorption/Effect Step 1:

the ejected electron (PE) imparts the atom with energy equal to the excess of the electron's binding energy

PE Absorption/Effect Step 2:

characteristic cascade
(outer shell electrons move in to fill opening)

PE Absorption/Effect Step 3:

-electron drop to void
-may shed excess energy (secondary photon)

PE Absorption/Effect Step 4:

Eb

Binding energy

1. Ei must be greater than the Eb of inner shell electron
2. A PE is most likely to occur when the Ei and Eb are nearer to each other in strength
3. A PE is more likely to occur when the electron is more highly bound in its orbit (high Z#)

Conditions for PE Effect to occur:

Ei

Energy of incident photon

Inverse relationship

Relationship between kVp and chances of PE interactions

Decreases

As photon energy (kVp) increases, chances of a PE interaction:

-Classical scatter
-Thomson scattering
-Unmodified scatter
-Rayleigh Scattering

Coherent Scatter AKA

Coherent scatter

X-ray photon is initially absorbed by the whole atom and is then ejected out with the same energy/wavelength/frequency, but in a different direction

With low-energy photons (below 10 keV)

When does coherent scattering occur?

Compton scatter

An incident x-ray photon interacts with a loosely bound outer-shell electron, removes the electron, and proceeds in a different direction

1. Recoil/Compton electron
2. Compton scattered photon

The process of the incident x-ray photon interacting with the outer shell electron creates 2 things:

The ejected electron and the scattered photon

The energy of the incident photon is distributed between:

Less energy, longer wavelength, and lower frequency

Compton scattered photon continues with:

occupational exposure and radiation fog

Compton scatter is a source of:

Direct relationship

Relationship between kVp and Compton scatter

Increases

As kVp increases, Compton scatter ________

-photons interact with outer shell electron
-photon imparts some energy to electron
-electron is ejected from orbit

Compton Effect (scattering) Step 1:

ejected electron (compton electron) leaves atom with energy equal to the excess imparted by photon

Compton Effect (scattering) Step 2:

-photon continues on altered path--scattered
-photon now has less energy and longer wavelength

Compton Effect (scattering) Step 3:

Inverse relationship

Relationship between scatter and contrast

Decreases

As scatter increases, contrast _______

Direct relationship

Relationship between scatter and IR exposure

Increases

As scatter increases, IR exposure _______

Photoelectric effect (absorption)

Total absorption of the incident x-ray photon is the result of:

Transmitted to create image

Most of the beam is attenuated (absorbed within the body)....what happens to the rest?

Direct relationship

Relationship between kVp and # of transmission with interaction

Increases

As kVp increases, the number of photons transmitted without interaction _______

Inverse relationship

Relationship between kVp and % of PE interactions

Decreases

As kVp increases, the percentage of PE interactions ________

Direct relationship

Relationship between kVp and % of Compton interactions

Increases

As kVp increases, the percentage of Compton interactions ________

Inverse relationship

Relationship between PE Absorption and Compton Scatter

Increases

As kVp increases, the percentage of scatter ________

Inverse relationship

Relationship between kVp and % of absorption

Decreases

As kVp increases, the percentage of absorption ________

Compton scatter

What type of interaction is the most predominant interaction through most of the diagnostic range?

1. Lower energy ranges (25-40 keV produced by 40-70 kVp)
2. In elements with higher z#

Photoelectric interactions predominate in what 2 circumstances?

High contrast

PE interactions result in what type of image contrast?

Increase in patient exposure

PE interactions result in what type of patient exposure?

Low contrast

Compton interactions result in what type of image contrast?

Decrease in patient exposure

Compton interactions result in what type of patient exposure?

1. thickness & density of part
2. kVp
3. Field size

What 3 factors control scatter?

Direct relationship
(More Matter = More Scatter!)

Relationship between part thickness and scatter

Increases

As part thickness increases, scatter ________

Direct relationship

Relationship between kVp and scatter

Increases

As kVp increases, scatter _______

Direct relationship

Relationship between field size and scatter

Increases

As field size increases, scatter ________

Inverse relationship

Relationship between collimation and scatter

Decreases

As collimation increases (smaller), scatter ________

Electrons striking other than focal spot

When does off-focus (or extrafocal) radiation occur?

Poorly defined soft tissue outside the collimation
-adds unwanted density and reduces contrast

Off-focus radiation results in what?

Secondary radiation

Radiation produced in interactions of x-rays with matter

Undercutting

-Poor collimation of primary beam
-Unattenuated beam strikes tabletop
-Scatter "undercuts" image

Control of Scatter Production

-Not much we can do/control
-No control over patient body habitus
-kVp limited: selection based on size of part
-We can collimate though

High

(High or Small) % of beam will be attenuated

Small

(High or Small) % of beam will reach the image receptor