Chapter 8 Flashcards
What factors affect beam attenuation?
Part thickness, atomic composition, and tissue density, and energy level of the beam
What type of radiation forms the latent image?
Exit radiation
What are the interactions that occur when the primary beam passes through the body?
Absorption
Scatter
Transmission
When does complete x-ray absorption occur?
When the xray photon has enough energy to eject an inner-shell electron from an atom
When does ionization occur?
When an electron is ejected
In the diagnostic energy range, absorption is known as:
Photoelectric effect
What happens during a photoelectric event?
An atom is ionized, creating a vacancy in the inner shell
Electrons drop down from the outer valence to fill the created vacancy creating secondary photons
What determines the probability of absorption?
Initial energy of the incoming x-ray photon
What must the photon energy be to cause ionization?
Equal to or greater than the inner shell binding energy
When does Coherent scatter occur?
When an incoming photon is not absorbed, but instead loses energy while interacting with an atom
What are the two forms of scatter?
Compton and Coherent
When does Compton scattering occur?
Incoming photon transfers some of it’s energy to an atom, causing ionization
Incoming photon moves past the atom, and continues on a path through the body with less energy
What is the ejected electron called during ionization?
Compton electron or secondary electron
What affects the probability of compton scattering?
Probability increases as energy increases
Probability is not dependent on tissue density or atomic structure
When does Coherent scattering occur?
With low-energy x-rays
What happens to the photon during Coherent scattering?
The photon interacts with the atom, exciting it
The photon does not lose energy, but it does change direction
How much does attenuation increase due to tissue thickness?
Attenuation increases 50% for each 4-5 cm of increases tissue thickness
How does tissue type affect attenuation?
Higher atomic number, such as bone, attenuate more radiation
Lower atomic number, such as fat, attenuate less radiation
How does atomic composition affect absorption and scatter?
The higher the atomic number, the more opportunities for scatter or absorption
How does tissue density affect attenuation?
The more dense the tissue, the greater chance of attenuation
How does tissue density affect absorption or scatter?
The more dense the tissue, the greater chance for absorption or scatter
What are the 4 types of tissue from least dense to most dense?
Air
Fat
Muscle
Bone
What is scattered radiation that contributes to an unwanted receptor exposure?
Fog
What creates the white of an image?
Absorbed radiation
What creates the black of an image?
Trasmitted radiation
What creates the gray of an image?
Scatter radiation
What percent of the primary beam reaches the IR?
Less than 5%
The latent image is processed into the:
Manifest image
Describe the cassette used in film-screen
Light-tight container that holds radiographic film
The film is sandwiched between two intensifying screens and the cassette
How is an image created with film-screen IRs?
Exit radiation interacts with the screen and is converted into visible light
Visible light interacts with the radiographic film, imprinting the latent image
The latent image is processed chemically to create the manifest image
What are the two types of digitial image receptors?
CR - computed radiography
DR - digital radiography
How is an image created using CR?
Cassette based system - radiographic film is replaced with an imaging plate that is removed the from cassette and scanned by a CR scanner
The CR scanner coverts the latent image contained in the IP into data that is turned into the manifest image seen on the computer
What is used in CR and DR to create the manifest images?
Algorithms
Digital image receptors are able to respond to a wide range of x-ray exposures - this is called:
Dynamic range
What is the benefit of a wide dynamic range?
Less radiation is needed to produce a diagnostic image
What is the disadvantage of dynamic range?
Dose creep - digital algorithms are able to correct overexposure, allowing techs to fall into a bad habit of using more radiation than needed
How are digital images displayed?
As a matrix
What is a matrix?
A combination of rows and columns made up of pixels
What is a pixel?
A small square representing a single numerical value, which translates to a single brightness for that one pixel in the image
Are digital images impoved with smaller or larger matrix sizes? Smaller or larger pixels?
Larger matrix, smaller pixels
Larger matrix sizes = more pixels
How is the numerical value determined for each pixel?
Attenuation of x-rays
Pixels that receive greater amounts of radiation correlate to darker areas
Pixels that receive less amounts of radiation correlate to lighter areas
What does bit depth determine in an image?
Amount of different shades of gray that can be displayed
Difference between static and dynamic?
Static - motionless
Dynamic - moving
How is fluoro different from general diagnostic radiography?
Fluoro uses dynamic imaging, and the x-ray tube is under the table with an image intensifier above the patient
