X-ray interactions - Ch 12

when x-ray beam passes thru matter, this is the reduction in the # of x-ray photons in the beam, and subsequent energy loss

x-ray photons interacting with matter and losing energy through these interactions – usually an orbital electron is what the photons interact with.

The whole atom, an orbital electron or directly with the nucleus, depending on the energy of the photon

the whole atom

orbital electrons

the nucleus

very high-energy photons

positively charged and contains protons and neutrons

negatively charged and fall in orbital paths around the nucleus

the energy required to remove an electron from a shell. It’s characteristic to a given shell and to a given atom.

possess the highest binding energy for a given atom and binding energies decrease progressively for successive shells. More tightly bound to the nucleus in high atomic number elements. The higher the number of the element, the more energy will be required to remove a K-shell electron from the atom.

They have the lowest energy total with the highest binding energy. Each successive shell, total electron energies increase and binding energies decrease.

.5 keV

are not bound as tightly and need less energy to remove them from their orbit. The further an electron is from the nucleus, the higher the total energy of the electron will be.

K, L, M, N, O, P, AND Q. – therefore, K-Shell electrons possess less total energy than outer shell electrons.

it will release energy equal to the difference b/w the binding energies of the two shells.

1. photoelectric absorption
2. Coherent scattering
3. Compton scattering
4. Pair production
5. Photodisintegration

when x-ray photons interact and change direction or are absorbed by the atom. Photons still exist but have less energy. They will continue on until their energy is absorbed or scatters again. # of interactions depends on the incident energy and atomic #.

when a photon is absorbed, all of the energy of the photon is transferred to the matter and the photon no longer exists.

most predominant in very low photon energy ranges

occur only at very high photon energy ranges

results when an x-ray photon interacts with an inner-shell electron. The incident photon possesses a slightly higher energy than the binding energy of the electrons in the inner (K or L) shells. The incident photon ejects the electron from its inner shell and is totally absorbed in the interaction.

The results is an ionized atom due to the missing inner-shell electron and an ejected electron. The incident must have a slightly greater energy than that of the binding energy of the electron for the interaction to occur.

The body has elements of low atomic number, so binding energies of the K-shell electrons are very low.

The ionized atom is unstable with an inner-shell electron missing. Instantly filled by an electron from the L-shell or M-shell (uncommon) or a free electron. Releases energy in the form of a characteristic photon AKA secondary radiation.

an ejected electron – travels with kinetic energy equal to the difference b/w the incident photon and the binding energy of the inner-shell electron. Ei= Eb + Eke.

is matter. It will not travel far. Absorbed within 1-2 mm in soft tissue, but this is a significant way in which x-ray energy can create biological changes.

are very low atomic number elements – binding energies of the K-shell electrons are very low.

In a photoelectric absoprtion interaction, when electrons transfer from outer to inner shells to fill vacancies – it has excess energy to release – in the form of secondary radiation

energy created at the x-ray target

from shell to shell until the atom returns to a normal state & is no longer a positive ion.

each shell electrons will fill lower shells with a corresponding emission of photons

AKA characteristic photon – energy produced is very low. Produced in photoelectric absorption interactions when electrons of outer shells drop to lower shells – When the electrons drop, they have excess energy to release.

secondary radiation energies are significantly higher for these elements.

1. The incident x-ray photon energy must be greater than the binding energy of the inner-shell electron.
2. A photoelectric interaction is more likely to occur when the x-ray photon energy and the electron binding energy are nearer to one another
3. A photoelectric interaction is more likely to occur with an electron which is more tightly bound in its orbit. Most interactions will occur with the K-shell electron with low atomic number elements. Probability of a photoelectric interaction increases dramatically as the atomic number increases. It’s for this reason that radiography is so useful in demonstrating the bones of the body.

the chance of a photoelectric interaction decreases dramatically – inversely proportional. Photoelectric effect = 1/energy3 – Important for establishing appropriate technical factors for specific body tissues

approximately proportional to the third power of the atomic number (photoelectric effect = atomic #3. It’s for this reason that radiography is so useful in demonstrating the bones of the body.

interaction which occurs b/w very low-energy x-ray photons and matter. “Classical” or “unmodified” scatter. When the very low-energy photon interacts with the electron in an atom, it may cause the electron to vibrate at the same freq. as the incident photon. It immediately releases this excess energy by producing a secondary photon which has the same energy and wavelength but travels in a direction different. The atom is not ionized in the process. Occurs in very low x-ray energy ranges, outside the usual range for diagnostic imaging. Some reaches the IR and has little significance.

Thomson and Rayleigh. Both have the same basic interaction results.

involves a single electron in the interaction

involves all of the electrons of the atom in the interaction

occurs when an incident x-ray photon interacts with a loosely bound outer-shell electron, removes the electron from the shell and then proceeds in a different direction as a scattered photon. Produces the Compton effect. Part of the incident photon’s energy removes the outer-shell electron and imparts kinetic energy to it.

The incident photon energy is divided b/w the ejected electron and the scattered photon.

The dislodged electron – available as a free electron to fill a shell “hole” created by another ionizing interaction.

The photon that exits the atom in a different direction. Possesses less energy than the incident photon and has lower freq and wavelength. Retains most of the energy because little energy is needed to eject an outer-shell electron due to its low binding energy. Will interact until absorbed photoelectrically.

The energy retained depends on the initial energy of the photon and angle of deflection. If 0°, no energy is transferred. 180° – more energy is imparted to the recoil electron and less remains with the scattered photon.

These photons can create a radiation hazard and impair image quality.

It’s the primary cause of occupational radiation exposure to the radiographer and is primary reason for wearing protective devices.

Ei = Es + Eb + Eke

when a scattered photon is deflected back toward the source – it travels in the direction opposite to the incident photon. Most go in a more forward direction, especially when photon energy increases. For this reason, scatter has a serious impact on image quality.

in the x-ray room

unwanted exposures caused by scattered photons.

devices designed to remove unwanted scatter and to improve radiographic image quality.

the energy of the x-ray photon is converted to matter in the form of two electrons. Needs a very high-energy photon with an energy of at least 1.02 mEv. (energy equivalent of the mass of one electron at rest is equal to .51 MeV). A high energy incident photon comes close to the strong nuclear field and loses all its energy. This energy is used to create a PAIR of electrons, one negative (negatron), and one positive (positron). The negatron is quickly absorbed by the other nearby atoms becuz it is negative. A positron is volatile. It combines with a negative electron nearly instantaneously. When the 2 particles combine, they disappear and give rise to 2 photons moving in opposite directions, each with .51 MeV. (annihilation reaction). Doesn’t become significant until 10 MeV, therefore it is not in the diagnostic x-ray imaging range.

reaction during a pair production interaction – matter is being converted back to energy. A positron is volatile. It combines with a negative electron nearly instantaneously. When the 2 particles combine, they disappear and give rist to 2 photons moving in opposite directions, each with .51 MeV.

an interaction b/w an extremely high-energy photon above 10 MeV and the nucleus. A high-energy photon strikes the nucleus and all of its energy is absorbed by the nucleus, exciting it. The nucleus emits a nuclear fragment. Not relevant to diagnostic imaging.

photoelectric absorption and Compton scattering

attenuated. Only a small % of the photons exit to create the image.

the total number of photons which are transmitted without interactions increases – therefore the probability of photoelectric and Compton interactions decreases with increasing kVp.

decreases with increased kVp – less absorption

increases with increased kVp – more scatter

predominant interaction through most of the diagnostic x-ray range.

two circumstances: 1. in the lower-energy ranges (25-56 kev) produced by 40-70 kVp and 2. when high atomic number elements are introduced, such as CM like iodine and barium. They absorb a greater % of the photons through photoelectric interactions – thus creating a visible radiograph

the resulting radiographic image will possess high contrast (lots of differences b/w densities). It is the result of the complete absorption of the incident photons without the creation of undesirable scatter to fog the image. Use low kVp/high mAs.

so does the absorption of radiation by the patient. It increases the likelihood of biological effects. So high-contrast, low kVp, high mAs techniques tend to result in higher pt doses.

the resulting radiographic image has lower contrast – small differences b/w densities, with more gray shades. Created by high kVp and low mAs because Compton interactions predominate as kVp increases. Scatter from Compton interactions is a significant cause of the lower-contrast images. However, low-contrast, high kVp/low mAs techniques reduce pt dose.

Centrifugal force and the attractive electrostatic force./

the number of protons and electrons is equal

different number of orbital electrons

so does the binding energy of a given electron, due to the increase in the positive charge in the nucleus

are bound more tightly than the electrons of lower atomic number elements

keVs – kiloelectron volts

requires much more energy than would be necessary to remove a k-shell electron from an atom of hydrogen, carbon or oxygen.

the atom is said to be ionized

the x-ray energy must be greater than the binding energy of the electron

the photon from the focal spot of the anode

transmission and photoelectric absorption. Not Compton scatter.

occur. But it is not useful. It shows up as a grainy look on the film – fog. Compton scatter obscures detail. It is random in direction. It is the primary source of the rad tech’s radiation

provides grays

Compton scatter

unwanted density on the film – it obscures detail

Compton scatter

the IR, patient, glass envelope, dielectric oil – all are examples

range of x-rays in the kev range.

pass through without interaction – AKA remnant, exit, image-forming radiation – these produce the image

1-2%.

the number of protons in the nucleus

they add mass to the nucleus

atomic mass

the atomic # of an element

the number of protons in the nucleus – how positive the nucleus is

the energy of the photon and the type and volume of the material (characteristics of the atom(s))

much bigger than the electron

centrifugal force

are held in orbit around the nucleus by electrostatic forces

binding energy – the energy required to remove an electron from a shell. It’s characteristic to a given shell and to a given atom.

highest binding energy and lowest total energy

lowest binding energy and the highest total energy

inversely related to binding energy

so does the binding energy.

require the greatest energy to remove them from orbit

because it has a higher atomic number, so it absorbs more photons

tungsten

the energy of a photon has to be slightly greater than the binding energy of the K-shell electron

binding energy

photoelectric absorption

at the focal spot

film will have a high radiographic contrast

with low kVp techniques and decreases as kVp increases

to have photoelectric absorption – because their atomic numbers are higher, they have greater binding energies (barium, lead, bone – all have higher atomic number than soft tissue, so they absorb more photons)

film will have a low radiographic contrast

It means the unique way (characteristic) an atom’s electrons will drop into lower shells in a photoelectric absorption – characteristic to each element

Compton scatter is the predominant interaction.

percentages of interactions that will occur

Compton Scatter increases (increased % of scatter), fog occurs, and a lower contrast film results. Photoelectric absorption decreases due to the increased force of the photons pushing thru.

Photoelectric absorption increases (more photons being absorbed by material because the photons have a lower energy and are not pushing right thru), and a higher contrast film results.