220 terms

Radiographic Equipment Operations

Chapter 8, 9 and 11 in Bushong, 10th Edition (X-Ray Emission, X-Ray Interaction With Matter, & Control of Scatter Radiation)
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X-ray intensity, x-ray quantity and radiation exposure are the same terms and are measured in what?
mGya - milligray in air (formerly milliroentgen - mR)
X-ray quantity is
The number of x-rays in the useful beam
Most general radiography tubes, when operated at 70 kVp, produce x-ray intensities of ____ uGya/mAs at a ____ 100cm SID
50, 100
Image contrast is ____ as kVp is ____
decreased, increased
Added filtration to the useful x-ray beam reduces what?
Patient dose
X-ray beam hardening increases the number of ____ by removing the ____
High energy x-rays, low energy x-rays
The penetrability of an x-ray beam is called the ____
X-ray quality
Penetrability is the ____
Ability of an x-ray beam to pass through tissue
X-rays of any given energy are more penetrating in material of ____ than in material of ____
low atomic number, high atomic number
Attenuation is the ____
Reduction in x-ray intensity
Attenuation is the result of ____
Absorption and scattering
The quality of x-rays is measured by the ____
HVL
The HVL of an x-ray beam is the ____
Thickness of absorbing material necessary to reduce the x-ray intensity to half its original value
How is the HVL theoretically determined?
Experimentally, using an x-ray tube, a radiation detector, and a graded thickness of filters, usually Aluminum
How is the HVL mathematically determined?
On a plot, find the number that is half the radiation dose of the dose from the filterless exposure. Draw a horizontal line from that number until it intersects the curve. Draw a line downward to the x-axis. Where it hits on the x-axis is the HVL
X-ray beam quality can be identified by kVp or filtration, but ____ is most appropriate
HVL
An increase in mAs has ____ effect on x-ray quality, and ____ x-ray quantity
no, increase
An increase in kVp ____ x-ray quality, and ____ x-ray quantity
increases, increases
An increase in distance has ____ effect on x-ray quality, and ____ x-ray quantity
no, reduces
An increase in filtration ____ x-ray quality, and ____ x-ray quantity
increases, reduces
What are the two factors that affect x-ray quality?
kVp and filtration
What are the three types of filtration?
Inherent filtration, added filtration, and compensating filtration
What is inherent filtration?
The glass or metal enclosure of an x-ray tube that filters the emitted x-ray beam and is usually 0.5mm Al equivalent
With age, inherent filtration tends to ____ due to ____
Increase, tungsten deposits
What is added filtration?
A thin sheet of Al positioned between the protective x-ray tube housing and the x-ray beam collimator
Added filtration ____ the HVL
Increases
Added filtration attenuates a greater number of ____ energy x-rays than ____ energy x-rays
low, high
The collimator contributes an additional ____ equivalent added filtration
1mm Al
What is a compensating filter?
A filter that compensates for differences in subject radiopacity
When is a compensating filter used?
When a body part is examined that varies greatly in thickness or tissue composition
Where is the thick portion of a compensating filter positioned?
At the thin portion of the body part
What are the five x-ray interactions with matter?
Coherent (Classical) scatter, Compton scatter, Photoelectric effect, Pair production, and Photodisintegration
____ and ____ are of particular importance to diagnostic radiology
Compton scatter, Photoelectric effect
Coherent scattering occurs with energies below ____ kVp and cause the atom to become ____. The atom immediately releases this excess energy as a _____ with a wavelength ____ to the ____ x-ray.
10, excited, scattered x-ray, equal, incident
What is different about Coherent scatter in comparison to the incident x-ray?
It changes direction
Most Coherent scattered x-rays are scattered in the ____ direction
Forward
X-rays with energy through ____ can experience Compton Scattering
The diagnostic range
In Compton scattering, the x-ray interacts with ____ electrons that not only scatters the x-ray but ____ its energy and ____ the atom as well.
outer-shell, reduces, ionizes
In Compton scattering, the x-ray interacts with an outer-shell electron and ____ it from the atom, thereby ____ the atom.
Ejects, ionizing
The ejected electron is called a ____ and continues in a ____ direction with less energy
Compton electron, different
The energy of the Compton-scattered x-ray is ____ to the ____ between the energy of the incident ray and the energy of the ____ electron
equal, difference, ejected
The energy of the ejected electron is ____ to its ____ energy plus the ____ energy with which it leaves the atom.
equal, binding, kinetic
What is the equation for the Compton effect?
E₁ = Es (Eb + Eke)
Where E₁ is energy of the incident ray, Es is energy of the scattered x-ray, Eb is electron binding energy, and Eke is kinectic energy of the electron
During Compton scatter, the ____ x-ray retains most of the energy
scattered
Both the scattered x-ray and the Compton electron have sufficient ____ to undergo additional ____ interactions before they lose all their energy
energy, ionizing
How dose the Compton electron lose its energy?
Through ionization and excitation and drops into an electron shell vacancy previously created by another ionizing event
Compton scattered x-rays can be deflected in ___ direction
Any
At a deflection of 0 degrees, ____ energy is transferred
No
As the angle of deflection increases to ____ degrees, more energy is transferred to the ____
180, Compton electron
At 180 degrees, the scattered x-ray retains at least ____ its original energy
Two thirds
X-rays scattered back in the direction of the incident x-ray beam are called ____
Back scatter
The probability of Compton scattering ____ as x-ray energy ____
decreases, increases
The probability of Compton scattering is _____ to x-ray energy and independent of _____
inversely proportional, atomic number
____ reduces image contrast
Compton scattering
In the Photoelectric effect, x-rays with energy through the _____ also undergo _____ interactions with ____ electrons
diagnostic range, ionizing, inner-shell
In the photoelectric effect, the x-ray is _____
Totally absorbed
The electron removed from the atom is called a _____
Photoelectron
The photoelectron escapes with kinetic energy _____ to the difference between the energy of the _____ and the _____
equal, incident ray, binding energy
What is the equation for the photoelectric effect?
E₁ = Eb + Eke
Where E₁ is the energy of the incident ray, Eb is the electron-binding energy, and Eke is the kinetic energy of the electron
The ____ is total x-ray absorption
Photoelectric effect
____ x-rays are produced after a photoelectric interaction
Characteristic
In the photoelectric effect, a ____ electron is ejected, resulting in a vacancy immediately corrected when a ____ electron drops into the vacancy
K-shell, L-shell
Characteristic x-rays have energy ____ to the binding energies of the shells involved
equal
Characteristic rays resulting from the photoelectric effect consist of _____ radiation and behave the same as _____ radiation
secondary, scattered
The probability of the photoelectric effect is _____ to the ____ power of the x-ray energy
inversely proportional, third
The probability of the photoelectric effect is _____ to the third power of the _____ of the absorbing material
proportional, atomic number
To remove a k-shell electron, the x-ray must have a keV that is ____ or ____ than the k-shell electron binding energy
equal to, higher
Differential absorption occurs due to what three factors?
Compton scattering, photoelectric effect, and x-rays transmitted through the patient
Which factor of differential absorption contributes no useful information to the image?
Compton scatter
Scattered x-rays result in ____
Image noise
What is image noise?
A dulling of the image by x-rays not representing diagnostic information
____ provides diagnostic information to the IR by representing anatomical structures that are ____
Photoelectric effect, radiopaque
The photoelectric absorption of x-rays produces the ____ areas in an image, such as ____
light, bone
Other x-rays penetrate the body and are transmitted to the IR with ____ interaction and produce the ____ areas in an image when they pass through ____ structures
no, dark, radiolucent
What is differential absorption?
The process of the difference between x-rays absorbed photoelectrically in the patient and those transferred to the IR resulting in an image
Approximately what percent of incident x-rays reach the IR?
1%
What percent of those x-rays that reach the IR interact to form an image?
0.5%
Differential absorption ____ as the kVp ____
increases, decreases
Name one negative to reducing kVp to increase differential absorption and image contrast.
Patient dose is increased
When kVp is increased, Compton scatter ____ but not as rapidly as ____
decreases, photoelectric effect
At low energies, most x-ray interactions with tissue are ____, and at high energies, most x-ray interactions are ____
photoelectric, Compton
To image small differences in soft tissue, a ___ kVp needs to be used to get maximum differential absorption
low
The crossover point between photoelectric effect and Compton scattering for bone is approximately ____
40 keV
Differential absorption between bone and soft tissue results from ____ interactions, which greatly depend on ____
photoelectric, anatomic number
The loss of contrast due to noise is caused by ____
Compton scattering
What is mass density?
The quantity of matter per unit volume in kg/m³ or g/cm³
The interaction of x-rays with tissue is ____ to the mass density of the tissue regardless of ____
proportional, the type of interaction
When mass density is doubled, the chance for x-ray interaction is _____
doubled
An interaction such as the photoelectric effect is called an ____
Absorption process
An absorption process is an ____ concept
all or nothing
The Compton interaction is a ____ process
partial absorption
What is attenuation?
The total reduction in the number of x-rays remaining in an x-ray beam after penetration through a given thickness of tissue (pg 159)
When a broad beam of x-rays is incident on any tissue, some of the x-rays are ____, and some are ____.
absorbed, scattered
What are the two types of x-rays responsible for the OD and contrast on an image?
Those that pass through the patient without interacting, and Compton scattered x-rays
X-rays that exit from the patient are ____ x-rays, and those that exit and interact with the IR are ____ x-rays
remnant, image forming
What reduces patient dose and improves contrast resolution?
Collimation
What are the three factors that influence the relative intensity of scatter radiation that reaches the IR?
kVp, field size, and patient thickness
Scatter radiation ____ as x-ray beam field size ____
increases, increases
Reduced scatter radiation results in ____ radiographic OD, which must be compensated for by ____ technique
lower, increasing
What are three positives of tissue compression?
Improves spatial resolution, contrast resolution and lowers the patient radiation dose
One of the most important characteristics if image quality is ____
Contrast
What is contrast?
The visible difference between the light and dark areas of an image
What is the difference between contrast and contrast resolution?
Contrast is the degree of difference in OD between areas of a radiographic image, and contrast resolution is the ability to image and distinguish soft tissues
Most ____ x-rays are scattered
Remnant
If you used only transmitted, unscattered x-rays, the image would be ____
Very sharp with high contrast
If you used only scatter radiation with no transmitted x-rays, the image would be ____
Dull grey with very low contrast
If you use both scatter and transmitted radiation, the image has ____
Moderate contrast
What are the two types of devices that reduce the amount of scatter radiation that reaches the IR?
Beam restrictors and grids
What are the three types of beam restrictors?
Aperture diaphragm, cones or cylinders, and the variable-aperture diaphragm
What is the aperture diaphragm?
A simple lead or lead-lined metal diaphragm that is attached to the x-ray tube head with an opening that covers just less than the size of the IR used
Which two dedicated systems use an aperture diaphragm?
The trauma unit and the chest radiography unit
What are cones and cylinders?
Extensions/modifications of the aperture diaphragm
What is a difficulty regarding cones and cylinders?
Alignment
What is cone cutting?
When the x-ray source, cone and IR are misaligned and one side of the radiograph may not be exposed due to cone interference
What is the variable-aperture diaphragm?
The light-localizing collimator
What type of beam restricting device is most common?
Variable-aperture diaphragm
What is off-focus radiation?
X-rays produced when electrons stray and interact at such positions on the anode other than the focal spot
What does off-focus radiation do to the image?
Increases blur
Off-focus radiation is controlled by ____ in the variable-aperture diaphragm
Shutters
The light field in a variable-aperture diaphragm is produced using a ____ and ____
small lamp, mirror
The light field must coincide exactly with the ____
X-ray beam
Automatic light-localizing collimators are called ____
Positive beam limiting devices (PBL)
What does the PBL do?
Restricts the x-ray beam to the size of the IR in use or smaller
What is the equation for total filtration?
Total filtration = inherent filtration + added filtration
What is a radiographic grid?
A carefully fabricated section of radiopaque material (grid strip) alternating with radiolucent material (interspace material)
Where is a grid positioned?
Between the patient and the IR
The grid is designed to transmit only x-rays whose direction is on a ____ from the x-ray tube target to the IR
Straight line
____ radiation is absorbed by the grid material which is called ____
Scatter, clean up
What is the equation for grid surface x-ray absorption?
% x-ray absorption = (width of strip ÷ {width of grid strip + width of grid interspace}) x 100
High quality grids can attenuate what percentage of scatter radiation?
80-90%
What are the three important dimensions of a grid?
The thickness of the grid strip (T), the width of the interspace material (D), and the height of the grid (h)
What is the equation for grid ratio?
Grid ratio = height of grid (h) ÷ width of interspace material (D)
Which grid is more effective with scatter cleanup, one with a high ratio or one with a low ratio and why?
High ratio because the angle of scatter allowed is less than that permitted by low ratio
High-ratio grids ____ patient radiation dose
Increase
___ ratio grids should be used with high kVp
High
What is grid frequency?
The number of grid strips per cm
Which grid frequency shows less distinct grid lines on an image, high frequency or low frequency?
High frequency
The use of high frequency grids require ____ radiographic technique and results in a ____ patient radiation dose
high, increased
What is the equation for grid frequency?
Grid frequency = 10,000 μm/cm ÷ ({T + D}μm/line pair)
Where T is the thickness of the grid strip, and D is the width of the interspace material
What is the purpose of interspace material?
To maintain a precise separation between the delicate lead strips of the grid
As the interspace material, which produces less visible lines on the image, aluminum or plastic fiber?
Aluminum
Which interspace material, aluminum or plastic fiber, absorbs more primary x-rays, especially at low kVp?
Aluminum
At a low kVp, aluminum interspace can increase the patient dose by approximately ____% because higher ____ is required
20%, mAs
What are two advantages of using aluminum as an interspace material over plastic fiber?
Aluminum is nonhygroscopic and it is easier to manufacture into high-quality grids
What is the most common material used for the grid strips?
Lead
What is the principal function of a grid?
To improve image contrast
What does the contrast improvement factor tell us?
The ability of the grid to improve image contrast
What is the equation for contrast improvement factor (k)?
k = image contrast with grid ÷ image contrast without grid
A contrast improvement factor of 1 indicates what?
No improvement
What is contrast improvement factor a function of?
The x-ray emission spectrum, patient thickness, and tissue irradiated
The contrast improvement factor is ___ for high-ratio grids
Higher
The quantity of image forming x-rays transmitted through a grid is much ____ than that of image-forming x-rays incident on the grid
Less
What is the Bucky factor (aka grid factor)?
The amount of increase in radiographic technique required to produce an equal image when a grid is used
What is the equation for the Bucky factor (B)?
B = incident remnant x-rays ÷ transmitted image-forming x-rays
or
B = patient dose with grid ÷ patient dose without grid
The higher the grid ratio, the ____ the Bucky factor
Higher
The Bucky factor ____ with increasing kVp
increases
As the Bucky factor increases, radiographic technique and the patient radiation dose increase ____
Proportionately
What are the four different grid types?
Parallel grid, Crossed grid, Focused grid, and Moving grid
Grid cut off is common with the parallel grid. What is grid cut off?
The undesirable absorption of primary x-rays by the grid
The attenuation of primary x-rays becomes ____ as the x-rays approach the edge of the IR
greater
Grid cut off is most pronounced with parallel grids when a ____ is used or a ____
short SID, large-area IR
What is the equation for grid cutoff?
Distance to cut off = SID ÷ grid ratio
Parallel grids clean up scatter radiation in ____ along the axis of the grid
One direction
Crossed grids have lead strips that run ____ to the long and short axis of the grid
Parallel
Which type of grid is usually made by sandwiching two parallel grids together with the strips perpendicular to each other?
Crossed grid
Crossed grids have a ____ contrast improvement factor than parallel grids
Higher
The main disadvantage of parallel and crossed grids is ____
Grid cutoff
What are the three main disadvantages of crossed grids?
The CR must coincide with the center, tilt-table techniques require the x-ray tube and table to be aligned, and higher exposure technique is required
Which grid is designed to minimize grid cutoff?
Focused grid
The focused grid has lead strips that lie on the _____
Imaginary radial lines of a circle
High ratio grids allow for ____ error than low ratio grids
Less
Which type of grid best removes visible grid lines from the image?
Moving grid (aka Potter-Bucky diaphragm)
What are the two types of moving grids?
Reciprocating and oscillating
What does a reciprocating grid move by?
A motor
What does an oscillating grid move by?
Springlike devices and an electromagnet
The reciprocating grid moves ____ while the oscillating grid moves ____
back and forth, in a circular fashion
What are the three disadvantages to moving grids?
Mechanism is subject to failure, magnification, and image blur
____ grids are the grids of choice
Moving
The most frequent error in grids is ____
Improper positioning
What are the four common grid misalignments?
Off-level grid, off-center grid, off-focus grid, upside-down grid
What occurs in an off-level grid?
The CR is incident on the grid at an angle and all incident x-rays will be angled, and grid cutoff will occur across the entire image
Aside from x-ray tube misalignment, when does off-level grid occur?
During horizontal beam radiation if the grid tilts, or when the grid sinks into a patient's bed during portables
What occurs in off-center grid?
The grid is perpendicular to the CR but produces grid cutoff because it is shifted laterally
Off-center grid occurs most often with which grid type?
Focused
Any lateral shift resulting in grid cutoff is called ____
Lateral decentering
Off-focus grid occurs when ____
Radiographs are taken at SIDs unspecified for that grid
What is the most common improper grid position?
Combined off-center, off-focus grid
What occurs to the image during combined off-center, off-focus grid?
Uneven exposure - dark on one side, light on the other
Between parallel or focused grids, which is more common?
Focused grids
Selection of a grid with the proper ratio depends on what three factors?
kVp, degree of scatter radiation reduction, and patient radiation dose
As grid ratio increases, scatter radiation attenuation ____
Increases
In general, grid ratios up to ____ are satisfactory at tube potentials below ____
8:1, 90 kVp
Grid ratios above 8:1 are used when kVp exceeds ____
90 kVp
The use of a moving grid instead of a similar stationary grid requires approximately ____ more patient dose
15%
When compared with the use of low-kVp and low-ratio grids, the use of high-kVp and high-ratio grids results in ____ patient radiation dose and ____ image quality
lower, equal
When using a 5:1 grid, mAs must increase by ____
2x
When using a ____ grid, mAs must increase by 4x
8:1
When using a 12:1 grid, mAs must increase by ____
5x
When using a ____ grid, mAs must increase by 6x
16:1
What is the Air-Gap technique?
10-15cm of space is left between the patient and IR
When the Air-Gap technique is used, the ____ must be increased ____ for every cm of gap
mAs, 10%
What is the major disadvantage of the Air-Gap technique?
Magnification with associated focal-spot blur
For every ____ of extra part thickness, ____ the mAs to compensate
3cm, double
Photoelectric effect is ____ times more likely in bone and contrast media than Compton scattering
4-7x
The use of k-edge filters is also known as ____
Beam shaping
Noise occurs when ____
mAs is too low
What are two positive contributors to image formation?
Transmitted radiation and scintillation (light)
What are three negative contributors to image formation?
Fog, noise and artifacts
Fog is less noticeable in ____ exposed areas of image
Heavily
HVL ____ as beam quality increases
Increases
Negligent MRTs use what to hide bad/no collimation?
Digital shuttering
What is the purpose of digital shuttering?
To remove the white background and make viewing easy on the eyes
What is the diamentor/DAP meter?
A radiation detector inside the collimator that measures the dose per cm x exposure area
DAP calculates the ____ to patient's skin surface
total radiation exposure
When using a grid, the mAs must be increased _____ to compensate for lost exposure to the lead strips
3-6x
What two grid ratios are most common?
8:1 and 12:1
What is the typical grid frequency?
25-45 lines per cm
High frequency grids have ____ on image quality
No effect
Crossed grids are ____ efficient at eliminating scatter
Most
What are two positives to using the air-gap technique?
Similar or better scatter clean up than grids and a lower radiation dose to the patient
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