X-ray beam Flashcards
The size of the focal spot is not altered with a rotating anode, but the
actual physical area of the target bombarded by electrons is constantly changing, causing a greater area—a focal track—to be exposed to electrons.
Because of the larger area of the target being bombarded during an exposure
the rotating anode is able to withstand higher heat loads produced by greater exposure factors
Rotating anode x-ray tubes are used in all applications in radiography, whereas stationary
anode tubes are limited to studies of small anatomic structures such as teeth.
Rotating anodes can withstand higher heat loads than stationary anodes because
the rotation causes a greater physical area, or focal track, to be exposed to electrons.
The components necessary for x-ray production are housed in
a glass or metal envelope
Metal envelopes are more commonly used because of
their superior electrical properties.
A disadvantage of a glass envelope x-ray tube is
that tungsten evaporated from the filament during exposure can be deposited upon the inside of the glass, especially in the middle portion of the envelope. This evaporation can affect the flow of electrons and cause the tube to fail. Replacing this section of glass with metal prevents these problems and extends the tube life.
An additional advantage of a metal envelope is
the reduction of off-focus radiation.
Off-focus radiation occurs when projectile electrons are
reflected and x-rays are produced from outside the focal spot.
The metal tube envelope can collect
electrons and conduct them away from the anode.
The envelope allows air to be completely evacuated from the x-ray tube, which in turn allow
an efficient flow of electrons from cathode to anode.
The envelope serves two additional functions: i
provides some insulation from electrical shock that may occur because the cathode and anode contain electrical charges and it dissipates heat in the tube by conducting it to the insulating oil surrounding the envelope
The purpose of insulating oil is to
provide more insulation from electrical shock and to help dissipate heat away from the tube.
All of these components are surrounded by metal tube housing on all sides except for
a port, or window, which allows the primary beam to exit the tubE
What does the radiographer observes and handles when moving the X-ray tube
It is the metal tube housing
Leakage radiation refers to
any x-rays, other than the primary beam, that escape the tube housing.
The tube housing is required to allow a leakage radiation of no more than
100 mR/hr to escape when measured at a distance of 1 m from the source
Production of X-rays
As electrons strike the target, their kinetic energy is transferred to the tungsten atoms in the anode to produce x-rays.
Two types of interactions produce x-ray photons
Brems and characteristic
Bremsstrahlung interactions occur when
a projectile electron completely avoids the orbital electrons of a tungsten atom and travels very close to its nucleus.
What range is brems interaction
30-150kVp
100% of the x-ray beam results from bremsstrahlung interactions.
Below 70 kVp (with a tungsten target),
approximately 85% of the beam results from bremsstrahlung interactions.
Above 70 kVp,
Characteristic interactions are produced when
a projectile electron interacts with an electron from the inner shell (K-shell) of a tungsten atom.
K-shell electrons in tungsten have the
strongest binding energy at 69.5 keV
X-ray energy is measured in
kiloelectron volts (keV) (1 keV = 1000 electron volts).
x-ray emission spectrum
The x-ray beam is polyenergetic (has many energies) and consists of a wide range of energieS
The lowest energies are always approximatel
15 to 20 keV
the highest energies are always equal to
the kVp set on the control panel
Deadman switches
A radiographic exposure is produced by a radiographer using two switches located on the control panel of the x-ray unit. These are sometimes combined into a single switching device that has two levels of operation corresponding to rotor preparation and x-ray exposure.
The first switch is usually called the
rotor, or prep button,
the second switch is usually called
exposure, or x-ray button.
Pushing the rotor, or prep button, causes an
electrical current to be induced across the filament in the cathode
filament current
approximately 3 to 5 A and operates at approximately 10 V.
The amount of current flowing through the filament depends on
the mA set at the control panel.
filament current heats the
tungsten filament.
Thermionic emission refers to
When the tungsten filament gains enough heat (therm), the outer-shell electrons (ions) of the filament atoms are boiled off, or emitted, from the filament
space charge.
The electrons liberated from the filament during thermionic emission form a cloud around the filament
The space charge effect refers to
the tendency of the space charge to prevent more electrons to be boiled off of the filament.
Tube current refers to th
the flow of electrons from cathode to anode and is measured in units called milliamperes (mA)
When the rotor, or prep button, is activated:
On the Cathode Side of the X-ray Tube
- The filament current heats up the filament.
- This heat boils electrons off of the filament (thermionic emission).
- These electrons gather in a cloud around the filament (space charge).
- The negatively charged focusing cup keeps the electron cloud focused together.
- The number of electrons in the space charge is limited (space charge effect).
On the Anode Side of the X-ray Tube
- The rotating target begins to turn rapidly, quickly reaching top speed.
As electrons strike the anode target, more than
99% of their kinetic energy is converted to heat, whereas less than 1% of their energy is converted to x-rays.
After activation of the rotor and the exposure is initiated:
On the Cathode Side of the X-ray Tube
- High negative charge strongly repels electrons.
2. These electrons stream away from the cathode and toward the anode (tube current).
On the Anode Side of the X-ray Tube
- High positive charge strongly attracts electrons in the tube current.
- These electrons strike the anode.
- X-rays and heat are produced.
The kilovoltage (kVp) that is set by the radiographer and applied across the x-ray tube at the
speed at which the electrons in the tube current move.
The speed of the electrons traveling from the cathode to the anode _____ as the kilovoltage applied across the x-ray tube______
Increase /increases
The speed of the electrons in the tube current determines the _____ or ____of the x-rays that are produced. The quality or energy of the x-rays in turn determines the _____ lof the primary beam.
Quality /energy
Penetrability
As kVp _____, beam penetrability ____
Increases/increases
kVp and X-ray Quality
- Higher kVp results in electrons that move faster in the tube current from the cathode to the anode.
- The faster the movement of the electrons in the tube current, the greater the energy of the x-rays produced.
- The greater the energy of the x-rays produced, the greater the penetrability of the primary beam.
- The quality of the x-ray beam refers to its energy level; hence, adjusting kVp affects the quality of the x-ray beam.
The term voltage ripple describes
describes voltage waveforms in terms of how much the voltage varies during x-ray production.
Milliamperage (mA)
is the unit used to measure tube current.
Quality-Control Check: Kilovoltage Accuracy
X-ray quality can be affected if the actual kilovoltage used is inaccurate.
• A digital kVp meter measures the actual kilovoltage, and a Wisconsin Test Cassette estimates the kilovoltage by measuring densities (blackness) produced on a film.
• The maximum variability of the kilovoltage is ±5%.
Voltage waveforms produced by various x-ray generators
A, Single phase. B, Three phase. C, High frequency.
The quantity (amplitude) and the quality (shift to the right) of the x-ray beam are_____when using high-frequency and three-phase generators because they are more efficient in x-ray production.
Increased
mA and X-ray Quantity
- Higher mA results in more electrons moving in the tube current from the cathode to the anode.
- The more electrons in the tube current, the more x-rays produced.
- The number of x-rays produced is directly proportional to mA.
The quantity of electrons in the tube current and quantity of x-rays produced are directly or indirect proportional to the milliamperage.
Direct
Exposure time determines the
the length of time over which the x-ray tube produces x-ray
Exposure Time and X-ray Quantity
- Longer exposure time results in more electrons moving in the tube current from the cathode to the anode.
- The more electrons in the tube current, the more x-rays produced.
- The number of x-rays produced is directly proportional to the exposure time.
Quality-Control Check: Exposure Timer Accuracy
- X-ray quantity can be affected if the actual exposure time used is inaccurate. A digital timer device measures the actual exposure time. A synchronous spinning top test device estimates the actual time by measuring the density (blackness) arc produced on film with a timer protractor or by counting the number of black dots expected for the type of x-ray generator.
- The maximum variability of the exposure timer is ±5% for times >10 ms and ±10% for times <10 ms.
The quantity of electrons flowing from the cathode to the anode and the quantity of x-rays produced are directly/indirect proportional to the exposure time.
Direct
The quantity of electrons flowing from the cathode to the anode and the quantity of x-rays produced are directly/indirectly proportional to mAs.
Direct
mAs and X-ray Quantity
- Higher mAs results in more electrons moving within the tube current from the cathode to the anode.
- The more electrons in the tube current, the more x-rays produced.
- The number of x-rays produced is directly proportional to the mAs.
- mAs affects only the quantity of x-rays produced; it has no effect on the quality of the x-rays.
Three quality-control tests are typically performed with a
Dosimeter
Dosimeter
a device that measures x-ray exposure) to evaluate the radiation output by measuring the radiation intensity: reproducibility of exposure, mAs reciprocity, and milliamperage and exposure time linearity.
• Reproducibility of exposure verifies the
consistency of radiation intensity for changes in mA and exposure time with constant mAs. The maximum variability of reciprocity is ±10%.
• Milliamperage and exposure time linearity verifies that proportional changes in mA or exposure time or both likewise change the radiation intensity. Doubling the mA or exposure time should double the radiation intensity. The maximum variability of linearity is ____
+/-10%
The line-focus principle describes the relationship between
the actual focal spot, where the electrons in the tube current bombard the target, and the effective focal spot, which is the same area as seen from directly below the tube.
The actual focal spot size refers to the size of the area on
the anode target that is exposed to electrons from the tube current
The effective focal spot size refers to
focal spot size as measured directly under the anode target
A tube’s focal spot is an important factor because a large focal spot can withstand the heat produced by large exposures, whereas a small one produces better image quality.
Quality
The line-focus principle demonstrates how, by angling the face of the anode target, the actual focal spot can remain relatively ____while the effective focal spot is ____ in size.
Large/Reduced
Based on the line-focus principle, the _____the anode target angle, the ______ the effective focal spot size.
Smaller/smaller
the anode heel effect occurs because of the
angle of the target.
The heel effect describes how the x-ray beam has ____ intensity (number of x-rays) on the cathode side of the tube but a ___ intensity toward the anode side
Greater/lower
Low-energy photons serve only to ___ patient dose and do not contribute to image formation.
Increase
Reduction of the low-energy photons requires that filtration be added to the x-ray beam to ___or___ these photons.
Attenuated, absorbed
___is the material primarily used for this purpose because it absorbs more low-energy photons while the useful higher-energy photons can exit (
Aluminum
Inherent filtration refers to the filtration that is permanently in the path of the x-ray beam. Three components contribute to inherent filtration:
1) the envelope of the tube, (2) the oil that surrounds the tube, and (3) the window in the tube housing.
The current guidelines state that x-ray tubes operating at above 70 kVp must have a minimum total filtration of ____mm of aluminum or its equivalent. I
2.5
Increasing the amount of tube filtration ____ the x-ray beam quality because there is a greater percentage of x-rays that have high energy rather than low energy.
Increase
increasing tube filtration ____ the quantity of x-rays or x-ray emission
Decreases
Quality-Control Check: Beam Filtration
- Half-value layer (HVL), the amount of filtration that reduces the intensity of the x-ray beam to one-half of its original value, is considered the best method for describing x-ray quality.
- The HVL can be used as an indirect measure of the total filtration in the path of the x-ray beam. It is expressed in millimeters of aluminum (mm-Al).
- During the HVL test, a radiation-measuring device, such as a dosimeter, is used to measure both the radiation intensity of the original exposure and that following the addition of increasing millimeters of aluminum filtration in the path of the primary beam. The radiation intensity can be graphed for the increasing levels of aluminum filtration to determine the HVL.
- According to the NCRP Report #102, for equipment operated at or above 70 kVp, the required minimum total filtration should be at least 2.5 mm, which indicates the total filtration in the x-ray tube is adequate to protect patients from unnecessary low-energy radiation.
- Normal HVL of general diagnostic beams is 3–5 mm aluminum.
Beam Filtration
Low-energy photons, created during x-ray production, are unable to penetrate the patient. Patients are protected from unnecessary exposure to this low-energy radiation by the placement of ___and ___ filtration in the path of the x-ray beam.
Inherent,added
___can be added to the primary beam to alter its intensity. These types of filters are used to image anatomic areas that are non-uniform in makeup and assist in producing more consistent exposure to the image receptor.
Compensating filters
The most common type of compensating filter is a simple ___
Wedge filter
The thicker part of the wedge is lined up to the __part of the anatomy, allowing __xray photons to reach the end of the part
Thinner, fewer
the trough filter has a ___ wedge. A trough filter may be used for an AP projection of the thorax to compensate for the easily penetrated air-filled lungs.
Double
The generator has 3 phases :
Single phase=1.00
Three phase=1.35
High frequency=1.40
What are tube rating charts considered to be
Single exposure rating charts
A few simple but important guidelines for x-ray tube operation should be consistently adhered to by the radiographer to extend tube life:
- If applicable, warm up the tube according to the manufacturer’s specifications, especially if it has not been energized for 2 h or more.
- Avoid excessive heat unit generation. Repeatedly using exposure techniques near an x-ray tube’s limit increases the total number of heat units. Figure 2-23 shows anode targets that have been damaged as a result of excessive heat loading.
- Do not hold down the rotor button without making an exposure. Unnecessarily holding down the rotor button causes excessive wear on both the filament and the rotor.
- Use lower tube currents with longer exposure times when possible to minimize wear on the filament.
- Do not move the tube while it is energized. This movement can cause damage to the anode and anode stem as a result of torque, the angular analog of force that acts to produce rotation.
- If the rotor makes noticeable noise, stop using the tube until it has been inspected by a qualified service personnel. Noises can be indicative of a potentially serious problem.
Which x-ray tube component serves as a source of electrons for x-ray production? A. Focusing cup B. Filament C. Stator D. Target
B
2. Electrons interact with the \_\_\_\_\_\_ to produce x-rays and heat. A. focusing cup B. filament C. stator D. target
D
The cloud of electrons that forms before x-ray production is referred to as \_\_\_\_\_\_. A. thermionic emission B. space charge C. space charge effect D. tube current
B
4. The burning or boiling-off of electrons at the cathode is referred to as \_\_\_\_\_\_. A. thermionic emission B. space charge C. space charge effect D. tube current
A
Which primary exposure factor influences both the quantity and the quality of x-ray photons? A. mA B. mAs C. kVp D. Exposure time
C
6. The unit used to express tube current is \_\_\_\_\_\_. A. mA B. mAs C. kVp D. s
A
What percentage of the kinetic energy is converted to heat when moving electrons strike the anode target? A. 1% B. 25% 41 C. 59% D. 99%
D
8. The intensity of the x-ray beam is greater on the \_\_\_\_\_\_. A. cathode side of the tube B. anode side of the tube C. short axis of the beam D. long axis of the beam
A
9. According to the line-focus principle, as the target angle decreases, the \_\_\_\_\_\_\_\_. A. actual focal spot size decreases B. actual focal spot size increases C. effective focal spot size decreases D. effective focal spot size increase
C
- ___________ extends x-ray tube life.
A. Selecting higher tube currents
B. Using small focal spots when possible
C. Producing exposures with a wide range of kVp values
D. Warming up the tube after 2 h of non-use
D
11. Which type of target interaction is responsible for most of the x-rays in the diagnostic beam? A. Characteristic interaction B. Thermionic emission C. Bremsstrahlung interaction D. None of the above
C
12. What value of mAs is produced when the radiographer sets a kilovoltage peak of 70 kVp, a milliamperage of 600 mA, and an exposure time of 50 ms? A. 3.5 mAs B. 30 mAs C. 300 mAs D. 350 mAs
B
13. Increasing the kVp results in \_\_\_\_\_\_. A. x-rays with higher energy B. x-rays with lower energy C. more x-rays D. A and C E. B and C
D
Total filtration in the x-ray beam includes \_\_\_\_\_\_. A. compensating filters B. inherent filtration C. added filtration D. B and C E. all of the above
D
How many heat units result from an exposure made on a single-phase x-ray unit using a beam current of 400 mA, an exposure time of 0.2 s, and a kilovoltage peak of 70 kVp? A. 5600 HU B. 7560 HU C. 7896 HU D. 8120 HU
A
