EXAM #3 Flashcards
What is attenuation?
Reduction in the number or intensity of x-rays as a result of absorption and scattering
Differences in attenuation cause subject contrast
Can x-rays be transmitted without any interaction?
yes
What can x-rays interact with?
- entire atom
- orbital electron
- nucleus of an atom
X-rays interact with the nucleus of an atom at _____ _____ energies
very high energies
What are the five interactions between x-rays and matter
- ) Coherent scattering
- ) Photoelectric (PE) absorption (2° radiation)
- ) Compton scattering
- ) Pair production
- ) Photodisintegration
http: //youtu.be/4p47RBPiOCo
What is secondary radiation?
Radiation produced using the patient’s atoms
What is off-focus radiation?
Radiation produced off the focal spot of the x-ray tube
What is photoelectric absorption?
- More photoelectric absorption at high/low kVp?
- Is photoelectric absorption ionizing?
- The incident electron interacts with and _____ the _____ shell electron
- The incident electron’s energy is divided into…?
What gives us those white areas in our image
- More absorption at a low kVp
- Photoelectric absorption is ionizing
- The incident electron interacts with and ejects the inner shell electron
- The incident electron’s energy is divided into breaking the binding energy and giving off kinetic energy to the ejected electron
When is an ion pair formed, and in what interaction?
- What is the product called?
- What happens to the affected atom?
An ion pair is formed when an electron is ejected from the atom in photoelectric absorption
- It becomes known as the [photoelectron]
- Remaining atom has a vacancy in its inner electron shell
When is photoelectric absorption most likely to occur?
Most likely to occur when x-ray photon has just slightly more energy than Eb of a K or L-shell electron
[*] Photoelectrons are related to what interaction?
Photoelectric absorption
Photoelectron energy can be determined with what equation ?
Ei = Eb + Eke
Ei = energy of the incident photon
Eb = binding energy of the electron
Eke = kinetic energy of the photoelectron
[*] Recoil electrons are related to what interaction?
Compton scatter
What are the three characteristics of a photoelectron?
• Kinetic energy (Eke)
• Mass
• Reabsorbs quickly
- Within 1-2mm of tissue
Three steps to characteristic cascade?
What interaction?
- Vacancy filled by an outer shell electron
- Electron undergoes change in energy level
• Emits characteristic photon
– (secondary radiation)
• Photoelectric
What is secondary radiation?
• When do characteristic photons emit?
Radiation that originates from irradiated material outside of x-ray tube (from the patient’s atoms)
• Characteristic photons emitted from atoms of patient after PE absorption interaction has occurred
_____ of resulting x-ray is dependent on the atomic (Z#) of the material.
Low Z# in tissue = _____ energy secondary radiation
Higher Z# with contrast agents = _____ energy secondary radiation
Energy of resulting x-ray is dependent on the atomic (Z#) of the material.
Low Z# in tissue = low energy secondary radiation
Higher Z# with contrast agents = higher energy secondary radiation
Photoelectric Absorption Condition #1
Incident photon energy (Ei) must be _____ than or _____ to binding energy (Eb) of inner-shell electron
Incident photon energy (Ei) must be greater than or equal to binding energy (Eb) of inner-shell electron
Photoelectric Absorption Condition #2
PE absorption interaction is more likely to occur if:
• Incident photon energy (Ei) and inner-shell electron binding energy (Eb) are _____ to each other
[*] As photon energy increases, chance of PE interaction _____ _____
– _____ _____ relationship (1/E^3)
PE absorption interaction is 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 (1/E^3)
Example: 1 / 2^3 = 8
If you double your energy, the probability of absorption goes down to 1/8
Photoelectric Absorption Condition #3
• Increased Z# has a dramatic impact on the amount of PE absorption, with a _____ _____ relationship
• Double Z#
Increase chance of PE absorption interaction by a factor of __.
Kerry: Denser things, things that have higher binding energies are most likely to absorb more
PP: PE absorption interaction is more likely to occur in elements with a higher Z#, and therefore higher binding energy (Eb) of inner-shell electrons
• Increased Z# has a dramatic impact on the amount of PE absorption
– Direct cubed relationship (Z^3)
• Double Z#
Increase chance of PE absorption interaction by a factor of 8.
PE absorption is most influenced by:
A. Subject volume
B. Subject thickness
C. Subject mass density
D. Subject atomic #
D. Subject atomic #
because atomic number is cubed and mass density is proportional
What is Coherent scatter
- Are electrons ejected?
- Does ionization take place?
- High/low energy photons?
- What are the two types and how are they produced?
- What percentage of x-ray interactions in patients? Does this affect our image?
Electrons are excited and vibrate at photon frequency (momentarily bound to orbital electron which excites the atom and is reemitted)
- No electrons are ejected
- No ionization takes place
- Involves low energy photons (below 10 keV)
- Two types with same result:
- Thompson (single outer-shell electron interaction)
- Rayleigh (all electrons of the atom interaction)
• less than 5% Slightly
What is Compton scatter?
- What is formed because of Compton scatter?
- Where is the incident photon’s kinetic energy transferred to?
- Does ionization take place?
- Increasing kVp _____ the likelihood of Compton scatter
- The amount of Compton scatter is _____ proportional to tissue density
- What does Compton scatter create?
Incident photon (Ei) interacts with outer-shell, loosely bound electron and ejects it
- Ion pair is formed
- Incident photon transfers some of its kinetic energy to the recoil (Compton) electron and continues on in a different direction (never leaves patient, and has no affect on image quality)
- Ionization takes place in Compton scatter
- Increasing kVp decreases the likelihood of Compton scatter
- The amount of Compton scatter is directly proportional to tissue density
- Compton scatter degrades image quality with a blanket of exposure called noise
What is the equation for Compton scatter?
Ei = Es + Eb + Eke
Incident photon energy (Ei) is distributed between recoil electron (Eke) and scattered photon (Es)
Energy transferred to recoil electron (Eke) affects angle and energy of scattered photon (Es)
[Compton Scatter]
Recoil electron travels until it…
Scattered photon continue to interact until absorbed _____.
Recoil electron travels until it fills a vacancy in another atom (never leaves patient, and has no affect on image quality)
Scattered photon continue to interact until absorbed photo-electrically
What is backscatter
- What is it’s energy in relation to the incident photon?
- What interaction?
Blast the patient with radiation and it bounces back to you
- 2/3 the energy of the incident photon
- Compton scatter
What is a source of occupational exposure and radiation fog?
Compton scatter
[Pair production]
• Most commonly found in what diagnostic areas?
[*] Incident photon energy must be _____ MeV or higher
• Photon energy absorbed by _____
[*] What is the annihilation reaction?
• Therapy and PET
- Incident photon energy must be 1.02 MeV or higher
• Photon energy absorbed by nucleus
- An electron and positron collide destroying each other and give off energy and mass as 511 keV x-rays
[*] What interaction creates positrons
Pair production
What is photo-disintegration?
[*] What energy level?
- What is released?
- Is it significant in diagnostic imaging?
Absorption of photon by nucleus
[*] Extremely high energy photon (10 MeV or greater)
- Excited nucleus releases alpha particle (2 protons, 2 neutrons)
- Not significant in diagnostic imaging range
[?] Which of the following interaction with matter produce a characteristic x-ray?
A. Pair production
B. Photodisintegration
C. Photoelectric absorption
D. Coherent Scatter
C. Photoelectric absorption
[?] Which of the following interactions with matter does not cause an ionization of the atom?
A. PE absorption
B. Compton’s scatter
C. Coherent scatter
D. All cause ionization
C. Coherent scatter
[?] Which of the following adds to occupational dose?
A. Pairs production
B. Photodisintegration
C. Compton’s scatter
D. Coherent scatter
C. Compton’s scatter
_____ of the x-ray beam is attenuated while _____ of the beam is transmitted.
Most of the x-ray beam is attenuated (90’s%) while some of the beam is transmitted.
As kVp increases the number of photons transmitted without interaction _____
_____ probability of PE absorption and Compton interactions
Vice versa is true, too
As kVp increases the number of photons transmitted without interaction increases
Decreased probability of PE absorption and Compton interactions
Vice versa is true, too
As kVp increases:
PE absorption _____
Compton effect _____
_____ percentage of scatter
_____ percentage of absorption
As kVp increases:
PE absorption decreases
Compton effect increases
Increases percentage of scatter
decreases percentage of absorption
Scatter becomes more problematic at _____ kVp
Scatter becomes more problematic at _____ mAs
Scatter becomes more problematic at higher kVp
Scatter becomes more problematic at lower mAs
Just remember: Scatter is a gray transparency/film over an otherwise good image
Scatter is a gray transparency/film over an otherwise good image
When PE absorption predominates:
Resulting image will have _____ scale contrast
_____ kVp, _____ mAs
When PE absorption predominates:
Resulting image will have short scale contrast (very black and white)
Low kVp, high mAs
[?] Which of the following interactions predominates within diagnostic x-ray energy range
A. Compton’s
B. Coherent
C. PE absorption
A. Compton’s
- The reduction in the number and energy of photons as radiation passes through matter is termed:
a. Irradiation
b. Deflection
c. Photoelectrolysis
d. Attenuation
d. Attenuation
- The production of scatter radiation wen x-radiation strikes an object principally results from:
a. Compton interaction
b. Brem’s event
c. Characteristic event
d. Photoelectric interaction
a. Compton interaction
- The type of interaction in the diagnostic range responsible for the radiographic contrast between soft tissue and bone is:
a. Compton interaction
b. Coherent scattering
c. Pairs production
d. Photoelectric absorption
d. Photoelectric absorption
- At photon energies above 1.02 MeV, the creation of a negatron and a positron may occur in an event termed:
a. Photodisintegration
b. Pairs production
c. Annihilation reaction
d. Van de Graff production
b. Pairs production
- The photoelectric interaction most commonly occurs when the incident photon strikes a/an:
a. Inner shell electron
b. Outer shell electron
c. Protons
d. Neutron
a. Inner shell electron
- The interaction of a x-ray photon and an orbital electron, which results in partial transfer of energy is :
a. Photoelectric effect
b. Pairs production
c. Compton scatter
d. Thomas scatter
c. Compton scatter
- At 70 KeV photon strikes and inner shell electron, all of the photon energy is transferred to the electron. This describes:
a. Coherent scattering
b. Compton scattering
c. Photoelectric effect
d. Photodisintegration
c. Photoelectric effect
- The incoming photon before any interaction with matter takes place is called the:
a. Incident photon
b. Scatter photon
c. Inherent photon
d. Recoil photon
a. Incident photon
- Which of the following interactions contributes to image noise?
a. Brems
b. Characteristic
c. Compton
d. Photodisintegration
e. Photoelectric effect
c. Compton
- Compton scatter is:
a. Independent of Z#
b. Inversely proportional to Z#
c. Proportional to E
d. Proportional to Z#^2
a. Independent of Z#
- The probability that a photon will undergo attenuation:
a. Decreases with increasing energy
b. Increases with decreasing electron energy
c. Increases with increasing electron energy
d. Increase with increasing x-ray energy
a. Decreases with increasing energy
- The photoelectric effect is principally associated with which of the following?
a. Absorption of an x-ray
b. Brems production
c. Characteristic production
d. Scattering of an x-ray
a. Absorption of an x-ray
- During photoelectric interactions:
a. An electron is emitted from an atom
b. An x-ray is emitted from the atom
c. Electron excitation results
d. The atom is made radioactive
a. An electron is emitted from an atom
- A 35 KeV x-ray will most likely undergo a k-shell PE interaction with which of the following?
a. Barium, BE of 37 keV
b. Calcium, BE of 4 keV
c. Iodine, BE of 33 keV
d. Tungsten, BE of 69 keV
c. Iodine, BE of 33 keV
- The probability of the PE effect varies as what function of x-ray energy?
a. E^-3
b. E^-2
c. E
d. E^2
e. E^3
a. E^-3
- A 39 keV photon interacts through PE absorption with a k-shell electron having a BE of 37 keV. Therefore:
a. The photoelectron will have 2 keV of energy
b. The photoelectron will have 37 keV of energy
c. The photoelectron will have 39 keV of energy
d. The scatter x-ray will have 2 keV of energy
a. The photoelectron will have 2 keV of energy
- Differential absorption between bone and soft tissue occurs principally for which of the following reasons?
a. The difference in Z#
b. The difference in mass density
c. The polychromatic x-ray beam
d. The monochromatic x-ray beam
a. The difference in Z#
1.The only non diverging ray in the primary x-ray beam is the __________.
central ray
2.A “latent image” or signal is carried to the receptor by the __________ x-ray beam.
remnant
3.What does OID stand for?
object to image distance
5.Even after the radiographic image is processed and stored, what other type of variables still can alter its quality?
postprocessing
6.Microscopic white spots are produced in the image by which interaction?
photoelectric absorption
7.The ideal conditions for the photoelectric effect to take place are created when the average kV of
incident x-ray photons is __________ than the inner-shell binding energies of tissue atoms.
greater (or equal to)
11.Which of the two major interactions can occur in a series within the patient from a single x-ray photon?
Compton interactions, end in a photoelectric interaction
What is differential absorption?
- Images are the result of __ % of the incident beam reaching the x-ray
- Areas where radiation is transmitted will appear _____.
- Areas of absorption will appear _____.
- Differential absorption is important because it results in _____.
The combination of Compton scatter and photoelectric absorption within the patient
- Images are the result of 0.5 % of the incident beam reaching the x-ray
- Areas where radiation is transmitted will appear dark.
- Areas of absorption will appear white.
- Differential absorption is important because it results in subject contrast.
As kVp increases, the probability of interactions, whether PE or Compton, _____.
As kVp increases, the probability of interactions, whether PE or Compton, decreases.
X-ray transmission is _____ as the mass density increases.
X-ray transmission is reduced as the mass density increases.
Attenuation is exponential/proportional for ticker tissues
• 5cm of tissue reduces x-ray intensity to about _____
Attenuation is exponential for ticker tissues
• 5cm of tissue reduces x-ray intensity to about one-half
Iodine attenuates x-rays _____ than soft tissues and is used for visualization of the patient _____
Iodine attenuates x-rays more than soft tissues and is used for visualization of the patient vasculature
mAs and distance do/don’t affect beam quality
mAs and distance don’t affect beam quality
The half-value layer is used to describe _____.
The half-value layer is used to describe beam quality
thickness of a material that will reduce the intensity of the beam to one-half its original value
What is transmission?
X-rays reaching the IR
97% of all scattered x-rays originate from _____ interactions.
97% of all scattered x-rays originate from Compton scatter interactions.
The occurrence of all interactions is _____ proportional to the physical density of the tissue through which the x-rays pass.
The occurrence of all interactions is directly proportional to the physical density of the tissue through which the x-rays pass.
(fairly extreme differences in physical density between tissues are necessary to result in high subject contrast)
The most likely amount of energy an x-ray photon loses in a coherent scatter interaction is __ %
A. 0
B. 5
C. 25
D. >25
A. 0
Compton interactions are most likely to occur with:
A. outer shell electrons
B. K-shell electrons
C. electrons in all shells
D. atomic nuclei
A. outer shell electrons
Backscatter is the term used to describe a photon deflected through an angle of:
A. 90°
B. 180°
C. 270°
D. 360°
B. 180°
When the density of an absorber is doubled, the number of Compton interactions is most likely to increase by __ %
A. 25
B. 50
C. 100
D. 200
C. 100
A 50 keV photon absorbed in soft tissue is most likely to produce an electron with an energy of about __ keV.
A. 50
B. 25
C. 5
D. 0
A. 50
To be absorbed by a K-shell electron in an iodine atom, an x-ray photon must have a minimum energy of __ keV.
A. 4
B. 25
C. 33
D. 70
C. 33
The probability of a photoelectric interaction with an x-ray photon energy E varies as ___.
A. E^3
B. E
C. 1/E
D. 1/E^3
D. 1/E^3
Attenuation by lead (Z=82) is greater than that in soft tissue (Z=8) by a factor of about:
A. 10
B. 100
C. 1000
D. 10,000
C. 1000
(82/8)^3
Remnant radiation is the radiation that:
A. enters the patient
B. exits the patient
C. is backscattered
D. is scattered through 90°
B. exits the patient
When the x-ray beam area doubles, the amount of scatter produced in the patient is most likely to increase by ___ %.
A. 10
B. 25
C. 50
D. 100
D. 100
Which x-ray examination is likely to result in the greatest amount of scatter radiation?
A. AP abdomen
B. PA chest
C. extremity
D. skull
A. AP abdomen
Which would likely be the best interspace material in an x-ray scatter removal grid?
A. aluminum
B. copper
C. silver
D. tungsten
A. aluminum
In soft tissue, at which photon energy (keV) are the photoelectric and Compton interactions equally likely?
A. 15
B. 25
C. 45
D. 65
B. 25
The percentage of the x-ray beam in a skull radiograph that reaches the image receptor is most likely __ %
A. 0.01
B. 0.1
C. 1
D. 10
C. 1
[Radiation protection]
- The passage of x-ray photons though a patient with-out interaction is called:
A. absorption
B. attenuation
C. scattering
D. direct transmission
D. direct transmission
[Radiation protection]
- A Compton-scattered electron:
A. annihilates another electron
B. is absorbed within a few microns of the site of the original Compton interaction
C. causes a pair production
D. engages in the process of photo-disintegration
B. is absorbed within a few microns of the site of the original Compton interaction
[Radiation protection]
- Within the energy range of diagnostic radiology that includes mammography (23 - 150 kVp), when kVp is decreased, the patient dose:
A. decreases
B. increases
C. remains the same
D. doubles
A. decreases
[Radiation protection]
- Of the following interactions between x-radiation and matter, which does not occur in the range of diagnostic radiology?
- photoelectric absorption
- pair production
- photodisintegration
A. 1 + 2
B. 1 + 3
C. 2 + 3
D. 1 + 2 + 3
C. 2 + 3
[Radiation protection]
- The quality, or penetrating power of an x-ray beam is controlled by:
A. the absorption characteristics of the patient being radiographed
B. fluorescent yield
C. mAs
D. kVp
D. kVp
[Radiation protection]
- kVp controls:
A. absorption characteristics of the body part being radiographed
B. fluorescent yield
C. random interaction of x-ray photons with the image receptor
D. quality, or penetrating power, of the photons in the x-ray beam
D. quality, or penetrating power, of the photons in the x-ray beam
[Radiation protection]
- Which of the following terms are synonymous?
- coherent scattering
- classical scattering
- unmodified scattering
A. 1 + 2
B. 1 + 3
C. 2 + 3
D. 1 + 2 + 3
B. 1 + 3
[Radiation protection]
- The process most responsible for the contrast between bone and soft tissue in a diagnostic radiographic image is:
A. coherent scattering
B. Compton scattering
C. photoelectric absorption
D. photodisintegration
C. photoelectric absorption
[Radiation protection]
TRUE/FALSE?
- The optimal x-ray image is formed when only indirect transmission of photons reach the image receptor
FALSE
[Radiation protection]
TRUE/FALSE?
- During the process of Compton scattering, an x-ray photon interacts with an inner-shell electron of an atom of the irradiated object
FALSE
outer shell, not inner shell
[Radiation protection]
TRUE/FALSE?
- In the radiographic kilovoltage range, compact bone with a high calcium content by weight undergoes much more photoelectric absorption than an qual mass of soft tissue and air
TRUE
[Radiation protection]
TRUE/FALSE?
- Compton scattering results in all-directional scatter
TRUE
[Radiation protection]
TRUE/FALSE?
- During the process of photoelectric absorption, the atom also emits primary radiation when the outer-shell electron fills the inner-shell vacancy
FALSE
it emits secondary only
[Radiation protection]
TRUE/FALSE?
22.The byproducts of photoelectric absorption include photoelectrons and characteristic x-ray photons
TRUE
In what interaction(s) is/are ion pairs formed?
Photoelectric absorption
Compton scatter
Scintillators, Phosphor, or Capture element are a part of the _____
_____ emitted from scintillators.
_____ patient exposures when scintillators are used vs. direct exposure systems.
Some loss of _____ when scintillators are used.
Scintillators, Phosphor, or Capture element are a part of the image receptor
Visible light emitted from scintillators.
Decreases patient exposures when scintillators are used vs. direct exposure systems.
Some loss of detail when scintillators are used.
Phosphor Layer (capture element)
this layer converts remnant x-rays into light.
Originally used to expose film. Now light is captured by a PMT/PD and digitized into an image.
Calcium tungstate was the original material used with film.
Currently rare earth materials are used.
Gadolinium (most common), lanthanum and yttrium
Barium and Cesium Iodide is also a popular materials.
Phosphor layer thickness, concentration and size affect conversion rate or speed (what if I have phosphor layer and I have a thin and thick one, thick one will absorb more, more concentration makes it a better absorber, the bigger the phosphor, the better the absorption)
WIP
[Luminescence]
_____ emitted from a material
_____ is produced when an _____ shell electron is excited, returns to its normal state and gives off the excess energy as _____.
Due to the excitement of _____ shell electrons the energy released is within a limited range, which provides a limited range of energy released or color of _____.
Visible light emitted from a material
Light is produced when an outer shell electron is excited, returns to its normal state and gives off the excess energy as visible light.
Due to the excitement of outer shell electrons the energy released is within a limited range, which provides a limited range of energy released or color of light.
[Screen Speed ]
A numeric value representing the efficiency of the screen, or conversion factor.
Calcium tungstate screens, par speed, are rated at 100, all others are compared to this, even CR/DR.
There were three speeds with film, with CR/DR the system is just one speed.
par speed, detail and high speed.
Intensification factor = exposure without screens / exposure with screens
50/5 = 10 intensification factor
(WIP)
WIP
[Factors Affecting Speed ]
Which factors increase speed for both categories?
Under our control:
• Radiation Quality - kVp (more kVp = more light)
— increase in kVp increase speed
Not under our control: • Phosphor type • Phosphor thickness • Reflective layer • Dye • Phosphor concentration
Under our control:
• Radiation Quality - kVp (more kVp = more light)
— increase in kVp increase speed, decreases detail
Not under our control: • Phosphor type • Phosphor thickness • Reflective layer (increases speed, decreases detail) • Dye (slower speed, better detail) • Phosphor concentration
Adding components to increase speed always _____ detail
Adding components to increase speed always DECREASES detail
What is Image Noise ?
What causes noise?
Often seen with low/high speed screens and low/high kVp exams.
Must be very careful in CR/DR.
Increases in image noise _____ image contrast.
Image noise is a grainy appearance, salt and peppered look.
Caused by a lack of a good signal, lack of mAs
Often seen with HIGH speed screens and HIGH kVp exams.
Must be very careful in CR/DR.
Increases in image noise reduce image contrast.
High spatial resolution imaging requires _____.
A. Reflective layer
B. Absorptive layer
C. Fast speed
D. None of the above
B. Absorptive layer
Dye is synonymous with _____ layer
Dye is synonymous with absorptive layer
What is DQE
What is the equation?
percentage of x-ray absorbed by screens
# of x-rays absorbed / # of incident x-rays
What is CE
increasing CE _____ noise, because less x-rays are used to create image
What is the equation?
amount of light emitted for each x-ray absorbed
increasing CE increases noise, because less x-rays are used to create image, quantum mottle (noise, bad)
Emitted light
/
X-rays absorbed
[?] Which of the following capture elements will have the highest DQE with a 112 kVp beam?
Calcium, Z#74, 69 keV BE
Gadolinium Z#64, 50 keV BE
Cesium Z#55, 35 keV BE
Gadolinium Z#64, 50 keV BE
because average of 112 kVp is about 50-60 and is closest to the 50keV BE
[Image Detail]
What is image detail?
What is spatial resolution?
• Scintillators have a(n) INCREASED/DECREASED spatial resolution verses direct exposure
What is the lines pairs test?
• more line pairs = _____ detail
(Is there more of a chance of interaction in thin or thick phosphors?)
Image detail is the scintillators ability to produce accurate blur free images.
Spatial resolution - how small of an object may be imaged.
• Scintillators have a DECREASED spatial resolution verses direct exposure
Lines pairs test- used to quantify spatial resolution
• more line pairs = more detail
(In thick phosphors, there is more of a chance of interaction)
[?] In an image 8LP/mm can be seen, a repeat image was taken now with 4LP/mm seen, what changed?
A. Kvp
B. mAs
C. Speed increased
D. Speed decreased
C. Speed increased
Does kVp or mAs affect spatial resolution?
no, neither do
[Scintillators Protection ]
Scintillators are _____ materials that could be ______ through use.
A _____ layer must be created between the scintillators and the outside world.
In CR imaging a _____ houses the scintillators, in DR a _____ or _____ is placed over the scintillators and detector element.
- The cover facing the x-ray source must be made of a _____ material, _____ and _____ fiber.
- Back covers are made of a metal or lead lined to limit _____. Backscatter from other objects.
Scintillators are delicate materials that could be damaged through use.
A protective layer must be created between the scintillators and the outside world.
In CR imaging a cassettes houses the scintillators, in DR a cover or housing is placed over the scintillators and detector element.
- The cover facing the x-ray source must be made of a radiolucent material, plastic and carbon fiber.
- Back covers are made of a metal or lead lined to limit scatter radiation. Backscatter from other objects.
[Terms to know]
Image receptor
Useful beam
Remnant beam
Image Forming X-rays
Image receptor- medium that transforms x-ray beam into a visible image.
Useful beam- emerges from tube housing, uniformly distributed.
Remnant beam- x-rays that remain after useful beam exits the patient.
Image Forming X-rays- those that interact with the image receptor to form an image.
[QUIZ] A photoelectric interaction is more likely to occur when the x-ray photon energy and the electron binding energy are farther from one another.
TRUE/FALSE
FALSE
[QUIZ] During photoelectric absorption, the ejected electron is called a/an
incident electron
photoelectron
characteristic electron
nuclear electron
photoelectron
[QUIZ] The photon that exits the atom after a Compton scattering is called the Compton
scattered photon
scattered electron
recoil electron
photoelectron
recoil electron
[QUIZ] During pair production, the positvely charged electron is termed a negatron.
TRUE/FALSE
FALSE
[QUIZ] When an x-ray photon with a slightly greater energy than the binding energy of the inner-shell electron interacts with that inner-shell electron, the following interaction results:
Compton scattering
coherent scattering
photoelectric absorption
characteristic radiation
photoelectric absorption
[QUIZ] Which of the following interactions has a signficant impact on the x-ray image?
Selected Answer:
Compton scattering
coherent scatter
pair production
photodisintegration
Compton scattering
[QUIZ] During coherent scattering, the scattered photon possesses ___ as the incident photon.
the same energy, the same frequency, and the same wavelength
the same energy, the same frequency, and a different wavelength
the same energy, a different frequency, and the same wavelength
a different energy, the same frequency, and the same wavelength
the same energy, the same frequency, and the same wavelength
[QUIZ] When a scattered photon is deflected back toward the source, it is traveling in the direction opposite to the incident photon and is called backscatter radiation.
TRUE/FALSE
TRUE
[QUIZ] What is the total differential absorption in bone compared to fat?
bone Z# 13.8 and mass density 1900 kg/m3
fat Z# 6.8 and mass density 910 kg/m3
- 51
- 05
- 02
- 5
17.5
[QUIZ] During which interaction with matter is the x-ray photon converted to matter in the form of two electrons?
pair production
Compton scattering
photoelectric absorption
coherent scattering
pair production
[QUIZ] In which element are the inner-shell electrons more tightly bound to the nucleus?
mercury (Z = 80)
tungsten (Z = 74)
lead (Z = 82)
chromium (Z = 24)
lead (Z = 82)
[QUIZ] Which of the following interaction has a significant impact on the x-ray image?
photoelectric absorption
coherent scatter
pair production
photodisintegration
photoelectric absorption
[QUIZ] Which atomic shell possesses the highest binding energy?
K
L
M
all atomic shells possess the same binding energy
K
[QUIZ] Which of the following interactions with matter results in a radiograph with a short scale of contrast(high contrast black and white image)?
Compton scattering
coherent scatter
photoelectric interactions
photodisintegration
photoelectric interactions
[QUIZ] When x-ray photons interact with matter and change direction, the process is called
absorption
scatter
radiation
binding energy
scatter
[QUIZ] Compton scattering occurs when an incident x-ray photon interacts with a ___ electron.
loosely bound inner-shell
tightly bound inner-shell
loosely bound outer-shell
tightly bound outer-shell
loosely bound outer-shell
[QUIZ] Which of the following interactions with matter results in a radiograph with a long scale of contrast (low contrast gray image)?
Compton scattering
coherent scatter
photoelectric interactions
photodisintegration
Compton scattering
[QUIZ] When an x-ray passes through matter, it undergoes a process called
radiation
filtration
attenuation
fluoroscopy
attenuation
[QUIZ] Secondary radiation is created after a ________ interaction.
Pairs production
Photoelectric absorption
Photodisintegration
Compton’s scatter
Photoelectric absorption
[QUIZ] As kVp increases, the total number of photons transmitted without interaction increases.
TRUE/FALSE
TRUE
[QUIZ] During Compton scattering, the incident photon energy is divided between the
ejected electron and the scattered photon
incident photon and the scattered photon
incident photon and the recoil electron
recoil electron and the photoelectron
ejected electron and the scattered photon
[QUIZ] The electron dislodged during Compton scattering is called the
photoelectron
recoil electron
incident electron
lost electron
recoil electron
[QUIZ] Some x-ray photons may pass through matter and not interact with that matter at all.
TRUE/FALSE
TRUE
[QUIZ] Secondary radiation energies are highest for which element?
hydrogen
carbon
barium
oxygen
barium
[QUIZ] During the photoelectric interaction, as an electron transfers from an outer shell to a vacant inner shell, energy is released in the form of
a characteristic photon
a primary radiation
an incident electron
an incident photon
a characteristic photon
[QUIZ] During photoelectric absorption, a/an ___ shell electron is typically ejected.
inner-
middle-
outer-
all of the above
inner-
[QUIZ] An interaction that occurs between very low energy x-ray photons and matter is called ___ scatter.
coherent
classical
unmodified
all of the above
all of the above
[QUIZ] Unwanted densities caused predominantly by scattered photons are called
kilovoltage
radiation fog
amperage
pair production
radiation fog
[QUIZ] During the process of attenuation, the x-ray photon may interact with
the nucleus
the entire atom
an orbital electron
all of the above
all of the above
Why do we use screens?
- How do screens work?
- What does it take in and give off?
Decreases patient dose
The PMT (photomultiplier tube), the PD (photodiode, takes in the light and gives it off as an electronic signal)
converts remnant x-rays into light
What were original screens made of?
• More modern scintillators are from what group?
ORIGINAL = calcium tungstate (CaW)
• MODERN = rare Earth screens are principally gadolinium, lanthanum, and yttrium
What material can the phosphor layer be made of?
• Is one better than the other and why?
CaW and rare Earth
rare Earth is better; lower atomic number and k-shell absorption edge
• What controls the speed of a screen?
o How can we build it differently to change its response to radiation in terms of:
- Thickness
- Size
- Concentration
- Reflective layer vs. absorbing dyes
THICKNESS = The thicker the phosphor layer, the higher is the detective
quantum efficiency. High-speed screens have thick phosphor layers; fine-detailed screens have thin phosphor layers.
SIZE = Larger individual phosphor crystals produce more light per x-ray interaction. The crystals of detail screens are approximately half the size of the crystals of
high-speed screens.
CONCENTRATION = Higher crystal concentration results in
higher screen speed.
REFLECTIVE LAYER = The presence of a reflective layer increases screen speed but
also increases image blur.
DYE = Light-absorbing dyes are added to some phosphors to control the spread of
light. These dyes improve spatial resolution but reduce speed.
The original screens, CaW were what speed?
• Regular screens with film were what speed?
– Faster or slower?
Calcium tungstate screens, par speed, are rated at 100, all others are compared to this, even CR/DR.
Three speeds with film
50 - 1200 speed
What is IF?
How do you calculate it?
Intensification factor, measure of speed in the screen
exposure without screens
/
exposure with screens
Are CR and DR set speeds or can that be changed?
CR and DR are set speeds and cannot be changed
Fluorescence vs. phosphorescence: which is which, which one do we want for imaging?
Fluorescence: light is emitted only while phosphor is stimulated.
Phosphorescence: light is emitted during and after stimulation.
• Also known as screen lag or afterglow. Not a good thing!
What are DQE and CE; do we want them to be high, low moderate?
• What do they affect in terms of technique and noise?
percentage of x-rays absorbed by the screen
we want high DQE and moderate CE
↑ CE = ↑ image noise
↑ DQE = no increase noise
What can we increase to fix a noisy image?
kVp
System speed: effect on resolution or detail
more speed = less resolution/detail
less speed = more resolution/detail
Why do we like Rare earth materials over CaW? (Hint: its about the k edge)
All rare earth screens have a lower atomic number and k-shell absorption edge.
Rare earth screens probability before and after the k-shell binding energies is lower than tungstate
however between the k-shell binding energies (35-70keV) it is much higher, absorbing five times as much radiation.
If given LP/mm, calculate object size
If: 10 LP/mm, then 1 object = 0.10mm
If: 5 LP/mm, then 1 object = 0.20mm
- The principle advantage of direct exposure verse scintillators imaging is:
a. Decreased patient dose
b. Shorter scale of contrast
c. Increased detail
d. Decreased technical factors (mAs)
c. Increased detail
- The emission of light from a phosphor after the incident radiation exposure is termed:
a. Fluorescence
b. Polarization
c. Phosphorescence
d. Incandescence
c. Phosphorescence
- The amount of x-ray absorption in a phosphor is primarily dependent upon the:
a. Pairs production
b. Photoelectric effect
c. Compton effect
d. Thomson effect
b. Photoelectric effect
- The effect of quantum mottle is most commonly seen when a _____ is employed.
a. Direct exposure system
b. Slow speed system
c. Fast speed system
d. None of the above
c. Fast speed system
- Rare earth phosphors are a significant improvement over calcium tungstate due in large part to the improved _________.
a. Conversion efficiency
b. Spectral matching
c. Phosphorescence
d. Quantum mottle
a. Conversion efficiency
- As phosphor size increases, recorded detail will tend to____________.
a. Increase
b. Decrease
c. Stay the same
b. Decrease
- The primary advantage of the rare earth-phosphors used in modern systems is:
a. Improved resolution
b. Reduced patient dose
c. Lower cost
d. Lower noise
b. Reduced patient dose
- The resolution of a scintillation layer may be improved by using
a. Thicker layers
b. Light absorbing dyes
c. Smaller phosphors
d. Thinner layers
d. Thinner layers
- For maximum efficiency of a scintillators, the light emitted by the phosphor should correspond to the PMT/PD’s light sensitivity. This is known as:
a. Spectral matching
b. Homogeneity
c. Conversion efficiency
d. Synergism
a. Spectral matching
- The principal function of a phosphor is to convert
a. Light to electrons
b. X-rays to visible light
c. Visible light to x-rays
d. Electrons to visible light
b. X-rays to visible light
- A high Z# material, lead foil is built into the back of a cassette to reduce
a. Reduced backscatter
b. Focus the x-ray beam
c. Create an air gap
d. Improve resolution
a. Reduced backscatter
- The resolution, detail, is generally measured in units of:
a. Lines pair per inch
b. Lines pair per millimeter
c. Candela per inch
d. Candela per millimeter
b. Lines pair per millimeter
Image noise appears on a radiograph as a speckled background. It occurs most often when _____ screens and _____-kVp techniques are used.
Image noise appears on a radiograph as a speckled background. It occurs most often when fast screens and high-kVp techniques are used.
