X-ray interactions / filtration / beam restriction / grids

attenuation

the reduction in the number of x-ray photons in the beam (getting soaked up in the body)

Attenuation is a result of

X-ray photons interacting with matter, therefore giving up their energy to the matter with which they interact

Exponential attenuation

50% for every 4-5cm (1-2in) of tissue thickness

(lose 50% of photons to attenuation)

to compensate, radiographic technique must be roughly doubled for every 4cm (2'') increase in part thickness

compensating for attenuation

to compensate, radiographic technique must be roughly doubled for every 4cm (2'') increase in part thickness

for every 4cm (2'') increase in part thickness

technique doubled

x-rays can be

transmitted without interaction

or

interact with

-entire atom (low energy)

-orbital electron (moderate energy)

-nucleus of an atom (high energy)

low energy photons interact

with whole atom (usually too low x-rays, arent useful enough for us)

moderate energy photons interact

with orbital electrons (mainly live here and useful for us)

high energy photons

interact with nucleus

dont deal w has as much in x-rays

atomic structure

nucleus

orbital electrons (bound vs more free, binding energy)

Higher atomic number

more energy required to remove a K-shell electron from atom

We would rather have

-photons fully pass through unscathed

or

-interact with tissue fully (fully attenuate)

X-ray interactions with matter

Coherent scatter/classic (low)

Photoelectric absorption (PE)

Compton Scattering

Pair production (high energy)

Photodisintegration (high energy)

low, moderate, moderate, high, high

Coherent scattering involves

low energy photons (below 10 keV)

we dont want these

types of coherent scattering

1. Rayleigh Scattering (all electrons of atom)

2. Thomson Scattering (single outer shell electron)

Produce same result

coherent scattering

electrons are excited and vibrate at photon frequency (makes atom wiggle)

no electrons are ejected

no ionization takes place

atom will stabilize itself by releasing a photon equal in energy to the incident photon, but in a different direction

so low energy, it will most likely not leave the patient

overall, mainly just waves at atom

Which x-ray interaction do we prefer to happenn

photoelectric absorption

What happens in photoelectric absorption?

Incident photon energy completely absorbed by inner shell electron

Ionizes so it causes a photoelectron

overall

-incident energy photon completely absorbed by inner shell electron

-emits photoelectron

When is photoelectric absorption most likely to occur?

When x-ray photon has just slightly more energy than the binding energy of a K or L shell electron (reminds of characteristic)

In photoelectric, an ion pair is formed when

-an electron is ejected from the atom (becomes a photoelectron)

-remaining atom has a vacancy inn its inner electron shell

this will cause a characteristic cascade

photoelectron energy equation

Ei = Eb + Eke

incident photon= BE + Kinetic energy

photoelectron characteristics

-Kinetic energy (Eke)

-Mass

-Reabsorbs quickly

>>Within 1-2mm of tissue

Ionized atom

inner shell electron vacancy makes atom electrically unstable

Causing

characteristic cascade

-vacancy filled by an outer shell electron

-electron undergoes change in energy level

-emits characteristic photon

for PE interactions, incident photon energy

must be greater than or equal to binding energy (Eb) of inner shell electron

PE absorption interaction more likely to occur

if incident photon energy (Ei) and inner shell electron binding energy (Eb) are close to each other

As photon energy increases

Chance of PE interaction decreases dramatically

INVERSE CUBED RELATIONSHIP

-cannot be too much

Photon energy and chance of PE interaction has what kind of relationship?

Inverse cubed relationship (1/n3)

example

Ei= 120 keV

Eb= 36 keV

PE interaction will likely not take place

Only transmission will occur due to too big of energy differences

Higher atomic numbers (so higher binding energies)

will result in higher PE absorption interactions

Atomic number and PE absorption relation

Direct cubed relationship (n3)

Higher likelihood of PE with increased atomic number

If you double the atomic number

You increase chance of PE absorption by a factor of 8

Lower atomic numbers experience PE absorption with the

K shell

Higher atomic numbers experience PE absorption in

K, L, or M shell

Secondary radiation

Radiation that originates from irradiated material outside of x-ray tube

Comes from characteristic photons emitted from atoms of patient after PE absorption has occurred

Will most likely not reach the detector, not really high energy, overall it is most likely getting absorbed in the body

Low atomic number in tissue

Low energy secondary radiation

High atomic number in contrast agents

Higher energy secondary radiation (more total absorption)

Compton scattering

incident photon (Ei) interacts with outer shell electron and ejects it

Ion pair is formed

Photon will transfer some of its kinetic energy to the recoil (compton) electron, and continues on in a different direction

Produces scatter

-which could hit detector but has less fidelity/truthfullness

97% comes from compton

Incident photon energy in compton is distributed

between recoil electron (Eke) and scattered photon (Es)

The angle and energy of scattered photon affected by

The energy transferred to recoil electron (Eke)

therefore, it affects the frequency and wavelength of the scattered photon

The recoil electron from compton will

travel until it fills a vacancy in another atom

Scattered photon will continue to interact until

aborbed photoelectrically

Compton scattering is the source of

occupational exposure and radiation fog

For a scattered photon

the amount of energy retained is dependent on the initial energy of photon and its angle of deflection from recoil electron

At lower energy (coherent)

scatter goes forward or backward (as back scatter) with less at a 90 degree angle

At moderate energy in diagnostic range (50 keV) so compton

less will be backscatter and more will be in forward direction

as energy increases outside of diagnostic radiography range

scatter is almost entirely in a forward direction

relative contribution of PE and compton on detector exposure

Pair production

high energy levels (radiation therapy)

The incident photon energy must be 1.02 MeV or higher

photon energy absorbed by the nucleus

-the nucleus will become unstable

What does the nucleus release as a result of pair production

a positron and negatron to stabilize itself

the positron will be attracted to some electron and annihilate

Negatron and positron both have

equal masses but will opposite charges

Negatron

acts like free electron and will combine with nearby atom

functions same way as normal electron

Positron

unstable antimatter

will combine with nearest electron and an annihilation reaction occurs

matter of particles is converted to energy

-results in two photons of .511 MeV (511 keV) traveling at 180 degrees to each other

Pair production is more significant in

radiation therapy

does not occur in diagnostic range of energies

Pair production is not a significant interaction until energies of

10 MeV are being used

Photodisintegration

Extremely high energy photon (10 MeV or greater)

Absorption of photon by nucleus

What does the excited nucleus do in photodisintegration?

Releases an alpha particle

Photodisintegration is not significant

in diagnostic imaging range

Most of the x-ray beam is

attenuated while some is transmitted

As kVp increases

the number of photons transmitted without interaction increases (reduces PE much faster than compton)

so it

-decreases probability of PE absorptionn and compton interactions

Within the attenuated beam as kVp increases

PE absorption decreases

Compton effect increassed

Increased percentage of scatter and decreased percentage of absorption

Compton scatter typically

predominates within diagnostic x-ray energy range

-reason as why we add grids when we increase in tissue density

If we increase kVp

we get more scatter

PE absorption interactions predominate in

-lower energy ranges (25-45 keV produced by 40-70 kVp techniques)

-in elements with higher atomic numbers

introduction of contrast agents result in increase of PE absorption as well

When PE absorption predominates

-resulting image will have short scale contrast (high contrast)

-low kVp

-high mAs

when compton interactions predominate

Resulting image will have long scale contrast

High kVp and low mAs

Photoelectric effect with its all or nothing absorption of x-rays is responsible for

PRODUCTION OF SUBJECT CONTRAST

Scatter radiation destroys

subject contrast in remnant beam

Photoelectric effect occurs only in

inner atomic shells when the energy of the incident x-ray photon is slightly higher than the shell binding energies

compton effect occurs

in outer shells when the energy of incident x-rays is much higher than binding energies

compton effect responsible for

97% of scattered x-rays

As the angle of deflection from scattered x-rays increases away from the central ray

we find higher amounts of scatter but at lower energies

coherent scatter can be produced by

thompson effect (single outer shell electron) or rayleigh effect (all electrons)

x-ray photon temporarily excited an entire electron or an entire atom and is then reemitted 3%

Coherent scatter

DOES NOT IONIZE so it only occurs at very low photon energies

Compton is more uniform because

it relies on electron density

probability remains relatively constant

Compton scatter contributes least

to patient dose and image formation

Increaassing patient thickness

increases the probability of attenuation

Filtration

Process of eliminating undesirable, low-energy photons from the primary beam

Provides dose reduction to patient

Filtration is also known as

hardening the beam

making it strong!!

What is considered the standard filtering material

Aluminum

How is filtration expressed

Aluminum/equivalence (Al/Eq)

Half-value layer (HVL)

amount of absorbing material that will reduce intensity of primary beam to one-half its original value

Required HVL

2.5 mm of Al

Types of filtration

Inherent

Added

Compound

Compensation/compensating

Total

Inherent filtration

filtration that is a result of the composition of the tube and housing

-glass or metal envelope x-ray tube

-insulating oil

Value of inherent filtration in general purpose tubes

0.5mm to 1.00 mm Al/Ew

Tube degradation will increase

the value of inherent filtration due to vaporization of tungsten, reducing efficiency

HVL tests recommended as part of QC program

Added filtration

Any filtration that occurs between outside of tube/housing and the patient

It is outside tube housing but before it reaches the patient

Added filtration composed of

1.0 mm Al/Eq from variable-aperture light-localizing collimator

-due to reflective silver surface of mirror in collimator

an additional 1.0 mm Al filter inserted between the tube housing or collimator (to meet required total filtration of 2.5 mm Al/Eq)

Technologies will have no control over these sources of filtration (cant change thickness of filtration)

Some systems have added filtration capabilities

Button/mechanism on collimator

Example: copper filtration

Compound filtration

composite filters using two or more materials to absorb photons of different energy levels

layered with highest atomic number material ON TOP

lowest atomic number will go on bottom

designed so that subsequent layers absorb characteristic photons produced by previous layer

example: Thoraeus filter

There has been a renewed interest in

copper filters for digital diagnostic imaging (our GE digital machine has Cu filtration capabilities)

Compensating filter

Shapes of aluminum mounted onto transparent panel that slides into grooves beneath the collimator

Purpose is to compensate for unequal absorption within target tissue (not used to reduce patient dose)

Compensating filters balance

the intensity of x-ray beam to deliver more uniform exposure to the detector

Wedge (spine and foot)

Trough (chest)

ClearPbTM: clear lead, some aspect of filtration

Thoraeus Filter

Tin

Copper

Aluminum

Total filtration

the sum of inherent and added filtration

does NOT include compound or compensating filters that may be added later

National council on radiation protection and measurement (NCRP) minimum recommendations for filtering

below 50 kVp: 0.5 mm Al/Eq

50-70 kvp: 1.5 mm Al/Eq

Above 70 kVp: 2.5 mm Al/Eq!!!!

Effect of filtration on output

increased filtration causes

-increase in X-ray beam energy

-increase in X-ray penetrability

-decreases total number of photons

Exposures must be increased to compensate for increased attenuation

But there is still a patient exposure savings due to getting rid of low energy useless photons

If above 3mm Al/Eq

the reduction in patient entrance skin dose does not improve enough to offset the necessary increase in technical factors or the tube loading increase

TAKING TOO MANY PHOTONS AWAY (MOTTLE, ADJUST MAS TO COMPENSATE)

Sole purpose of protective filtration

Spare unnecessary patient exposure from primary beam

If filtration affects the remnant beam exposure to the detector

Too much filtration has been utilized