CH 10 xray interaction with matter

Electromagnetic radiation may be described as?

Wavelike fluctuations of electric and magnetic fields

What does the EMS illustrate?

That visible light, microwaves, and radio waves are within the EMS, and that all EM radiations have the same velocity, ie, 186,000 miles per second; however, they differ greatly in wavelength

EM energy exists over a wide range called an?

Energy continuum

What is the only section of the EM continuum apparent to us?

The visible light segment

What is a photon?

The smallest quantity of any type of EM radiation

How may a photon be pictured?

As a small bundle of energy or quantum, traveling through space at the speed of light

What are the properties of a photon?

Frequency, wavelength, velocity, and amplitude

Ionizing radiation contains _ energy than visible light photons or an RF photon

More

The frequency of x-radiation is much _ and the wavelength is much ___

Higher; shorter

When we set a 80 kVp, what do the x-rays produced contain?

Energies varying from 0-80 keV

What is wavelength?

Refers to the distance between two consecutive wave crests

What is frequency?

Refers to the number of cycles per second (cps); its unit of measure is the hertz (Hz), which is equal to 1 cps

What can x-rays be measured in?

Angstroms (x-ray 0.1 to 1.0 angstroms)

What does 1 angstrom equal?

1 x 10^-10 meter

Why are some forms of radiation called ionizing?

Bc they have the energetic potential to break apart electrically neutral atoms, resulting in the production of negative and positive ions

When an electron is added or removed from the atom is it?

Ionized

An electron must _ energy to move to outer shells

Gain

An electron __ energy to move to inner shells

Loses

The closer the electron is to the nucleus of an atom the ___ its binding energy and vice versa

Higher

Interactions in the body begin at the __ level

Atomic

What can x-ray photons change?

Cells

What happens when an x-ray doesn't interact with matter?

The x-ray passes completely through the tissue and into the IR

What happens with an x-ray is completely absorbed with matter?

The x-ray energy is completely absorbed by the tissue with resulting in no imaging information

What happens when x-rays are partially absorbed and scattered with matter?

Scattering involves a partial transfer of energy to tissue, with the resulting scattered x-ray having less energy and a different trajectory. Scattered radiation tends to degrade image quality and is the primary source of radiation exposure to operator and staff

Where is coherent scattering important?

Not important in any energy range

What interactions are important is diagnostic radiology?

Compton scattering and photoelectric absorption

What interactions are important in therapeutic radiology?

Pair production and photodisintegration

What are the other names for Coherent scattering?

Classical or Thompson scattering

Coherent scattering occurs with energies below _ keV

10

What happens in coherent scattering?

The incident x-ray interacts with an atom of matter, causing it to become excited. Immediately the atom releases this excess energy and the scattered x-ray; no ionization

In coherent scattering, the energy in equals the?

Energy out

What is the only difference in coherent scattering?

The direction of travel

Coherent scattering occurs primarily with __ energy x-rays and throughout the diagnostic range

Low

Coherent scattering contributes slightly to _ and reduces

Film fog; image contrast

What happens in compton scattering?

The incident x-ray interacts with the outer electron shell on an atom of matter, removing it. Not only ionizing it but scatters the incident x-ray causing a reduction in energy and the change of direction

What is released during a compton interaction?

A compton electron

What does the scattered compton photon contribute to?

Film fog and to personnel dose (as in fluoroscopic procedures)

What can both the scattered compton photon and the compton electron cause?

More ionization before loosing all their energy

What happens to the scattered compton photon in the end?

It is absorbed photoelectribally

The probability of Compton effect __ as photon energy increases

Increases

What does not affect the chances of compton effect?

Atomic number

What type of tissue can compton effect occur with?

Any type of tissue

What provides no useful diagnostic information?

Scattered photons

Scattered radiation produces a uniform optical density on the radiograph that ___ image contrast

Reduces

During fluoroscopy, what is the largest scattering object?

The patient

During fluoroscopy, where is the least exposure located in relation to the patient?

90 degrees to the patient

During fluoroscopy, where is the most exposure located in relation to the patient?

At the head or feet of the patient

What happens with photoelectric effect or absorption?

A low energy (low kVp) x-ray photon uses all its energy (true absorption) to eject an inner shell electron leaving an orbital vacancy. An electron from the shell above drops down to fill the vacancy and, in doing so, gives up energy in the form of a characteristic ray

The photoelectric effect is more likely to occur in tissues of ____ atomic number (bone and positive contrast media)

High

What interaction contributes significantly to patient dose?

Photoelectric effect

Photoelectric effect is responsible for the production of what kind of contrast?

Short-scale contrast

What are electron transitions accompanied by?

The emission of more x-rays called secondary radiation

What is secondary radiation?

Radiation that behaves much like scatter radiation and contributes nothing to the image

The probability that any given photon will undergo a photoelectric interaction is dependent on?

The photon energy and the atomic number of the atom

Photoelectric absorption provides the ____ on the film

Contrast

What two x-ray tube interactions are important?

Bremstrauhlung and characteristic interactions

The probability for compton scatter to occur is dependent on what?

kVp or x-ray energy

How does the photoelectric effect represent anatomic structures on an image?

Appear as white due to anatomic structures with high x-ray absorption characteristics; radiopaque structures; tissues with high atomic number; or tissue with high mass density

How do photoelectric photons provide information to the IR?

Bc photons do not reach the IR

What is attenuation?

The total reduction in the number of photons remaining in an x-ray beam after penetration through tissue

What is absorption?

The disappearing of an x-ray as seen in photoelectric absorption, pair production, and photodisintegration

What is scattering?

Partially absorbed, x-ray emerges from the interaction traveling in a different direction sometimes with less energy

What makes up attenuation?

Absorption and scattering

What is differential absorption?

The difference between those x-rays absorbed and those transmitted to the IR

What are the 3 types of x-rays important for image formation?

Compton scattering (no useful information), photoelectric absorption (produces the light areas on the image), and transmitted x-rays (produces the grey/dark areas on the image)

The probability of radiation interaction is a function of?

Tissue electron density/atomic number, tissue thickness/density, and x-ray energy (kVp)

What provides the contrast necessary to form an image?

The differential rate of attenuation

Differential absorption _ as the kVp is reduced

Increases

How many photons that interact with the patient (primary beam) reach the IR?

Approx. 1%

Of the 1% of photons that reach the IR, how many interact to form the image?

0.5%

Photoelectric absorption is more predominant __ 80 kVp

Below

Compton scatter is more predominant __ 80 kVP

Above

What does high atomic number mean for an atom?

Atoms are larger

What is mass density?

How tightly the atoms of tissue are packed

Since the Z number for air and soft tissue are about the same, what changes are due to mass density differences?

Optical density